A 3 phase wye-connected, 115 V, 400 Hz generator is used to power a test stand in which a motor controls a wiper arm that operates a three phase Variac to control the voltage across each of the phases. A DVM is used to measure the voltage across one of the phases with respect to earth ground.

The problem is that at the end of a test, the computer controller activates the motor to bring the phase voltage down to 0 V, but the meter measures 3 to 7 volts ac. This causes the test to fail.

The neutral doesn't appear to be connected to earth ground. A resistance measurement between the neutral and earth ground gives about 500 kohm.

Any insight as to why there would be such a high resistance between neutral and ground and what I can do to eliminate the residual voltage being measured from the meter?

  • 2
    \$\begingroup\$ A schematic would be useful. (There's an app on the editor toolbar.) There are a few details missing: (1) Is the secondary wye point connected to building neutral? (2) Is your neutral grounded? (3) If either or both of the previous are not true why would you expect 0 V between phase and ground? \$\endgroup\$ – Transistor Oct 3 '17 at 21:44
  • \$\begingroup\$ Neutral to earth connection needs to be checked at source and corrected. The variacs are supposed to be wye as well with a common to neutral . Test with breakers off. \$\endgroup\$ – Tony Stewart Sunnyskyguy EE75 Oct 3 '17 at 21:54
  • \$\begingroup\$ Sorry, I don't have the authority to provide a schematic. I'm not sure if the secondary wye point is connected to building neutral. I'll check that tomorrow. I assumed that the neutral would be grounded, which I why I was surprised to measure 500 kohms between neutral and ground. Is there any reason why the neutral isn't typically connected to earth ground? \$\endgroup\$ – pr871 Oct 3 '17 at 22:08

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