Electromotive force

Electromotive force, also called EMF, is the voltage developed by any source of electrical energy such as a battery or dynamo.

EMF in electric motors

In case of electric motor EMF is the voltage generated by motor when motor is rotating. All permanent magnet motors act also as generators and they produce voltage that is directly proportional to rotation speed. EMF is generated no matter if motor is being used as generator or motor.

Typically EMF voltage constant is denoted as K_v which has units of V/krpm. I.e. some motor may have K_v of 30 V/krpm which means that motor generates 30 volts when rotating at 1000 rpm and 60 volts at 2000 rpm etc.

EMF and drives

To drive motor, motor driver must output voltage greater than EMF of motor in order to be able to drive some current to the motor.

Maximum speed obtainable from motor is mostly determined by EMF and supply voltage of drive. I.e. if DC motor with K_v of 30 VDC is driven by motor drive that has HV DC bus of 320 VDC, then the maximum obtainable speed would be 320 VDC/30 VDC/krpm = 10667 rpm. In practical case there are also other losses in equation so actual obtainable speed under mechanical load will be less than the ideal 10667 rpm.

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