»Frequency inverter control

Within each period of the alternating current the stator’s magnetic field rotates forward for one pole pair p.

Synchronous speed: nF = (f * 60) / p

Rotating magnetic field is followed by a squirrel-cage rotor only, if it spins slower than with synchronous speed nF. Namely at equal speed there is no voltage induction in the squirrel-cage rotor.
With the input voltage frequency conversion the rotation speed is changed, which is the basic operating principle of frequency inverters.
The torque provided by an asynchronous motor is related to its magnetic field. If an optimal torque is desired magnetic flux should be constant. The magnetic flux is proportional to stator winding voltage and inversely proportional to power supply frequency. This means that when changing the frequency, power supply voltage has to be changed proportionally as well. For complicated starts additional starting voltage U0 is required. The term I x R compensation (torque boost) is used here.

Frequency inverter cannot provide a higher voltage to its output than the input voltage itself – that is why at a speed higher than the nominal speed the U/f relation is decreased, which results in the weakening of the magnetic field and by this as well of the torque provided by the motor. In this phase of motor operation the electric power is constant.
U/f = constant
U/f = decreasing

konst. moment = constant torque
moment pada = torque decreasing

Marjan Bitenc