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.
- Relative movementThe field moves relative to rotor conductors.
- Induced currentsCurrents flow in the rotor circuit.
- TorqueTheir magnetic interaction produces turning effort.
Slip describes the speed difference.
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.