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What Is a Repulsion Motor?

By Ray Hawk
Updated May 17, 2024
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A repulsion motor is a type of electric motor that is designed to provide a high level of torque or rotational force upon start up, and to have the capability of easily reversing the direction of rotation. It is an alternating current (AC) motor that uses a series of contact brushes which can have a varied angle and level of contact for changing torque and rotational parameters. These motors were widely used in early industrial equipment, such as drill presses until the 1960s that required a large amount of slow rotational force, and in micro-control systems, such as for traction motors on model railroads. As of 2011, they have mostly been replaced by less complex induction motor designs with circuitry controls that are more reliable and easier to manufacture and maintain.

The design of a repulsion motor has both an electrical winding for the stator and rotor assembly and no permanent magnets to generate an electro-magnetic field. Electrical brushes are positioned over the rotor assembly through a commutator, and current is passed through them to the rotor while in contact to start the motor. Once the repulsion motor reaches a high rate of speed, the brushes are usually withdrawn and the motor acts as a typical induction motor. This gives the repulsion motor high torque at low speeds and standard motor performance at high speeds. A shorting mechanism is also built into the motor to break the connection to the commutator so that it can operate as an induction motor and also have the ability to reverse rotation.

The drawbacks to the design of the repulsion motor include the complex mechanical design of the contact brushes and the fact that it was modeled after early direct current (DC) motor functionality. It is a single-phase motor, meaning that it uses AC current that is run through a stator assembly with one electrical winding, but the stator itself has up to eight magnetic poles. The rotor assembly resembles the way that an armature is built into a DC motor, so it is often referred to as an armature in engineering fields, and this is where the commutator and brushes come into contact to control torque and direction of rotation.

The direction in which the brushes approach or contact the commutator and, therefore, the rotor, as well as their physical proximity to it, determines the motor's speed by creating a repulsion effect with competing magnetic poles. The armature and stator each have their own sets of magnetic poles and are offset by roughly 15 electrical degrees from each other, which creates a magnetic repulsion effect that starts the rotor rotating. The location of the brushes is critical in the proper function of the repulsion motor, because, if the brushes are at direct right angles to the stator assembly, the poles cancel each other out preventing magnetic flux, and no rotation torque exists.

While modern electrical circuitry has replaced many repulsion motors with induction motors that have similar control features, the repulsion motor is still used in some fields due to its ability to generate a large amount of torque at slow speeds. These include such applications as printing press drives and ceiling fans, or blowers for environmental controls that have slowly rotating fan assemblies. Variations on the original design of the repulsion motor include incorporating typical induction performance principles into it, such as the repulsion start induction motor, repulsion induction motor, and compensated repulsion motor.

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