AC Motor Control circuits

Transcription

1 AC Motor Control circuits This part of document only provides brief definitions of the key terms and concepts that is just a part of the complete document. You may download the complete document from website just by clicking on: Symbol to download the PDF file. Electronic Controls Using SCRs Except for the Ward Leonard system, where we had a special dc generator to supply voltage to the motor, we have assumed a source of constant dc voltage. Because most of the commercial power furnished in industrial plants is ac, the problem of providing the necessary dc voltage for motor operation must be considered. One of the reason why the Ward Leonard system is so popular is that the dc excitation supply must usually be furnished anyway, so the controlling generator is not necessarily an extra piece of equipment. But there are electronic devices that may be used to deliver large amounts of dc power from ac supply lines. These devices fall into two main categories: gas-filled tubs and thyristors. In addition to being able to produce dc power from commercial ac power lines, these devices can be easily controlled to provide exact amounts of voltage or current with accuracy and precision. For this reason, many motor control systems have been developed using electronic control for use with both small and large motors. These electronic systems can respond rapidly to small signals, provide excellent automatic speed or torque regulation, and handle large amounts of power. They are also more economical and efficient than the best combination of motorgenerator sets and electromechanically control systems. Gas-filled Tubes For many years, gas-filled tubes were used to control the large currents in motor control circuits. The most popular of these gas-filled tubes was the thyratron. The thyratron is somewhat similar to the triode vacuum tube. Like the triode, the thyratron has an anode, a heated cathode, and a control grid. The difference between the two tubes is that the thyratron's envelope is filled with gas. Thyratron tubes have been valuable in industrial application because the grid allows large currents to be 1

2 controlled by a comparatively small signal voltage. The control characteristics of he thyratron, however, are entirely different from those of a vacuum tube. A thyratron is basically a "controlled gas rectifier". Triode, tetrode and pentode variations of the thyratron have been manufactured in the past, though most are of the triode design. Because of the gas fill, thyratrons can handle much greater currents than similar hard vacuum valves/tubes since in the ionized gas electron multiplication occurs (each electron leaving the cathode may generate 4 more electrons) by collisions of electrons with gas atoms, using the phenomenon known as a Townsend discharge. The average speed of the ions in the gas is much lower than that of the electrons, so that the ions may only account for 10% of the total current. Gases used include mercury vapor, xenon, neon, and (in special high-voltage applications or applications requiring very short switching times) hydrogen. Unlike a vacuum tube, a thyratron cannot be used to amplify signals linearly. Figure 46. An hydrogen thyratron, used in pulsed radars, next to miniature 2D21 thyratron used to trigger relays in jukeboxes THYRISTORS A thyristor is a solid-state equivalent of the thyratron tube. Most jobs that previously required thyratrons are now being accomplished more efficiently with thyristors. There are two common types of thyristors: the silicon-controlled rectifier (SCR) and the TRIAC. Figure 47 illustrates the schematic symbol for the thyratron, the SCR, and the TRIAC. The characteristics of the SCR are similar to those of the thyratron. Both are unilateral devices; that is, they pass current in only one direction. Both are turned on by applying a positive voltage to their control elements 2

3 Figure 47. A comparison of the schematic symbols for (A) a thyratron, (B) and SCR, and (C) a TRIAC. The control element of the SCR is called the gate and is equivalent to the thyratron grid. Once turned on, or fired, the control element loses all control, and current continues through the device as long as the anode current is above its holding current. The SCR was especially designed to act like the thyratron, and its characteristics are quite similar. But the SCR has several advantages over the thyratron. For example, the voltage drop across the device when it conducts is usually on the order of 0.6 to 1 volt about one-tenth of the forward voltage drop of the thyratron. Also, because the SCR requires no filament, it operates more efficiently and significantly cooler. In addition, the SCR can be turned off in a fraction of the time it takes to turn off the thyratron. Finally, the SCR is considerably smaller and more rugged than the thyratron. The Silicon-Controlled Rectifier The operation of the SCR is normally explained in terms of the two-transistor analogy. In this analogy, the transistor equivalent of the SCR is used to explain its unique characteristics. Figure 48 shows the construction and physical shape of a typical SCR. Figure 48. A cutaway view of an SCR. 3

4 A thyristor is a solid-state semiconductor device with four layers of alternating N and P-type material. They act as bistable switches, conducting when their gate receives a current trigger, and continue to conduct while they are forward biased (that is, while the voltage across the device is not reversed). Some sources define silicon controlled rectifiers and thyristors as synonymous. Other sources define thyristors as a larger set of devices with at least four layers of alternating N and P-type material. The first thyristor devices were released commercially in Because thyristors can control a relatively large amount of power and voltage with a small device, they find wide application in control of electric power, ranging from light dimmers and electric motor speed control to high-voltage direct current power transmission. Originally thyristors relied only on current reversal to turn them off, making them difficult to apply for direct current; newer device types can be turned on and off through the control gate signal. A thyristor is not a proportional device like a transistor. It operates only fully on or fully off, making it unsuitable as an analog amplifier. Figure 49. An SCR rated about 100 amperes, 1200 volts mounted on a heat sink - the two small wires are the gate trigger leads. The SCR is ideal for controlling the amount of ac power supplied to a motor. If an SCR is connected in series with the motor, and the gate and anode are driven in phase, the SCR acts like a half-wave rectifier. This arrangement is shown in fig. 50 (A). in this example, the trigger circuit is simply a voltage divider that applies a fraction of the input voltage to the gate. By comparing the ac applied to the anode, shown in figure 50 (B), with the ac applied to the gate, shown in fig. 50 (C), we can see that the gate and anode are driven in phase. When the anode and gate both swing positive ant the same time, the SCR conducts. When an SCR conducts, it acts almost like a short circuit. During this time, the voltage across the SCR is practically zero, as shown in figure 50 (E). this means that most of the entire supply voltage is across the motor. The voltage across the motro is shown in fig. 50 (D).this condition continues for as long as the SCR conducts. But the SCR stops conducting as soon as the anode swings negative. When the current in the circuit attempts to reverse, the SCR becomes reverse biased and blocks current. In this 4

5 condition, the SCR acts like an open circuit. The entrie input voltage is now across the SCR and no voltage is across the motor. By observing the waveforms we can see that the SCR is acting like a simple half-wave rectifier. This mode of operation is call Zero switching control and is characterized by the fact that the SCR conducts for the entire 180 of each cycle. The voltage waveforms for a typical phase-control circuit are shown in figure 51. Compare the motor voltage curve in figure 51 (B) with the one in figure 50 (D). Note that with phase control, current flows through the motor for less than 180 of each cycle. That portion of the cycle in which current is flowing through the motor is called the conduction angle. The firing angle is controlled by the gate potential. In the circuit shown in fig. 50, the gate potential becomes positive ant the same time as the anode potential. Therefore, the firing angle is zero degrees and the conduction angle is 180 degrees. In phase-controlled circuits, the gate potential is held below the firing point until the anode is at some positive value. One way to accomplish this is to shift the phase of the ac applied to the gate. In this way, the anode can be made to swing positive a number of degrees before the gate. A more common method is to place a triggering device in the gate circuit. The triggering device can be a simple neon bulb or a trigger diode. On the other hand, some triggering circuits are quite complex and use many components. Let's consider some of the most common triggering methods. Figure 50. Zero switching control. 5

6 Figure 51. Phase control waveforms. Triggering Devices We have seen that if the gate and anode are driven in phase, the SCR conducts as soon as both swing positive. In this case, the SCR acts like a half-wave rectifier, conducting for the entire positive half-cycle. Bu using certain triggering devices, the gate current can be held off until the anode potential is several degrees into the positive half-cycle. In this way, conduction angles considerably less than 180 degrees can be achieved. There are many different devices suitable for triggering SCRs. Symbols for several types of commonly used trigger devices are shown in figure 52. Figure 52. Common trigger devices. Neon Lamps The neon lamp is a small gas-filled bulb that is often used as a pilot or indicator light. This device will conduct current equally well in either direction. The neon gas in the bulb, however, will not ionize below a certain voltage. The voltage at which the gas ionizes is called the ionization potential, breakdown voltage, or starting voltage. At 6

7 voltages below the ionization potential, the bulb acts like an open circuit. But once the ionization potential is exceeded, the bulb conducts quite readily. It is this characteristic that makes the neon bulb suitable for triggering an SCR. The ionization potential varies from about 50 to 120 volts, depending upon the type of bulb used. A circuit containing a neon bulb trigger is shown in figure 53 (A). The load is placed in series with the SCR. C1, R1 and L1 make up the trigger circuit. Q1 is initially cut off because no gate current can flow through L1 until it ionizes. Let's assume that the ionization potential of L1 is 100 volts. When power is applied to the circuit, C1 will start charging toward the peak voltage of the AC sine wave. As the voltage swings positive, C1 charges through R1, as shown by the solid arrow in Fig. 53 (A). The voltage across the capacitor is shown in Fig. 53 (B). When the charge on the capacitor reaches 100 volts, the neon bulb, L1, ionizes and C1 discharges through the cathode-gate junction of Q1 and the neon bulb, as shown by the dashed arrow in fig. 53 (A). this triggers Q1 into conduction. As you can see from Fig. 53(B), the SCR conducts for considerable less than 180 degrees. By making R1 variable, we can control the conduction angle over a broad range. Figure 53. (A) a neon bulb trigger. (B) voltage across the capacitor Neon bulbs have the advantages of being inexpensive and reliable. On the other hand, precise control cannot be easily attained because of wide variations of the ionization potential of the neon bulbs. Variations of +/-30 percent or greater are common. Also, the ionization potential is affected by the amount of light falling on the neon bulb. In other words, a neon bulb exposed to light will ionize at a direct voltage than the same bulb in darkness. These disadvantages are overcome by the trigger diode. 7

8 The main voltage monitor The simplest application is the main voltage monitor that is simply a lamp that glows when the main voltage is present. To obtain such a monitor it's enough to connect a resistor in series with the bulb and connect at a main outlet. The resistance of that resistor may vary on the type of bulb and the main voltage but it's not critical: about 150 KΩ for Vac and about 39 KΩ for Vic. Figure 54 application of a neon lamp as a main voltage monitor. Trigger Diode. The trigger diode is the solid-state equivalent of the neon bulb as far as its breakdown characteristics are concerned. It is a four-layer avalanche device, similar to an SCR, which breaks down when its voltage rating is exceeded. Typical breakdown voltages for trigger diodes range from about 20 volts to 120 volts with a tolerance of +/- 10 % or better. This device will pass current flow in either direction. Before breakdown, trigger diodes offer high impedance. At the breakdown voltage, the diode conducts and the potential across it drops to a low value. The trigger diode could replace the neon lamp in fig. 53 (A). The circuit would work equally well. Two-Transistor Switch The two-transistor switch requires two complementary transistors. When properly connected in a circuit, the switch is turned on by forward biasing the base-emitter junction of the NPN transistor. This transistor then conducts through the base-emitter junction of the PNP transistor, which in turn increases the base current of the NPN transistor. As you can see, each transistor conducts through the base of the other. The feedback is regenerative and bother transistors become completely saturated within a few microseconds after a trigger is applied. Later, we will study a circuit that uses the two-transistor switch. Speed Regulation Figure 55 (A) shows a simple circuit for controlling the speed of a universal motor. The circuit requires only a few components and yet is quite effective. It uses the counter EMF (cemf) developed by the motor as a feedback voltage to indicate how fast the motor is turning. As we have learned, the counter EMF is directly proportional to the speed of rotation of the motor. If the speed of the motor changes, then the counter EMF also changes. In the circuit shown in fig 55 (A), we are concerned with the counter EMF produced driving that portion of the cycle in which the SCR in not conducting. 8

9 You may wonder how the counter EMF is produced during this period when no voltage is applied to the motor. Remember that the motor is turning and that a slight magnetic field exists because of the residual magnetism of the field core. Therefore, a counter EMF is produced by the armature windings, even though there is no current through the armature. The counter EMF is of the polarity shown in fig. 55 (B). that is, the end the armature, which is connected to the cathode of the SCR, is positive. This small positive voltage holds the SCR cut off until the gate swings more positive than the cathode. The gate voltage, Eg, is determined by the setting of the arm of R1. D2, R1, and R2 make up the gate firing circuit. D2 acts as a half-wave rectifier, and R1 and R2 act as a voltage divider. During the negative half-cycle of the input voltage, D2 is cut off. But as the Input voltage crosses zero volts and starts to swing positive, D2 conducts and a positive voltage is developed at the arm of R1. Figure 55 This voltage, Eg, is shown in Figure 55 (B), where it is compared with the counter EMF on the cathode of the SCR. Note that from point A to point B on the diagram, the gate voltage is less positive than the cathode voltage. During this time, not gate current can flow and the SCR is held cut off. However, the gate voltage continues to increase. At point B, the gate voltage is equal to the SCR cathode voltage. As the gate becomes more positive than the cathode, gate current starts to flow through D1, turning on the SCR. Once the SCR fires, the gate loses control and the SCR continues to conduct for the remaining portion of the positive half-cycle. As the ac supply voltage swings negative once more at point C, the current through the motor is blocked by the reverse-biased SCR. Current through the motor is from point B to point C of each positive half-cycle of the input voltage. The voltage applied to the motor is shown in fig 55 (C). note the relationship between point B on the gate voltage curve and the same point on the motor voltage 9

10 curve. At this point, the gate voltage is equal to the counter EMF. This point can be varied by adjusting potentiometer R1. R1 is the speed control adjustment for the motor. To see how it works, let's assume that R1 is set to midrange and that the motor is turning at a constant speed. At this speed, a certain value of counter EMF is applied to the cathode of the SCR. As we saw earlier, the SCR fires any time its gate goes more positive than its cathode. Now let's move the arm of R1 up and see what effect this has on the speed of the motor. When the arm of R1 is moved up, Eg increases as shown in fig. 56 (A). This means that Eg becomes equal to the counter EMF earlier that when R1 was set at midrange. Consequently, the SCR fires earlier in the cycle. As you can see by referring to Fig 56 (A), the firing point has moved form point B1 to point B2. Figure 56 (B) shows that the conduction angle has been increased by the same amount. Therefore, the average value of the voltage across the motor has been increased. This results in increased armature current and a high motor speed. Figure 56. The effects of R1 on gate and motor voltages. Moving the arm R1 down from midrange has just the opposite effect. When the arm of R1 is moved toward D2, Eg decreases, as shown in fig 56 (C). this means that Eg becomes equal to the counter EMF at a later point in the cycle and the SCR fires later. The average voltage applied to the motor is reduced as shown in fig 56 (D), and the motor slows down. 10

11 Direction control Figure 57 shown a circuit in which both the speed and direction of a motor can be controlled. It does not provide speed regulation. By using a shunt-wound dc motor, however, a near constant speed can be maintained under most load conditions because the shunt motor has excellent speed regulation. The circuit has two manual controls: R1 is the speed control, and S1 is a switch that determines the direction of motor rotation. The switch s shown in the clockwise, CW position. In this position, the motor rotates clockwise. If S1 is moved to the CCW position, the motor rotates is reversed by reversing the direction of the current through the armature of the motor. Figure 57. A direction and speed control circuit for shunt-wound dc motor. Position Servo Control Figure 58 shows a position servo that can be used to position the motor at precise points. A dc shunt motor is used because it can easily be reversed by reversing the armature current. The field winding is supplied with a constant de voltage by the half-wave rectifier, D1. C1 helps filter the pulsating dc by discharging through the field winding when D1 is not conducting. 11

12 Figure 58. A position control servo using SCRs. The armature if the motor is connected so that current can flow through it in either direction. If both SCRs are conducting, current will flow through Q2, the armature, and D2 during the positive half-cycle and through Q1, the armature, and D3 during the negative half-cycle. In this condition, the current through the armature is reversed 60 times a second. This is the same as applying ac to the armature. AC on the armature of the shunt dc motor locks the armature in one position. This is actually a type of dynamic breaking that holds the armature fast in its present position. Electronic Controls Using TRIACs The TRIAC or bidirectional thyristor can be considered as two pnpn structures connected in parallel. By orienting the two structures in opposite directions, identical bidirectional electrical characteristics are achieved. Figure 59 is a cutaway view of a TRIAC pellet that shows the two PNPN structures. The half of the pellet on the right is similar to the conventional SCR. Current flow in this half of the TRIAC is from anode 1 to anode 2. This current is turned on by forward biasing the p2-n2 junction. Like the SCR, this half of the TRIAC is fired by applying a positive potential to the gate. Once this side fires, the TRIAC acts exactly like the SCR. It conducts a heavy current from anode 1 to anode 2 until the applied voltage is reversed or at least reduced to a low value. The left half of the TRIAC is also similar to an SCR. In this half, however, current flow is from anode 2 to anode 1 because an extra layer of n-type material (n3) has been added to the left side of the pellet. The result is a five layer device that operates like a conventional SCR, but has a different type of firing mechanism. Figure 60 shows an exploded view of the same TRIAC structure shown in figure 59. Referring to these two figures, notice that the gate terminal connects to both the n2 layers. The five layers (n2, p2, n1, p1, and n3) make up the second SCR. Figure 61 (A) and (B) show how the transistor equivalent circuit is developed. The basic fivelayer device is shown in figure 61 (A). 12

13 Figure 59. A cutaway view of a TRIAC. Figure 60. An exploded view of the TRIAC structure shown in figure

14 Figure 61. The transistor equivalent of the left side of a TRIAC. For the purpose of explanation, let's consider this device as three separate transistors. The three-transistor equivalent circuit is formed by separating sections p1, n1, and p2 at the dotted lines, as shown in figure 61 (B). Sections n2, p2, and n1 from an NPN transistor, which we will call Q1. Sections p2, n1, and p1 from a PNP transistor, Q2. Finally, sections n1, p1, and n3 form an NPN transistor, Q3. In figure 61 (C), we have merely replaced the three structures with their transistor symbols. The various layers are labeled for easy reference. Now let's see how this device operates. Let's assume that anode 1 is positive with respect to anode 2. Without a signal at the gate, no current flows in the device. When a negative signal is applied to the gate (emitter of Q1), Q1 conducts, providing a base current for Q2. This causes Q2 to conduct. Notice that Q2 conducts through the emitter-base junction of Q3. This triggers Q3 into conduction. But the current path for Q3 is through the emitter-base junction of Q2. This greatly increases the conduction of Q2, which in turn increases the conduction of Q3. As you can see, the action is regenerative and the current will rapidly increase until both transistors are saturated. This regenerative action is exactly like that in the conventional SCR except that current flow in from anode 2 to anode 1. Transistor Q1 is used only to start the regenerative conduction of Q2 and Q3. The only difference in the operation of the two sides of the TRIAC is the method in which they are fired. As you see, the TRIAC can conduct current in either direction and can be triggered on by either polarity of gate voltage. In other words, if anode 2 is positive with respect to anode 1, the TRIAC is turned on by a negative voltage on the gate. Figure 62(A) shows a simple TRIAC circuit that provides full-wave control for a small universal (series) motor. You may recall that a universal motor will operate with ac or dc. The TRIAC is in series with the motor and controls the average current to the motor. R1 is a manual speed adjustment. C1 and D1 make up the TRIAC trigger circuit. This circuit differs from the SCR circuits studied earlier in that the TRIAC is triggered on during both the positive and the negative half-cycles. Let's see how the circuit operates. When the ac supply voltage is at 0 volts, as shown by point A in figure 62 (B), the TRIAC is cut off because there is no difference in potential between anode 1 and e voltage across the TRIAC rises, as shown in fig 62 (D). However the TRIAC remains cut off because there is no path for gate current until the trigger diode, D1, breaks down. During the time from point A to point B, C1 is charging 14

15 through R1 toward the peak positive voltage. But just as the input ac peaks at point B, the charge on C1 exceeds the breakdown voltage of D1. Figure 62. A full-wave control for a universal motor using a TRIAC. When D1 conducts, it allows C1 to discharge partially through the anode 1 gate junction of the TRIAC. The TRIAC fires, passing a heavy current from anode 1 to anode 2 through the universal motor. For the reminder of the positive half-cycle, the TRIAC acts like a closed switch. The voltage across the TRIAC drops to a low level, leaving almost the entire line voltage for the motor. Form point B to point C of the supply waveform, current flows from right to left through the motor. When the input voltage returns to 0 volt at point C, the current through the TRIAC drops to 0 and the TRIAC cuts off. The TRIAC will remain at cutoff until it is triggered again by the trigger circuit. As the ac input voltage reveres in polarity, anode 2 becomes negative with respect to anode 1. At point D, the TRIAC is dropping the entire peak negative voltage. During the time from point C to point D, C1 has been charging. At point D, the charge on C1 again exceeds the breakdown voltage of D1. This time the capacitor discharges from the upper plate, through D1 and the gate anode 1 junction of the TRIAC. The TRIAC fires and passes a heavy current from anode 2 to anode 1. This time current through the motor is from left to right. This is where the TRIAC differs from the SCR. Figure 62 (C) shows that the voltage across the motor is a chopped ad. In this example, the conduction angle is 90 degrees. However, this angle can be changed by adjusting R1. Moving the arm of R1 up decreases the resistance in the charge path of C1, allowing it to charge faster. C1 charges to the breakdown voltage of D1 earlier in each half-cycle. This increases the conduction angle of the TRIAC and increases the motor speed. While this circuit works quite well for universal motors, it will not work for shunt dc motors nor for certain types of ac motors. We saw earlier that the armature of a shunt 15

16 dc motor is locked in a stationary position when ac is applied to it. Only limited control is possible with synchronous ac motors because the speed of the synchronous motor is almost completely independent of the average voltage and the current applied to it. if the power supplied is adequate, the motor turns at its synchronous speed. If the power is not adequate, the motor simply does not turn at all. The speed of motor is determined by the line frequency, the number of poles, and the slip. Obviously, it would be quite expensive to provide a variable line frequency. Also, the number of poles is determined by the manufacturer when the motor is mad e and this number can not easily be change. If we are to control the speed of the synchronous motor, we must do it by controlling the slip. The slip of a synchronous motor is influenced by factor such as the motor load, friction, and the shape of the applied waveform. While we can do little to control the load or friction, we can readily modify the power waveform by using TRIACs. Even so, the range of speed control is limited. For this reason, the TRIAC is normally used only with the universal motor. Key terms discussed on the download file: Stepper Motors, Special-purpose motors, variable-reluctance, permanent-magnet types, variable reluctance, Variable-Reluctance Stepper Motor, Permanent-Magnet Stepper Motor, clockwise or counterclockwise rotation is produced, Motor Starters, Starting Characteristics, Automatic-Resistance Starters, Braking, Mechanical braking, Dynamic braking, Clutches, Electromechanical Control Systems, In addition, conveyor systems and other types of machinery all have different requirements regarding rotation, position, speed, and torque, Motor characteristics, dc shunt motor is the easiest to control over wide ranges of speed and torque, shunt motor's direction of rotation may be reversed easily by reversing either the field connections or the armature connections, single-phase ac motor is naturally a constant-speed motor, frequency of the voltage supply, the number of poles, and the amount of slip, Twophase ac motors are used in some control applications, 3-phase ac induction motors are also generally considered to be constant-speed motors, Speed, Speed is the most commonly controlled motor characteristic and is also the easiest to understand, Torque, Torque is the twisting or turning force of the motor that tries to produce rotation, The torque measured in this fashion with the motor unable to turn is called stall torque, Horsepower, Horsepower is a unit of measure used to determine the rate of doing work, Horsepower may be expressed in either electrical terms or mechanical, Hp = (2 pi TS) (33,000) = 6.28 TS (33,000) = TS, Elementary Control Circuits, Direction of Rotation, A DPDT switch for reversing dc motor, A motorrevising circuit provides automatic delay, Control of position, A position control using polarized relay as a sensing device, WARD LEONARD SYSTEM, A schematic of Ward Leonard Motor Control system, 16

17 AC Motor Control circuits

18 Introduction Welcome to another course offered from educational department of Plc-doc.com. These technical educational programs are designed to educate those who come to checkout tutorial materials related to single base computers or PLCs. This course shall cover AC Motor Control circuits and some related components or products. Table of content Special-purpose Motors, Stepper motors Variable-reluctance Stepper Motor Permanent-Magnet Stepper Motor Motor Starters Automatic-Resistance Starters Braking, Mechanical Braking Dynamic Braking Cluches Electromechanical Control Systems, Motor Characteristics Speed, Torque Horsepower Elementary Control Circuits Direction of Rotation Control of Position WARD LEONARD SYSTEM

19 Special-Purpose Motors Stepper Motors There are applications in electronics in which discrete steps are required rather than uniform rotation produced by the motors discussed earlier. One method to accomplish this is with stepping switches or stepping relays. These devices use a relay that is mechanically coupled to a ratchet wheel and pawl. Each time the relay energizes, the ratchet wheel turns one space. This system works reasonably well if the stepping rate is fairly low. However, for stepping rates above a few steps per second, other methods must be found. A method in wide use today involves the use of stepper motors. A stepper motor is a special type of dc motor that rotates or steps in discrete increments. Stepper motors are driven by electronic pulses produced by a drive circuit. Each pulse causes the motor to increment on step. By applying pulses in the proper sequence, the motor can be made to step clockwise or counterclockwise. Stepper motors come in several types and many sizes. The two most common types are the variable-reluctance and permanent-magnet types. The variable reluctance type is needed when high pulse rates are used and small stepping angles are acceptable. Typical rates and increment angles for the variablereluctance stepper motor are 1200 pulses per second (maximum) and angles of 15 and 7.5. The permanent-magnet stepper motor is popular in applications where larger angles such as 90 and 45 are acceptable. Because of the larger steps, a lower pulse rate (300 pulses per second, maximum) is required to drive the motor. Variable-Reluctance Stepper Motor A simplified diagram of a variable-reluctance stepper motor is shown in fig.30. This motor has six field windings spaced 60 apart. The rotor has four teeth spaced 90 apart. In fig.30, a positive pulse has just been applied to line A, as indicated in waveform A, so current flows from ground through winding 4 and winding 1 to point A. Winding 1 develops a magnetic north pole, while winding 4 develops a magnetic south pole. Figure 30 19

20 The magnetic lines of flux seek the path of least reluctance between the north and south poles. The reluctance of the soft iron rotor is much less than the reluctance of air. Seeking the position of least reluctance, the teeth of the rotor line up with the energized poles. Rotor tooth a lines up with field core 1 and rotor tooth c lines up with field core 4. After the rotor is aligned as shown in fig 30, the pulse on line A ceases. If a positive pulse is now applied to line B, current will flow from ground through winding 5 and winding 2 to point B. A magnetic north pole is developed at winding 2 and a south pole is developed at winding 5. These magnetic poles attract the nearest rotor teeth. Rotor tooth b lines up with winding 2 and rotor tooth d lines up with winding 5 that is, the rotor moves 30 in the counterclockwise direction. After applying the pulse to line B, a third positive pulse on line C will pull rotor teeth a and c into line with poles 6 and 3, producing another 30 counterclockwise step. By applying pulse to lines A, B, and C in the proper sequence, the motor can be made to rotate clockwise or counterclockwise in discrete steps. Notice that the size of the steps is equal to the difference between the rotor teeth spacing (90 ) and the field winding spacing (60 ), or = 30 steps, 12 pulses applied in the correct sequence are necessary for one complete revolution. Many other combinations of rotor teeth and field windings are possible. Step angles of 15 can be achieved by using rotor teeth spaced 45 apart and 12 field windings spaced 30 apart. Notice also that the same result can be achieved by using six rotor teeth spaced 60 apart and eight field windings spaced 45 apart. One disadvantage of this stepper aside from the small residual fields of the rotor and poles, is that when there is no excitation, the rotor is not firmly locked to its new position. Some sort of detent or brake must be used to ensure accurate positioning when the motor is not being pulsed. The permanent-magnet stepper motor dos not have this limitation. Permanent-Magnet Stepper Motor The field windings of the permanent-magnet stepper motor are similar to those used in the variable-reluctance type. The rotor, however, is quite different. A powerful permanent magnet is used for the rotor. When the field winding is energized, the poles of the magnet line up with the poles of the field. The direction of rotation is determined by the sequence in which the field windings are energized. Once the rotor is stepped into a position, it remains there until a different field winding is energized. That is, the permanent magnet is attracted to the core of the field winding even after the field winding is de-energized. This is called magnetic detenting and requires no ratchets, brakes, or other wearing parts. In certain applications, this is a definite advantage over the variable-reluctance motor, which requires mechanical detents. The principle of the permanent-magnet stepper motor can best be described by looking at fig.31. This is the simplest version of this type of motor. The field is composed of four windings spaced 90 apart. Each winding is energized separately. The rotor is a specially shaped permanent magnet having two poles. Let's examine the operation of the motor. With no pulse applied to either winding, the rotor is magnetically detented in the position shown. This is the position the rotor was in when the last pulse was removed during a previous operation. We can cause the rotor to rotate 90 clockwise by applying a positive pulse to point B. 20

21 Figures 31 and 32 A positive pulse at point B causes current flow through field winding 2, which establishes a north magnetic pole in the core of the winding. This north pole attracts the south pole of the rotor. Subsequently, the rotor turns 90 to align the two unlike poles. If a positive pulse is then applied to point C, the rotor rotates another 90. By applying positive pulses in the sequence A, B, C, D, A, B, C, D, etc., the rotor rotates clockwise in 90 steps. The rotor can be made to rotate counterclockwise by reversing the polarity of the pulses or by reversing the sequence of positive pulses. Smaller angles are possible with the arrangement shown in fig.32. Here, eight field windings and a four-pole rotor are used. This arrangement will produce 45 steps. For clarity, the actual field windings are not shown. A positive pulse applied to winding 2 and 6 produces south magnetic poles that attract the north poles of the rotor. Therefore, the rotor rotates clockwise 45 from the position shown. A pulse applied to windings 3 and 7 causes an additional 45 step. By applying pulses in the proper sequence, clockwise or counterclockwise rotation is produced. Motor Starters In this method, some voltage-dropping resistances are placed in series with the motor during starting. The impedance seen by the power system then is that of the resistances plus that of the motor. Starting Characteristics: 1. Motor terminal voltage is reduced from line voltage. 2. Motor current equals line current. 3. Starting torque is reduced by the square of the terminal voltage. 21

22 Applications: 1. Usually on low voltage (less than 600 v). 2. Where current reduction requirements are low, or where load torque during acceleration is minimal. 3. Not often used with large motors because of the high heat loss in the resistors. 4. May be used for full acceleration or for system voltage recovery. Figure 33 Automatic-Resistance Starters We can easily adapt the principle of resistance starting to automatic control through the use of relays or other electromechanical devices. By doing this, we gain much closer control of motor acceleration, making the motors come up to speed more smoothly. A typical automatic-resistance starter is shown in the following illustration. The power circuit is fairly straight forward. Contactor K1 closes and the supply is connected to the motor via the starting resistors. After a time delay, contactor K11 closes, shorting out the starting resistors and connecting the motor directly to the supply. 22

23 Figure 34 Functioning of the control circuit begins with the pressing of momentary start button. This energizes K1, which is then latched and power is supplied to the motor via the resistors. After a time delay, a K1 delayed contact energizes K11, shorting out the resistors. Pressing the stop button removes all power and stops the motor. Braking We have already mentioned that the inertia of the large motor or heavy load will often cause the motor to continue turning long after it is disconnected from the line. In many industrial applications, it is necessary to stop the motor and its load quickly, with a great deal of accuracy with regards to the position of the load. In such cases, it is impractical, if not impossible, to let the motor coast to a stop. For this reason, many types of brakes and braking systems have been developed for use with motors of all types and sizes. Mechanical braking One of the best known types of brake is the common friction brake. It consists of two round discs, or rings, with flat faces. One of these face plates is attached to the motor shaft and rotates with it. The other is fixed in position so that it cannot rotate. The braking action is caused by brining the discs together with enough force so that the rotating disc rubs against the stationary disc. The friction caused by this rubbing rapidly slows down and finally stops the rotating element. 23

24 Nearly all brakes operate on this simple principle. There are, however, many different methods of brining the two surfaces together. In some applications, mechanical linkages are used to position the face plates. But in most electric motor installations, the brakes are operated electrically or magnetically. A cross-sectional view of a typical braking system for small motors is shown in the following illustration. Figure 35 In this particular brake, the rotor and field poles of the motor itself from part of the braking system. When the motor is de-energized, no current flows through the field winding and there is little or no magnetic field surrounding the field poles. The spring at the right-hand end of the motor shaft forces the rotor itself against the left end of the motor frame. The friction between the cork disc on the motor frame and the end of the rotor holds the motor in position. When the motor is energized, the same field poles that create the torque-producing magnetic field or rotating the motor also create a pulling force on the rotor. This pulling force has the same effect on the rotor of the motor as a solenoid winding has on a plunger. The rotor is pulled to the right against the force of the spring until it is centered between the field poles. The rotor no longer touches the cork disc and the motor turns and rapidly comes up to speed. When the motor is de-energized again, the magnetic centering force of the filed poles is removed and the spring again forces the rotor against the cork disc. This slows down the motor to a full stop and holds the shaft in position. Dynamic braking A motor can be made to supply a large part of its own braking action by changing the connections so that it functions as a generator. When this is done, the inertia of the motor and the load becomes a driving force tat causes the motor to generate electricity. The energy expended in this generator action quickly uses up most of the momentum of the motor and slows down rapidly. This type of braking is not effective for completely stopping the motor because as the armature slows down, less current is generated, and therefore less braking action is 24

25 produced. Consequently, dynamic braking is used to slow the motor down to a low speed, and then a typical friction brake is used to slop the motor completely. Figure 36 shows the schematic of a shunt-wound motor and the circuitry required for dynamic braking. With the switch in the run position, R, as shown, fig 36A, both the motor field and the armature are energized. The braking field is energized, which releases the friction brake, and the motor runs normally. To stop the motor, the switch is thrown to the brake position, B, as shown in the fig 36B. The friction brake field and the motor field are still energized, but the armature is disconnected from the line and connected to the braking resistor. Figure 36 Because the braking field is still energized, the friction brake remains released and the armature rotates in the motor field. This generates a current in the armature that flows through the braking resistor. The work performed by the armature in generating this current rapidly expends the momentum of the motor and the load, which is what causes the armature to turn in the first place. This generation of current causes the motor to slow down rapidly. By adjusting the resistance of the braking resistor, we control the amount of current that the armature generates which in turn controls the amount of work being done and the force of the braking action. As the motor slows down to a speed where the amount of current generated is small and the dynamic braking force is rapidly falling off, we can throw the switch to the off position, O. This de-energizes the motor field, the braking field, and the armature. 25

26 When the braking filed is de-energized, the friction brake is set finally to stop and hold the motor. In this way, the dynamic braking action does most of the work in stopping the motor, saving wear and tear on the friction brake. The dynamic braking connections for a series-wound dc motor must be slightly different, but the barking action is essentially the same. In a series motor, disconnecting the armature also disconnects the motor field. If we simply disconnected the armature from the line and connect it to the braking resistor, there would be no appreciable barking current generated because there would be no magnetic field. The motor would merely continue to coast to a stop. In the dynamic braking of a series motor, therefore, the field is disconnect from the armature and connected in series with a resistance across the line. The series motor is temporarily converted to a shut-wound motor generator by circuit such as that shown in fig37. The switch is in the run position and the motor is connected and runs as a typical series motor. But, if the switch is thrown to the braking position, B, you can see that the armature will be connected across the braking resistor and the motor filed will be connected across the line in series with the special current-limiting field resistor. Because this effectively makes the series motor a shunt-wound generator, the dynamic braking action will be the same as it was with the shunt motor. Although dynamic braking is effective, it is seldom used except for extremely large motors and loads. Remember that in the braking circuits of fig 36 and fig.37, we have shown only the connections necessary for the braking action. In practice, the circuits will probably be modified to include the starting and speed control circuits as well. The actual connection may vary somewhat from those that we have shown, but the principle will still be the same. Figure 37 26

27 Clutches Clutches are often used between motors and their loads to allow easier starting and braking. Where large motors are used to drive heavy loads, a clutch to disengage the load during starting reduces both the time and starting current needed to bring the motor up to speed. Disengaging a motor from its load during braking allows either the motor or the load to be braked separately as desired. In many instances, only the load needs to be braked and the use of a clutch permits the braking force to be applied only to the load while the motor continues to coast to a stop. Although there are many different types of clutches, the friction clutch is the most common. These clutches are inexpensive, efficient and easily maintained. While they are basically mechanical, they can be controlled by mechanical levers and linkages, electromechanical devices such as the solenoids, or electromagnetic devices like the brakes we just studied. A typical electromagnetically operated friction clutch is shown in fig. 38. You will notice that it is the same as a brake except that both face plates can rotate and are connected to separate shafts. When the clutch is de-energized, the two face plates are separated and each shaft will be completely independent of the other. But, when power is applied to the field coils, the two face plates will be magnetically attracted to each other and the two shafts will be joined together. Any force applied to one shaft will be transmitted through the face plates to the other shaft, and they will operate as though they were a single shaft. Clutches of this type or of a similar type will be found in most power-driven machinery. While they may not be at the motor itself or at the load, they will be somewhere in the system of shafting and gearing between the two. Figure 38 27

28 Electromechanical Control Systems The control of motors includes many factors besides safe starting and efficient braking. For example, the direction of rotation of a passenger elevator drive motor must be controlled accurately to send the elevator either up or down as desired. It also must bring the elevator to a stop at the selected floors in such a way that the floor of the elevator is carefully positioned with respect to the floor level. Between floors, the elevator must be driven at nearly constant speed, whether it carries one passenger or several. When either starting up or stopping, the acceleration or deceleration must be rapid but smooth. On the other hand, the motor used for driving the take-up reel of a magnetic tape deck must meet entirely different requirements. In winding the tape, the tape must be pulled at a constant tension and at a certain rate of speed. As the tape is wound, the diameter of the take-up spool hub is effectively increased by the layers of tape. As this happens, the motor torque must be increased and its speed decreased to maintain the proper tape speed. In addition, conveyor systems and other types of machinery all have different requirements regarding rotation, position, speed, and torque. To meet these widely varying requirements, motors may be either manually or automatically controlled, depending on the motor and the application. We will consider some of the characteristics of these various types of motors and discuss some of the more common electromechanical control systems. Motor characteristics The dc shunt motor is the easiest to control over wide ranges of speed and torque. The shunt motor's direction of rotation may be reversed easily by reversing either the field connections or the armature connections. On the other hand, the single-phase ac motor is naturally a constant-speed motor. Its speed is determined by the frequency of the voltage supply, the number of poles, and the amount of slip. The only two possible variables are the supply frequency and the slip. Neither of these provides an adequate method of speed control. To vary the frequency for speed control, we would need a variable-speed generator of some sort or a power oscillator. Although these devices provide reasonably good speed control, they are relatively expensive and require a lot of extra equipment. The slip can be varied to some extent by varying the load or the supply voltage to vary the speed. But this greatly reduces the efficiency of the motor and the motor tends to overheat. Two-phase ac motors are used in some control applications because the speed may be varied by relatively small amounts by changing the current in the control field. The direction of rotation can also be reversed by shifting the phase of the control field 180. But even the two-phase motor is relatively insufficient at low speeds and its use is limited to light loads in speed control applications. 28