Before connecting a three-phase generator to bus bars already connected to one or more generators, you must make sure that certain conditions prevail. A synchroscope is the device you use to find out if the following required conditions have been met:

1. Phase sequence for the both generator and bus bars must be the same.

2. The generator and the bus-bar voltages must be the same.

3. The generator and bus-bar frequency must be the same.

4. The generator frequency must be practically constant for an appreciable period of time.

5. The generator and bus-bar voltages must be in phase. They must reach their maximum voltages at the same time; therefore, when connected, they will oppose excessive circulation of current between the two machines.

Figure 11-20 shows a synchroscope. It is basically a power factor meter connection to measure the phase relationship between the generator and bus-bar voltages. The moving element is free to rotate continuously. When the two frequencies are exactly the same, the moving element holds a fixed position. This shows the constant phase relationship between the generator and bus-bar voltages. When the frequency is slightly different, the phase relationship is always changing. When this happens, the moving element of the synchroscope rotates constantly. The speed of rotation is equal to the difference infrequency; the direction shows whether the generator is fast or slow. The generator is placed on line when the pointer slowly approaches a mark. This mark shows that the generator and bus-bar voltages are in phase.

PHASE-SEQUENCE INDICATORS

A phase-sequence indicator (fig 11-21) is used to determine the sequence in which the currents of a three-phase system reach their maximum values.

Ships have phase-sequence indicators installed in switchboards that may be connected to shore power. These instruments indicate whether shore power is in

Figure 11-20.-Synchroscope.

the correct phase sequence with the ship before shipboard equipment is connected to shore power. Three-phase motors, when connected to incorrect phase-sequence power, rotate in the opposite direction.

The phase-sequence indicator has three neon lamps that light when all three phases are energized. A meter connected to a network of resistors and condensers shows correct or incorrect sequence on a marked scale.