A dc separately excited motor has constant field current. The armature is fed fr

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Question
A dc separately excited motor has constant field current. The armature is fed from a single phase supply through a full converter. When a = 0, speed is 500 rpm. If a = 45�, the speed is likely to be
Options
A. about 350 rpm
B. about 250 rpm
C. about 175 rpm
D. about 125 rpm

Correct Answer

about 125 rpm

Explanation

Average output voltage ? cos a and therefore reduced to 0.707 of the value for a = 0. Speed is nearly proportional to voltage.

Power Electronics problems

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1. The dynamic equalising circuit consists of a series combination of capacitor C and resistor R_C across each thyristor. This resistance R_C (along with parallel connected diode)
Options
A. limits charging current
B. limits discharging current
C. limits both charging and discharging currents
D. none of the above

2. In a single phase full wave regulator, the firing angles in the positive and negative half cycles are generally
Options
A. equal
B. different
C. equal or different
D. different but sometimes equal

3. In a 3 phase fully controlled converter the firing frequency is
Options
A. 3 times the line frequency
B. 6 times the line frequency
C. 9 times the line frequency
D. 12 times the line frequency

4. The terminals of a power MOSFET are called
Options
A. emitter, base, collector
B. source, gate, drain
C. source, base, drain
D. emitter, gate, drain

6. Two thyristor of same rating and same specifications
Options
A. will have equal turn on and turn off periods
B. will have equal turn on but unequal turn off periods
C. may have equal or unequal turn on and turn off periods
D. will have unequal turn on and turn off periods

7. In single phase half wave regulator, the average current over one full cycle
Options
A. is always positive
B. may be positive or negative
C. may be negative
D. is always negative

9. If duty cycle is a and the load is a resistance R, the rms value of output voltage is
Options
A. aV
B. a V
C. a²V
D.

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