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1 DC Principles Study Unit DC Motor and Generator Theory By Robert Cecci

2 iii Preview DC motors and generators are widely used in industrial applications. Both motors and generators are devices that produce energy. A motor converts electrical energy from a power source into mechanical energy. A generator converts mechanical energy into electricity. DC motors are commonly used in industrial equipment. One reason is that DC motors are generally easier to control than AC motors. DC motors provide the mechanical energy needed to drive cranes, hoists, elevators, material handling equipment, conveyors, precision machinery, and measuring equipment. While storage batteries can be used to provide current for lighting and motors, they need continual recharging in order to remain effective. In contrast, a DC generator can provide a more economical source of electric power to run industrial devices. In this text, you ll learn how DC motors work and how they re controlled. Then, you ll learn about generator design and construction. When you complete this study unit, you ll be able to Describe the function of a commutator and brush assembly in a DC motor Explain how permanent magnet DC motors and stepper motors operate Identify series-wound, shunt-wound, and compound-wound motors and discuss their applications List the steps used to reverse a DC motor s direction Describe how the speed of a DC motor is controlled Explain the basic principle used to generate direct current List the factors that affect the strength of an induced voltage Explain how the field connections of series-wound, shunt-wound, and compoundwound generators differ Explain why it s necessary to shift brushes in a DC generator Discuss how interpoles and compensating windings can produce better generator operation List the various types of machine losses and calculate machine efficiency

3 v Contents DC MOTORS DC Motor Construction How a DC Motor Works Motor Types DC Motor Connections Interpoles Reversing a DC Motor Controlling a DC Motor GENERATOR BASICS Generating a Voltage by Electromagnetism Direction of Current Generator Construction Generator Operation Variations in Induced Voltage Commutator and Brush Action Field Connections Field Excitation Armature Reaction Counteracting Armature Reaction Eddy Currents Hysteresis Loss DC GENERATORS IN INDUSTRY Types of Machine Losses Machine Efficiency Voltage Regulation Operating Generators in Parallel POWER CHECK ANSWERS EXAMINATION

4 1 DC Motor and Generator Theory DC MOTORS DC Motor Construction When you walk through an average industrial plant, you ll very likely see many different motors used in a variety of applications. DC motors are used on cranes, elevators, conveyors, and on various process and batch control manufacturing equipment. A working knowledge of different types of DC motors, and how DC motors are controlled, is very important to an industrial electrician or electronic technician. While the construction of the different types of DC motors varies somewhat, all DC motors contain the same principal moving parts. Understanding what these parts are and how they work together will enable you to troubleshoot or repair a motor quickly and easily. Generally speaking, an electric motor converts electrical energy into turning motion. A motor uses a DC power supply and electromagnetic forces to cause an armature to turn, producing turning force, or torque. The turning force is then used to drive another machine or piece of equipment, such as a crane, hoist, pump, elevator, or even a generator. The principal parts of a motor are the following: A housing Field poles and field windings A rotating armature A commutator and brush assembly An output shaft Figure 1 shows an external and a cutaway view of a typical, small DC motor. This type of motor provides the turning motion to run a small machine. In Figure 1A you can see the outer case or housing that contains the rotating armature and the field poles. The front and rear covers of the housing are called the end bells. The end bells contain the bearings that hold and support the shaft on which the armature is mounted.

5 2 DC Motor and Generator Theory FIGURE 1 Figure 1A shows an external view of a DC motor. Figure 1B shows the rear end bell, the brushes, and brush holders. The manufacturer s nameplate is typically located on the side of the motor. The nameplate lists the manufacturer s specifications, such as the armature voltage, current, and revolutions per minute (RPM). This information is useful when repairs are needed. The output shaft is the turning part of the armature that s connected to another machine. The base is attached to a work surface to anchor the motor. The brushes and brush holders are located in the rear end bell. The brush holders located on each side of the motor housing hold the brushes in a stationary position inside the housing. A view of the rear end bell and the brush holders is shown in Figure 1B. Now, examine the simplified cutaway drawing of the motor in Figure 2A. Note the position of the armature, the field poles and windings, the commutator, and the brushes. The field windings are conductive coils wound around metal cores called the field poles. The field poles are securely mounted to the inside of the motor housing. The field windings are insulated from the housing and are connected to terminals that extend out through the housing. When current is applied to the field windings, a stationary magnetic field is generated between the field poles. The field windings are wound around each pole in the same direction to create a stationary magnetic field.

6 DC Motor and Generator Theory 3 FIGURE 2 Figure 2A is a cutaway view of a DC motor. Figure 2B is a cross-sectional view of the motor. Figure 2B is a cross-sectional view of a motor showing the field poles, field coils, armature, and armature windings. Note how the magnetic field (represented by the dashed lines) flows through the armature and into the motor housing. The armature (sometimes called the rotor) is a rotating device that s mounted on a shaft and positioned in the magnetic field between the field poles. The armature contains current-carrying conductors called windings wound around a laminated steel core. The armature windings fit into the slots in the surface of the armature core. Note that the windings are represented by circled dots and circled plus signs in Figure 2B. The windings represented by the circled dots are carrying current out of the page toward you. The windings represented by the circled plus signs are carrying current into the page away from you. The commutator is a round device made up of copper segments separated from each other by an insulating material (such as mica). The ends of each armature winding are connected to the copper segments of the commutator. The brushes are floating electrical contacts that slide over the surface of the commutator as the armature rotates. Most DC motors have between two and six brushes. Brushes are often made from natural graphite, a soft type of carbon.

7 4 DC Motor and Generator Theory FIGURE 3 This figure shows a side view and an end view of the armature from a typical DC motor. The brushes are held in place around the commutator by the brush holders. Springs inside the brush holders hold the brushes against the commutator with the correct pressure. The armature and commutator are illustrated in Figure 3. Figure 3A shows a side view of a typical armature. In this view, you can see the rear shaft that fits into the bearing in the rear end bell. Note the position of the commutator near the rear shaft. On the opposite end of the armature, you can see the output shaft. The output shaft passes through the bearing in the front end bell, through a hole in the end bell, and then out of the motor. This shaft can then be connected to a machine or load. Figure 3B is a rear view of the armature showing the commutator segments, the laminated steel core, and the armature slots. Figure 4A is a close-up view of the commutator and the armature windings. Figure 4B is a cross-sectional view of the armature, armature windings, and commutator segments. Note how the end of each armature winding is connected to a commutator segment.

8 DC Motor and Generator Theory 5 FIGURE 4 Figure 4A shows a close-up view of the commutator and armature windings. Figure 4B is a cross-sectional view of the armature showing the armature slots, armature windings, and the connections of the windings to the commutator. How a DC Motor Works A DC motor operates on the principles of electromagnetism. The field poles are positioned inside the motor housing with their opposite magnetic poles across from each other. A stationary magnetic field exists between the field poles (Figure 5). When current flows into an armature winding, the winding becomes energized, and a magnetic field develops around it (Figure 6). The magnetic field is positioned perpendicular to the winding. The winding then develops a north and a south pole. FIGURE 5 A stationary magnetic field exists between the field poles in a motor.

9 6 DC Motor and Generator Theory FIGURE 6 When an armature coil is energized, it develops a magnetic field all around it. The winding also develops a north pole and a south pole, as indicated in the figure. Now, let s combine all the forces together. Figure 7 shows an illustration of the field poles and field windings, the commutator and brushes, and one armature winding. The combination of electrical and magnetic forces shown here is known as the motor action of electromagnetic induction. To understand the motor action of electromagnetic induction, remember that opposite magnetic forces attract each other and like forces repel each other. In Figure 7, the north pole of the armature winding is attracted (pulled) to the south pole of the field magnet. At the same time, the south pole of the armature winding is attracted (pulled) to FIGURE 7 This simplified illustration shows a motor with one armature winding. Note the positions of the brushes, commutator, field poles, and armature winding. The interaction of the magnetic fields generated by the field winding and the current in the armature winding causes the armature to rotate.

10 DC Motor and Generator Theory 7 the north pole of the field magnet. These pulling forces cause the armature to rotate one half-turn in the direction shown by the arrows (clockwise). When the armature has rotated one half-turn, the north pole of the armature winding faces the south pole of the field magnet. Opposing magnetic forces are holding the armature winding in place. Thus, unless the magnetic poles are somehow reversed at this point, the armature will remain locked in this position. And, unless the armature turns continuously, the motor can t provide the turning force needed to drive a machine. So, what keeps the armature turning? The answer is the commutator and brush assembly. The commutator s function is to continuously reverse the direction of current flow into the armature winding. When the current is reversed, the magnetic poles of the armature winding reverse (north becomes south and south becomes north). Since like magnetic forces repel each other, the field magnet poles will repel the winding, forcing the armature to turn. Thus, while the field pole s magnetic field stays constant, the armature s magnetic field constantly switches polarity due to the action of the commutator. Look at the commutator assembly in Figure 7 again. Each end of the armature winding is connected to one set of commutator segments. These segments are located 180 degrees away from each other. Note that the brushes are also located 180 degrees away from each other. The purpose of the brushes is to conduct current from the external power source into the armature winding only when the armature winding is located next to the field poles. As the armature winding rotates away from the field poles, its commutator segment disconnects from the brushes. At the same time, the next commutator segment and its attached winding becomes connected to the brushes. Therefore, the brushes and commutator only allow current to flow in one direction in the armature winding that s adjacent to the field poles. The commutator action keeps a constant force on the armature windings, and the armature winding rotates continuously. In an actual motor, the connection of the armature windings is more complicated than the connections shown in Figure 7. Remember that a real DC motor contains many armature windings and could contain more than two field poles. The interaction of all the magnetic fields around these components creates the powerful turning force needed to run a motor. Fleming s left-hand motor rule can be used to illustrate the forces in a DC motor. Position your left hand as shown in Figure 8. In this position, the thumb points in the direction of the motion of the armature winding. The first finger points in the direction of the magnetic field, and the middle finger points in the direction of the current flow.

11 8 DC Motor and Generator Theory FIGURE 8 Fleming s left-hand motor rule can be applied to the forces in a DC motor. Motor Types Many different types of DC motors are used in industrial applications, from very small motors rated in ounce/inches of torque to large, powerful 1,000 horsepower (HP) motors. In this text, we ll group DC motors into two basic categories: precision and nonprecision motors. Nonprecision DC motors provide the turning force needed to operate cranes, elevators, and heavy conveyors. All of these machines need simple turning force to run, but they require little close control of speed, starts, and stops of the motor. In contrast, the speed and positioning of a precision motor can be accurately controlled through the use of an electronic control system. Precision motors are used on machine tools, inspection equipment, and on some process control equipment. Now, let s look at some special types of DC motors. Permanent Magnet Motors All DC motors require a magnetic field to operate. Most larger DC motors count on field windings to provide a stationary magnetic field. However, some motors contain strong permanent magnets bonded to the inside of the motor housing instead of field windings. These permanent magnet motors are usually found in small cordless tools and appliances. The power source in such a cordless appliance is a battery pack containing rechargeable nickel cadmium cells (NiCad cells). The output voltage of one NiCad cell is 1.2 V. Therefore, the armature voltage in these motors is either 2.4 VDC, 3.6 VDC, 4.8 VDC, 6.0 VDC, or 12.0 VDC (all multiples of 1.2 V). A small permanent magnet DC motor is shown in Figure 9.

12 DC Motor and Generator Theory 9 FIGURE 9 This permanent magnet DC motor contains four permanent magnets bonded to the motor s case instead of field windings. One advantage of a permanent magnet motor is that there are no field windings to energize with electricity. Current is delivered directly to the armature through the commutator and brushes. Thus, the motor draws less current from the battery. This allows for longer periods of operation between chargings. Permanent magnet DC motors aren t limited to cordless tools and appliances. Some newer-model permanent magnet motors are used as automotive starters. High-technology permanent magnet DC motors are also used in DC servo systems that control the positions of machine tools and other process equipment. In such a servo system, a tachometer and position feedback devices are connected to the motor to precisely control the motor s position and speed. Stepper Motors For many years, stepper motors were used to provide the precise motion needed to position machine tools and process equipment. A stepper motor is very similar to an electrical ratchet. Each pulse from the motor driver causes the motor s shaft to turn a set amount of degrees. Usually, the amount of movement is either 1.8 degrees (full step) or 0.9 degrees (half step). In recent years, stepper motors have generally been replaced by DC and AC servo systems. DC Motor Connections In different DC motors, the number of windings and brushes may differ. In addition, the electrical connections between the armature windings and the field windings may vary. These windings may be connected in series, in parallel (shunt), or in a combination of both (compound). The different connections provide different types of motor function. Let s take a closer look at each of these connection types. Series-Wound DC Motors In a series-wound or series-connected DC motor, the field windings are connected in series with the brushes connected to the armature windings.

13 10 DC Motor and Generator Theory A series-wound motor has a very high torque at start-up, and the speed of the motor is easy to control. Series-wound motors are widely used in cranes, elevators, and other heavy lifting equipment where lots of power is needed at the start-up to get the load moving. However, series-wound motors tend to slow down as the load increases. Also, if a load is suddenly removed, the motor turns at a higherthan-normal rate of speed. A schematic diagram of a series-wound DC motor is shown in Figure 10. The field windings in this type of motor consist of a few turns of very heavy gage wire around each pole shoe. Heavy gage wire is needed for the field windings and the armature windings because a large amount of current must pass through the windings and through the series-connected armature. FIGURE 10 This illustration shows the connection of the field winding and the armature in a series-wound motor. + Shunt-Wound (Parallel) DC Motors Shunt-wound or parallel-wound DC motors operate much differently than series-wound motors. In a shunt-wound motor, the field windings are connected directly across the armature windings (Figure 11). The field windings in a shunt-wound motor are made up of many turns of thin gage wire. The armature windings are also smaller gage wire. The parallel connection of the windings causes the current to be split between the field winding and the armature windings. Shunt-wound motors have a lower starting torque than series-wound motors, but shunt-wound motors have better speed control. So, when the load on a shunt-wound motor changes, the motor speed remains FIGURE 11 A shunt-wound DC motor has a shunt field connected in parallel with the armature.

14 DC Motor and Generator Theory 11 almost constant. Shunt-wound motors are used in applications that require constant speed, such as drilling machine spindles, lathes, and conveyor drives. One variation of the shunt-wound motor is the stabilized shunt motor. This motor contains an additional field winding connected in series with the parallel field winding and armature winding. This additional winding helps to further regulate motor speed against varying load conditions. Compound-Wound DC Motors A compound-wound DC motor contains both a series field winding and a parallel field winding connected to the armature. (This configuration is very similar to the stabilized shunt motor discussed in the previous section.) Both the series winding and the parallel winding are wound on the same field pole (not on separate field poles) so that the magnetic fields from each winding add together. The series winding is heavier gage wire than the parallel winding since the largest amount of current flows through the series winding. Compound-wound motors provide the advantages of fairly high starting torque and good speed regulation for varying loads. Figure 12A shows a schematic diagram of a compound-wound motor while Figure 12B shows a pictorial view. FIGURE 12 This figure contains both a schematic and a pictorial diagram of a compound-wound motor.

15 12 DC Motor and Generator Theory Interpoles One other connection variation you should be aware of is the addition of interpoles to a DC motor. In a motor, as one commutator segment moves out from under a brush and another makes contact with a brush, sparking can occur at the commutator. To reduce this sparking, interpoles can be placed in the motor. Interpoles are additional field windings placed on poles located midway between the field poles (Figure 13). FIGURE 13 Shown here is the location of the interpoles in a larger DC motor. The interpole coils are connected in series with the armature; therefore, they carry current that s proportional to the armature current. The interpole coils are wound so as to produce a magnetic field with a polarity opposite to the polarity of the magnetic field created by the armature current. Now, take a few moments to review what you ve learned by completing Power Check 1.

16 DC Motor and Generator Theory 13 Power Check 1 At the end of each section of your DC Motor and Generator Theory text, you ll be asked to check your understanding of what you ve just read by completing a Power Check. Writing the answers to these questions will help you review what you ve learned so far. Please complete Power Check 1 now. Write a short answer for each of the following questions. 1. What type of pole can be added to the field windings to prevent brush sparking in a DC motor? 2. The commutator of a DC motor is made up of many segments of copper. What type of insulating material is used between these segments? 3. What is another name for the armature? 4. Why are the field windings of a series-wound motor made up of a few turns of heavy gage wire? 5. What is the step angle of a DC-powered stepper motor if it s being driven at half step? Check your answers with those on page 49.

17 14 DC Motor and Generator Theory Reversing a DC Motor During the operation of a motor in an industrial setting, it s frequently necessary to reverse the direction of the motor. When a motor is reversed, the armature spins in the opposite direction. This reversal of the armature is periodically necessary to change the direction in which a piece of equipment is moving. Thus, for example, a motor runs in one direction to lift a crane; the armature direction must then be reversed to lower the crane. In DC motors that contain field windings, the armature direction can easily be reversed by reversing the polarity of either the field windings or the armature windings. This action is shown in Figure 14. Figure 14A shows standard electrical connections in a DC motor. The positive terminal of the power supply is electrically connected to field winding 1 and brush 2. The negative terminal of the power supply is electrically connected to field winding 3 and brush 4. The armature turns in a clockwise direction. In Figure 14B, the polarity of field windings 1 and 3 has been reversed. Field winding 3 is now connected electrically to brush 2, and field winding 1 is connected to brush 4. The armature turns in a counterclockwise direction. FIGURE 14 Shown here are two different methods of reversing a DC motor.

18 DC Motor and Generator Theory 15 In contrast, in Figure 14C, the polarity of the armature windings has been reversed instead of the field windings. This action also causes the motor to reverse and turn in the counterclockwise direction. In practice, it s much easier to reverse the polarity of the armature windings than to reverse the connections to the field windings in a typical compound DC motor. On older industrial equipment, motor reversing is done with special switches called drum switches, or with reversing relays called contactors. In modern DC-powered equipment, motors are reversed by electronic control systems that reverse the polarity of the voltage that s fed to the armature windings. In a permanent magnet DC motor, the polarity of the permanent magnets can t be changed. Thus, the motor can only be reversed by reversing the polarity of the voltage to the armature. When a permanent magnet DC motor is placed in a cordless appliance, a double-pole, double-throw switch is used to reverse the motor (Figure 15). By changing the switch position from the right to the left, you change the polarity of the voltage to the armature. An on-off switch is used to energize or deenergize the motor. Controlling a DC Motor FIGURE 15 By using a reversing switch, it s easy to make a permanent magnet DC motor reverse direction. In precision industrial applications, motor speed control is very important. There are many different methods of controlling the speed of a DC motor. The most common method is to control (vary) the current to the armature windings and to maintain a fixed current in the field windings.

19 16 DC Motor and Generator Theory Rheostat Control Systems On older industrial equipment, armature voltage was controlled by a high-power variable resistor known as a rheostat. The rheostat was connected in series with the armature (Figure 16). FIGURE 16 One method of controlling the speed of a DC motor is to place a rheostat in series with the armature as shown here. In this type of system, alternating current (AC) is supplied to a rectifier assembly. The rectifier converts the AC voltage to a smooth DC voltage. Part of this DC voltage is tapped to provide power for the field windings. The rest runs through a rheostat to the armature windings. The rheostat can be adjusted to allow varying amounts of current to enter the armature. When the rheostat is adjusted to a low resistance setting, a large amount of DC current passes into the armature windings and the motor turns at high speed. When the rheostat is adjusted to a high resistance setting, only a small amount of current passes into the armature winding, and the motor turns much slower. In some older industrial systems, many rheostats were placed in the armature circuit. Relay contacts or switch contacts were connected in series with the rheostats to select which rheostat would be in the armature circuit. In this type of arrangement, the control system was a simple multispeed DC motor drive system. There are many disadvantages to a rheostat control system. First, some of the electric energy is dissipated (lost) as heat. Second, the rheostat system doesn t allow the motor speed to be adjusted in increments it s either slow or fast. No smooth speed changes are possible. Finally, rheostat systems require too much maintenance to the relay or switch contacts and to the rheostat s wiper and windings. Silicon-Controlled Rectifier (SCR) Systems Later, another type of motor control system was developed called an infinite or stepless speed control system. The first type of stepless system used vacuum tubes to control the armature or field winding

20 DC Motor and Generator Theory 17 FIGURE 17 A simplified diagram of an SCR speed controller is shown here. currents. These systems have now largely been replaced with modern, solid-state stepless motor drive systems. Modern DC motor controllers use solid-state switching devices known as silicon-controlled rectifiers (SCRs), triacs, or transistors to control either the armature or field current. A block diagram of an SCR speed controller is shown in Figure 17. In an SCR speed control system, the field windings are powered by a constant voltage from a DC power supply. The armature voltage, however, is controlled by a triggering device and an SCR that s connected in series with the armature. If the SCR is triggered early or often in the AC cycle, a large amount of voltage reaches the armature. As a result, the armature turns fast. If the SCR is triggered late in the AC cycle, or at irregular intervals, a small amount of voltage reaches the armature, and it turns much slower. H-Bridge Control Systems A more elaborate speed controller called an H-bridge motor controller is used in permanent magnet servo motors. These motors must be very closely controlled so that they can offer exact positioning of machine tools, inspection, and process control equipment. An H-bridge system is shown in Figure 18.

21 18 DC Motor and Generator Theory FIGURE 18 Here s a simplified drawing of an H-bridge DC motor controller. Each transistor acts as a high-speed switch. An H-bridge motor controller is a type of switching transistor driver. Figure 18 shows the power driver stage of the motor controller. This stage contains four power transistors or high-current switching transistors. The base leads of the power transistors are located at points A, B, C, and D. These inputs are low-voltage inputs that turn the transistor on to apply power to the motor. When transistors A and D are turned on, the left side of the armature becomes positive and the right side becomes negative, causing the armature to turn in one direction. If transistors A and D are turned off and transistors B and C are activated, the left side of the armature becomes negative and the right side becomes positive, causing the armature to turn in the opposite direction. An H-bridge motor driver uses pulse-width modulation (PWM)to control speed. In PWM, the period of time that a group of two transistors is activated is varied to change the speed. This allows the armature speed to be closely controlled while maintaining full voltage to the armature windings. PWM signals for slow and fast speeds are shown in Figure 19. PWM signals are created at a special circuit within a servo motor controller system. If a motor needs to run at high speed, the motor controller first issues a PWM signal like the one in Figure 19A, then the controller increases the on time of the signal until it looks like the signal in Figure 19B.

22 DC Motor and Generator Theory 19 FIGURE 19 When comparing the two PWM signals, notice that the voltage remains constant while the on time of the voltage changes. PWM signals are applied to the base terminals of the power transistors. Special circuits within the motor driver allow only two transistors to be turned on at one time (such as A and D or B and C). This prevents a short circuit across the power supply. Field Weakening A final type of motor speed control is called field weakening. This type of control is generally offered for motors of two horsepower or more. In a field weakening system, the field winding voltage is varied instead of armature voltage to provide speed control. A steady DC voltage is fed to the armature at all times. This system works opposite from the way you think it should. When full voltage is applied to the armature and the field, the motor turns slowly. As the field voltage is reduced, the motor gains speed. This weakening action continues until the field voltage reaches a cutoff voltage at which no further speed increase can occur. Field weakening is often used to control spindle systems in machine tools such as boring machines and lathes. Now, take a few moments to check your learning by completing Power Check 2.

23 20 DC Motor and Generator Theory Power Check 2 Fill in the blanks in each of the following statements. 1. An H-bridge motor driver uses to control speed. 2. In a speed control system, the field winding current is varied instead of the armature voltage. 3. A/An motor controller is sometimes used in permanent magnet servo motors. 4. A motor control system doesn t allow the motor speed to be controlled in increments. 5. In DC motors that contain field windings, the armature direction can be reversed by reversing the polarity of either the or the. 6. A permanent magnet DC motor can only be reversed by reversing the polarity of the current to the. Check your answers with those on page 49.

24 DC Motor and Generator Theory 21 GENERATOR BASICS Generating a Voltage by Electromagnetism Generators and motors are very similar machines. The basic parts of a motor and a generator are the same. In fact, if the armature of a DC motor is turned mechanically, the motor will act as a generator and produce a voltage. Or, if a voltage is applied to the armature of a generator, it will begin to rotate and the machine will act as a motor. Therefore, the basic difference between a motor and a generator is that a generator converts mechanical energy into electrical energy, while a motor converts electrical energy into mechanical energy. In simple terms, electric current is produced inside a generator by the relative movement of a conductor through a magnetic field. When a conductor is moved through a magnetic field, a voltage is induced in the conductor. By relative movement we mean that either the conductor moves or the magnetic field moves in the generator. (In a real generator, the necessary relative movement of the magnetic field and the conductor is produced by some outside source of mechanical power, such as a gasoline or diesel engine.) The relative movement of the conductor and magnetic field induces a voltage on the conductor. This process is called the generator action of electromagnetic induction. The voltage produced in a conductor by electromagnetic induction can be used to perform useful work. If the energized conductor is connected to a complete electrical circuit, the voltage becomes moving current. The current can then be used to run other machines or equipment. The current produced by a generator in this manner is called the load current. Figure 20 shows a simplified diagram of a generator mechanism. A conductor is being moved upward through the magnetic field produced by the two field magnets. The galvanometer that s connected to the conductor indicates that a voltage is being induced on the conductor. Note that when the conductor is moved upward through the magnetic field, the current produced moves in a counterclockwise direction. If the conductor is moved downward through the magnetic field instead of upward, the current will flow in the opposite direction clockwise. Note that the current produced by the rotating coil-and-magnet device in a generator is always alternating current (AC). If direct current (DC) is needed for a particular application, the current output from the generator must be rectified to convert it from AC to DC. Most industrial applications require DC power.

25 22 DC Motor and Generator Theory FIGURE 20 The upward motion of the conductor in the magnetic field causes a current to be induced on the conductor. Direction of Current In a generator, the direction of the motion of a coil, the direction of the magnetic field, and the direction of conventional current flow (positive to negative instead of negative to positive), in the conductor are all related in a fixed way. If any two of these directions are known, the third direction can be determined by applying Fleming s righthand rule of electromagnetic induction (the generator rule). The rule is illustrated in Figure 21 and is stated as follows: Rule: Extend the thumb, the forefinger, and the middle finger of your right hand so that the middle finger is at a right angle to the thumb and forefinger. Point the thumb in the direction of the coil motion and the forefinger in the direction of the magnetic field. The middle finger now points in the direction of the induced voltage. The induced current flows in the same direction as the induced voltage.

26 DC Motor and Generator Theory 23 Note that according to this rule, if you reverse either the direction of the magnetic field or the direction of the motion of the coil, the direction of the induced voltage (and the resultant current) will also be reversed. You can understand Fleming s right-hand rule better if you apply it to Figure 20. In the figure, the coil is moving upward, as indicated by the thumb, and the magnetic field is directed from right to left, as indicated by the forefinger. The current flows toward you, as indicated by the middle finger. FIGURE 21 Fleming s right-hand rule shows the relationship between the direction of coil motion, the direction of the magnetic field, and the direction of conventional current flow in a generator. Generator Construction DC generators and DC motors are very similar in construction. Both types of machine contain the same basic parts (housing, armature, commutator, brushes, and so on). A simplified drawing of the parts of a DC generator is shown in Figure 22. The figure shows the position of the field poles and field windings, the commutator, brushes, and armature coil. Note that the field poles shown in this drawing are electromagnets conductive coils wound on iron cores. While permanent magnets can be used to generate the magnetic field in small generators and motors, they have disadvantages that limit their use in larger machines. For example, the magnetic strength of a permanent magnet decreases over time. Also, the strength of a permanent magnet can t be varied or controlled. For these reasons, electromagnets are used to supply the magnetic field in most DC generators. The coil of wire around the north field pole (N) and the south field pole (S) forms a field winding. When this winding is connected to a DC power source, current will flow into the winding and produce a magnetic field between the field poles. (Note that while this figure contains only two field poles, a real generator contains many field poles and windings equally spaced around the frame that holds the armature.)

27 24 DC Motor and Generator Theory The magnetic field that the electromagnets produce is called the main field. The current required to energize the electromagnets is called the field excitation. This current is provided by a device called an exciter. In most machines, the armature of the generator itself is the exciter. However, a generator s exciter may also be another generator, a battery, or a supply of rectified AC voltage. In Figure 22, the exciter connected to the field winding is a battery. FIGURE 22 The field winding in this generator produces the magnetic field. The strength of the magnetic field produced between the field poles depends on two factors: the number of turns in the field windings and the amount of current applied to the windings. Also, a field pole with an iron core produces more magnetic flux than a field pole without an iron core. FIGURE 23 The armature core of a DC generator is made of laminated steel punchings that are 1 64 in. (0.4 mm) thick. (Courtesy of Allis- Chambers Corporation)

28 DC Motor and Generator Theory 25 The rotating part of the generator, the armature, is shown in Figure 23. The armature core is made of laminations or punchings of special steel or iron. The core contains slots in which the armature conductors (the windings) are placed. The armature windings are spaced around the armature core in fixed positions. The ends of the windings are connected to the commutator. The armature is mounted on a shaft and positioned inside the magnetic field generated by the field poles. The armature shaft is held at each end by bearings in the end bells of the generator housing. These bearings must be strong enough to support the armature and also withstand the force exerted by the gear or belt used to drive the armature. The bearings may be ball or roller bearings (Figure 24)or sleeve bearings. Roller bearings are used for heavy-duty applications where the stress is too great for a ball bearing. A sleeve bearing is a cast-iron or steel shell with a Babbitt lining. Babbitt is a soft metal often used for bearing linings because it prevents cutting of the shaft. Bearing care is a major part of any generator maintenance program. FIGURE 24 Although ball and roller bearings are similar in construction, the roller bearing is used for heavier duty applications. (Courtesy of SKF Industries) The armature is turned by an outside mechanical force. In a very small generator, the turning force could be provided by a hand crank. However, the armatures of larger industrial generators are turned by steam or water turbines, gasoline or diesel engines, or even electric motors. A generator s commutator is a band divided into segments. The ends of the armature windings are connected to the commutator segments. The segments are insulated from each other and from the shaft by an insulating material, such as mica. Brushes are positioned around the

29 26 DC Motor and Generator Theory commutator to make a sliding electrical contact with the commutator surface. Adjustable springs are used to maintain proper brush pressure against the commutator. The brushes provide a conductive path for the current generated between the rotating and stationary parts of the machine. The brushes connect the commutator electrically to the external circuit. Low-voltage, high-current generators use brushes made of pulverized copper mixed with carbon. The brushes must be uniform in quality and must not groove the commutator segments. The brushes must be strong enough to prevent them from chipping or breaking due to machine vibrations. As the armature turns, the brushes should polish the commutator to a chocolate brown color, indicating that good contact is being maintained between the brushes and the commutator. Brush holders support and hold the brushes alongside the commutator. One brush holder may contain a set of several brushes if the current generated is too high for just one brush to handle. The entire assembly of brushes and brush holders is often called the brush rigging. Since the proper operation of DC machines depends greatly upon the brush rigging, it s vital that the person responsible for generator maintenance have a good understanding of brushes and brush holders. FIGURE 25 Two types of brush holders are shown here. Figure 25A is the box type and Figure 25B is the reaction type. The proper operation of DC generators depends greatly on the brushes and brush holders. (Courtesy of D. B. Flower Manufacturing Company)

30 DC Motor and Generator Theory 27 There are two basic types of brush holders: the box type and the reaction type. The box-type brush holder (Figure 25A) consists of a box, open at both ends, in which the brush is free to slide. If the box is too big, the brush will be loose in the box and vibrate; if the box is too small, the brush may stick. A reaction-type brush holder is shown in Figure 25B. The brush used in this type of holder must be set at an angle against the commutator; it s held against the commutator in either direction of rotation. The top of the brush (not shown) is beveled at a sharp angle, so that the spring which presses it against the commutator also holds it against one face of the holder. Brush pressure can be changed by adjusting the spring. Because the brush isn t as free to move up and down, reaction-type holders are seldom found on machines that vibrate considerably. Generator Operation The basic generator mechanism shown in Figure 26 contains one conductor loop called a single-turn coil. The single-turn coil is a basic armature that rotates within the magnetic field. The curved arrow at the top of the illustration indicates the clockwise rotation of the armature. The magnetic lines of force (imaginary lines that make up a FIGURE 26 As this single-turn armature is rotated in the magnetic field, an AC current is generated and measured by the galvanometer.

31 28 DC Motor and Generator Theory magnetic field) are indicated by the horizontal arrows pointing from the north field pole to the south field pole. The long straight sections of the coil are called coil sides. As the loop rotates, the coil sides cut across the magnetic lines of force, causing a current to flow in the coil. Essentially, each coil side operates as a separate straight conductor moving through a magnetic field. When the coil rotates in a magnetic field, a voltage is generated on each side of the coil. The ends of the coil are connected to slip rings that rotate along with the coil. A slip ring is different from a commutator in that the slip ring is one continuous band of conductive metal (such as copper) that s insulated from the armature shaft. The stationary brushes slide over the slip rings and conduct the generated current to an external circuit. The voltage that appears at the slip rings of this generator is an alternating (AC) voltage. When the coil in Figure 26 is rotated, each of the two coil sides cuts across the magnetic lines of force at the same speed. Thus, the strength of the voltage induced in one side of the coil is always the same as the strength of the voltage induced in the other side of the coil. If you think carefully about this action, however, you can see that each of the coil sides cuts the lines of force in a different direction. For example, as the loop rotates in a clockwise direction, the upper coil side cuts down through the magnetic lines of force. The lower coil side cuts up through the magnetic lines of force. The voltage induced in one coil side, therefore, is opposite to the voltage induced in the other coil side. However, since the two coil sides are connected in a closed loop, the voltages add together. The result is that the total voltage generated by one full turn of the coil is equal to two times the voltage generated in each coil side. The total voltage is collected by the brushes and is applied to an external circuit. Now, suppose that one end of the coil in Figure 26 is bent around to make another full turn before connecting to the slip ring. This new coil contains two conductor loops and is called a double-turn coil. The double-turn coil has four coil sides. The total voltage generated by the double-turn coil is therefore equal to two times the total voltage generated by a single-turn coil. Similarly, if the number of conductor loops in a coil is increased to three, the coil would have six coil sides and would generate three times as much voltage as a single-turn coil. Thus, to generate the high voltages needed to run machinery, the armatures in real generators contain many coil turns. The strength of the voltage induced in a generator coil depends on three factors: the strength of the magnetic field, the length of the coil,

32 DC Motor and Generator Theory 29 and the speed of rotation. First, the greater the number of magnetic lines of force in the field through which the coil moves, the more lines the coil will cut in a given time period. Second, the longer the coil sides, the greater the number of magnetic lines of force cut. Third, the faster the conductor moves, the more magnetic lines it will cut in a given period of time. So, by increasing any of these factors the strength of the magnetic field, the length of the coil sides, or the speed of armature rotation you can increase the amount of voltage induced in the generator. Now, take a few moments to review what you ve learned by completing Power Check 3.

33 30 DC Motor and Generator Theory Power Check 3 Fill in the blanks in each of the following statements. 1. In an electric generator, a relative movement between magnetic lines of force and a cutting across the lines generates a voltage. 2. In an armature having a single-turn coil, the voltages produced in each coil side because the coil sides are connected in a closed loop. 3. The process of producing a voltage in a generator is called the. 4. The supply the magnetic field or flux in DC generators. 5. The connect the commutator of a DC generator to the external circuit. Check your answers with those on page 49.

34 DC Motor and Generator Theory 31 Variations in Induced Voltage When a coil rotates in a magnetic field, the voltage generated doesn t have a steady fixed intensity or value. Nor does the induced voltage always flow through the coil in the same direction. The value of induced voltage changes as the coil rotates. For some coil side positions, a great many magnetic lines are cut; for other positions, few or no magnetic lines are cut. The direction of voltage changes because at one instant a coil side travels downward through lines of force, while a half-turn later, it travels upward through the same lines. Figure 27 shows an end view of the sides of an armature coil. One of the coil sides has been highlighted so we can track its movement through the magnetic field. In Figure 27A, the armature coil is in a vertical (up-and-down) position between the field poles. The highlighted coil side is positioned parallel to the magnetic lines of FIGURE 27 The maximum number of magnetic lines of force are cut when the conductor moves at right angles to the magnetic field. Since the direction of the conductor motion reverses with respect to the magnetic field at views (A) and (C), the direction of the induced voltage reverses.

35 32 DC Motor and Generator Theory force. Therefore, the coil side isn t cutting through any magnetic lines, and no voltage is induced in the coil side at this point. In Figure 27B, the armature coil is in a horizontal (side-to-side) position between the field poles. The highlighted coil side now cuts the magnetic lines at right angles. At this point, the maximum number of magnetic lines is being cut, so the maximum possible voltage is being induced in the coil side. In Figure 27C, the armature coil continues to rotate toward the vertical position, and the coil side cuts fewer lines of force. Therefore, the amount of voltage generated decreases toward zero. In Figure 27D, the armature coil rotates again toward the horizontal position and the generated voltage increases back to maximum. However, note that the coil side now cuts the lines in the opposite direction from that in which it cut them during the first half of the rotation. Finally, the armature coil returns to the original vertical position shown in Figure 27A, and the generated voltage decreases again to zero. The voltage generated by one rotation of the armature coil is shown in the voltage waveform graph in Figure 28. The voltage waveform or curve, also called a cycle, represents the voltage wave during one revolution of the coil. The curve moves up from the zero voltage point to indicate the increase in voltage in one direction, falls back to the zero point, then slopes down below the zero point to indicate the increase in voltage in the opposite direction. The positions of the armature coil shown in Figure 27 can be related to this graph. The position shown in Figure 27A (zero voltage) is the starting point (0) on the graph. As the armature coil begins to turn, the voltage waveform begins to rise upward from the zero point. When the armature coil is horizontally aligned with the field poles, as in Figure 27B, the maximum voltage is induced in it. This is the high point of the curve in the graph in Figure 28. The armature coil continues to turn until it reaches the position shown in Figure 27C, and the voltage waveform slopes downward until it again reaches the zero position. As the coil continues to turn, FIGURE 28 The current or voltage produced in a generator coil during a complete revolution is actually an alternating current or voltage.

36 DC Motor and Generator Theory 33 the waveform moves below the zero point. When the coil reaches the position shown in Figure 27D, the waveform reaches its lowest point. Finally, as the coil rotates back to its original position, the waveform slopes back upward to the zero point on the graph. You can also relate the waveform to the generator shown in Figure 26. Since the generated voltage changes its direction every half cycle, the current also changes direction every half cycle. The current flows through the external circuit, from one slip ring to the other, then reverses and flows in the opposite direction. Commutator and Brush Action Remember that the voltage that s generated at the slip rings of the generator in Figure 26 is an AC voltage; that is, the voltage is moving in two directions. In order to be used in DC applications, this AC voltage must be converted to DC voltage (a voltage moving in only one direction). The commutator performs this conversion in a DC generator. Without a commutator, all generators would be AC generators (that is, alternators). Figure 29 shows a basic DC generator with commutator and brushes. The ends of the armature coil are connected to the commutator FIGURE 29 In the basic DC generator shown, the commutator consists of two segments of a slip ring. Each end of the single-turn coil connects to a commutator segment. The segments are insulated from each other and from the shaft by an insulating material, such as mica.

37 34 DC Motor and Generator Theory segments. The brushes are stationary and are positioned at either side of the commutator. The brushes collect the generated current from the commutator segments and provide a connection to the external circuit and the load. Just as the direction of the voltage generated in the armature coil reverses direction, each commutator segment moves out from under one brush and moves in under the other brush. It can be seen from Figure 29 that the brush marked with a plus sign (+) only collects current from the armature coil side that moves upward along the south pole of the permanent magnet. Similarly, the brush marked with a minus sign ( ) only collects current from the armature coil side that moves downward along the north pole of the permanent magnet. Therefore, the polarity of each brush stays the same, and the current supplied to the external circuit and load is in one direction. Note that inside the armature coil, the direction of current flow is from minus ( ) to plus (+). In summary, the commutator and brush assembly interchanges the connections to the ends of the coil at the instant the polarity (the voltage direction) reverses. The action of reversing the connections to the coil to obtain a DC rather than AC voltage is called commutation. The current and voltage at the brushes are actually voltage pulses (pulses of voltage), as shown in Figure 30. The resulting output voltage is a pulsating DC voltage. FIGURE 30 The action of reversing the connections to the coil to obtain a direct rather than an alternating voltage is called commutation. The resulting output voltage of a DC generator is a pulsating DC voltage. Field Connections Generators, like motors, may have their components connected in series, in parallel (shunt), or in a combination (compound). Figure 31A shows the connection diagram of a shunt-wound generator. The field winding, called the shunt field, is connected directly in parallel to the armature. The line voltage across the line terminals is equal to the voltage across the armature and the voltage across the shunt field, as in any parallel-connected circuit. The connections of a series-wound generator are shown in Figure 31B. The field winding is connected in series with the armature. Since all the elements in a series circuit receive the same amount of current, the

38 DC Motor and Generator Theory 35 FIGURE 31 The field windings in generators may be connected in parallel or in series with the armature. In compound-wound machines, both series and parallel connections are used. current through the armature is the same as the current through the series field, and is also equal to the line current. Generators containing a combination of series and parallel connections are called compound-wound generators. The compound-wound generator in Figure 31C contains a shunt field winding connected in parallel with the armature and in series with a series field winding. The resulting connection is called a short-shunt compound-wound connection. The generator in Figure 31D contains a shunt field winding connected in parallel with the armature and a series field winding. This connection is called a long-shunt compound-wound connection. Field Excitation DC generators are often classified according to the way in which their field windings are excited. If the current used to excite the field windings is drawn from the armature of the machine, the field is termed self-excited. The schematic drawings in Figure 31 show various types of self-excited field connections. When the field windings are excited by current produced by an outside source (an exciter), the field is said to be separately excited. The schematic drawings in Figure 32 show two types of separately excited field connections. In both connections, the shunt field is energized by an exciter. However, in Figure 32B, the series field is connected in series with the armature and receives the same line current as the armature.

39 36 DC Motor and Generator Theory FIGURE 32 Separately excited machines are either shunt-wound or compound-wound, but they can t be simply series-wound. Armature Reaction Whenever current flows through a conductor, a magnetic field develops around that conductor. In a DC machine, the magnetic field needed to operate the machine is produced by a direct current in the field windings. A simplified drawing of a generator with two field poles is shown in Figure 33. The field windings and armature coils are shown as small circles in the figure. The circles with dots are carrying direct current toward you. The circles with X s are carrying direct current away from you. Figure 33A shows the magnetic flux produced by the field poles when the field windings are energized. The energized field poles act as north and south poles (N and S). The direction of the magnetic lines of force between the poles is indicated by the arrows. Note that the armature coils aren t energized in Figure 33A. The brushes are located exactly midway between the poles, or in the neutral position (indicated by the vertical broken line). In Figure 33B, the field poles aren t energized. However, the armature windings are supplied with direct current, and a magnetic field develops around the armature windings. The direction of this magnetic field is indicated by the circular broken lines and arrows, from the bottom to the top. The armature develops a magnetic field with the north pole at the top (N) and the south pole at the bottom (S). Figure 33C shows the magnetic flux that results when current is applied to both the field poles and the armature windings. This is the magnetic field produced during normal operation of a DC generator.

40 DC Motor and Generator Theory 37 FIGURE 33 The uniform magnetic field produced by the field windings is affected by the varying magnetic field of the armature. The armature field causes a reaction which shifts the neutral position. The magnetic fields of the armature windings and the field combine and interact, and the resulting magnetic lines of force flow in the direction indicated in the figure. The effect that the armature s magnetic field produces on the main field is called the armature reaction. This armature reaction opposes the main magnetic field and reduces its strength. In addition, the armature reaction causes a shift of the neutral position of the main field. Since the neutral position must be located at right angles to the magnetic lines of force, the armature reaction causes the original neutral position (the dashed line in Figure 33A) to be shifted in the direction of armature rotation. The new neutral position is shown in Figure 33C. When the neutral position of the main field shifts, the position of the brushes must shift also. If the brushes remain in the original position, dangerous sparking will occur at the commutator.

41 38 DC Motor and Generator Theory How does sparking occur? Well, ideally, the coil that s short-circuited by the brush (that is, the coil being commutated) shouldn t cut across any magnetic lines at the moment that the segment to which it s connected is under the brush. However, if the neutral position is shifted due to armature reaction, the coils will cut across some magnetic lines during commutation. As a result, the cutting of the lines induces a voltage in the coil being commutated and causes heavy currents to flow through the coil and into the brush. These currents often cause sparking at the commutator. Sparking can damage the surface of the commutator and the brushes. Counteracting Armature Reaction There are several methods used to counteract armature reaction and prevent sparking at the commutator. One method is to shift the brushes to a new neutral position, as shown in Figure 33C. In this new brush position, no voltage is induced in the coils being commutated, and consequently no sparking. On a generator, the brushes are shifted in the direction of armature rotation, which is shown to be clockwise in Figure 33C. Depending upon the application, the shift may be up to 20 degrees. Brush shifting has several disadvantages, however. The magnetic field strength needed varies with the armature current; therefore, for best results, the brushes should be shifted for each new armature current value. This shifting is seldom practical. Moreover, the direction of shift changes with the direction of rotation. Thus, the position of the brushes should be changed with every change in rotation direction. Another method used to overcome the effects of armature reaction is using commutating poles or interpoles. The interpoles are additional field windings placed on poles located midway between the field poles, and directly over the armature coils being commutated. The interpole coils are connected in series with the armature; therefore, they carry current that s proportional to the armature current. The interpole coils are wound so as to produce a magnetic field with a polarity opposite to the polarity of the magnetic field created by the armature current. For example, Figure 33B shows the location of the two brushes, one over the north pole of the armature and the other over the south pole. When interpole coils are used, one is placed so that it forms a north pole at the top of the armature; the other forms a south pole at the bottom. Therefore, each interpole is midway between the field poles. The interpoles prevent armature reaction because there s no magnetic field perpendicular to the magnetic field created by the field windings and the brushes. A third method of counteracting armature reaction is to use compensating windings, which are coils embedded in the surface of the pole

42 DC Motor and Generator Theory 39 next to the armature. These windings are connected in series with the armature so that their strength is proportional to armature current. The effects of compensating windings are shown in Figure 34. The magnetic field in the armature conductor opposes the magnetic field in the compensating winding; therefore, the effect of armature reaction is eliminated. FIGURE 34 The result of the two opposing magnetic fields is such that, at every point under the poles, the two fields balance, and the effects of armature reaction are eliminated.

43 40 DC Motor and Generator Theory Eddy Currents The steel of the armature surface that s near current-carrying conductors and (to a lesser extent) the steel of the face of the field pole are subject to magnetic field variations. Remember that a voltage is induced in a coil when the coil cuts through magnetic lines of force or is subject to a magnetic field change. Therefore, when a voltage is induced in an armature winding, a voltage is also induced in the steel of the armature slot that holds the winding. These voltages cause currents called eddy currents to flow in the steel, resulting in an energy loss. To reduce these eddy currents, the steel is alloyed with silicon to increase its resistance. It s also laminated (composed of thin insulated sheets) to decrease the total length of the paths that the currents may travel (Figure 35). FIGURE 35 To reduce the flow of eddy current, an armature core is made up of steel laminations about 1 64 in. (0.4 mm) thick. The armature shown has four laminations, and the power is one-fourth that of a solid steel core. Hysteresis Loss As the armature rotates, all points on it are exposed to alternating magnetic field directions. The power used or lost in reversing the magnetic field direction in the armature core is called hysteresis loss. This loss varies with the number of reversals, the density of the magnetic field, and also the material of the armature (the loss is higher for some steels than for others). Therefore, a special steel with a low hysteresis loss is used for armature cores. Take a few moments now to check your learning by completing Power Check 4.

44 DC Motor and Generator Theory 41 Power Check 4 Fill in the blanks in each of the following statements. 1. In a generator, the converts a generated AC voltage to a DC voltage. 2. The current through the armature coil and the current through the field coils combine to produce a distorted field. This distorted field effect is called. 3. Your foreman asks you to see that the compensating winding in a DC generator is connected properly. You do this by making sure that it s connected in with the armature. 4. are embedded in the pole face of the field coil. 5. reduce the flow of eddy current in an armature. 6. are placed midway between the main field coils to reduce commutator arcing. Use the illustrations provided to answer the following questions. 7. The connection diagram shown here for a self-excited generator represents which type of connection? (Continued)

45 42 DC Motor and Generator Theory Power Check 4 8. In which of the following figures is the field winding connected directly in parallel with the armature? A C B. D. 9. Which of the diagrams shown in Question 8 represents a generator with a separately excited field? 10. Look at the position of the brushes in the drawing shown below. In what direction is the generator s armature rotating? Check your answers with those on page 49.

46 DC Motor and Generator Theory 43 DC GENERATORS IN INDUSTRY Types of Machine Losses The cost of operating machinery and equipment is a major concern in all types of industry. The energy lost during the production of current is one major source of increased operation costs. This is true whether the voltage is produced in a plant or by a utility company. The energy used by a machine is always greater than the energy it produces. For instance, the mechanical energy required to turn a generator is always greater than the electrical energy produced. The difference between the energy used by a machine (input power) and the energy produced (output power) is called the total machine losses. There are two types of total machine losses. One type is mechanical loss, a loss that occurs whether or not the machine is connected to a load. (For instance, a generator could be driven while not electrically connected to an external circuit.) Mechanical losses include bearing friction, windage, and brush friction losses. The other type of loss is an electrical loss. An electrical loss is dependent on (or proportional to) the load applied to a machine. Electrical losses include brush contact losses, copper losses or I2R losses (the square of the current times the resistance), field and armature winding losses, iron losses, and stray load losses. Let s take a closer look at some of these machine losses to see how they affect overall machine efficiency. Bearing Friction and Windage Losses Bearing friction losses and windage losses (losses due to wind or air turbulence caused by the rotating armature) are mechanical losses that consume the power required to rotate the armature at normal speed, with no armature current flowing and no field flux. Factors that affect bearing friction include the type of lubrication used and the area of the internal surfaces contacting the bearings. Windage losses are affected by the shape of the moving surface. The more wind created by the moving surfaces, the greater the windage losses. Brush Friction Losses The brushes sliding on a commutator surface act like brakes on the armature, due to the friction between the commutator surface and the brushes. The magnitude of these brush friction losses depends on the speed with which the commutator is moving, the total brush area, and the material the brushes are made of.

47 44 DC Motor and Generator Theory Brush Contact Loss Brush contact losses occur as a result of electrical resistance. This loss is equal to the voltage drop that occurs between the brush and the commutator surface multiplied by the current. (With an open circuit, therefore, brush contact losses are zero.) The loss increases with the load, but it remains constant once the load is applied. Thus, the loss is said to be a load dependent loss. In practice, the voltage value depends upon the type of brush material, and whether shunts (also called pigtails) are used with the brushes. Copper Losses Copper losses or I2R losses in the armature and field windings depend on the amount of current flow and the resistance of the windings. In practice, the resistances used are corrected to 75 C (167 F) since this is the average expected temperature of such machine windings. Although some machines have different winding temperatures (depending on the load and on whether the armature is specially cooled), the important point to remember is that I2R losses vary with the square of the current. For example, if the current triples, the I2R losses increase by 3 2, or nine times. Iron Losses Iron losses in an armature core are also called core losses, since they represent power used up in the core. These losses consist of eddy current losses and hysteresis losses. Eddy current losses occur as a result of the armature material acting as a conductor. For this reason, the armature is laminated to reduce the effective conductor length. Hysteresis loss is the power required to reverse the direction of the magnetic flux in iron or steel. Stray Load Losses When generators are tested, machine losses can be determined by measuring the difference between the power input and the power output. The difference can also be calculated without actually measuring these quantities. However, the actual measured loss will always be greater than the calculated loss. This extra loss is called a stray load loss. A stray load loss can result from eddy current losses in the armature conductors, or losses caused by magnetic field distortion resulting from armature reaction. Machine Efficiency In all types of equipment, efficiency is very important. The efficiency of a generator can be calculated easily. First, you must determine the

48 DC Motor and Generator Theory 45 total power loss of the machine by adding together all the various losses. The total power input is equal to the power output plus the total loss, as illustrated in the equation below: power input power output power loss The calculated efficiency of a machine is equal to the ratio between the power output and the power input, and is usually expressed as a percent. You can use the following formula to calculate the efficiency of a machine as a percent: power output efficiency 100 power input To use this formula, the power output and the power input must be expressed in the same units; that is, both must be expressed in either watts, kilowatts, or horsepower. So, let s calculate the efficiency of a generator with a power output of 90 kw and power losses of 10 kw. power input power output power loss power input 90 kw 10 kw power input 100 kw First, determine the total power input. Add the power output and the power loss. The power input is 100 kw. power output 100 efficiency power input Write the efficiency formula. 90 kw 100 Substitute 90 kw for power output and 100 efficiency kw for power input. 100 kw Multiply efficiency = , % Divide. Remember that the answer is a percent. Answer: The efficiency of this machine is 90%. Voltage Regulation The amount of voltage produced by a generator depends on the armature speed and on the strength of the generator s magnetic field. Since the magnetic field is produced by the current that flows through the field winding, we can also say that the amount of voltage produced depends on the armature speed and on the value of the field current. If the armature speed or field current or both increase, the voltage increases. Likewise, if one (or both) of these factors decrease, the voltage decreases.

49 46 DC Motor and Generator Theory As a rule, the voltage produced by a generator isn t controlled by increasing or decreasing the armature speed: it s best to run the generator at the manufacturer s rated speed. Therefore, in practice, the voltage is controlled by increasing or decreasing the field current (the excitation) as needed. Various types of voltage controllers are used. Many are completely automatic, and use electrical and electronic control circuits to adjust or regulate the voltage to the desired values. Operating Generators in Parallel In order to supply greater current, DC generators in power stations are connected in parallel through common bus lines. For shuntwound generators, the load is kept in balance by rheostats or by electronic controllers that are connected in series with each shunt field. The voltage output is increased (while current is kept equal) by increasing the field excitation of both generators. For generators of unequal capacities, the loads are generally divided in proportion to their ratings. Now, take a few moments to review what you ve learned by completing Power Check 5.

50 DC Motor and Generator Theory 47 Power Check 5 Fill in the blanks in each of the following statements. 1. If you increase the load on a certain DC generator in your plant, the brush-contact loss will. 2. If the current in the armature increases from 10 A (amperes) to 20 A, the copper losses in the armature would be times as much as they were before the increase. 3. Your foreman asks you to calculate the output power of a DC generator. He then asks you to divide that value by the input power and multiply the result by 100. You ve just calculated the of the generator. 4. The output voltage of a DC generator is usually controlled by adjusting the. 5. losses include bearing-friction losses, brush-friction losses, and windage losses. 6. losses are equal to the difference between measured input power and measured output power. 7. losses are made up of brush-contact losses, copper losses, iron losses, and stray-load losses. Check your answers with those on page 49.

51 48 DC Motor and Generator Theory NOTES

52 49 Power Check Answers An interpole 2. Mica 3. The rotor 4. Because a large current will flow through these windings degrees 1. pulse-width modulation (PWM) 2. field weakening 3. H-bridge 4. rheostat 2 1. commutator 2. armature reaction 3. series 4. Compensating windings 5. Laminated cores 6. Interpoles 7. A series connection 8. A 9. D 10. The armature rotation is clockwise. 5. field windings, armature windings 6. armature 3 1. conductor 2. add together 3. generator action of magnetic induction 4. field poles and windings 5. brushes 1. increase 2. four 3. efficiency 4. field excitation 5. Mechanical 6. Total machine 7. Electrical 5

53 50 Power Check Answers NOTES

54 Examination 51 DC Motor and Generator Theory EXAMINATION NUMBER: Whichever method you use in submitting your exam answers to the school, you must use the number above. For the quickest test results, go to When you feel confident that you have mastered the material in this study unit, complete the following examination. Then submit only your answers to the school for grading, using one of the examination answer options described in your Test Materials envelope. Send your answers for this examination as soon as you complete it. Do not wait until another examination is ready. Questions 1 25: Select the one best answer to each question. 1. Which of the following DC motors provides the highest starting torque? A. Series-wound C. Compound-wound B. Shunt-wound D. Generator-wound 2. What is the efficiency of a DC generator that requires 100 watts of energy to supply 80 watts of output power? A. 8% C. 80% B. 12.5% D. 125% 3. Which of the following DC motors acts as a form of electrical ratchet? A. Servo C. Stepper B. Permanent-magnet D. Syncro

55 52 Examination 4. Why is steel alloyed with silicon used as the core material for the armature of a DC generator? A. To reduce eddy-current flow in the armature core B. To increase the hysteresis loss in the core C. To decrease the resistance of the armature conductors D. To reduce the resistance to current flow in the core 5. Which of the following losses can happen even when a DC generator isn t connected to a load? A. I2R loss C. Copper loss B. Brush friction loss D. Iron loss 6. If a DC generator with an efficiency of 94% has an output of 240 volts at 100 amperes, the machine power losses for this generator would be A watts. C watts. B watts. D watts. 7. What happens when the speed of a generator s armature increases? A. The load decreases C. The windage loss decreases B. The field current reverses D. The induced voltage increases 8. To get better commutation in a DC generator, you should move the brushes A. in the direction of rotation. B. in the direction of field polarity. C. against the direction of rotation. D. against the direction of field strength. 9. Which of the following DC motor drive systems controls motor speed by controlling the period of the waveform to the drive s output transistor? A. Frequency-effect transition (FET) B. Frequency-drift commutation (FDC) C. Pulse-controlled modulation (PCM) D. Pulse-width modulation (PWM) 10. In a generator, the voltage induced in a rotating armature coil is at maximum when the coil A. moves parallel with the magnetic lines of force. B. cuts the fewest magnetic lines of force. C. is at right angles to the magnetic lines of force. D. is in a vertical position between the field poles. 11. Which of the following methods can be used to cut down on brush sparking in a DC generator? A. Install interpoles in the armature core. B. Install compensating coils in the field pole faces. C. Move the brushes against the armature rotation. D. Connect commutating coils in parallel with the armature coils.

56 Examination Technician A is told to inspect and clean the brush holders and replace the brushes in a DC generator. Which of the following areas would the technician be working on in this case? A. The base C. The rigging B. The commutator D. The shaft 13. Which of the following devices did older DC motor speed controllers often use to control armature voltage? A. A battery C. A potentiometer B. A rheostat D. A diode 14. What is the efficiency of a DC generator that requires 200 watts of energy to supply 160 watts of output power? A. 40% C. 90% B. 80 % D. 125% 15. In a field weakening DC motor speed controller system, what happens when the voltage to the field is decreased? A. The motor speed will increase. B. The motor will slow down. C. Sparking will occur at the commutator. D. Excess heat will be generated. 16. What type of generator connection is shown in the diagram below? A. Separately-excited series-wound C. Separately-excited compound-wound B. Separately-excited shunt-wound D. Self-excited series-wound 17. What part of a motor s armature is in direct contact with the brushes? A. The interpole C. The commutator B. The end bell D. The case 18. The turning force produced by a motor is called A. excitation. C. SCR. B. horsepower. D. torque.

57 54 Examination 19. A DC generator s load loss from hysteresis is one type of A. bearing friction loss. C. copper loss. B. brush-contact loss. D. iron loss. 20. When using an SCR motor speed controller, the speed of the motor depends upon the A. voltage on the field windings. B. resistance in the armature circuit. C. triggering timing of the SCR. D. AC input voltage to the rectifier. 21. What type of generator connection is shown in the diagram below? A. Self-excited compound-wound C. Separately-excited series-wound B. Self-excited series-wound D. Separately-excited compound-wound 22. In which of the following figures is the maximum voltage being induced in the coil? A. C. B. D.

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