Power electronics and drives miscellaneous
- Consider the following statements regarding speed control of induction motors by means of external rotor resistors :
1. Reduction in speed is accompanised by reduced efficiency.
2. With a large resistance in the rotor circuit, the speed would vary considerably with variation in torque.
3. The method is very complicated.
The disadvantage of such a method of speed control would include
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T4 > T3 > T2 > T1
It is seen from the figure that, variation in the speed, as the torque varies between T1 and T2, is more on curve 2 than that on curve 1.Correct Option: A
T4 > T3 > T2 > T1
It is seen from the figure that, variation in the speed, as the torque varies between T1 and T2, is more on curve 2 than that on curve 1.
- In machine tool drive application, the speed of separately excited dc motor is required to be controlled both below and above the rated speed of the motor. Which one of the following methods is best suited for this purpose ?
(Va = armature voltage, lf = field current and If ,rated = field current at rated speed)
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With If > I f rated , speed is reduced and with If < I f rated, the speed is increased above normal. Also, with Va greater than Va . rated; speed falls, and with Va greater than Va rated ; speed rises above normal. To avoid overloading of either the field finding; or the armature winding, though little expensive; hence choice (c) will be most suitable.
Correct Option: C
With If > I f rated , speed is reduced and with If < I f rated, the speed is increased above normal. Also, with Va greater than Va . rated; speed falls, and with Va greater than Va rated ; speed rises above normal. To avoid overloading of either the field finding; or the armature winding, though little expensive; hence choice (c) will be most suitable.
- The speed/torque regimes in a dc motor and the control methods suitable for the same are given respectively in Group II and Group I
Group I Group II P. Field Control 1. Below base speed Q. Armature Control 2. Above base speed
3. Above base torque 4. Below base torque
The match between the control method and the speed/torque regime is
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NA
Correct Option: B
NA
- An ac induction motor is used for a speed control application. It is driven from an inverter with a constant V/f control. The motor name-plate details are as follows :
V = 415 V, Ph = 3, f = 50 Hz, N = 2850 rpm
The motor is run with the inverter output frequency set at 40 Hz, and with half the rated slip. The running speed of the motor is
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Given : Ns = 3000 rpm
∴ s = 3000 - 2850 = 150 = 0.05 3000 3000 Now , Ns = Ns 40 = 2400 rpm 50 ∴ Running speed of the motor, Nr = 
1 - s 
. Ns' = (1 - 0.025) . 2400 = 2340 rpm 2
Correct Option: C
Given : Ns = 3000 rpm
∴ s = 3000 - 2850 = 150 = 0.05 3000 3000 Now , Ns = Ns 40 = 2400 rpm 50 ∴ Running speed of the motor, Nr = 1 - s . Ns' = (1 - 0.025) . 2400 = 2340 rpm 2
- A single-phase full-bridge voltage source inverter feeds a purely inductive load, as shown in the given figure, where T1, T2, T3, T4 are power transistors and D1, D2, D3, D4 are feedback diodes. The inverter is operated in square-wave mode with a frequency of 50 Hz. If average load current is zero, what is the time duration of conduction of each feedback diode in a cycle?
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NA
Correct Option: D
NA
