Engineering · Motors & Drives · Stage 1
Induction Motor Torque–Speed Characteristics
Why does a motor start, settle at one speed, or stall?
The torque–speed curve is the motor’s whole personality on one graph: how hard it pulls at standstill, how hard it can pull at most, and where it runs when a load pushes back. Put the load’s own curve on top and the crossing point is the speed you actually get. This page lets you move every part: slip, supply voltage, rotor resistance, load type and drive frequency. The curves come from a simple per-phase model tuned to typical figures; check the maker’s curve for a real motor.
From standstill the torque rises to a peak, the breakdown torque, then falls to zero at synchronous speed. Left of the peak the motor is still accelerating; right of it is the stable working region, where more load means slightly lower speed. The rated point sits on the steep, nearly straight part close to synchronous speed, which is why speed changes so little with load.
| Point | Where | Typical value (% of rated torque) |
|---|
Slip is the speed gap between the rotating field and the rotor, as a fraction of synchronous speed: s = (Ns − Nr) ÷ Ns. This chart is the same curve drawn against slip instead of speed, so standstill (slip 100%) is on the right and synchronous speed (slip 0) on the left. In the normal working range, up to a little past rated slip, torque is almost proportional to slip: double the slip, double the torque. The straight dashed line shows that approximation.
| Supply voltage | Torque factor (V²) | Starting torque | Breakdown torque |
|---|
Torque goes with the square of the voltage. At 90% voltage every torque on the curve is 81% of its normal value; at 80% it is 64%; at 70% only 49%. Speed at the peak does not move, so a heavily loaded motor on a weak supply slows down, draws more current, and can stall.
Adding resistance in the rotor circuit of a wound-rotor motor does not change how high the peak is; it slides the peak toward standstill. The breakdown torque stays the same while the breakdown slip grows in proportion to the rotor resistance. With enough resistance the peak sits at standstill and the starting torque equals the breakdown torque. The price is a larger slip at the working torque, and the rotor loss is slip times the air-gap power, so the extra resistance is paid for in heat. A squirrel-cage rotor gets its resistance from its design instead (deep-bar and double-cage rotors raise starting torque).
| Load type | Torque versus speed | Typical machines |
|---|
The motor accelerates while its torque is above the load torque, and stops accelerating where the two curves cross: that is the operating point. If the load curve is above the motor curve at standstill the motor will not start. Constant-torque loads (conveyors, hoists, positive-displacement pumps) are the hardest on starting; fans and pumps need little torque at low speed; constant-power loads (spindles, winders) ask for more torque as speed falls.
A drive running at constant V/f slides the whole curve to the left: the same shape, centred on a lower synchronous speed. The peak torque stays the same down to a few hertz, then drops as the stator resistance takes a bigger share of the voltage; a voltage boost wins it back. This is why a drive can start a motor at full torque with no current surge, and why V/f drives add boost. See VFD Control Methods.
| Load | Slip | Speed (rpm) | Speed drop from no load |
|---|
Speed regulation = (NNL − NFL) ÷ NFL × 100, where NNL is the no-load speed and NFL the full-load speed. Slip grows roughly in step with load, so the speed falls on a nearly straight line from no load to full load. Standard motors regulate to about 2 to 5%; large motors are under 1%.
| Parameter | Typical value for standard induction motors |
|---|
The charts come from a per-phase equivalent-circuit model with typical parameters, so they show the right shapes and trends, not the exact curve of any one motor. Design classes (NEMA A to D, IEC N and H) differ widely: always use the maker’s curve and datasheet.