Medium Voltage (MV) motors require special consideration compared to low voltage AC motors. For one thing, medium voltage AC motors are usually high horsepower power motors, and normally operate in critical applications.Here at EMA, our primary focus is medium voltage VFDs, and we do not offer repair services on medium voltage (MV) motors. However, we have been forced to develop expertise in this area, and often find ourselves both supplying and assisting in troubleshooting recurring problems with medium voltage motors.
Here are a few issues that we’ve run into with medium voltage AC motors operating from medium voltage VFDs.
DVDT issues: All medium voltage VFDs provide a variable frequency, variable voltage to the medium voltage motor. This is a synthesized medium voltage AC waveform. Since the switching devices are turning off and on, the result can be significant spikes on the medium voltage motor cables and stator. This has caused motor damage in a number of cases. The medium voltage VFD manufacturers have taken great strides to prevent this, and each manufacturer has a recommended maximum motor cable length. To exceed this length without additional filtering is to invite medium voltage motor damage.
Two companies with which we are impressed in their DVDT suppression efforts are Toshiba and LSIS. Both of these VFDs allow substantial cable lengths without filtering. (See the Toshiba VFD here)
Starting issues: In theory, a medium voltage AC motor starting on a medium voltage VFD should have better starting torque than an across the line start, not to mention considerably less mechanical and electrical stress. But, many earlier medium voltage VFDs were designed primarily for centrifugal loads, where there is very little starting torque needed. As medium voltage VFDs are used in more diverse applications, this becomes an issue. More modern medium voltage VFDs have resolved this issue.
As a side note: Many medium voltage VFDs are used strictly as starters for medium voltage motors in areas where the power line simply will not support an across the line start. Sometimes a medium voltage softstarter can do this, but not always. (Read “when should you use a VFD as a softstarter?“)
Monitoring: It is imperative that you use more sophisticated monitoring on medium voltage motors being driven by MV VFDs. The one we see used most commonly, and one that we sell, repair, and program on a regular basis is the GE Multilin. (Click HERE for details) These and other similar units motor stator temps, number of starts, voltage fluctuations, and other data.
IF you need to talk with us about medium voltage VFDs or medium voltage motors, then hit any of the contact methods on the right of this page, or just call 770-448-4644.
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