Motor Control Resistors

Motor Control Resistors:

DC Series Wound Motor Resistor
The resistor limits torque by limiting current flow to the motor. It is used for starting and stopping the motor.

Initial information needed for sizing your resistor:

  • Application
  • Horsepower
  • Line voltage
  • Full load amps
  • Starting torque
  • Number of speeds
  • Duty/NEMA class (see tables)
  • For dynamic lowering applications, include manufacturer of control

AC Squirrel Cage Induction Motor Resistor

Commonly known as a "reduced-voltage" or "ballast" resistor, this resistor acts as a voltage divider for soft starting of the motor.

Initial information needed for sizing your resistor:

  • Application
  • Horsepower
  • Line voltage
  • Full load amps
  • Duty cycle/NEMA class (see tables)

AC Wound Rotor Induction Motor Resistor (Slip Ring Motor)

The resistor is wired into the motor secondary slip rings, where it dissipates torque energy in the form of heat. It also provides "soft starting" as resistance is removed in steps.

Initial information needed for sizing your resistor:

  • Application
  • Horsepower
  • Secondary volts
  • Secondary amps
  • Starting torque
  • Number of speeds
  • Duty/NEMA class (see tables)

More Information:
WOUND ROTOR MOTOR RESISTORS TECH SHEET MC105-07

Wye-Delta Closed-Transition Starting Resistor

Wye-delta is a reduced-inrush starting method in which 3 phase motor stator coils are initially connected in a wye configuration and switched to delta as the motor's speed increases. The resistor works as a reduced voltage starting resistor by starting a motor at 58% reduced voltage.

Initial information needed for sizing your resistor:

  • Application
  • Horsepower
  • Line voltage
  • Full load amps
  • Duty/NEMA class (see tables)

CRANE CONTROL RESISTORS

Information needed for sizing your resistor:

  • Application
  • Horsepower
  • Line voltage
  • Full load amps
  • Starting torque
  • Number of speeds
  • Duty/NEMA class (see tables)
  • For dynamic lowering applications, include manufacturer of control package.

When overhead cranes are used in industrial facilities, crane resistor banks are frequently used for control of hoisting and lowering speeds, as well as positioning the crane itself. Resistors are usually mounted in, or on top of, the bridge structure.

Many steel mills use DC motors because, in the past, DC power was the best way to hoist heavy loads. Newer plants are increasingly utilizing more readily available three-phase power, and wound rotor motors are often employed.

Brake resistor control techniques — In hoisting and lowering operations, some special considerations are involved. Hoisting is straightforward — the load is resisting the motor, and positive torque against gravity is required. In lowering an empty hook, it may be necessary to drive the hook down, since gravity may be insufficient to lower it at the desired speed.

But, when the hook is loaded, the load may tend to overhaul the motor, which then may have to apply a retarding torque as if it were hoisting. Dynamic braking ensures the load won’t run away and crash instead of making a soft landing.

All of these conditions require modifications of the speed-torque curve of the hoist motor. This is readily achieved by applying the proper amount of external armature and field resistance.

A resistor bank is the practical device available for the purpose. Operators effect changes in motor speed with various loads and operate a bank of magnetic contactors to achieve the desired operation. The contactors change taps on the resistor banks as required to maintain appropriate rotor circuit resistance.

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