The simplest arrangement consists of a lamp or preferably a voltmeter connected across one pole of a two pole switch connecting the incoming machine to the busbars, the other pole of the switch being already closed.
If the machines be out of step, the lamps will fluctuate in brightness, or the voltmeter pointer will oscillate, the pulsation becoming less and less as the incoming machine approaches synchronous speed. Synchronism is shown by the lamp remaining out, or the voltmeter at zero.
Fig. 2,571.—General Electric synchronism indicator. The synchronism indicator is a motor whose field is supplied with single phase current from one of the machines to be synchronized, and its armature from the other. The armature carries two inductance coils placed at a large angle, one supplied through a resistance, the other through an inductance. This arrangement generates a rotating field in the armature, while the stationary field is alternating. The armature tends to assume a position where the two fields coincide when the alternating field passes through its maximum; hence, the armature and pointer move forward or backward at a rate corresponding to the difference of frequency, and the position when stationary depends on the phase relation. When the machines are running at the same frequency and in phase the pointer is stationary at the marked point. In construction, it is like a small, two phase, bipolar synchronous motor, the field being supplied with alternating instead of direct current. The armature is mounted in ball bearings in order to make it sufficiently sensitive and smooth in operating. The armature coils are not exactly 90 degrees apart, since it is not possible to get the current in the two coils exactly in quadrature without introducing condensers on other complicated construction. Standard ratings are for 110 and 220 volt circuits. Synchronism indicators should be ordered for the frequency of the circuit on which they are to be operated, although the instruments may be used on circuits varying 10 per cent to 15 per cent from the normal. The words "Fast" and "Slow" on the dial indicate that the frequency on binding posts E and F is respectively higher or lower than that on A and B; or, in other words, clockwise rotation of the pointer means that the incoming machine is running at too high speed, counter clockwise rotation indicating too low speed.
Ques. How does the resonance type of synchronism indicator operate?
Ans. On the same principle as the resonance type of frequency indicator, already described.
Ques. What is the principle of the rotating field type of synchronism indicator?
Ans. Its operation depends on the production of a rotating field by the currents of the metered circuits in angularly placed coils, one for each phase in the case of a polyphase indicator. In this field is provided a movable iron vane or armature, magnetized by a stationary coil whose current is in phase with the voltage of one phase of the circuit. As the iron vane is attracted or repelled by the rotating field, it takes up a position where the zero of the rotating field occurs at the same instant as the zero of its own field. In the single phase meter the positions of voltage and current coils are interchanged and the rotating field is produced by means of a split phase winding connected to the voltage circuit.
