Back-EMF
The voltage a turning DC motor produces in its windings, opposing the applied voltage. It grows in direct proportion to speed and is the main quantity limiting the armature current. It falls to zero when the motor stops; this is the mechanism that explains why starting and stall currents are so high.
The armature current is the difference between the applied voltage and the back-EMF divided by the winding resistance. If the motor produces 21 V of back-EMF on a 24 V supply and the winding resistance is 0.4 ohm, the current is roughly 7.5 A. With the same motor at standstill the back-EMF is zero, so the current rises to the order of 60 A.
Consequences that matter in the field:
- Speed drops as load increases: When the speed drops the back-EMF falls, so the current and therefore the torque rise. The motor balances itself.
- Energy flows back during deceleration: If the back-EMF rises above the supply voltage, the current turns back towards the source and the line voltage rises.
- Braking: When the motor terminals are shorted, the back-EMF drives current through the winding and produces an opposing torque.
To measure the motor speed roughly you can read the voltage at the terminals at the moment the supply is removed; this value is the back-EMF at that instant and is converted to speed with the EMF constant.
The context in which this term is used: DC Motor Driver Selection Guide