# DC Motor Control Circuits

## DC Electric Circuits

• #### Question 1

How is it possible to electrically measure the torque output by a permanent-magnet DC motor? Hint: it is very simple, and for large electric motors it involves the use of a shunt resistor. Modify this circuit diagram to include a meter that provides indirect indication of motor torque:

• #### Question 2

What will happen to the stall torque of this DC motor if the “field control” resistor value is suddenly decreased? Explain your answer.

• #### Question 3

What will happen to the counter-EMF of this DC motor if the “field control” resistor value is suddenly decreased (while it is running)?

What effect will this change in field excitation do to the operating speed of the motor?

• #### Question 4

Series-wound DC motors have very different operating characteristics than either shunt-wound or permanent magnet DC motors. Describe what happens in a series-wound motor when a mechanical load is placed on the motor, causing it to slow down and the counter-EMF to decrease:

Contrast this behavior against that of a permanent magnet DC motor under the same conditions (increased mechanical load, causing counter-EMF to decrease).

• #### Question 5

Describe what a compound-wound DC electric motor is, and how it compares with the other motor types you’ve learned about so far.

• #### Question 6

This motor-start circuit reduces the amount of “inrush” current when starting by inserting a resistance in series with the motor for a few seconds, then removing that resistance after the time delay to allow full speed operation. A time-delay relay provides the reduced-speed control.

The relay labeled “M1” is a large “contactor” designed to shunt the motor’s current around the start-up resistor. It requires at least a few amps of current through its coil to energize.

The relay labeled “CR1” is a much smaller “control relay,” and its turn-on time is controlled by the charging of an electrolytic capacitor.

What could be adjusted in this circuit to make it switch to full-speed operation sooner after start-up?

• #### Question 7

There is more than one way to electrically “brake” (slow down) an electric motor. Three methods in common use are:

Dynamic braking
Regenerative braking
Plugging

Describe how each of these methods work.

• #### Question 8

Identify the method of electrical braking used in this motor control circuit:

• #### Question 9

Suppose someone wires a DPDT switch to an electric motor like this, hoping to achieve forward/reverse control:

Unfortunately, this switch arrangement will not reverse the motor!

Explain why the motor will not reverse, and determine a correction to the circuit that will allow the switch to function as a forward/reverse control.