Applications Considerations While Appling Frequency Inverter (VFD) (1)

PRLog (Press Release) - Jul 13, 2010 -
Today Frequency Inverters are widely used in a variety of applications for various reasons. And in pump and fan application, variable frequency drives (VFDs, variable speed drives, VSDs) are the most effective energy savers. (visit http://www.inverter-china.com/frequency-inverter.htm)

A thorough understanding is essential while appling frequency inverters.

The output voltage of a Pulse Width Modulation (PWM) variable frequency drive (VFD) (http://www.inverter-china.com/variable-speed-drive.htm)consists of a train of pulses switched at the carrier frequency. Because of the rapid rise time of these pulses, transmission line effects of the cable between frequency inverter (vfd) and the motor must be considered. Since the transmission-line impedance of the cable and motor are different, pulses tend to reflect back from the motor terminals into the cable. The resulting voltages can produce up to twice the rated line voltage for lone cable runs, putting high stress on the cable and motor winding and eventual insulation failure.

Increasing the cable or motor size/type for long runs and 480v or 600v motors will help offset the stresses imposed upon the equipment due to the VFD (modern 230v single phase motors not effected). At 460 V, the maximum recommended cable distances between VFDs and motors can vary by a factor of 2.5:1. The longer cables distances are allowed at the lower Carrier Switching FrequencY (CSF) of 2.5 kHz. The lower Carrier Switching FrequencY (CSF) can produce audible noise at the motors.

For applications requiring long motor cables, variable frequency drive (VFD, variable speed drive, VSD) manufacturers usually offer du/dt filters that decrease the steepness of the pulses. For very long cables or old motors with insufficient winding insulation more efficient sinus filter is recommended. Expect the older motor's life to shorten. Purchase VFD rated motors for the application. (To be continued)

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