Electric motors, both AC and DC have common characteristics whereby they produce high inrush currents on starting and during their run up.
These high inrush currents risk overloads on the system and before power electronics were used to prevent this, in AC systems the use of primary and reduced voltage starting resistors were common place, for DC systems the provision of similar starting resistors/divert resistors were used.
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 have been made redundant with the introduction of soft start systems and variable speed drives, for which Cressall can supply torque increasing resistors or dynamic braking resistors.
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.
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.
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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 – such 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 Starter
Star-delta starting is a commonly used method to provide reduced current starting whereby the stator windings of the motor are initially connected in a star configuration and switched to delta as the motor speed increases. With this method a voltage spike arises when the motor windings are briefly open-circuit, so to eliminate this three resistors are put into the circuit (via a third contactor) for a short time.
Squirrel Cage AC Motor Resistors
Primary resistors can be used upstream of these motors to reduce inrush currents on starting.
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 long travel, hoist, and cross-travel motions. For these applications, standard switchgear and stepping cover most common operating conditions. Cressall’s 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 over-voltages if not dissipated safely. The field discharge resistor limits the current to the required value, usually 20% of nominal current.