The stationary and rotating parts.

The stator is stationary; the rotor turns. Magnetic interaction between them produces torque on the shaft. Bearings support the rotor while permitting rotation.

Windings create magnetic fields when current flows. The rotor field may come from induced currents, permanent magnets or supplied windings, depending on the motor.

Follow the energy path.

  1. Electrical inputSupplies the motor.
  2. Magnetic interactionProduces turning effort.
  3. Shaft outputDrives the connected load.
Motoring operation. Heat and other losses mean useful output is less than input.

A changing field sustains rotation.

In a three-phase AC motor, suitably arranged windings and phase-shifted currents create a rotating magnetic field. Frequency and pole count determine its rotational speed.

The rotor’s relationship to that field depends on motor type. The next two lessons separate induction motors from permanent-magnet synchronous motors.

1. First instant

Stator field at 0 degreesFixed winding axes A, B and C surround the rotor space. Relative currents A +1, B −0.5, C −0.5 produce the orange resultant field at 0 degrees. The stator stays still.ABCField direction: 0°

Relative phase currents:
A +1 · B −0.5 · C −0.5

2. One-third cycle later

Stator field at 120 degreesFixed winding axes A, B and C surround the rotor space. Relative currents A −0.5, B +1, C −0.5 produce the orange resultant field at 120 degrees. The stator stays still.ABCField direction: 120°

Relative phase currents:
A −0.5 · B +1 · C −0.5

3. Two-thirds cycle later

Stator field at 240 degreesFixed winding axes A, B and C surround the rotor space. Relative currents A −0.5, B −0.5, C +1 produce the orange resultant field at 240 degrees. The stator stays still.ABCField direction: 240°

Relative phase currents:
A −0.5 · B −0.5 · C +1

  1. The windings stay still. A, B and C mark the positive magnetic axes of three winding groups, 120° apart in this simplified two-pole motor. The opposite ends are shown in grey.
  2. The currents change. +1 means the positive peak current; −0.5 means half the peak in the opposite direction. These are relative values, not amperes.
  3. The combined field rotates. The orange arrow is the resultant magnetic-field direction. It moves anticlockwise through these snapshots; the central circle only marks the rotor space and does not show rotor position.
Ideal balanced sinusoidal currents. The field turns continuously between these three instants. This is a magnetic-axis sketch, not a winding or wiring drawing. More pole pairs change the relationship between electrical cycles and mechanical rotation.

Rotation can move different machinery.

Traction and hydraulic lifts

A traction motor turns the drive sheave, directly or through gearing. A hydraulic power-unit motor drives a pump, which supplies fluid to the lifting circuit.

The motor shaft speed is not automatically the car speed. Gearing, sheave size, roping or hydraulic flow connects one to the other.

Torque, speed and temperature all matter.

A motor must meet the load’s requirements through starting, travel and stopping. Producing torque at low speed can still create considerable heat.

Motor, drive, controller and brake have distinct roles. Understanding a motor does not make every motor interchangeable: supply, cooling, duty and mechanical fit also matter.

A closer look

When the machinery drives the motor and electromagnetic torque opposes rotation, energy can flow from the shaft into the electrical system. This generating operation is developed later in the module.

Check your understanding

Does a motor’s rated power alone tell you how fast the lift car travels?

Reveal the explanation

No. Motor speed and the connected mechanical or hydraulic arrangement determine car movement. Power alone does not specify either.

Further reading.

Optional reading to explore this lesson in more detail.