Shunt-Wound Generators in Parallel. A and B of Fig. 8-59 are two similar generators feeding the same bus bars C and D. If A tends to take more than its share of the total load, its voltage falls and more load is automatically thrown on B.

Also, if the driver of one of the generators slows down to stop, the emf of the machine falls until the other generator starts to drive it as a motor. This continues until its driver takes over again.

The external characteristics of the two machines are shown in Fig. 8-60. At voltage E, the currents in the generators are Ia and Ib, and the line current is Ia # Ib. To make machine A take more of the load, its excitation must be increased to raise its characteristic curve.

If a 1000-kW generator and a 500-kW machine have the same regulation curves, the machines will divide the load according to their respective capacities, as shown in Fig. 8-61.

Compound-Wound Generators in Parallel. A and B of Fig. 8-62 are two compound-wound machines. If A tends to take more than its share of the load, the series excitation of A increases, its voltage rises, and it takes still more of the load.

Thus, the operation is unstable. If this continues until A takes all the load and the voltage of B drops to the point that A reverses the current in B, B will be driven as a motor. With the reversed current in the series field of B it becomes a differentially compounded motor, and the series weakens the flux to speed up the motor.

This may progress to a point at which the unit may be damaged mechanically and electrically. To prevent this, a bus bar of large section and of negligible resistance, called an equalizer bus, is connected from e to f (Fig. 8-62).

Points e and f are then practically at the same potential. Therefore, the current in each series coil is independent of the current in its particular generator, is inversely proportional to the resistance of the coils, and is always in the same direction.

When a single compound generator has too much compounding, a shunt in parallel with the series field coils will reduce the current in these coils and so reduce the compounding.

When compounded generators are operating in parallel using an equalizer bus, the current in the series field coils depends only on the resistance of the coils and a shunt connected across one of them is actually across all of them, reducing the compounding of all but not disturbing the relative compounding between the machines.

To reduce the compounding of a single machine, it is necessary to place a resistance in series with the coils. This may require a large resistor to handle the large load current it must carry.

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