85 Frequency Converters and Motors However, the slip will be approximately 50% of the required speed if the FC is controlling a motor at 300 RPM (10 % of the rated synchronous speed of 3000 RPM). If the FC has to control the motor at 5 % of the rated speed, the motor will stall if it is loaded. This load dependence is undesirable, and the FC is able to fully compensate for this slip by eff ectively measuring the active current to the motor. The FC then compensates for the slip by increasing the frequency according to the actual measured current. This is called active slip compensation.
The FC calculates the slip frequency (f slip ) and the magnetisation or no-load current (I ) from the motor data. The slip frequency is scaled linearly to the active current (diff erence between no-load and measured current). Example A 4-pole motor with a rated speed of 1455 RPM has a slip frequency of 1.5 Hz and a magnetisation current of approximately 12 A. With a load current of 27.5 A and 50 Hz, the FC will output a frequency of about 51.5 Hz. At a load current between I Φ (12 A) and I N (27.5 A), the frequency will be adjusted accordingly between zero and 1.5 Hz.
As demonstrated in the example, factory setting of slip compensation is often scaled such that the motor runs at the theoretical synchronous speed. In this case, 51.5 Hz - 1.5 Hz = 50 Hz. 3.2.5 PM Motor and SynRM Compensations For Permanent Magnet motors the start and slip compensations are irrelevant, but other parameters are essential. The magnetising profi le diff ers of course from the asynchronous motor, but other important data and compensations are: • Nominal motor speed and frequency • Back EMF • Max speed before back EMF damages the FC • Field weakening • Dynamic details relevant for the control For SynRM motors other parameters are essential, for instance:
• Stator resistance • d and q axis inductances • Saturation inductances and • Saturation point