The stator produces a rotating field.

Three-phase currents in distributed stator windings create a rotating magnetic field. Its synchronous speed is nₛ = 120f ÷ p, where f is frequency in hertz and p is the total number of poles.

At 50 Hz, a four-pole motor has a synchronous field speed of 1,500 rpm. This describes the field, not necessarily the shaft.

The rotor receives energy by induction.

1. Starting reference

Induction rotor marker and rotating fieldTwo-pole illustration. The orange field direction is 0 degrees and a dark physical marker on the rotor is at 0 degrees. The field advances 120 degrees per interval while the rotor advances 116 degrees.Field 0° · mark 0°

2. One interval later

Induction rotor marker and rotating fieldTwo-pole illustration. The orange field direction is 120 degrees and a dark physical marker on the rotor is at 116 degrees. The field advances 120 degrees per interval while the rotor advances 116 degrees.Field 120° · mark 116°

3. Two intervals later

Induction rotor marker and rotating fieldTwo-pole illustration. The orange field direction is 240 degrees and a dark physical marker on the rotor is at 232 degrees. The field advances 120 degrees per interval while the rotor advances 116 degrees.Field 240° · mark 232°
  1. Orange arrow: field direction. It advances 120° during each equal time interval.
  2. Dark dot: a mark on the rotor body. It advances 116° per interval. The gap grows by 4° each time, so the field keeps moving past the rotor conductors.
  3. Relative movement sustains induction. The speed difference induces rotor voltage and current; the resulting magnetic interaction produces torque.
Simplified two-pole squirrel-cage illustration at steady motoring speed: (120 − 116) ÷ 120 ≈ 3.3% slip. The dark dot is a physical reference mark, not the direction of the rotor’s magnetic field. The four-pole worked example below has the same slip percentage.

Slip describes the speed difference.

1,500 rpm field, 1,450 rpm rotor

Slip = (1,500 − 1,450) ÷ 1,500 = 0.0333, or approximately 3.3%.

In ordinary steady motoring operation, the rotor runs below synchronous speed. At exactly synchronous speed, the ideal rotor sees no changing field and produces no induction torque.

Load affects the operating point.

With a fixed-frequency supply, increasing load normally increases slip over the stable operating region. A controlled drive can adjust its output to meet the required torque and speed.

Slip is not a fault by itself. Comparing a nameplate speed with synchronous speed often reveals the normal difference.

The drive and motor must work together.

Induction motors are used in many lift machines and hydraulic pump units. Variable-frequency control can provide controlled acceleration and deceleration where the installation is designed for it.

The drive needs an appropriate motor model and ratings. Frequency alone does not guarantee an exact shaft speed under every load.

A closer look

If the rotor is driven above synchronous speed, slip becomes negative and induction generating operation is possible with suitable excitation and an electrical energy path. The normal motoring example is not the only operating condition.

Check your understanding

Is a four-pole induction motor supplied at 50 Hz necessarily turning at exactly 1,500 rpm?

Reveal the explanation

No. That is the synchronous field speed. The rotor normally runs slightly slower during motoring, with slip allowing torque production.

Further reading.

Optional reading to explore this lesson in more detail.

  • ABB: induction-motor slip (PDF)

    Technical reading: Guide 7, chapter 4, printed pages 11–12 (PDF pages 277–278). The equations and torque–speed curves develop synchronous speed and slip.

  • Microchip: induction-motor slip and torque (PDF)

    AN984, page 7, relates rotor speed, rotating-field speed and slip. The torque curve also shows why the normal motoring region is only part of the full operating picture.