79 Frequency Converters and Motors U [V] T [Nm] 800 T U x l f 1 f max. torque decrease with factor T Load 400 10% M N 230 0 50 50 f [Hz] 0 T motor f [Hz] a) U/f characteristic control (ideal) b) T/n characteristic (ideal) Fig. 3.1 Principle U/f characteristic and torque The ideal curve of the U/f characteristic for a star connected 50 Hz motor is shown in Fig. 3.1 Principle U/f characteristic a) applied motor voltage b) resulting torque. Up to 50 Hz the FC applies a constant U/f ratio to the motor which result in the possibility to get a constant torque out of the motor.
For operating the motor at 100 Hz ideally the output voltage should be increased to 800 V to maintain a constant U/f ratio (dotted line in Fig. 3.1a Principle U/f characteristic and torque). As this high voltage is critical for the motor insulation this is not an applied strategy. Typically the FC limit its output voltage to the value of the input (for example 400 ±10%) This means that the FC can maintain a constant U/f ratio to a certain frequency only. After this frequency it can continue to the frequency but not the voltage anymore. As this is aff ecting the U/f ration the magnetic fl ux density is reduced. Therefore this speed range is also called fi eld weakening area (Fig.
3.1b Principle U/f characteristic and torque). The reduced magnetic fi eld results in a reduced maximum motor torque. While the nominal torque is reduced by 1/f the stall torque decreases by 1/f 2 . Please note that the shown curves are ideal and require some compensation which are described in the following sections.