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DC Motor Control Circuits
9 questions By Tony R. Kuphaldt
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Question 7 of 9
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.
Reveal answerDynamic braking is re-connecting the motor as a generator, and dissipating the generated power into a resistive load. Regenerative braking is an extension of dynamic braking, using the generated power to do something useful. Plugging is the temporary application of reverse polarity to the motor to forcefully stop it.
Notes:All three methods of motor braking are used in industry, each with its own benefits. Discuss the relative merits of each method with your students, with regard to simplicity, braking power, power consumption, etc.
Be sure to discuss the advantages and disadvantages of electrical braking over mechanical braking. What advantage(s) does electrical braking (using the motor as a brake) have over mechanical braking (using a separate brake mechanism attached to the motor shaft)? Which type of braking do you think might be more reliable?
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Question 8 of 9
Identify the method of electrical braking used in this motor control circuit:
Reveal answerThis circuit provides dynamic braking.
Notes:It is apparent from the schematic diagram that the motor’s field winding continues to receive power when the switch is in the “brake” position. Ask the students why this is necessary for dynamic braking to work. Ask them what would happen if the field winding were de-energized as well as the armature, as the switch moved to the “brake” position.
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Question 9 of 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.
Reveal answerThis switching circuit will not reverse the motor, because it reverses polarity on both the armature and the field.
Follow-up question: what does this fact tell us about the motor’s ability to operate on alternating current (AC)?
Notes:There is more than one solution for the reversing problem. Discuss your students’ solutions, and encourage the submission of multiple ideas! One thing you might want to mention is that the field winding of a shunt-wound DC motor like this typically draws far less current than the armature winding (especially under full load). Ask your students how this fact might influence their decision on how to re-design the switch circuit.
