A repulsion motor provides high torque and easy reversibility. It uses contact brushes to change torque and rotation parameters. It has been replaced by induction motors, but is still used in some applications. The position of the brushes is critical for correct operation.
A repulsion motor is a type of electric motor designed to provide a high level of torque or rotational force at start-up and to have the ability to easily reverse the direction of rotation. It is an alternating current (AC) motor that uses a series of contact brushes that can have a variable contact angle and level to change the torque and rotation parameters. These motors were used extensively in early industrial equipment, such as drill presses up until the 1960s, which required a large amount of slow rotational force, and in microcontrol systems, such as for traction motors on model railways. As of 2011, they have mostly been replaced by less complex induction motor designs with more reliable loop controls and easier to manufacture and maintain.
A repulsion motor design has both an electrical winding for the stator and rotor assembly and no permanent magnets to generate an electromagnetic field. The electric brushes are placed on the rotor assembly through a commutator and current is passed through them to the rotor as they are in contact to start the motor. Once the repulsion motor reaches high speed, the brushes are usually pulled out and the motor acts like a typical induction motor. This gives the pusher motor high torque at low speeds and standard motor performance at high speeds. A short circuit mechanism is also built into the motor to break the connection to the commutator so that it can work as an induction motor and also have the ability to reverse rotation.
Disadvantages of the repulsion motor design include the complex mechanical design of the contact brushes and the fact that it was modeled on the functionality of the early direct current (DC) motor. It is a single-phase motor, which means it uses alternating current which is passed through a stator assembly with one electric winding, but the stator itself has up to eight magnetic poles. The rotor assembly resembles the way an armature is built into a DC motor, hence it is often referred to as an armature in engineering fields, and this is where the commutator and brushes 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 speed of the motor by creating a repulsion effect with the competing magnetic poles. The armature and stator each have their own sets of magnetic poles and are offset by approximately 15 electrical degrees from each other, which creates a magnetic repulsion effect that initiates rotation of the rotor. The position of the brushes is critical to the correct operation 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 there is no rotational torque.
While modern electrical circuits have replaced many repulsion motors with induction motors that have similar control characteristics, the repulsion motor is still used in some fields due to its ability to generate a large amount of torque at low speeds. These include applications such as printing press drives and ceiling fans or environmental control fans with slowly rotating fan assemblies. Variations on the original repulsion motor design include the incorporation of typical induction performance principles, such as the repulse start induction motor, the repulsion induction motor, and the compensated repulsion motor.
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