I have 380V 3 phase with the following line to neutral voltages: L1: 238V, L2: 234V, L3: 224V. The system has a single phase servo driver that takes in L3 and N, however when hooked up L1, L2, and L3, change to the following: 394V, 400V, 0V. I'm not sure why this discrepancy is happening.

Thanks in advance!



simulate this circuit – Schematic created using CircuitLab

Figure 1. You have a broken neutral.

  • \$\begingroup\$ In slightly more detail: you're using three voltmeters with roughly balanced impedances, or there's some conductance and/or capacitance from your local neutral to the neutral reference of your power coming in that keeps things balanced. So when you put that super-high impedance voltmeter on the wires, you read roughly what you should with a connected neutral. But when you hook up an actual load, it overwhelms whatever wimpy connection there is, and just holds L3 at zero volts. \$\endgroup\$ – TimWescott Nov 16 '18 at 19:50

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