Unit-IV. 1. Explain the operation, characteristics and application of DC and AC servo motor.

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1 Unit-IV Special Machines - Servo motor DC and AC servomotors; stepper motors variable reluctance and permanent magnet stepper motors; single phase synchronous motor reluctance motor and hysteresis motor universal motor Repulsion motor synchronous motor. 1. Explain the operation, characteristics and application of DC and AC servo motor. They are also called control motors and have high-torque capabilities. Unlike large industrial motors, they are not used for continuous energy conversion but only for precise speed and precise position control at high torques. Of course, their basic principle of operation is the same as that of other electromagnetic motors. However, their construction, design and mode of operation are different. Their power ratings vary from a fraction of a watt upto a few 100 W. Due to their low-inertia, they have high speed of response. That is why they are smaller in diameter but longer in length. They generally operate at vary low speeds or sometimes zero speed. They find wide applications in radar, tracking and guidance systems, process controllers, computers and machine tools. Both dc and a.c.(2-phase and 3-phase) servomotors are used at present. Servomotors differ in application capabilities from large industrial motors in the following respects : DC Servomotors 1. They produce high torque at all speeds including zero speed. 2. They are capable of holding a static (i.e. no motion) position. 3. They do not overheat at standstill or lower speeds. 4. Due to low-inertia, they are able to reverse directions quickly. 5. They are able to accelerate and deaccelerate quickly. 6. They are able to return to a given position time after time without any drift. These motors are either separately-excited dc motors or permanent-magnet dc motors. The schematic diagram of a separately-excited d.c. motor alongwith its armature and field MMFs and torque/speed characteristics is shown in Fig.(a)

2 The speed of d.c. servomotors is normally controlled by varying the armature voltage. Their armature is deliberately designed to have large resistance so that torque-speed characteristics are linear and have a large negative slope as shown in the Fig.(c).The negative slope serves the purpose of providing the viscous damping for the servo drive system. As shown in Fig.(b), the armature m.m.f. and excitation field mmf are in quadrature. This fact provides a fast torque response because torque and flux become decoupled. Accordingly, a step change in the armature voltage or current produces a quick change in the position or speed of the rotor. Advantage of DC servomotor 1. Higher output than from a 50 Hz motor of same size. 2. Linearity of characteristics is achieved easily. 3. Easier speed control from zero speed to full speed in both directions.

3 AC Servomotors Most of the ac servomotors are of the two-phase squirrel-cage induction type and are used for low power applications.however, recently three-phase induction motors have been modified for high power servo systems which had so far been using high power d.c. servomotors. (a) Two-phase AC Servomotor Such motors normally run on a frequency of 60 Hz or 400 Hz (for airborne systems). The stator has two distributed windings which are displaced from each other by 90º (electrical). The main winding (also called the reference or fixed phase) is supplied from a constant voltage source, V m 0º (as shown in the below Fig.). The other winding (also called the control phase) is supplied with a variable voltage of the same frequency as the reference phase but is phase-displaced by 90º (electrical). The control phase voltage is controlled by an electronic controller. The speed and torque of the rotor are controlled by the phase difference between the main and control windings. Reversing the phase difference from leading to lagging (or vice-versa) reverses the motor direction. (b) Three-phase AC Servomotors A great deal of research has been to modify a three-phase squirrel-cage induction motor for use in high power servo systems. Normally, such a motor is a highly non-linear coupled-circuit device. Recently, this machine has been operated successfully as a linear decoupled machine (like a d.c.machine) by using a control method called vector control or field oriented control. In this method,the currents fed to the machine are controlled in such a way that its torque and flux become decoupled as in a dc machine. This results in a high speed and a high torque response.

4 Fig. AC servo motor Advantages of AC servomotors 1. Low cost. 2. Higher efficiency and less maintenance (because no commutator and brushes). Disadvantages of AC servomotors 1. Non-linear characteristics. 2. More difficult to control in positioning applications.

5 2. Explain about the working of different types of stepper motor. These motors are also called stepping motors or step motors. The name stepper is used because this motor rotates through a fixed angular step in response to each input current pulse received by its controller. In recent years, there has been widespread demand of stepping motors because of the explosive growth of the computer industry. Their popularity is due to the fact that they can be controlled directly by computers, microprocessors and programmable controllers. Stepping motors are ideally suited for situations where either precise positioning or speed control or both are required in automation systems. precise The unique feature of a stepper motor is that its output shaft rotates in a series of discrete angular intervals or steps, one step being taken each time a command pulse is received. When a definite number of pulses are supplied, the shaft turns through a definite known angle. The only moving part in a stepping motor is its rotor which has no windings, commutator or brushes. This feature makes the motor quite robust and reliable. Step Angle: The angle through which the motor shaft rotates for each command pulse is called the step angle β. Smaller the step angle, greater the number of steps per revolution and higher the resolution or accuracy of positioning obtained. The value of step angle can be expressed either in terms of the rotor and stator poles (teeth) N r and N s respectively or in terms of the number of stator phases (m) and the number of rotor teeth Resolution is given by the number of steps needed to complete one revolution of the rotor shaft. Higher the resolution, greater the accuracy of positioning of objects by the motor. Resolution = No. of steps / revolution = 360º / β

6 Advantages of stepper motor 1. The rotation angle of the motor is proportional to the input pulse. 2. The motor has full torque at standstill (if the windings are energized) 3. Excellent response to starting/stopping/reversing. 4. Very reliable since there are no contact brushes in the motor. Therefore the life of the motor is simply dependant on the life of the bearing. 5. The motors response to digital input pulses provides open-loop control, making the motor simpler and less costly to control. 6. A wide range of rotational speeds can be realized as the speed is proportional to the frequency of the input pulses. Disadvantages of stepper motor 1. Resonances can occur if not properly controlled. 2. Not easy to operate at extremely high speeds. Applications Such motors are used for operation control in computer peripherals, textile industry, IC fabrications and robotics etc. Applications requiring incremental motion are typewriters, line printers, tape drives, floppy disk drives, numerically-controlled machine tools, process control systems and X-Y plotters. It includes commercial, military and medical applications where these motors perform such functions as mixing, cutting, striking, metering,blending and purging. They also take part in the manufacture of packed food stuffs, commercial endproducts and even the production of science fiction movies. Types of Stepper Motors Stepper motors which can be divided into the following three basic categories 1. Variable Reluctance Stepper Motor 2. Permanent Magnet Stepper Motor 3. Hybrid Stepper Motor

7 Variable Reluctance Stepper Motors A simple circuit arrangement for supplying current to the stator coils in proper sequence is shown in below Fig.(e). The six stator coils are connected in 2-coil groups to form three separate circuits called phases. Each phase has its own independent switch.diametrically opposite pairs of stator coils are connected in series such that when one tooth becomes a N-pole, the other one becomes a S-pole.Although shown as mechanical switches in Fig.(e) Energising one or more stator coils causes the rotor to step forward (or backward) to a position that forms a path of least reluctance with the magnetized stator teeth. The step angle of this three-phase, four rotor teeth motor is β = 360/ 4 3 = 30º.

8 Working: The motor has following modes of operation (a) 1-phase-ON or Full-step Operation In the above Fig.(a) shows the position of the rotor when switch S 1 has been closed for energising phase A. The magnetic field with its axis along the stator poles of phase A is created. The rotor is therefore,attracted into a position of minimum reluctance with diametrically opposite rotor teeth 1 and 3 lining up with stator teeth 1 and 4 respectively. Closing S 2 and opening S 1 energizes phase B causing rotor teeth 2 and 4 to align with stator teeth 3 and 6 respectively as shown in Fig. (b). The rotor rotates through full-step of 30º in the clockwise (CW) direction. As each switch is closed and the preceding one opened, the rotor each time rotates through an angle of 30º. By repetitively closing the switches in the sequence and thus energizing stator phases in sequence ABCA etc. This mode of operation is known as 1-phase-ON mode or full-step operation. The stator phase switching truth table is shown in the above Fig.(f) (b) 2-phase-ON Mode In this mode of operation, two stator phases are excited simultaneously. When phases A and B are energized together. If the stator phases are switched in the sequence AB, BC, CA, AB etc., the motor will take full steps of 30º each (as in the 1-phase-ON mode) but its equilibrium positions will be interleaved between the full-step positions.

9 (c) Half-step Operation Half-step operation or half-stepping can be obtained by exciting the three phases in the sequence A, AB, B, BC, C etc. i.e. alternately in the 1-phase-ON and 2-phase-ON modes. It is sometime known as wave excitation and it causes the rotor to advance in steps of 15º i.e. half the full-step angle. The truth table for the phase pulsing sequence in half-stepping is shown in Fig. Half-stepping can be illustrated with the help of the above Fig.

10 (d) Microstepping It is also known as mini-stepping. It utilizes two phases simultaneously as in 2-phase-ON mode but with the two currents deliberately made unequal (unlike in half-stepping where the two phase currents have to be kept equal). The current in phase A is held constant while that in phase B is increased in very small increments until maximum current is reached. The current in phase A is then reduced to zero using the same very small increments. In this way, the resultant step becomes very small and is called a microstep. Stepper motors employing microstepping technique are used in printing and phototypesetting. Multi-stack VR Stepper Motor Multi-stack motors are also available which provide smaller step angles. The multi-stack motor is divided along its axial length into a number of magnetically-isolated sections or stacks which can be excited by a separate winding or phase. Both stator and rotor have the same number of poles The stators have a common frame while rotors have a common shaft as shown in Fig.(a) which represents a three-stack VR motor and angular displacement as shown in the developed diagram of Fig.(b) for phase excitation.

11 Permanent-Magnet Stepping Motor Construction Its stator construction is similar to that of the single-stack VR motor discussed above but the rotor is made of a permanent-magnet material like magnetically hard ferrite. As shown in the Fig. the stator has projecting poles but the rotor is cylindrical and has radially magnetized permanent magnets. The rotor has two poles and the stator has four poles. Since two stator poles are energized by one winding, the motor has two windings or phases marked A and B The step angle of this motor β = 360º / mnr = 360º / 2 2 = 90º or β = (4 2) 360º / 2 4 = 90º. Working When a particular stator phase is energized, the rotor magnetic poles move into alignment with the excited stator poles. The stator windings A and B can be excited with either polarity current (A + + refers to positive current i A in the phase A and A to negative current i A ).Fig. (a) shows the condition when phase A is excited with positive current i + A. Here, θ = 0º. If excitation is now switched to phase B as in Fig. (b), the rotor rotates by a full step of 90º in the clockwise direction. Next, when phase A is excited with negative current i A, the rotor turns through another 90º in CW direction as shown in Fig.(c). Similarly, excitation of phase B with i B further turns the rotor through another 90º in the + same direction as shown in Fig.(d). After this, excitation of phase A with i A makes the rotor turn through one complete revolution of 360º It will be noted that in a permanent-magnet stepper motor, the direction of rotation depends on the polarity of the phase currents as tabulated below : i + A ; i + B ; i A ; i B ; i + A,... A + ; B + ; A ; B ; A + ;... for clockwise rotation i + A ; i B ; i A ; i + B ; i + A ;... A + ; B ; A ; B + ; A + ;... for CCW rotation

12 when only one phase is energized at a time in 1-phase-ON mode giving step size of 90ºas shown in the table and it represents 2-phase-ON mode when two phases are energised simultaneously. The resulting steps are of the same size but the effective rotor pole positions are midway between the two adjacent full-step positions. It represents half-stepping when 1-phase-ON and 2-phase-ON modes are used alternately.

13 Advantages and Disadvantages Since the permanent magnets of the motor do not require external exciting current, it has a low power requirement but possesses a high detent torque as compared to a VR stepper motor. This motor has higher inertia and hence slower acceleration. However, it produces more torque per ampere stator current than a VR motor. Since it is difficult to manufacture a small permanent-magnet rotor with large number of poles, the step size in such motors is relatively large ranging from 30º to 90º. However, recently disc rotors have been manufactured which are magnetized axially to give a small step size and low inertia.

14 3. Describe the constructional features and principle of operation Reluctance Motor. Reluctance Motor It is a single phase synchronous motor which does not require d.c. excitation to the rotor. It is also known as synchronous reluctance motor. Reluctance motor operates on the following principle: Whenever a piece of ferromagnetic material is located in a magnetic field, a force exerts upon the material, tending to bring it into the position of the densest portion of the field. The force tends to align the specimen of material so that the reluctance of the magnetic path passing through the material will be at minimum. Construction A reluctance motor consists of: A stator carrying a single phase winding along with auxiliary winding to produce a synchronously revolving magnetic field. A squirrel cage rotor having un symmetrical magnetic construction this is achieved by symmetrically remaining some of the teeth from the squirrel cage rotor to produce salient poles on the rotor. As shown in fig. A salient poles are available on the rotor. The salient poles created on the rotor must be equal to the poles on the stator. Fig. Reluctance motor

15 Operation When a single phase supply is given to the stator, the auxiliary winding gets energized hence a synchronously revolving magnetic field is produced. The motor starts as a standard squirrel cage induction motor and will accelerate to its near synchronous speed. As the rotor approaches synchronous speed, the rotating stator flux will exert a reluctance torque on the rotor poles tending to align the salient pole axis with the axis of the rotating field. Consequently the motor will continue to run at the speed of revolving flux. i.e.,at the synchronous speed. Types of reluctance motors Following are the types of reluctance motors. Synchronous reluctance. Switched reluctance motor. Variable reluctance stepper motor. Advantages of reluctance motor Construction of the motor is simple Brushes, commuatators, permanent magnets are absent The motor do not require dc excitation Starting torque is quite good Accurate speed control is possible Cost effective and easy maintenance Disadvantages of reluctance motor Efficiency is less Power factor p.f is lower than that of equivalent induction motor Maximum output is reduced to great extent Size of this motor is larger than equivalent synchronous motor Applications The constant-speed characteristic of a reluctance motor makes it very suitable for such applications as signalling devices, recording instruments, many kinds of timers and phonographs etc.

16 4. Describe the constructional features and principle of operation Hysteresis Motor. It is a single phase motor whose operation depends upon the hysteresis effect. (i.e., magnetisation produced in a ferromagnetic material lags behind the magnetising force) and on the presence of continuously revolving magnetic flux. Construction It consists of: A stator designed to produce a synchronously revolving field from a single phase supply. This is accomplished by using permanent split capacitor type construction. Consequently the starting as well as main winding remain connected in the circuit during running operation as well as starting. The value of capacitance is so adjusted as to result in a flux revolving at synchronous speed. A rotor consisting of a smooth cylinder of magnetically hard steel, without winding or teeth. Operation When the stator is energised from a single-phase supply, a synchronously revolving field (assumed in anticlockwise direction) is produced due to split phase operation. The revolving stator flux magnetises the rotor. Due to hysteresis effect, the axis of magnetisation of rotor will lag behind the axis of stator field by hysteresis lag angle as shown in fig. If the rotor is stationary the starting torque produced is given by, Where, T s ϕ s ϕ r sin ϕ s - stator flux ϕ r - rotor flux Now, the rotor accelerates to synchronous speed with a uniform torque.

17 Fig. Hysteresis motor After reaching synchronism, the motor continues to run at synchronous speed and adjusts its torque angle so as to develop the torque required by the load. Advantages of hysteresis motor As rotor has no teeth, no winding, there are no mechanical vibrations. Due to absence of vibrations, the operation is quiet and noiseless. Suitability to accelerate inertia loads. Possibility of multispeed operation by employing gear train. Disadvantages of hysteresis motor The output is about one-quarter that of an induction motor of the same dimension. Low efficiency Low power factor Low torque Available in very small sizes Applications Due to their quiet operation and ability to drive high-inertia loads, hysteresis motors are used for driving i) electric clocks ii) timing devices (iii) tape decks (iv) turn tables and other precision equipment.

18 5. Describe the constructional features and principle of operation Repulsion Motor. Repulsion motor works on the principle of repulsion between two magnetic fields. These motors give excellent performance characteristics. Repulsion principle For understand the torque production by motor using repulsion principle consider a two pole salient pole motor having magnetic axis horizontal. The armature of the machine consists of d.c. windings having commutator and brushes. The brushes are short circuited by a low resistance jumper. The stator winding is given excitation in such away as to form the poles as shown in the Fig.(a). The brushes are aligned in the same direction of the field axis. The stator winding will produce alternating flux which will induce e.m.f. in the armature conductors by transformer action. The direction of induced e.m.f. can be found using Lenz's law. The direction of induced current will depend on position of brushes, These currents will lag behind the induced voltage by almost 90 o. Because of the current flowing through the armature, it will produce its own magnetic field with the poles as shown in the Fig.(a). Thus equal force of repulsion exists between like poles which will not produce any torque. Fig.(a) Alternatively it can be also explained as the armature to be divided into four quadrants producing four torques T 1, T 2, T 3 and T 4 which are equal and hence the net torque is zero. If brushes are shifted by 90 o, so the conductors undergoing short circuit are also changed.

19 The induced emf are in the same direction as before. The arrangement is shown in the Fig.(b). Fig. (b) Apart from the coils undergoing short circuit, the remaining armature winding gets divided into two parallel paths. It can be seen that the induced emfs are balanced and the resultant emf is zero. Thus no current flows through the brushes and the resultant torque is also zero.if the brushes are in the position shown in the Fig.(c). In this case, the brushes axis is not in the line of main field or at an angle of 90 o to main field but it is at an angle of α with the main field. Fig.(c)

20 Again the emf will be induced in the armature conductors and there will be net voltage across brush terminals which will produce current in the armature. Thus the armature will also produce its own magnetic field with the poles as shown in the Fig.(c). The north and south poles of stator and rotor will attract each other and there will be net torque available which will run the motor in the clockwise direction. Alternatively we can say that the north pole formed by armature winding will be repelled by the north pole formed by the main field winding and similarly the south pole will be repelled by south pole formed by the main field winding and the motor runs in clockwise direction. As the forces are of repulsion which contributes in the motion so the name of the motor is repulsion motor. Disadvantages of Repulsion Motor Occurrence of sparks at brushes. The power factor is poor at low speeds. No load speed is very high and dangerous. Applications Farm Motor Applications, Hoists, Floor Maintenance Machines, Air Compressors, Laundry Equipment, Mining Equipment.

21 6. Describe the constructional features and principle of operation Universal Motor. There are small capacity series motors which can be operated on d.c. supply or single phase alternating supply of same voltage with same characteristics, called universal motors. The general construction of such motor is similar to that of a.c. series motor as discussed in last article. It is manufactured in two types. i) Non compensated, low h.p ii) Compensated type, high h.p. Non compensated type pole has 2 poles, having entire magnetic path as laminated. Armature is wound type similar to the normal d.c. motor. Such non compensated construction is shown in the Fig. 1. Fig. 1 Cross-section of non-compensated universal motor While in compensated type, the motor has distributed field winding consisting of main field and compensating winding. This is somewhat similar to the stator of split phase single phase induction motor type construction. This also has a wound armature similar to the normal d.c. motor. Fig.2 shows the connection diagrams for both the types of universal motor. Fig. 2 Connection diagrams for a universal motor

22 Speed torque characteristics: The speed - torque characteristics for both the types of universal motor are shown in the Fig.3. Fig. 3 Speed-torque characteristic of universal motor Compensated type universal motor has better speed - torque characteristics i.e. the characteristics are same for the operation of motor on a.c. or d.c. supply. The motors are generally designed for full load operation speeds ranging between 3000 to r.p.m. Applications : Though compensated type characteristics are better, the non compensated type are more preferred for low h.p. applications. While compensated type of universal motors are preferred for h.p. applications. High starting torque is the important feature of universal motors. The universal motors are used for domestic applications like vacuum cleaners, food processor and mixers, hair driers, coffee grinders, electric shavers etc. Their other applications are blowers, portable tools like drilling machines and small drivers. Reversal of Rotation of Universal Motors By reversing the flow of current through the armature or field windings, the direction of rotation can be reversed for salient pole non compensated type universal motor. This is possible by interchanging the terminals on brush holders as shown in the Fig. 4. Fig. 4 Reversal of rotation of universal motors

23 Advantages of universal motor It is cheapest Running at high speeds Very small weight and size Disadvantages of universal motors Frequent maintenance Short operating life Applications Universal motors find their use in various home appliances like vacuum cleaners, drink and food mixers, domestic sewing machine etc. The higher rating universal motors are used in portable drills, blenders etc.

24 7. Explain about Synchronous motor and its V- Curves. Synchronous motor and induction motor are the most widely used types of AC motor. A synchronous motor is similar to an alternator (AC generator), and just like a DC machine, a same synchronous machine can be used as a synchronous motor or as an alternator. Synchronous motors are available in a wide range, generally rated between 150kW to 15MW with speeds ranging from 150 to 1800 rpm. Construction The construction of a synchronous motor (with salient pole rotor) is as shown in the fig. Just like any other motor, it consists of a stator and a rotor. The stator core is constructed with thin silicon lamination and insulated by a surface coating, to minimize the eddy current and hysteresis losses. The stator has axial slots inside, in which three phase stator winding is placed. The stator is wound with a three phase winding for a specific number of poles equal to the rotor poles. The rotor in synchronous motors is mostly of salient pole type. DC supply is given to the rotor winding via slip-rings. The direct current excites the rotor winding and creates electromagnetic poles. In some cases permanent magnets can also be used. The figure above illustrates the construction of a synchronous motor very briefly. Fig.Synchronous motor

25 Working The stator is wound for the similar number of poles as that of rotor, and fed with three phase AC supply. The 3 phase AC supply produces rotating magnetic field in stator. The rotor winding is fed with DC supply which magnetizes the rotor. Consider a two pole synchronous machine as shown in figure below. Now, the stator poles are revolving with synchronous speed (lets say clockwise). If the rotor position is such that, N of the rotor is near the N of the stator (as shown in first schematic in figure), then the poles of the stator and rotor will repel each other, and the torque produced will be anticlockwise. The stator poles are rotating with synchronous speed, and they rotate around very fast and interchange their position. But at this very soon, rotor can not rotate with the same angle (due to inertia), and the next position will be likely the second schematic in figure. In this case, poles of the stator will attract the poles of rotor, and the torque produced will be clockwise. Hence, the rotor will undergo to a rapidly reversing torque, and the motor will not start. Now, if the rotor is rotated upto the synchronous speed of the stator by means of an external force (in the direction of revolving field of the stator), and the rotor field is excited near the synchronous speed, the poles of stator will keep attracting the opposite poles of the rotor (as the rotor is also, now, rotating with it and the position of the poles will be similar throughout the cycle). Now, the rotor will undergo unidirectional torque. The opposite poles of the stator and rotor will get locked with each other, and the rotor will rotate at the synchronous speed.

26 Characteristic Features (i) Synchronous motor will run either at synchronous speed or will not run at all. (ii) The only way to change its speed is to change its supply frequency. (As Ns = 120f / P) (iii) Synchronous motors are not self starting. They need some external force to bring them near to the synchronous speed. (iv) They can operate under any power factor, lagging as well as leading. Hence, synchronous motors can be used for power factor improvement. Advantages of synchronous motor There motors can be made to operate at a leading power factor and thereby improve the power factor of an industrial plant from lagging to one (i.e.close to unity). It gives constant speed from no load to full load. Electro-magnetic power varies linearly with voltage. These motors can be constructed with wider air gaps than induction motors, which make them better mechanically. These motors operate at higher efficiency, especially in the low speed unity power factor range. Disadvantages of synchronous motor It cannot be used for variable speed job as there is no possibility of speed adjustment. It requires d.c. excitation which must be supplied from external source. It cannot be started under loaded condition. Its starting torque is zero. It has a tendency to hunt. It is not possible for places where frequent starting is required. It may fall out of synchronism and stop when over loaded. Collector rings and brushes are required. Application It is used where high power at low speed is required. Such as rolling mills, chippers, mixers, pumps, pumps, compressor etc. V-Curves and Inverted V-Curves of Synchronous motor if excitation is varied from very low (under excitation) to very high (over excitation) value, then current Ia decreases, becomes minimum at unity p.f. and then again increases.

27 But initial lagging current becomes unity and then becomes leading in nature. This can be shown as in the below Fig. Excitation can be increased by increasing the field current passing through the field winding of synchronous motor. If graph of armature current drawn by the motor (Ia) against field current (If) is plotted, then its shape looks like an english alphabet V. If such graphs are obtained at various load conditions we get family of curves, all looking like V. Such curves are called V-curves of synchronous motor. These are shown in the below Fig.(a). If the power factor (cos Φ) is plotted against field current (If), then the shape of the graph looks like an inverted V. Such curves obtained by plotting p.f. against If, at various load conditions are called Inverted V-curves of synchronous motor. These curves are shown in the Fig.(b). Fig. V-curves and Inverted V-curves