Applications for resistors in motor control
Wound Rotor Motors with Slip Rings
This type of motor is still used for high power (600kW+) or medium voltage (3.3kV+) applications in arduous operating conditions – as pumps and cement plants – where the combination of exact control and very high reliability is hard to achieve with variable speed drives. The rotor windings are brought out to slip rings which are short-circuited during running. During start-up multiple sets of resistors are connected across the slip rings to control the starting current. As the motor speeds up the resistances are removed in timed steps by the use of short-circuiting contactors.
Closed Transition Resistors for Star-Delta Starters
Star-delta is a commonly-used and economical reduced current starting method in which the stator coils of the motor are initially connected in a star configuration and switched to delta as the motor speed increases. A voltage spike would arise motor as the motor coils are briefly open-circuit, so to eliminate this three small resistors are put in circuit (via a third contactor) for a short time.
Squirrel Cage AC Motor Resistors
Commonly known as a “reduced-voltage” or “ballast” resistor, this resistor acts as a voltage divider across the supply for soft starting of the motor.
DC Series Wound Motor Resistors
The resistor limits torque by reducing the current to the motor. It is used for starting and stopping the motor.
Sets of resistors are used in many gantry crane drives for the drive, hoist, and cross-travel motions. For these applications, standard switchgear and stepping cover most common operating conditions. Cressall’s HPR steel grid resistors are particularly suited for these applications.
Field discharge resistors
Iron-cored magnetic coils cannot simply be switched off because the residual magnetic field would generate overvoltages if not dissipated safely. The field discharge resistor limits the current to the required value, usually 20% of nominal current.
The resistance and heat capacity of a conductive electrolyte can be used as a starter for a slip ring induction motors. Moveable electrodes or variable electrolyte level control the start. As the electrolyte concentration continually changes due to evaporation, liquid starters require continuous maintenance and control. However, they offer a low-cost solution for multi-megawatt motors.