Wednesday, 1 April 2015

Electric Motors And Generators


If an electrical conductor moves across a magnetic field in it an electrical voltage is induced; it is all the higher, the faster the movement. Accordingly, the magnetic field in the stator rotating rotor of an electric motor or magnetic rotor of a generator in its windings induces a voltage. This voltage is called for motors back EMF. It does not matter what voltage is actually applied to the motor or generator - the difference between the two voltages drops at the ohmic resistance of the windings or caused by leakage currents.

Increases the speed of a DC motor to the extent that the EMF approaches the applied voltage, the current consumption and the speed drops no longer go higher. With regard to the back-emf of a DC motor can thus calculate the limit speed for a given voltage.

The back EMF of a DC motor and other motors can be used for its control and speed control. This approach is used, but also in electronically commutated motors and drives for asynchronous motors in modern, for example, the case of small permanent-magnet motors for driving cassette tape recorders.

Separately excited direct-current motors can be increased by field weakening in its rotational speed - the back EMF now requires a higher speed, to achieve the value of the operating voltage.

And asynchronous motors induce a back EMF - here induces the squirrel-cage rotor rotating magnetic field in the stator windings an AC voltage that opposes the current consumption when the rotor reaches the rated speed.

The emf of stepper motors limited their dynamics and torque at high speeds.

The electromotive force is in generators nearly equal to the open circuit voltage. The generated voltage or the EMF of generators may be varied by varying the rotational speed or of the excitation field.