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5.8.7Slip Compensation
The motor speed is reduced depending on the load, due to the slip as described earlier. Slip can cause a speed regulation by as much as 10% with small motors. The inverter can compensate for this by increasing the output frequency slightly as the load increases. The inverter measures the current and increases the output frequency to compensate for the expected slip. This can give speed holding of better than 1%.
Slip compensation has no effect during Sensorless Vector Operation, as compensation is inherent.
Slip compensation is a positive feedback effect (increasing load increases output frequency), and too much compensation may cause slight instability. It is set up on a trial and error basis.
Torque/Current
Load Change
Speed
Speed Change without
Slip Compensation
Figure
5.8.8Pulse Frequency selection
The switching, or pulse width modulation frequency does not change with output frequency. See section 1.2. The switching frequency of the inverter can be selected between 2 and 16 kHz. A high switching frequency has higher losses and produces more Electromagnetic Interference (EMI). A lower switching frequency may produce audible noise. The switching frequency can be changed to suit the application, but some derating (as described in the Reference Manual), may be necessary on certain units.
The acoustic noise generated has a frequency of twice the switching frequency, except at light loads, where there is some fundamental frequency content. Therefore a switching frequency of 8 kHz will often be inaudible.
MICROMASTER Applications Handbook | 27 |