Rectifiers

21. In a rotary converter armature currents are

1. d.c. only
2. a.c. only
3. partly a.c. and partly d.c.
4. Any of the above

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22. In a rotary converter I2R losses as compared to a D.C. generator of the same size will be

1. same
2. less
3. double
4. three times

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23. In a single phase rotary converter the number of slip rings will be

1. two
2. three
3. four
4. six

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24. In a three-phase mercury arc rectifiers each anode conducts for

1. one-third of a cycle
2. one-fourth of a cycle
3. one-half a cycle
4. two-third of a cycle

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25. In mercury arc rectifiers having grid, the arc can be struck between anode and cathode only when the grid attains a certain potential, this potential being known as

1. maximum grid voltage
2. critical grid voltage
3. any of the above
4. none of the above

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26. In phase-shift control method the control is carried out by varying the of grid voltage.

1. magnitude
2. polarity
3. phase
4. any of the above

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27. In selenium rectifiers efficiencies ranging from _________ to _________ percent are attainable

1. 25, 35
2. 40, 50
3. 60, 70
4. 75, 85

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28. In which of the following appUcations, direct current is absolutely essential?

1. Illumination
2. Electrolysis
3. Variable speed operation
4. Traction

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29. In which of the following equipment direct current is needed?

1. Telephones
2. Relays
3. Time switches
4. All of the above

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30. It is the _________ of the transformer on which the magnitude of angle of overlap depends.

1. resistance
2. capacitance
3. leakage reactance
4. any of the above

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