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A tough VFD

Unlike DC drives, very few AC drives will withstand a phase to phase short across the output phases of the drive without catastrophic damage. The MX Plus variable frequency drive, which is made by Vatech in Austria and now part of Schneider Power Drives, was able to withstand the abuse of running in current limit with all three output phases shorted together.

On a recent start up of a Herrenknecht Tunnel Boring Machine (TBM) in New York City, a wiring error in the junction box of a 700 HP Elin motor resulted in a phase to phase short

on the U,V, and W phases of the VFD. This was not readily apparent, because there are parallel wires on each phase, due to the current.

During the troubleshooting process I ran the VFD with the motor connected and it stayed in current limit putting out roughly 1050 Amps. Without the motor the VFD ran fine, and all output voltage waveforms looked good and balanced. With a motor attached the VFD went into current limit with 2 volts on the output phases regardless of the speed reference. The current went to 1050 Amps.

Herrenknecht Company Headquarters

Tunnel Boring Machines (TBMs) are very tough applications. (see EMA’s underground business) The environment is challenging, space is limited, and downtown is incredibly costly. The Schneider Power Drives equipment ( formerly known as VaTech) does a wonderful job. EMA is a service and sales representative of Schneider Power Drives in North America. EMA is also an electrical service center for Herrenknecht Tunneling Equipment.

To talk with us about new equipment, field service, spare parts, or shop repairs, just click the “click here” icon in the top right of this page, or see the local telephone numbers at the bottom of this page. (See EMA Field Service )


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