acinsulationmotorneutralthree phase
When meggering a three phase motor, could someone please help me understand the values we would expect to see for a motor with no insulation or ground faults.
From my understanding:
Phase-GND: should see a very high resistance (in MegaOhms)
Phase-Neutral: should see a very low resistance (near zero) since all the phases are tied together at the neutral
GND-Neutral: honestly I'm not sure…
The circuit will both be at higher voltage (well the parts of it) and will self-adjust. The reasoning behind this is pretty simple: First, the phases are out of phase, or to explain it simply, their voltage peaks happen at different times, so while one is at its maximum voltage, the two other phases will be at lower voltage than the one at maximum is, so basically, the load will be at the voltage between phases. Since we don't have the neutral conductor, the current which was going through it will have to go through one of the phases and that's how the circuit will "self-adjust". You can see that from the simulation here.
When the switch is closed, all 3 loads are at the same voltage of 5 V compared to the ground. Once the switch is open, the voltages at the loads will change. So the lightest load will drop the least voltage and the greatest load will drop the most voltage.
No guarantees as to correctness, but this shows the general method.
By convention, phase rotation is anti-clockwise. An inductive load will make the current lag behind the voltage. (Remember, inductors resist changes in voltage dv/dt.)
Diagrams below are not to scale.
Best Answer
If you follow your own logic through you'll have the answer.
Since your phase to neutral resistance is close to zero then you can consider the phase and neutral as joined together (for the purpose of this exercise).
Now if phase-GND is high resistance then netural-GND must be high too.
This is correct. There should be no connection between the windings and the machine frame.
For the benefit of the unexperienced reading this post, all these measurements are carried out with the motor disconnected from the supply.