The ideal condition for an AC motor is when its neutral point is at or near ground potential. This would mean that the motor windings are only subjected to the stresses created by the phase to phase and phase to ground supply voltages.
The primary cause for the neutral point to be something other than zero to ground, is what’s termed common mode voltage. In general, common mode voltage is voltage that is common to both sides of a differential pair.
For a motor running across the line, its neutral is at zero volts assuming that all of the incoming phases are balanced. This is seldom the case, even without a VFD between the power supply and motor. In low voltage motor applications, the common mode voltage, while sometimes an annoyance, isn’t usually too much of an issue.
However, the common mode voltage is of particular concern when driving motors from medium voltage variable frequency drives, or VFD s. These are not insurmountable problems, and given the terrific energy savings and control improvement when using a drive, are worth addressing.
There are several ways of combating this problem. For a very informative (and detailed) technical paper on this subject, click HERE to download.
EMA offers repair services and new equipment on medium voltage drives and motors. No-One, Anywhere, is better at drives than we are. Call us today at 800-848-2504. (or click the CONTACT US at the top right of this page to contact us online)
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