Solid-State Dc Drives:Speed Control

Speed Control

The greatest advantage of using direct current motors is their variable speed characteristic. Although the ability to change motor speed is often desirable, it is generally necessary that the motor maintain a constant speed once it has been set. For example, assume that a DC motor can be adjusted to operate at any speed from 0 to 1800 rpm. Now assume that the operator has adjusted the motor to operate at 1200 rpm. The opera- tor controls are connected to the phase shift control unit (Figure 34 – 10). If the operator desires to change speed, a signal is sent to the phase shift control unit and the phase shift control permits the voltage applied to the armature to increase or decrease.

DC motors, like many other motors, will change speed if the load is changed. If the voltage connected to the armature remains constant, an increase in load

will cause the motor speed to decrease, or a decrease in load will cause the motor speed to increase. Since the phase shift unit controls the voltage applied to the armature, it can be used to control motor speed. If the motor speed is to be held constant, some means must be used to detect the speed of the motor. A very common method of detecting motor speed is with the use of an electrotachometer (Figure 34 – 11). An electro- tachometer is a small, permanent, magnet generator connected to the motor shaft. The output voltage of the generator is proportional to its speed. The output voltage of the generator is connected to the phase shift control unit (Figure 34 – 12). If load is added to the motor, the motor speed will decrease. When the motor speed decreases, the output voltage of the electrotachometer drops. The phase shift unit detects the voltage drop of the tachometer and increases the armature voltage until the tachometer voltage returns to the proper value.

If the load is removed, the motor speed will in- crease. An increase in motor speed causes an increase in the output voltage of the tachometer. The phase shift unit detects the increase of tachometer voltage and causes a decrease in the voltage applied to the armature. Electronic components respond so fast that there is almost no noticeable change in motor speed when load is added or removed. An SCR motor control unit is shown in Figure 34 – 13.

Review Questions

1. What electronic component is generally used

to change the AC voltage into DC voltage in large DC motor controllers?

2. Why is this component used instead of a diode?

3. What is a “freewheeling” or “kickback” diode?

4. Name two methods of sensing the current ﬂow through the shunt ﬁeld.

5. Name two methods of sensing armature current.

6. What unit controls the voltage applied to the armature?

7. What device is often used to sense motor speed?

8. If the motor speed decreases, does the output volt- age of the electrotachometer increase or decrease?