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DC Motor Theory
12 questions By Tony R. Kuphaldt
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Question 10 of 12
As the armature coils in a DC motor rotate through the magnetic flux lines produced by the stationary field poles, voltage will be induced in those coils. Describe how this phenomenon relates to Faraday’s Law of electromagnetic induction, specifically in regard to what variables influence the magnitude of the induced voltage:
e = N dφ dtThe self-induced voltage produced by a rotating armature is often called the counter-voltage, or counter-EMF. Why would it be called “counter”? What is implied by this terminology, and what electromagnetic principle is illustrated by the “counter” nature of this induced voltage?
Reveal answerCounter-EMF varies directly with armature speed, with the number of turns in the armature windings, and also with field strength. It is called “counter-” EMF because of Lenz’ Law: the induced effect opposes the cause.
Notes:The principle I wish to communicate most with this problem is that every motor, when operating, also acts as a generator (producing counter-EMF). This concept is essential to understanding electric motor behavior, especially torque/speed curves.
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Question 11 of 12
The amount of voltage applied to a permanent-magnet DC motor, and the amount of current going through the armature windings of a permanent-magnet DC motor, are related to two mechanical quantities: maximum speed, and torque output (twisting force).
Which electrical quantity relates to which mechanical quantity? Is it voltage that relates to speed and current to torque, or visa-versa? Explain your answer.
Reveal answerThe amount of voltage applied to a permanent-magnet DC motor determines its no-load speed, while the amount of current through the armature windings is indicative of the torque output.
Notes:This question asks students to relate concepts of electromagnetism and electromagnetic induction together with voltage and current. While the permanent-magnet style of DC motor exhibits almost linear relationships between these variables, all DC electric motors exhibit the same general pattern: more voltage, more speed; more current, more torque; all other variables being equal.
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Question 12 of 12
A problem has developed in this motor circuit. When the switch is turned “on”, the motor does not turn. It does, however, draw a lot of current (several times the normal operating current) as indicated by the ammeter:
Based on this information, what do you think may be wrong with the circuit? Is there anything we know for sure is not failed in the circuit? Explain your answers.
Reveal answerOne likely cause is either the field winding or something in the armature (a brush, perhaps) failed open. Internal motor problems are not the only possibilities, however!
Notes:This question is an exercise in diagnostic thinking. Always challenge your students to try diagnosing the nature of a problem with the given information before taking further measurements or observations. Far too often people take more measurements than necessary to troubleshoot electrical systems, because they do not think carefully enough about what they are doing.
