Back EMF Constant (Ke)
It is the amount of back EMF the motor produces per unit of speed. In catalogues it is usually given in V/1000 rev·min⁻¹ or V·s/rad. This constant determines the no-load speed reachable with an applied voltage and is the most practical way to calculate speed from a measured voltage.
Example: a motor with a back EMF constant of 8 V/1000 rev·min⁻¹ reaches roughly 3000 rev/min no-load speed on a 24 V supply. Because of the drop across the winding resistance and friction, the real value stays a little below this.
Areas of use:
- Speed measurement: If you let the motor run free and read the voltage at its terminals, you find the speed by dividing that voltage by the back EMF constant. No tachometer is needed.
- Speed estimation with PWM: If the duty cycle is 50 percent the average armature voltage halves and the no-load speed roughly halves as well.
- Regenerative risk: When the load turns the motor faster than the supply does, the voltage produced is calculated from the back EMF constant and the speed and compared with the supply limit.
In the international system of units the back EMF constant and the torque constant are numerically equal; if you know one you can obtain the other by unit conversion. This helps you when only one of them is given in the catalogue.
The context in which this term is used: DC Motor Driver Selection Guide