Repulsion Start Motors

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The repulsion start motor was the most common single-phase motor in use prior to the squirrel-cage motor. After the 1960s very few repulsion start motors were installed because they require brushes and a commutator to operate. The rotor for this type of motor is slightly different from the rotor of the squirrel-cage rotor since it uses copper wire to make its magnetic field. Another feature that makes the rotor different is that it has a wire that connects the commutator segments with a shorting mechanism, which is used in conjunction with the brushes. Since this motor was designed before squirrel-cage motor theory and technology became prevalent, the rotor was patterned after the wound rotor that is used in DC motors. The rotor was made of laminated sections with coils of wire pressed into place and their terminal ends brought out to commutator segments.

When the motor was being started, current was directed to the rotor coils through the brushes. After the rotor was spinning fast enough, the brushes were disconnected from the applied voltage and shorted so that the rotor would act like an inductive rotor. In some motors, the brushes remained connected to the applied voltage, hut they were lifted slightly so that they would not make contact with the commutator. At the same time a shorting mechanism would short the commutator segments to complete the circuit on each coil so that it could conduct the induced current like a squirrel-cage rotor in an induction motor.

In both of these types of motors the rotor would start the motor as a repulsion start motor, and after the rotor came up to speed the motor would operate like an induction motor. This would give the motor the maximum amount of starting torque.

Since the rotor required brushes and some kind of lifting or shorting mechanism, it would require an excessive periodic maintenance. This made the motors too expensive to maintain and they were soon replaced with squirrel-cage motors.

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