Multiple choice questions on Electrical Machines - 5

Important Multiple Choice Questions On Electrical Machines

1. Supply to one terminal of delta-star three-phase core type transformer which is on no-load fails. Assuming magnetic circuit symmetry, voltages on the secondary side will be

1. 230, 230,115
2. 230,115, 115
3. 345, 115, 115
4. 345, 0, 345

2. A 220/440 V, 50Hz, 5kVA single phase transformer operates on 220V, 40 Hz supply with secondary winding. Then

1. The eddy current loss and hysteresis loss of the transformer decrease
2. The eddy current loss and hysteresis loss of the transformer increase
3. The hysteresis loss of the transformer increases while eddy current loss remains the same
4. The hysteresis loss remains the same whereas eddy current loss decreases

3. The voltage regulation of a large transformer is mainly influenced by

1. No-load current and load power factor
2. Winding resistances and load power factor
3. Leakage fluxes and load power factor
4. Winding resistances and core losses

4. If a sinusoidal voltage source is connected to a power transformer, its no-load current would be

1. Sinusoidal and lagging the voltage by ninety degrees
2. Sinusoidal and lagging the voltage by less than ninety degrees
3. Rich in third harmonic and its fundamental would lag the voltage by ninety degrees
4. Rich in third harmonic and its fundamental would lag the voltage less than ninety degrees

5. The utility of delta-connected tertiary windings in star-star connected power transformer is that it

1. Makes supply available for small loads
2. Provides low reactance paths for zero sequence currents
3. Is used to suppress harmonic voltages
4. All of these

6. A 200/100V, 50 Hz transformer is to be excited at 40 Hz from the 100V side. For the exciting current to remain the same, the applied voltage should be

1. 150 V
2. 125 V
3. 100 V
4. 80 V

7. A 2-phase, 4 pole permanent magnet stepper motor has a step of

1. 90 degrees
2. 45 degrees
3. 30 degrees
4. 22.5 degrees

8. The magnetizing current in a transformer is rich in

1. Third harmonics
2. Fifth harmonics
3. Seventh harmonics
4. Thirteen harmonics

9. An auto-transformer having a transformation ratio of 0.8 supplies a load of 3kW. The power transferred conductively from primary to secondary is

1. 0.6 kW
2. 2.4 kW
3. 1.5 kW
4. 0.27 kW

10. While performing the open-circuit and short-circuit tests on a transformer to determine its parameters the status of the low voltage (L.V) and high voltage (H.V) will be such that

1. In Open circuit L.V is open and in short circuit H.V is shorted
2. In Open circuit H.V is open and in short circuit L.V is shorted
3. In Open circuit L.V is open and in short circuit L.V is shorted
4. In Open circuit H.V is open and in short circuit H.V is shorted