Wound rotor motors are an extremely versatile breed of induction
motors. Featuring a rugged design, these machines provide the
unique ability to gradually bring up to speed high-inertia equipment
and large loads smoothly and easily. Wound rotor motors also can
develop high starting torque at standstill - while maintaining
low inrush. Long motor life is ensured with the use of external
resistor banks or liquid rheostats that dissipate heat build-up
generated during motor start-up.
What makes the wound rotor motor a unique induction machine is
its rotor. Instead of a series of rotor bars, a set of insulated
rotor coils is used to accept external impedances. The rotor windings
are similar to those found on a DC armature, with the coils connected
together to a set of rings that make contact with carbon-composite
brushes. The circuit is completed by connecting the brushes to
a set of impedances such as a resistor bank or liquid rheostat.
This rotor construction design allows for a varying resistance
from almost short-circuit condition to an open-circuit condition
with infinite external resistance. By modifying the resistance,
the speed-torque characteristics can be altered. This allows for
the torque to remain high, the inrush low, and the speed varied.
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