Question

Describe W.T.A. Of Sketch the operation of a synchroscope

State the information obtained from it

Suggest a substitute in the event of sychroscope and stand by light failure

Figure a

The operation of synchronising an alternator before paralleling with another machine could be carried out with the type of synchroscope shown in Figure a. With its use, two phases of the incoming machine can be matched with the same two switchboard phases. The synchroscope is a small motor with coils on the two poles connected across red and yellow phases of the incoming machine and the armature windings supplied from red and yellow switchboard bus-bars. The latter circuit incorporates a resistance and an inductance coil in parallel. The inductance has the effect of delaying current flow through itself by 90° relative to current in the resistance. The dual currents are fed via slip-rings to the two armature windings and produce in them a rotating magnetic field.

Polarity of the poles will alternate north/south with changes in red and yellow phases of the incoming machine, and the rotating field will react with the poles by turning the rotor clockwise or anticlockwise. Direction is dictated by whether the incoming machine is running too fast or too slow. Normal procedure is to adjust alternator speed until it is running very slightly fast and the synchroscope pointer turning slowly clockwise. The breaker is closed just before the pointer reaches the twelve o’clock position, at which the incoming machine is in phase with the switchboard bus-bars.

Information obtained from it. Ideal time for closing the breaker when paralleling the generator when the voltage are in phase and the point reaches just before 12 oclock position. Since the dial of synchroscope is marked with fast and slow direction it is possible to ascertain whether the incoming machine is running too fast or too slow.

In the event of synchroscope and stand by light failure

Connect a voltmeter as shown in Fig. across one pole of the open in coming generator circuit breaker. This procedure is more easily (and safely) performed at the synchroscope terminals behind the door of the synchronising panel at the front of the main switchboard. Check the circuit diagrams before such testing. Adjust the generator speed until the voltmeter very slowly fluctuates from zero to maximum. Close the breaker when the voltmeter indication passes through zero. Note, for this operation, an analogue (pointer and scale) meter is easier to follow than a digital type.