Case History

Three large variable frequency drives were installed on HVAC equipment in a medical facility in addition to a Magnetic Resonance Imaging (MRI) machine. The facility consisted of an old building that had been renovated for use as a medical facility. However, much of the original wiring had been maintained.

The MRI machine was found to be giving false readings, causing the doctors and facilities to be sued for malpractice.

The Data

The MRI manufacturer had specified that ground to neutral voltage was not to exceed 3 volts. However, the ground to neutral voltage measured 22 volts at the MRI input.


As an immediate response, output reactors were installed on the variable frequency drives. The ground to neutral voltage then dropped to 6 volts. The result of the reactors was to slow the effective dv/dt output from the drive and substantially reduce the generation of common mode noise.

Rather than allowing more time to implement EMI/RFI filters, common mode chokes and other solutions, it was decided to install a separate service entrance for the MRI machine. The new electrical service isolated the MRI from the HVAC equipment and resolved the problem.

For a discussion of EMI/RFI see AC Drives and EMI/RFI Mitigation.