In this article we will discuss about:- 1. Speed Control of DC Motor Using Feedback Loops 2. Speed Control of Single Phase Induction Motor Using Triac (Fan Regulator) 3. Electric Braking of Rectifier Controlled Separately Excited DC Motor.
Speed Control of DC Motor Using Feedback Loops:
The systems of speed control are open- loop systems. An open-loop system can be converted into a closed-loop system by employing a feedback loop, as illustrated in Fig. 3.29. The tacho-generator is a permanent magnet dc generator whose output emf is directly proportional to the speed. The difference of input speed setting and speed feedback provides the error signal. The error signal is amplified by the amplifier and supplied to the firing circuit of the converter. Thus the firing of converter thyristors is advanced or delayed depending on whether the actual speed is less than or more than the desired speed.
Let the desired speed of motor be 1,200 rpm, error signal be 0.1 V, converter output be 240 V and output of tacho- generator be 10 V per 1,000 rpm. If the input setting is 12.1 V, the converter output will be 240 V and motor speed will be 1,200 rpm. If the load torque causes the decrease in speed and let the speed become 1,190 rpm, the output of tacho-generator will be 11.9 V increasing the error signal to 0.2 V.
The converter output voltage will increase causing the speed to increase to 1,200 rpm. If the speed tends to increase beyond 1,200 rpm, the error signal will become less than 0.1 V and the output voltage of the converter will decrease, thus reducing the motor speed. Thus negative feedback is employed for control of motor speed.
A change in motor speed causes a temporary increase in motor and thyristor currents. Excessive current, however, can be avoided by an automatic current limiting circuit using another feedback loop, as shown in Fig. 3.30. The dc current transformer (dc CT), measures the converter output current and develops a voltage proportional to current.
This signal is fed back, as illustrated at a position between the two amplifiers. If the converter current tends to increase, the output of dc CT increases thus reducing the current error. Thus the output current of the converter is reduced to the safe value.
Speed Control of Single Phase Induction Motor Using Triac (Fan Regulator):
The conventional speed regulator for a fan uses a resistance regulator. The regulator resistance is in series with the fan motor. The speed of the fan is reduced, whenever desired, by increasing the regulator resistance. Thus the voltage drop across the regulator resistance increases and, therefore, voltage applied to the motor is reduced and so the speed. The resistance regulator causes loss of energy. This loss of energy becomes significant at low speeds.
Figure 3.55 shows a speed control circuit using a diac-triac pair. The diac is meant to trigger the triac into conduction. It is preferable to use a matched diac-triac pair. The R-C circuit forms the triggering network. By adjusting the resistor R, the voltage across capacitor C can be adjusted.
When voltage across capacitor C exceeds breakdown voltage of diac, it is triggered into conduction and sends a triggering signal to the triac gate. When triac is turned on the motor starts. By varying resistance R, The firing angle of triac can be altered and thus the voltage applied to the motor is changed and consequently the motor speed is changed. Since resistance R, carries very small current, the loss of energy is very small. Sometimes an R-C snubber circuit is added in parallel with the triac to protect it from high dv/dt.
Electric Braking of Rectifier Controlled Separately Excited DC Motor:
Figure 3.31 (a) illustrates a simplified representation of a single phase fully controlled rectifier supplying a separately excited dc motor. The polarities of voltages and directions of currents are also marked. The firing angle of the rectifier is in the range of 0 ≤ α ≤ 90°.
For regenerative braking, the direction of power flow is to be reversed. The motor has to operate as a generator and supply power to converter which now operates as an inverter and feeds back power to the supply mains. The circuit diagram for this operation is illustrated in Fig. 3.31 (b). Since the inverter output is connected to the supply mains, natural commutation is possible. It is seen that direction of flow of current in both of the circuits is the same. It has to be so since the current through a thyristor can flow only from anode to cathode. For this mode of operation 90° ≤ α ≤ 180°.
It is seen from Fig. 3.31 that direction of induced emf (back emf) in circuit shown in Fig. 3.31 (b) is reverse of that in circuit shown in Fig. 3.31 (a).
The reversal of induced emf can be had by any of the following methods:
1. By reversing the direction of flow of field current. This provides forward regeneration.
2. By reversing the armature terminals (keeping direction of rotation unchanged). This also provides forward regeneration.
3. By driving the motor by an active load coupled to it in reverse direction. No change in connection is required. This provides reverse regeneration.
If the motor is supplied by a semiconverter regenerative braking is not possible because a semiconverter operates only in a first quadrant (i.e., cannot operate as an inverter). However, plugging is possible with both fully controlled and semiconverter. In this case it is necessary to reverse direction of induced emf by any of the above three methods. However, plugging needs an external resistor to be connected to the machine (to dissipate energy) and results in wastage of energy. Therefore, plugging is generally not employed in converter fed dc motors.