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DC Motor Control Circuits
9 questions By Tony R. Kuphaldt
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Question 4 of 9
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).
Reveal answerAs a series-wound motor is slowed down by a mechanical load, its field strength increases, causing the torque to increase dramatically.
Notes:Ask your students to contrast this motor behavior against that of a permanent magnet or shunt-wound motor. What happens to the stationary magnetic field strengths in those motor types when mechanically loaded? Ask your students what applications might be best for series motors, and what applications might be best for shunt or permanent magnet motors.
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Question 5 of 9
Describe what a compound-wound DC electric motor is, and how it compares with the other motor types you’ve learned about so far.
Reveal answer“Compound” electric motors have both a series and a shunt field winding, giving it a hybrid operating characteristic.
Notes:Ask your students to draw the schematic diagram of a compound DC motor, showing both field winding sets. Also, ask them if there is any substantial difference between the construction of the two field windings (shunt versus series). One is definitely different from the other, and with good reason!
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Question 6 of 9
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?
Reveal answerThe potentiometer could be adjusted to provide less resistance, in order to hasten the switch to “full-speed” mode after startup.
Challenge question: what else could be changed in this circuit to provide a shorter “reduced speed” time period?
Notes:This general principle is useful for the start-up of many different electric motors (including most AC motors). Circuits like this are sometimes referred to as soft start controls.
Ask your students to describe which way the potentiometer wiper needs to be moved in order to accommodate the resistance change.
