HOW A DC HIPOT WORKS
DC voltage is applied to open (disconnected) windings by the Hipot tester or motor analyzer. The DC voltage potential in the windings is rapidly raised to a predetermined level, or raised in steps up to this level, depending on what test method is used.
As the voltage is raised, several currents will flow into and possibly out of the windings to ground, and the combined total of these currents are measured by the Hipot tester. The currents are the same currents present in an insulation resistance test:
- IC– Capacitive current (or geometric capacitive current): This is also called inrush current. The windings have capacitance. Current is required to elevate its voltage potential. This current typically drops to zero within seconds after the test voltage provided by the motor tester is stable.
- IA– Absorption current: This current is present during the atomic and any molecular polarization of the insulation, and is the current one is interested in during a PI test. This current will drop to zero, or near zero, over a period of time that varies by motor. It can happen in seconds or may take 10 minutes or more. Click here for more information on PI tests.
- IG– Volume Conduction current: This is the current that flows through the entire volume of the insulation between ground and the conductors. In good windings, this current is usually zero or near zero, and depends on the composition and condition of the insulation system. People sometimes think of this current as the “leakage” current. It certainly “leaks” through the insulation, but the next current is usually the main “leakage” in a used motor.
- IL– Surface conduction current: This is often referred to as surface leakage current. The surface conduction current runs over the end winding surfaces of the insulation.
- It is a result of surface contamination, dirt and moisture on the windings that are connected to ground.
- As the contamination level increases, the resistance of the contamination drops, and the current increases.
- As the voltage increases, the current increases more or less proportionally with the voltage applied by the motor tester.
- For used, good motors, this current will dwarf the absorption and volume conduction currents because of the relatively lower resistance in the surface contamination.
- For new, totally clean, and dry motors this current should be zero or near zero.
The motor tester measures the sum of all the above currents in micro Ampere (µA) and displays it along with the test voltage. To add to the list of what “leakage current” may mean to people, the total current measured by IR/Megohm and Hipot testers is also often called “leakage current”. IEEE 95 calls this the “measured current”.
Measured current = IC + IA + IG + IL = C x (dV/dt) + (absorption current; dropping with time) + (volume conduction current through the insulation) + (surface current; depends on levels of surface contamination)
The higher the total measured current is, the dirtier the motor is and/or the weaker the insulation is. Sometimes the question becomes, which is it, dirt or weak insulation? To learn more see the Hipot Step Voltage Test page.
Definitions from IEEE 95 Recommended Practice for Insulation Testing of AC Electric Machinery (2300V and Above) With High Direct Voltage
Breakdown current: “The current discharged as a result of insulation failure. The peak value of this current may be very high, reflecting the energy stored in the capacitance of the winding. Normally, this current cannot be accurately measured.”
Breakdown voltage: “The voltage at which a disruptive discharge takes place through the volume or over the surface of the insulation.”
High direct voltage (also referred to as over-voltage): “A unidirectional voltage whose magnitude is greater than the peak value of the nominal RMS line-to-ground rating of the insulation system under test.”
Electrical insulation failure or breakdown is usually indicated by an arc, a sharp capacitive discharge, at the failure location. There are times when failure or partial failure is indicated by a large abnormal change in the measured current or by erratic behavior of the measured current.
Warnings of insulation breakdown by accelerating measured current can start within as little as 5% of the breakdown voltage, however, it can also start much earlier.