back-EMF

When a motor spins, it fights the very current that is driving it. The turning coil generates its own voltage that pushes back against the supply. That opposing voltage is the back-EMF. It answers a puzzle every motor shows: why does a motor draw less current the faster it spins?

Precisely, a running motor is also a generator: its rotating coil induces an EMF that opposes the applied voltage, exactly as Lenz's law demands. The current is set by the difference between the supply and this back-EMF: I = (V_supply - EMF_back) / R, where R is the coil's resistance. At start-up the coil is not yet turning, so EMF_back = 0 and the current is large; as the motor speeds up, the back-EMF grows and the current falls.

This explains start-up current surges, why a stalled motor overheats (its back-EMF collapses and the current soars), and why motors run efficiently once up to speed. The honest point is that back-EMF is not wasted energy — it is the signature of mechanical work being done; the more work the motor does, the more it is loaded down and the more current it must draw.