84 Frequency Converters and Motors U Motor [V] 800 Without load 400 230 Load compensation Compensating voltage f [Hz] T [Nm] 100 50 100% 50% n [min -1 ] Fig. 3.3 Real U/f and T/n characteristic In Fig. 3.3, additional voltage is supplied to the motor at low speeds for the purpose of compensation. 3.2.3 Load Compensations The motor voltage is increased under load ascertained from the measured motor current. The output voltage receives a voltage boost which eff ectively overcomes the infl uence of the DC resistance of the motor windings at low frequencies and during start. An increase in output voltage leads to over-magnetisation of the motor.
This increases the thermal load on the motor such that a reduction in torque is to be expected. The motor voltage is reduced in no-load operation. 3.2.4. Slip compensation The slip of an asynchronous motor is load-dependent and typically amounts to some 5% of the rated speed. For a two-pole motor, this means that the slip will be around 150 RPM.