Question

Back e.m.f reduces the effective voltage in the circuit." What is your opinion about this statement? Substantiate.

Answer 1

The electromotive force (voltage) that opposes the change in current that caused it is known as counter-electromotive force or back electromotive force (back EMF). The EMF produced by magnetic induction is known as CEMF. The self-induced voltage appears across an inductor or coil as a result of a change in current that induces a change in the magnetic field within the coil. To maintain the current constant, the polarity of the voltage at any one time opposes that of the change in applied voltage.

Complete answer:
When there is relative motion between the armature and the magnetic field produced by the motor's field coils or permanent magnet field, the term back electromotive force is also commonly used to refer to the voltage that occurs in electric motors where the armature and the magnetic field produced by the motor's field coils or permanent magnet field, thus acting as a generator while running as a motor.

This effect is caused by a distinct phenomena, not the motor's inductance, which creates a voltage in opposition to a changing current via Faraday's law. That is, the back-EMF comes from geometric considerations of an armature spinning in a magnetic field and is caused by inductance and Faraday's law. It happens even when the motor current is not changing.

This voltage is called "back-electromotive force" (per Lenz's law) because it is in series with and opposes the initial applied voltage. The current flowing into the motor drops as the total voltage across the motor's internal resistance lowers as the motor rotates faster.

Because the back-EMF is proportional to the armature's rotational speed, this phenomenon may be used to indirectly assess motor speed and position. Back-EMF is a term used in motor control and robotics to describe the use of the voltage created by a spinning motor to infer the speed of the motor's rotation, which may then be used to better control the motor in particular ways.

Note: To see how back-EMF affects a motor, try this simple experiment: turn on an incandescent light and start a big motor like a drill press, saw, air conditioner compressor, or vacuum cleaner. As the motor begins, the light may fade temporarily. There is no back-EMF when the armature is not spinning (known as locked rotor), yet the motor's current demand is extremely high. If the motor's beginning current is strong enough, it will lower the line voltage to the point where the light will fade noticeably.