Synchronous motors were first used because they were capable of raising the power factor of systems having large induction-motor loads. Now they are also used because they can maintain the terminal voltage on a weak system (high source impedance), they have lower cost, and they are more efficient than corresponding induction motors, particularly the low-speed motors.

Synchronous motors are built for operation at pf = 1.0, or pf = 0.8 lead, the latter being higher in cost and slightly less efficient at full load. The selection of a synchronous motor to correct an existing factor is merely a matter of bookkeeping of active and reactive power.

The synchronous motor can be selected to correct the overall power factor to a given value, in which case it must also be large enough to accomplish its motoring functions; or it can be selected for its motoring function and required to provide the maximum correction that it can when operating at pf = 0.8 lead.

In Fig. 20-8, a power diagram shows how the active and reactive power components Ps and Qs of the synchronous motor are added to the components Pi and Qi of an induction motor to obtain the total Pt and Qt components, the kVAt, and the power factor.

The Qs of the synchronous motor is based on the rated kVA and pf = 0.8 lead, rather than the actual operating Kva. The synchronous motor can support the voltage of a weak system, so that a larger rating synchronous motor can be installed than an induction motor for the same source impedance.

With an induction motor, both the P and Q components produce voltage drops in the source impedance. With a synchronous motor operating at leading power factor, the P component produces a voltage drop in the source resistance, but the Q component produces a voltage rise in the source reactance that can offset the drop and allow the terminal voltage to be normal.

If necessary, the field current of the synchronous motor can be controlled by a voltage regulator connected to the motor bus. The leading current of a synchronous motor is able to develop a sufficient voltage rise through the source reactance to overcome the voltage drop and maintain the motor voltage equal to the source voltage.

FIGURE 20-8 Power diagram of induction motor and synchronous motor operating in parallel, showing component and net values of P and Q.

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