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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