1. A PMG with its appropriate regulator can enhance a generator's transient

performance because it will provide a constant AC voltage input to the

automatic voltage regulator (AVR) regardless of the generator’s terminal

voltage.

A synchronous generator whose AVR is taking its input power from the

generator is termed a shunt excited generator. If the generator’s terminal

voltage is reduced as is seen with transient voltage dips upon the application

of significant block loading or motor starting, the input voltage to the AVR is

reduced by the percentage of transient voltage dip. This reduction in input

voltage to the AVR reduces its ability to force swift recovery of the generator’s

terminal voltage, and may well impact the generator’s ability to start relatively

large motor loads.

2. A PMG with its appropriate regulator can also provide 300% short circuit

support for ten seconds at 60 Hz, without the addition of current boost/series

boost systems.

When a down stream bolted fault is seen at any generator’s terminals, the

generator output voltage will drop to zero. For shunt excited generators, this

will result in zero voltage input to the AVR, and therefore, the generator may

be unable to force current flow long enough for down stream circuit

interrupters to clear the fault, and the entire electrical system will be shut

down. A PMG excitation support system will continue to provide input power

to the AVR, and provide the excitation necessary to clear the fault.

3. When the load on the generator is nonlinear due to thyrister (SCR) power

supplies such as UPS Systems, variable speed drives, etc., the load may

produce notches on the voltage severe enough to cause misfiring of the

power rectifiers in a shunt excited generator’s AVR. When this occurs, the

generator’s terminal voltage will become unstable.

If the generator is fitted with a PMG, the power input to the AVR is isolated

from these disturbances, and no voltage unstability will occur.

4. A PMG is constructed with a permanent magnet rotor which has a very strong

magnetic field. This eliminates the need for field flashing which is sometimes

necessary with shunt type synchronous generators.