Siemens MICROMASTER Eco Reference Manual page 9

This field crosses the air gap between the stator
and rotor and causes currents to flow in the rotor
windings. This produces a force (torque) on the
rotor as the current interacts with the changing
magnetic field, and the rotor turns.
If the windings are arranged in several pairs (or
poles), the frequency of the rotating field will be
less than the applied frequency (e.g. two pole =
50/60Hz = 3000/3600 rpm, but four pole =
50/60Hz = 1500/1800 rpm).
Torque
Torque Speed Characteristic of An Induction Motor
The speed of the motor depends on the applied
frequency, as well as the winding arrangement,
and to some extent on the load.
Therefore in order to control the motor speed it is
necessary to control the frequency of the supply.
If the frequency is reduced, the voltage must be
reduced, or the stator current and magnetic flux
will be too high and the motor's magnetic field will
saturate. Hence the voltage must be controlled as
well.
3-2
However, if the rotor runs at the same speed as
the rotating field, there will be no changing
magnetic field, and therefore no torque.
Since rotor currents must be induced in order to
create output torque, the rotor always runs a little
slower than the rotating field. This difference in
speed is known as slip and is generally about 3%.
If the frequency is increased above normal, more
voltage would normally be needed to maintain
maximum flux; this is not usually possible, so less
torque is available at high speed, (i.e. speeds
above the supply frequency).
ECO REFERENCE MANUAL
Pull out
(maximum) torque
Normal operating
point
Variable frequency
operation
Speed
Slip