Alternating-Current Motors

Alternating-current Motors







On alternating-current motors, the polarity of the alternating current from the stator coil reverses the polarity of the rotor magnetic field automatically (see illustration).







The slip rings or carbon brushes required on the direct-current motor are omitted as a result of the omission of the commutator and current supply to the stator coil. If the rotor has only just started moving, the rotation speed depends on the frequency of the alternating current, whose voltage level defines the maximum power that can be generated. However, the alternating current itself is not sufficient to achieve the curve shown in the image above. The so-called starting motor requires an initial impulse ("starting impulse") in order to initiate a clockwise or counter-clockwise rotation.







The number of poles is increased by distributing the coil around the stator. The nominal speed for all alternating-current/three-phase motors is calculated using the following formula:







f = frequency and p = number of pole pairs. At a frequency of f = 50 Hz, the nominal speed of a motor with two pairs of poles would be -> n = 1500 rpm. The speed of an alternating-current or three-phase motor is therefore modified by varying the frequency.

To allow an alternating current motor to start autonomously, a rotary field must be generated that carries the rotor along. On simple alternating-current motors, this is always achieved during an auxiliary phase generated by a capacitor. The "second" generated operating voltage is applied to another stator coil, which allows the motor to start automatically and determines the direction of rotation. The "capacitor motor" is used in almost all electric devices operated with 240 V AC power supply. The design of the motor is simple but unsuitable for applications requiring high levels of power.