When the 3-phase a.c. supply voltages are connected to the three stator phase windings, the resulting phase currents produce a multi-pole magnetic flux.
This flux is physically rotated around the stator core by the switched sequence of the R-Y-B currents at a speed called synchronous speed.
The value of synchronous speed depends on how many magnetic pole-pairs (p) fixed by the stator winding arrangement and by the frequency (f) of the voltage supply connected to the stator winding.
Synchronous speed of a 6-pole motor supplied at 60 Hz is 20 rev/s or 1200 rev/min
The stator rotating magnetic flux cuts through the rotor conductors to induce an alternating EMF into them.
Since the rotor conductors are connected together at the ends, the induced emf’s set up rotor currents.
The rotor currents also produce a magnetic flux which interacts with the stator rotating flux which produces a torque (T) on the rotor conductor bars.
The rotor reactance varies with the rate of cutting flux which depends on the rotor speed.
The direction of the rotor torque causes the rotor to rotate in the same direction as the rotating magnetic field.
Rotor direction reversed is simply by swapping over any two supply line connections at the stator terminal box.
This reverses the direction of the rotating magnetic field.
An induction motor cannot run normally at synchronous speed.
This is because the rotor conductors would then be stationary with respect to the rotating magnetic field.
No emf would be induced in the rotor and there would be no rotor current and no torque developed.
Even when the motor is on no-load the rotor speed has to be slightly less than the synchronous speed so that current can be induced into the rotor conductors to produce the torque to overcome the mechanical rotational losses of friction and windage.
Slip speed is the difference between the synchronous speed of the rotating magnetic flux and actual rotor speed.
Slip is usually expressed as a percentage of the synchronous speed.
If a 6-pole motor is supplied at 60 Hz and runs with a slip of 5%, what is the actual rotor speed is:
The synchronous speed is 1200 rpm, and the rotor slips by 5% of 1200 by 60 rpm so the rotor runs at 1140 rpm.
If the load torque on the motor shaft is increased, the rotor will tend to slow down (increasing the slip) which allows the rotor conductors to cut the flux at an increased rate.
This causes more current to flow in the rotor which is matched by more stator supply current to meet the increased shaft torque demand.
The motor will now run at this new, slightly reduced, speed.
The fall of motor speed between no-load and full-load is very small (between 1% and 5%) so induction motors are considered to be almost constant speed machines.
At start-up the motor develops more torque than is necessary to turn the load so the motor and load accelerate.
The speed increases until, at the intersection of the two characteristics, the torque developed by the motor is the same as the torque required by the load at that speed. The motor and load will then run at this steady speed as the torque supplied exactly matches the demand.