83 Frequency Converters and Motors Example A motor which is much smaller than the recommended motor size of an FC may require an additional, manually adjustable voltage boost in order to overcome static friction or ensure optimum magnetisation in the low speed range. If several motors are controlled by one FC (parallel operation), it is recommended to de-activate the load-independent compensation. The load-independent supplement (start voltage) ensures an optimum torque during start. 3.2.2 Load-dependent Start Compensations The load-dependent voltage supplement (start and slip compensation) is determined via the current measurement (active current).
This compensation is normally called the I R compensation, boost, torque increase, or, – at Danfoss, – start compensation. This type of control reaches its limits when the disturbances are diffi cult to measure and the load is highly variable (for example in motors with operational change in the winding resistance of up to 25 % between the hot and cold states). The voltage increase may have diff erent results. Under no-load operation, it may lead to saturation of the motor fl ux. In the event of saturation, a high reactive current will fl ow that leads to heating of the motor.
If the motor is operating with a load, it will develop little torque because of the weak main fl ux and may stop running. The real U/f and T/n characteristics are generally as shown in Fig. 3.3 Real U/f and T/n characteristic.