Difference in VFDs

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Please let me know about the difference in principle of operation and the physical component difference in the VFD's used for Asynchronous and Synchronous motors.

What is the special technology invovled in the VFDs used for the permenent magenet synchronous motors

Regards S.Sivakumar

-- S.Sivakumar (ssivakumar@sesmt.com), December 21, 2004


For asynchronous (induction) motors the VFD controls rotor slip and current to get torque. Speed ~ frequency +/- slip (& number of poles, etc). An important feedback therfore is actual rotor speed as measured by an incremental encoder so that the VFD can calculate actual slip.

For asynchronous (PM) motors the VFD controls the magnitude and position of the magnetic force that produces torque by manipulation of the magnitude and phase position of statur current relative to the position of PM magnets on the rotor. Rotor speed simply follows VFD output frequency. An important feedback is therefore actual rotor position. This often requires a resolver rather than a less expensive incremental encoder.

So the 'magic' for PM operation is knowing how to predict the required amplitude and phase of stator current in order to produce the desired torque from the motor without cogging or oscillation. A minor artifact is that even though the PM machine is more efficient, it usually operates at lower frequency. This requires operation of the VFD at super low output frequencies (including zero) for longer periods of time than when used with induction machines. This may require a larger ampere sized VFD/inverter hardware as most inverters are not designed to be operated at full amperes near DC.


-- Don Vollrath (dvollrath@magnetek.com), December 21, 2004.

In the reply (2nd paragraph) mentions PM machine as an asynchronous machine by typing mistake (For asynchronous (PM) motors ) . It should be corrected as synchronous machine.



-- javeed (javeed_skh@yahoo.com), February 22, 2005.

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