These resistors are used to control motor starting and stopping anywhere from reduced voltage motors of low horsepower up to large motor applications such as materials handling, mine hoists, cranes etc.

The most common applications are:

AC Wound Rotor Induction Motors – where the resistor is wired into the motor secondary slip rings and provides a soft start as resistance is removed in steps.
AC Squirrel Cage Motors – where the resistor is used as a ballast for soft starting also known as reduced voltage starting.
DC Series Wound Motors – where the current limiting resistor is wired to the field to control motor current, since torque is directly proportional to current, for starting and stopping.