A factory, which receives a 400 volt, 50Hz, three phase supply, has its load made up of the following circuits:

Circuit 1 which takes 180 kVA at a lagging power factor of 0.7,

Circuit 2 which takes 208 kVA at a lagging power factor of 0.75,

Circuit 3 which takes 245 kVA at a lagging power factor of 0.8, and

Circuit 4 which takes 153 kVA at a lagging power factor of 0.83,

An expansion project involves the addition to the circuits quoted above, of a three phase synchronous motor which delivers 103.5kW. The synchronous has an efficient of 90% and can be considered to be circuit5in the factory. With the synchronous motor added and operating the power factor of the complete factory is 0.872 lagging.

1. Determine the

1. Percentage change in the power factor of factory caused by the addition of the synchronous motor; treat the power factor of the factory before the synchronous motor was added as the base,
2. The power factor at which the synchronous motor operates and
3. kVA taken, from the supply, by the synchronous motor.

2. The synchronous motor in (a), above, runs at 1200 r/min and has 198slots with four conductors per slot.

Determine its pole flux density. When it operates at a leading power factor of 0.95

[20]

QUESTION 2

1. 250kVA, three phase, 50Hz, 400V alternator has 108slots, with four conductors per slot and runs at 1000r/min. The windings are connected in star. Coil span is 75% of pole pitch. When the alternator delivers its rated load at a lagging power factor of 0.8 the value of the flux, per pole, is 0.018Wb. The impedance angle of each phase of the lagging power factor of 0.8 the value of the flux, per pole, is 0.018 Wb. The impedance angle of each phase of the alternator is 86.

Determine the resistance and synchronous reactance, per phase, of the alternator windings. (12)

2. Sketch, neatly and representatively, with ALL values shown, the phasor diagram of the alternator when it delivers 75% of its rated output at a power factor of 0.75 leading. (3)