Poor Bonding between the PE Terminal and Motor Frame a big risk.
An insulation resistance test is conducted under IEEE 43-2000 where a test DC Voltage charges the Motor winding until saturation.
I have witnessed electricians press the instrument test button and as soon as the 2MOhm is reached they lift their finger and consider the Motor a Pass.
While this method is likely to consider a motor to be safe, it’s is returning very limited information.
The correct method takes a little longer and probably why short cuts are made, the charging of the winding/cable must follow a charge curve meaning the like a capacitor (Insulation has Capacitance) the rush of charge initially is rapid and slows as the winding saturates.
The insulation material has dipoles which must align with the charge and this takes time. The higher the test voltage the faster this alignment happens. The insulation material while under charge should hold onto the outer electrons in the Material (Property of an Insulator), any leakage from these electrons jumping the insulation materials atoms is measured by the instrument and the MOhm reading drops.
For this method to be effective a positive test lead is attached to a conductor and the ground attached to an PE or Protective Earth. If you forget to attach this earth lead the returned reading will jump immediately to the instruments maximum limit as test voltage is unable to align the dipoles.
The method requires the Positive charge and the Earth to be close proximity to each other (eg Across Winding Insulation) for the charging and alignment of dipoles to commence.
If the PE point is a poor connection meaning that the Earth isn’t in close proximity to the charged winding the charging is instant like if you have forgotten to connect the Earth lead.
This can happen when a painted surface or poor bonding exists and the Stator Core (Closest to the Motor Winding) is not bonded to the PE termination in the Motor Terminal Box.
If you repeated the test by moving the earth test lead eg The motor frame, and the charging curve is present then the Earth Terminal on the Motor is likely to be high resistance to the Motor frame.
If the repeated test returns an instant Instrument maximum reading then it is likely the Stator Core is not bonded to the Motor frame.
If a motor does not charge to saturation following a curve then suspect the Motor PE has a problem. When a motor in the state fails the fault current is impeded through the PE and the Touch Voltage requirements are likely to be breached.
Touch Voltage is a mandatory regulation in most countries which ensures Assets under fault conditions do not rise above 50 Volts within the operating time (Normally 0.04 Seconds).
The risk is the fault current partly finds an alternative route and that elevates personnel injury risk.
The other regulated test is Earth Loop Impedance to ensure Earthing will be safe, but this test is often not completed at the motor terminals. Hence other methods of Motor Testing is applicable. Eg ALL TestPro
The issue arises is that different sized motors charge at different rates, and in small motors that is much more rapid but still follows a curve.
In larger Motors the curve is likely to have an inflection point, the initial charge returns a high Insulation Resistance reading then drops, only to recover to saturation. This happens because the volume of insulation is larger and the dipoles aligning consume charge, the instrument sees this as leakage and the reading drops. This is quite normal and is why you must continue with the test until saturation at the correct test voltage. (eg The reading has plateaued and is stable).
Another test that indicates a poor earth terminal is when Dissipation Factor goes Out of Range.
The Capacitance measurement also is very low as the test leads are separated.
Conclusion: 3Phi Reliability recommend Acceptance Testing of New & Overhauled Motors and rejecting the defects. These test methods and Specifications are available to all our clients.
Be mindful a Motor under Insulation Resistance test should follow a charge curve, if it doesn’t move the earth lead and repeat the test. If that repeat fails to follow a charge curve suspect the PE point to be defective and the Motor could be Non Compliant.
Reject it and Return it!!!
Special Note: These types of defects are a leading root cause of bearing current attack.
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