Motor slip & torque-speed
Why an induction motor must run slightly slower than its field, and how its torque changes from start to full speed.
Slip
An induction motor's stator creates a magnetic field that rotates at synchronous speed, set by the supply frequency and the number of poles. The rotor is dragged along by that field — but it can never quite catch it, because if it did there'd be no relative movement to induce rotor current and so no torque.
The small gap between the field speed and the rotor speed is the slip. At no load it's tiny; as load increases the rotor falls back a little more, slip rises, and the motor develops more torque to match. Slip is what makes an induction motor self-regulating.
The torque-speed curve
From standstill the motor develops its starting (locked-rotor) torque while pulling a heavy inrush current. As it speeds up the torque rises to a peak — the pull-out torque — then falls steeply to the running point near synchronous speed, where it settles to balance the load.
This shape explains a lot of motor behaviour: a load that needs more than the starting torque won't break away (it hums and draws locked-rotor current); a load that briefly exceeds pull-out torque will stall. Reduced-voltage starters cut the inrush but also cut the starting torque, so they only suit loads that start lightly.
Safety first
A stalled motor draws locked-rotor current continuously and overheats fast — the overload must clear it. Never bypass or oversize motor overload protection to 'cure' a motor that won't start.
Isolate, lock out / tag out, and prove dead before working unless a live test is specifically required, authorised, and carried out under proper supervision. Always follow local regulations, your site procedures, and the equipment manufacturer's documentation.
Related faults
Motor hums but won't start to turn
On start the motor hums or buzzes but won't rotate — it may draw heavy current and trip protection. Often a sign it's only getting two phases or is mechanically locked.
Motor trips protection on start but runs fine if it gets going
Protection trips during the start/inrush, but on the rare occasion it gets running it's fine — pointing at starting current, settings, or load inertia rather than a running fault.
Motor running hot / overheating
The motor runs but gets excessively hot — too hot to touch, smell of hot insulation, or thermal protection cutting in after a while.
Related definitions
Induction motor
The workhorse AC motor — a rotating magnetic field in the stator drags the rotor around with it.
Star-delta starting
Starts a motor in star (lower current) then switches to delta (full power) once it's up to speed.
Soft starter
Reduces motor starting current by ramping the voltage up, then often hands over to a bypass contactor.