Wound Rotor Control Replacement
Copyright © 2003 Francis J. Martino
* Solid state controls are available for wound rotor motors that are found
on machinery, pumps, blowers, hoist traverses and bridge trolleys.
* High inertia loads will require the first step of resistance to be
maintained. Without the first step of resistance, a high inertia load will
cause the speed of a wound rotor motor to be unstable during start. The
instability will be caused by torque variations which occur as the motor
operates along the torque vs. speed curve. The instability will be in
the region from the zero speed point to the point on the curve at which
the torque increases continuously with speed. Determine the value of
the existing resistance for inclusion of a new resistor in the
* Low inertia loads will not need a step of resistance. Before replacing
the control, determine the value of the first step of resistance and leave
the slip rings on the motor so that a step of resistance may be added later
if necessary. When using the new controller, operate the motor with the
slip rings short circuited together. Once it is determined that the motor
will function satisfactorily under full load starting conditions without
the need of resistance, the slip rings may then be removed.
* A solid state reduced voltage starter may be used to start and ramp up
to full speed and may be used with a step of resistance if necessary. A
reduced voltage starter may also vary the speed of the motor using
several steps of resistance. The starter may be programmed to switch the
resistors at the required speed point or points.
* A variable frequency drive may be used to vary the speed of the motor
and may be used with a step of resistance if necessary.
* A variable frequency drive will introduce harmonic current into the
motor which will be approximately five to six per cent of normal motor
amps. Motor heating increases proportionally with the increase in current
which leads to the necessity of motor derating. A 100 HP motor will then
be capable of developing a maximum of 95 HP.
* If the motor was originally used at maximum speed only and it is now
desired to operate that motor at variable speed, then a drop in motor
performance due to decreased motor cooling at constant lower speeds
must be taken into account. There will be a linear reduction in motor
horsepower capability of approximately twenty percent as the motor
speed is lowered from 60 Hz to 20 Hz. Below 20 Hz the torque will
drop off rapidly.
* Derating at low speeds is applicable only to motors running at low
speeds continuously. A variable frequency drive will have the capability of
starting a high inertia load without the low speed derating if the motor is
ramped up to full speed.
Power Quality and Drives LLC