Back-EMF

Back-EMF is the voltage a spinning motor generates that pushes back against the supply driving it — the motor fighting its own power source. The moment a motor starts turning, its windings move through a magnetic field, and (by Faraday's law) any conductor moving through a field generates a voltage. That generated voltage opposes the applied voltage, like a cyclist freewheeling downhill who starts to feel they barely need to pedal.

Back-EMF rises in proportion to speed, so it acts as nature's own speed governor: at standstill it is zero, so huge current rushes in (the inrush that dims your lights when a fridge starts); as the motor speeds up, back-EMF grows and chokes the current down until it only draws what the load demands. A motor's no-load top speed is essentially the speed at which back-EMF nearly equals the supply voltage, leaving almost no net voltage to push current.