Introduction | International English |
This field crosses the air gap between the stator and rotor and causes currents to flow in the rotor windings. This produces a force 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 = 50/60 Hz = 3000/3600 rpm, but four pole = 50/60 Hz = 1500/1800 rpm). However, if the rotor runs at the same speed as the rotating field, there will be no changing magnetic field, and therefore no torque. Therefore the rotor always runs a little slower than the rotating field in order to generate torque. This difference in speed is known as slip.
You can see that the speed of the motor depends on the applied frequency, as well as the winding arrangement, and a little 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 magnetic flux will be too high and the motor will saturate so the voltage must be controlled as well. If the frequency is increased above normal, more voltage would normally be needed to maintain maximum flux. Since this is not usually possible, less torque is available at high speed.
Torque | Pull out (Maximum) |
| Torque |
Normal Operating
Point
Speed
Variable frequency operation | Slip |
Figure
2 | MICROMASTER Applications Handbook |