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A three-phase, 440 V, 50 Hz ac mains fed thyristor bridge is feeding a 440 V dc, 15 kW. 1500 rpm separately excited dc motor with a ripple free continuous current in the dc link under all operating conditions. Neglecting the losses, the power factor of the ac mains at half the rated speed, is
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- 0.354
- 0.372
- 0.90
- 0.955
Correct Option: A
Back emf of separately excited motor, Ea ∝ N (speed of the motor).
As losses are neglected, Ia2 ra ≈ O
then, by Vt – Ia2 ra = Ea
we get, Vt ≈ Ea ∝ N
When the speed is reduced by half of rated speed output voltage of the bridge also get reduced by half
| RMS value of the supply current, Is = Ia√ | ||
| 3 |
Power delivered to motor, Po = Vo Ia
Input power to thyristor, Pin = √3 Vs Is ,
| Hence, Input power factor, | ||
| Pin |
= 0.354
