Motor Control

We can design and build motor control resistors of any type for any application, including:
  • Wound-rotor AC motors with slip ring starters
  • Crane control systems
  • DC motor starters
  • Field discharge resistors
  • Torque-increasing resistors for use with soft starters
  • Wauchope and closed-transition starters

Why use resistors for motor control?

Electric motors of all types – AC single-phase or three-phase and DC – have two common characteristics during starting and run-up. Without some form of control, the motor will take very high currents, and the switching may itself cause voltage spikes.

The high currents risk overloading the supply and the overvoltages can damage the electrical insulation of the supply or the motor itself. Before the advent of power electronics these over-currents and over-voltages were usually controlled by the temporary introduction of resistors into the motor supply, and sometimes (for three-phase motors) also reduced-voltage switching using a star-delta starter.

Any electrical engineering textbook will have several chapters devoted to the many ingenious methods of simultaneously keeping starting currents and voltages at acceptable levels while creating sufficient torque to accelerate the motor to operating speed. Most of these techniques are now obsolete because they have been supplanted by variable speed drives, for which Cressall’s extensive range of dynamic braking resistors can be used for controlled and safe stopping.

However, when spares or replacements are wanted for existing installations and motors of all types (such as DC motors, slip-ring motors, speed-controlled motors or closed transition star-delta starters) Cressall’s engineers are always able to design and manufacture suitable resistors. Our expertise in application engineering means that we can offer you a complete range of dedicated resistors for starting, braking and speed control that meet European, international and bespoke specifications.

We can produce cost-effective designs for new or replacement motor control applications, particularly for customers with legacy systems for which the original equipment supplier no longer exists or has ceased manufacture.


For breakdowns and emergencies we can offer next-day service of appropriate replacements from our large stocks of pre-manufactured grids and coils.


Motor Control for crane control

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.

Crane resistors

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.

Liquid starters

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.