The Induction Motor

The three-phase induction motor drives the overwhelming majority of industrial machinery. It is simple, rugged, cheap and has no electrical connection to the rotor at all.

The rotating field

Three-phase current in the stator windings produces a magnetic field that rotates at synchronous speed, set by the supply frequency and the number of poles. A two-pole motor on a 50 hertz supply has a synchronous speed of 3000 revolutions per minute; a four-pole motor, 1500.

Slip

The rotating field cuts the rotor conductors, inducing current in them, and the interaction produces torque. For induction to occur there must be relative motion, so the rotor always runs slightly slower than the field. That difference is slip, typically one to five per cent at full load. A motor with no load runs close to synchronous speed; as load is applied it slows slightly, slip increases, rotor current increases and torque rises to match.

This gives the induction motor its most useful property: it is self-regulating. It draws the current the load requires without any control system.

The torque-speed curve

• Locked rotor torque — torque at standstill, which must exceed the load's breakaway torque.
• Pull-up torque — the minimum during acceleration; a dip here can stall the motor on a heavy load.
• Breakdown torque — the maximum the motor can produce. Loading beyond it stalls the motor.
• Full load torque — at rated slip.

Starting current is typically six to eight times full load current and stays high until the motor approaches speed. On a high-inertia load the motor therefore spends a long time drawing heavy current, and it is thermal damage during starting — not running — that limits how many starts per hour are permitted.

Other motor types

Synchronous motors run at exactly synchronous speed and can be operated to generate reactive power, improving site power factor. They are used for large, constant-speed drives. Wound rotor motors allow external resistance in the rotor circuit for high starting torque with low current. Single phase motors serve small loads and need an auxiliary means of starting.