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Technical note - Back EMF explained

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Back electromotive force (EMF) is known under a variety of other names. The most common alternative name is counter electromotive force. It is a voltage that opposes the change in current that induced it, as is described by Lenz’s law.

In a motor, the changing of a magnetic field, or change in current, is caused by a relative movement of its coil, with respect to alternating magnetic poles. This can either be by moving the coil past a static magnet (linear motors) or moving magnets past a static coil (torque motors). Back EMF can also be viewed in the context of generators, as motors and generators are very similar. The former converts electrical energy into mechanical energy and the latter vice versa. When the motor is for instance moving at a certain speed, a voltage is generated that is proportional to this speed. 

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