AC Motor Control circuits

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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 motor-generator 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 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.

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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

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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.

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Figure 48. A cutaway view of an SCR.

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 1956. 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.

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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 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.

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Figure 50. Zero switching control.

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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.

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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 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.

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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.

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 220..230 Vac and about 39 KΩ for 110..120 Vic.

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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.

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.

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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 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.

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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.

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.

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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.

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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 five-layer device is shown in figure 61 (A).

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Figure 59. A cutaway view of a TRIAC.

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Figure 60. An exploded view of the TRIAC

structure shown in figure 59.

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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 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.

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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 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

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