DC Motor Control Circuits
DC Electric Circuits
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:
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).
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?
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.
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