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Induction Motor Fundamentals
How does a motor with no electrical connection to its rotor still turn?
The three-phase induction motor does most of the world’s industrial work. Its stator makes a magnetic field that rotates; that field induces current in the rotor; and the rotor current, acting in the field, produces torque. The rotor always turns a little slower than the field, and that small difference, called slip, is what makes it all work. This page builds the idea step by step with things you can move: a rotating-field viewer, calculators for speed and slip, and a torque–speed curve you can load until the motor stalls. Typical figures; check your motor’s nameplate and datasheet.
Three coils sit 120° apart around the stator. Each carries one phase of the supply, and the three currents peak one after another. At every instant the three coil fields add up to one field of constant size that points in a steadily turning direction. Swap any two supply leads and the order reverses, so the field, and the motor, turns the other way. With more poles the field turns slower: one electrical cycle moves it 2 ÷ P of a turn.
Ns = 120 × f ÷ P, where f is the supply frequency in hertz and P is the number of poles. The rotor can never quite reach this speed on its own. A VFD changes f, which is how it changes the speed: see VFD Control Methods.
| Condition | Rotor speed | Slip | Rotor current frequency |
|---|
Slip s = (Ns − Nr) ÷ Ns. The rotor sees the field sweep past at the slip speed, so the current in it alternates at only fr = s × f. At standstill the slip is 1 and the rotor frequency equals the supply frequency; at full load it is only a few hertz. That is why the rotor iron can be simple and why rotor losses stay low. Typical full-load slip is about 0.5 to 5%: large motors are often under 1%, small motors can be above 5%.
The curve comes from the standard per-phase equivalent circuit (magnetising branch ignored), which captures the real shape. Starting torque is where speed is zero, the peak is the breakdown torque, and the motor runs on the steep falling part to the right of the peak: more load slows it slightly, raises slip, raises rotor current and so raises torque until it balances the load. That is the self-regulating action. Load it past the breakdown torque and the motor stalls. A deep-bar or double-cage rotor raises the starting torque. On a wound rotor, adding external resistance does the same and moves the peak towards low speed, at the price of heat in the resistor. Real curves vary by design; take figures from the datasheet.
| Load (% of rated torque) | Speed (rpm) | Slip | Rotor frequency (Hz) |
|---|
More load, more slip, more torque. The speed drop is small, a few percent from no load to full load, which is why an induction motor behaves almost like a constant-speed machine on a fixed supply. The table uses the settings above.
| Feature | Squirrel cage | Wound rotor (slip ring) |
|---|
Figures are typical for standard industrial motors. Real motors differ in design class, size and efficiency, so use the nameplate and the maker’s curves for any real selection.