Glossary · Motion control and drives
Back EMF
Also known as: Back electromotive force, Counter-EMF
German: Gegen-EMK
In electric motors, back EMF is the voltage induced in the windings by the motion of the rotor's magnetic field, which opposes the supply voltage and rises with speed; drives use it to estimate speed, rotor position or mechanical load without extra sensors.
- Motion control
In one sentence
Back EMF is the voltage a turning motor induces in its own windings; it rises with speed and lets drives estimate speed and load.
Example
At high speed the back EMF of a stepper motor approaches the 24 V supply, so the driver can no longer push full current into the windings and torque drops.
How it applies
- Engineering: Back EMF limits the speed a motor reaches at a given supply voltage; a higher supply voltage extends the usable speed range. The back EMF constant is a datasheet value.
- Commissioning: Sensorless methods need a minimum speed to see a usable back EMF signal, so they don't work at standstill or very low speeds.
- Operation: A motor turned by the load (for example a falling axis) generates back EMF and can feed energy back into the supply, raising its voltage.
- Documentation: Explain that sensorless functions based on back EMF have a speed range in which they work, and state it. Warn that a motor turned by hand or by the load produces voltage at its terminals even when the drive is off.