April 12, 2010 Precise speed control of AC motors is typically accomplished with a variable frequency drive (VFD) connected to a speed or position feedback sensor. In certain situations, however, it's possible to achieve a similar level of precision speed control without the need for a feedback sensor via the high frequency signal injection method.

To implement this method, the VFD must use open loop control, and it must be able to inject the required high frequency signal. The method only works with salient motors such as internal permanent magnet (IPM) synchronous motors. Read Full Article