Applications Considerations While Appling Frequency Inverter (VFD) (part 2)

Aug. 1, 2010 - PRLog -- Further, the rapid rise time of the pulses may cause trouble with the motor bearings. The stray capacitance of the windings provide paths for high frequency currents that close through the bearings. If the voltage between the shaft and the shield of the motor exceeds few volts the stored charge is discharged as a small spark. Repeated sparking causes erosion in the bearing surface that can be seen as fluting pattern. In order to prevent sparking the motor cable should provide a low impedance return path from the motor frame back to the inverter. Thus it is essential to use a cable designed to be used with variable frequency drives (VFDs, variable speed drives, VSDs).

In big motors, a slip ring with brush can be used to provide a bypass path for the bearing currents. Alternatively isolated bearings can be used.

The 2.5 kHz and 5 kHz CSFs cause less motor bearing problems than caused by Carrier Switching Frequencies (CSF) at 20 kHz. Shorter cables are recommended at the higher Carrier Switching Frequency (CSF) of 20 kHz. The minimum CSF for synchronize tracking of multiple conveyors is 8 kHz.

The high frequency current ripple in the motor cables may also cause interference with other cabling in the building. This is another reason to use a motor cable designed for VSDs (VFDs) (Visit http://www.inverter-china.com/frequency-inverter.htm for frequency inverters) that has a symmetrical 3 phase structure and good shielding. Further, it is highly recommended to route the motor cables as far away from signal cables as possible.

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