PLANT

TECH TIP: Grounding variable frequency drives

How to avoid the most common issues.


A small VFD. PHOTO: CJ COWIE

Maintenance crews frequently run into troubles with variable frequency drives (VFDs), including windings and bearings. There are two common issues: the drives were never tuned, or were not tuned following motor or drive repairs or replacement; and ground leads are in the same conduit as standard leads from the drive to the motor.

Follow manufacturers’ installation instructions. They contain directions on how to tune the drive and recommendations for cabling and grounding. Improper grounding will cause problems with how the drive operates.

One trouble spot is loose ground leads in the conduit combined with motor cables from the drive. This causes electrical unbalance between the phases and ground leading to a transformer effect (EMI) that generates voltage and current in the ground lead. Heating, potential fire or shock hazards occur, depending on the size of the ground lead and amount of current.

Use special inverter cables that either balance the phases to grounding circuit, shield cables grounded at both ends, or distance the output cables from control and ground cables.

Source: MotorDoc LLC newsletter, Howard Penrose. Visit www.motordoc.org.

This article appears in the April 2019 print issue of PLANT Magazine.

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