Electrical Machines-I: Transformers, DC Motors, Induction & Synchronous Machines: 30 flashcards
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1. On which side is the Open Circuit test performed and why?
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On the LV side at rated voltage, because core loss depends on flux which depends on applied voltage.
2. What does the Short Circuit test measure?
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Full-load copper loss and series branch parameters $R_{eq}$ and $X_{eq}$.
3. State the condition for additive polarity in a transformer polarity test.
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The voltmeter reads $V = V_1 + V_2$.
4. List the conditions for parallel operation of three-phase transformers.
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Same voltage ratio, same polarity, same percentage impedance, same phase sequence, same vector group, and similar X/R ratio.
5. When does a transformer operate at maximum efficiency?
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When copper loss equals core loss.
6. Write the DC motor torque equation.
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$$T = \frac{P \phi Z I_a}{2\pi A}$$
7. What is the speed equation for a DC motor?
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$$N \propto \frac{V - I_a R_a}{\phi}$$
8. What is armature reaction and its main effect?
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The armature's own magnetic field distorts the main field, shifting the MNA and causing sparking at brushes.
9. Why must a DC series motor never be started unloaded?
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Because $N \propto 1/I_a$, so at no load the speed becomes dangerously high and the motor can run away.
10. Write the torque-slip equation of a 3-phase induction motor.
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$$T = \frac{K s E_2^2 R_2}{R_2^2 + (sX_2)^2}$$
11. At what slip does maximum torque occur in an induction motor?
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At $s = R_2/X_2$.
12. How much does star-delta starting reduce starting current and torque?
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Both are reduced to one-third of their direct-delta values.
13. What does the no-load test on a 3-phase induction motor measure?
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Core loss plus friction and windage loss.
14. What does the blocked-rotor test on an induction motor measure?
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Full-load copper loss.
15. Write the synchronous speed formula.
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$$N_s = \frac{120f}{P}$$
16. Write the rotor frequency formula.
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$$f_r = s f$$
17. What is the synchronous impedance method used for?
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To find voltage regulation of an alternator using OCC and SCC.
18. What do V-curves of a synchronous motor plot?
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Armature current $I_a$ versus field current $I_f$, with minimum at unity power factor.
19. What do inverted-V curves plot?
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Power factor versus field current, peaking at unity power factor.
20. Write the EMF equation of a synchronous generator.
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$$E_{ph} = 4.44 f \phi T_{ph}$$
21. Compare salient-pole and cylindrical rotor alternators.
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Salient-pole: low speed, many poles, non-uniform air gap. Cylindrical: high speed, few poles, uniform air gap.
22. What are the two capacitors in a capacitor-start capacitor-run motor used for?
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Large starting capacitor gives high starting torque; small running capacitor improves power factor and efficiency.
23. What is a universal motor?
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A series-wound motor that runs on both AC and DC supplies.
24. State the double-field revolving theory.
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A pulsating field can be resolved into two equal fields rotating in opposite directions at synchronous speed.
25. How does a shaded-pole motor produce starting torque?
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A copper shading ring makes flux in the shaded portion lag, creating a sweeping field from unshaded to shaded side.
26. What is the main drawback of a shaded-pole motor?
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Very low efficiency and power factor.
27. What is the significance of the 4.44 factor in the EMF equation?
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It comes from the RMS value of a sinusoidal EMF: $\sqrt{2}\pi \approx 4.44$.
28. Why is the SC test done at reduced voltage?
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To circulate full-load current without core loss, since flux is tiny at low voltage.
29. What happens if two transformers of opposite polarity are paralleled?
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A short circuit is created through the secondaries.
30. What are the remedies for armature reaction?
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Compensating windings and interpoles.
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