Types of electric motors

Almost everything that moves in a factory is driven by an electric motor. Knowing which type you are looking at tells you how it is started, controlled and protected.

3-phase induction motor (squirrel cage)

The workhorse of industry: pumps, fans, conveyors, compressors. Three-phase current in the stator windings creates a rotating magnetic field. The field induces currents in the rotor bars, and the rotor is dragged round, always slightly slower than the field. This difference is called slip (a 4-pole motor on 50 Hz: field 1500 rpm, rotor about 1440–1480 rpm).

Rugged, cheap and almost maintenance-free. Swap any two phases and it runs in reverse. Speed is fixed by the supply frequency unless you use a VFD.

Single-phase motor

Single-phase current makes a pulsating field, not a rotating one, so the motor needs help to start: a start capacitor and auxiliary winding, a run capacitor, or a shaded pole. Used for small loads where only 230 V is available: small pumps, fans, workshop machines.

DC motor

A brushed DC motor has a commutator and brushes that switch the current in the rotor. Speed is proportional to voltage and is easy to control; reversing is done by reversing the polarity. Small 24 V DC motors drive light conveyors, gates and actuators. Brushes wear and need maintenance.

Brushless DC (BLDC) and permanent-magnet motors

Permanent magnets on the rotor, and electronics instead of brushes. Efficient, compact and long-lived. Used in fans, small conveyors, and as the basis of most servo motors. Needs a matching drive.

Servo motor

A servo is a permanent-magnet motor with a built-in encoder, driven by a servo amplifier in a closed loop. The amplifier constantly compares the actual position with the target and corrects it, giving precise position, speed and torque control. Used for axes, robots, cut-to-length, pick and place.

Stepper motor

Moves in fixed steps (typically 200 per revolution, or more with microstepping) for each pulse from its driver. Simple positioning without an encoder (open loop), good holding torque at low speed. If overloaded it can lose steps without knowing. Used in small positioning tasks, labellers and 3D printers.

Comparison

Starting and controlling induction motors

• Direct on-line (DOL) with a contactor and overload: simple, but 6–8 × starting current.
• Star-delta: reduces starting current to about one third.
• Soft starter: ramps the voltage up smoothly.
• VFD (inverter): varies frequency and voltage for full speed control, soft start and energy saving.

Reading the nameplate

Power (kW), voltage and connection (e.g. 400 V Δ / 690 V Y), full-load current (A), speed (rpm), frequency, power factor (cos φ), insulation class and IP rating. The full-load current is what you set on the overload relay.