Motor Generator Set:
ü Motor-generator (M-G) sets come in a wide variety of sizes and
ü This is a mature technology that is still useful for isolating
critical loads from sags and interruptions on the power system.
ü A motor powered by the line drives a generator that powers the
load. Flywheels on the same shaft provide greater inertia to increase
ü When the line suffers a disturbance, the inertia of the machines
and the flywheels maintains the power supply for several seconds.
ü This arrangement may also be used to separate sensitive loads
from other classes of disturbances such as harmonic distortion and switching
ü While simple in concept, M-G sets have disadvantages for some
types of loads:
1. There are losses associated with the machines, although they are
not necessarily larger than those in other technologies described here.
2. Noise and maintenance may be issues with some installations.
3. The frequency and voltage drop during interruptions as the
machine slows. This may not work well with some loads.
ü Another type of M-G set uses a special synchronous generator
called a written-pole motor that can produce a constant 60-Hz frequency as the
ü It is able to supply a constant output by continually changing
the polarity of the rotor’s field poles.
ü Thus, each revolution can have a different number of poles than
the last one. Constant output is maintained as long as the rotor is spinning at
speeds between3150 and 3600 revolutions per minute (rpm).
ü Flywheel inertia allows the generator rotor to keep rotating at
speeds above 3150 rpm once power shuts off.
ü The rotor weight typically generates enough inertia to keep it
spinning fast enough to produce 60 Hz for 15 s under full load.
ü Another means of compensating for the frequency and voltage drop
while energy is being extracted is to rectify the output of the generator and
feed it back into an inverter.
ü This allows more energy to be extracted, but also introduces
losses and cost.