We can measure the insulation resistance and measure the polarization index of L1-L2, L2-L3 and L3-L1. Is it important to measure these three, or is it practically and theoretically sufficient to just measure one of them to understand the characteristics of a motor? Why or why not?
No need to measure 'all lines'...
Provided that
In this case, for the motor side a single isolation resistance measurement is sufficient. On the power supply side things are eventually different: depending on the toology, an isolation resistance measurement may be required for each supply line individually. Everything depends on whether the lines are connected or not.
Take insulation resistance measurements phase to phase and phase to ground for each line.
Good results:
• Balanced comparative low resistance values on all three stator phases
• High resistance values on the phase to ground insulation test
Problems:
• Gross resistance deficiencies, such as a phase on phase short.
• Any winding to winding resistance imbalance. If the readings differ by more than a few percent, the motor is probably unsafe to energize.
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