Back-EMF & inductive kick
Why a spinning motor generates its own voltage, and why switching a coil produces a damaging spike.
A motor is also a generator
As a motor turns, its windings move through a magnetic field — exactly the condition that generates voltage. So a running motor produces a voltage of its own, opposing the supply: the back-EMF. It rises with speed and is what limits the running current to a sensible value.
At the instant of starting there's no back-EMF, so only the winding resistance limits the current — which is why starting current is so high until the motor speeds up and the back-EMF builds. It also means a motor can feed voltage back for a moment after the supply is removed, while it's still spinning.
Inductive kick when you switch a coil
Any coil — a contactor, relay or solenoid — stores energy in its magnetic field. When you break the circuit, that field collapses and tries to keep the current flowing, producing a sharp high-voltage spike across the opening contacts. That spike causes contact arcing and erosion, and can damage nearby electronics or false-trigger inputs.
It's tamed with suppression across the coil: a freewheel diode on DC coils, or an RC snubber or varistor on AC. A relay contact that pits and welds early, or a PLC input that flickers when a nearby coil drops out, is often an unsuppressed inductive kick.
Related faults
Motor won't start and makes no sound at all
Press start and nothing happens — no hum, no movement, no attempt to turn. The motor is completely dead rather than struggling.
Contactor coil overheating / burning smell
The contactor coil runs hot, discolours, or gives off a burning smell, and may eventually fail. It might still operate for now but won't last.
PLC input not reading despite a signal present
A field device is clearly providing a signal, but the PLC input bit/LED doesn't come on — the program never sees the input, so logic that depends on it won't run.
Related definitions
Induction motor
The workhorse AC motor — a rotating magnetic field in the stator drags the rotor around with it.
Inrush current
The brief, high current many loads draw at switch-on — and why it trips protection if not allowed for.
Relay
A small electrically-operated switch — like a miniature contactor — used to switch or route control signals.