Discussion
Consider the following statements about self-excitation.
1. Residual magnetism is essential.
2. Field resistance must be below critical.
3. Speed must be above critical.
Which of the statements are correct?
Answer: C
Correct answer: 1, 2 and 3
All three conditions must hold together. Residual magnetism supplies the small initial emf without which nothing starts. The field resistance must be below the critical value, or the resistance line will not cut the magnetisation curve. And the speed must be above the critical value, since a low speed depresses the magnetisation curve until the same resistance line no longer meets it. Failure of any one of the three leaves the machine unexcited.
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