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DC Generator Theory
14 questions By Tony R. Kuphaldt
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Question 13 of 14
Suppose a generator is mechanically coupled to an internal combustion engine in an automobile, for the purpose of charging the starting battery. In order that the battery not be over-charged by the generator, there must be some way of controlling the generator’s output voltage over a wide range of engine speeds.
How is this regulation of generator output voltage typically achieved? What variable within the generator may be most easily adjusted to maintain a nearly constant output voltage? Express your answer in relation to Faraday’s Law of electromagnetic induction.
Reveal answerThe most common method of generator voltage control is adjustment of field winding excitation.
Notes:Although adjustable field winding excitation is the most popular form of generator output voltage control, it is not the only means. Challenge your students with inventing other means of charge control for the battery in this automotive electrical system, besides field winding excitation control. What else can we do to the generator, or to the circuit it is within, to achieve charge control for the battery?
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Question 14 of 14
In most high-power DC generator and motor designs, the wire used to make the field winding is much thinner gauge than the wire used to make the armature winding. This indicates the relative magnitude of current through these respective windings, with the armature coils conducting much more current than the field coils.
That the armature conducts more current than the field is no small matter, because all current through the armature must be conducted through the brushes and commutator bars. The more current these components have to carry, the shorter their life, all other factors being equal.
Couldn’t the generator be re-designed so that the field conducted most of the current, with the armature only conducting a small amount? This way, the brushes and commutator bars would only have to carry a fraction of their normal current, making them less expensive and longer-lived. Explain why this is impossible to do.
Hint: consider the design of a permanent-magnet generator.
Reveal answerIt is impossible for the field winding to conduct more current than the armature in a functioning DC generator, because the armature has to be the source of electrical power, while the field is only a load.
Notes:Being that brush and commutator wear is the main reason AC motors and generators are favored over DC, any idea that may potentially reduce the “wear and tear” on DC motor or generator brushes is worth considering. However, the idea proposed in this question will never work. This is not necessarily an easy question to answer, as it tests the students’ comprehension of generator theory. The hint given in the question (“consider a permanent-magnet generator”) is intended to force students to simplify the problem, by considering a working generator design that only has one winding (the armature). By simplifying the problem in this way, students should see that the armature winding has to carry the bulk of the current in a DC generator.
