Insulation Resistance

Insulation Resistance (IR) is among the most common motor test. It is also has more types of currents than some users realize.  In its most basic form, an insulation resistance test is done with a hand-crank meter that measures megohms . An advanced tester plots megohms over a period of 10 minutes or more and displays voltage, leakage current, DAR, and PI ratios.  Learn more about DAR and PI ratios.

In an IR or megohm test the voltage applied and the total leakage current are measured between the windings and the motor frame/ground. Ohms law is applied to calculate the resistance in megohms.

R=V/I

Where R is resistance in megohms, V is voltage applied in Volts, and I is the total resulting current in micro Ampere (µA).

A temperature correction factor is applied to correct the megohm measurement at present temperature to what it would be at a standard temperature.  According to IEEE 43 and ANSI/EASA standards the standard temperature is 40°C.

The leakage current of a used motor is often surface current running in the dirt on the outside of the windings. The dirt contains particles of dust, oil, grease, moisture, etc.  Conduction current that flows through weak ground insulation to ground is often dwarfed by the surface currents. Therefore, the insulation resistance test or megohm measurement is sometimes referred to as the dirt test.  Megohms tend to drop with increasing amounts of dirt.

A megohm measurement on new motors is often not interesting other than to check that there are no direct shorts to ground. Users will often go directly to a hipot test.

Currents Involved In A Megohm, DAR and PI Test

  1. IC– Capacitive: A capacitive inrush current brings the potential in the motor up to the test voltage by charging it.  This current drops quickly and reaches zero within a few seconds after the test voltage is reached. For large motors with high capacitance, the inrush current is high. Total leakage current failure limits must be set high enough to avoid tripping the limit during this initial phase of the test. For more information on the subject of capacitive inrush current and how to avoid tripping a limit, see Hipot Test.
  2. IA – Absorption: Absorption current polarizes the insulation.  This current also goes to zero or very near zero within 30 seconds to 1 min in random wound motors.  Form wound motors take much longer due to the layers of insulation used between turns.  The change in absorption current over time is what is used to calculate PI and DAR ratios in an insulation resistance test.
  3. IG – Conductance: Conductance current flows between the copper conductors and ground through the bulk of the insulation. This current is usually zero if the motor is new or undamaged. As the motor insulation ages and cracks or is damaged conductance current may flow depending on the test voltage applied. Conductance current tends to accelerate with increasing voltage. This current is sometimes referred to as leakage current or as part of the leakage current.
  4. IL – Surface Leakage: According to IEEE 43, surface leakage is the current flowing in the dirt on the surface of the windings to ground. It is called surface conduction current in other standards. A dirtier motor has higher leakage current and a lower megohm result.  There may be an increase in the surface leakage current on motors with a stress-control coating on the the end-windings. After 1 minute with a random wound motor or 5-10 minutes with a form wound motor, the surface leakage current is typically the only current remaining unless the insulation is weak or damaged.
  5. IT – Total: The total current is the sum of the 4 currents.  An motor and insulation tester measures total current.  The total current equals or is very close to the surface leakage current at the end of the insulation resistance test. This gives the operator a good measure of how dirty or contaminated the motor.  It also alerts the operator to a possible catastrophic connection from the windings to ground.

Leakage Current As A Function Of Time

Leakage current as a function of time

Leakage current as a function of time

To determine if the leakage current is primarily a surface current or also contains conductance current, one must do a step voltage test or ramp test.  See information below on minimum megohm levels.  Note that these tests can be done at voltages lower than the normal DC hipot test voltage in order to find conduction current.

Tracking megohm measurements over time

Megohm measurements are tracked over time to help determine when a motor or generator should be reconditioned. This is done automatically with the iTIG III motor analyzer.  Especially for larger motors, other insulation resistance tests such as DAR or PI tests are used in reconditioning assessments.  Additional tests are DC hipot, step voltage/ramp tests, surge tests and partial discharge measurement.

Standards and Temperature Compensation

ANSI/AR100-2015 and IEEE 43-2013 make the same recommendations as follows. It is recommended that motors with low Insulation Resistance test readings not be subjected to high-voltage testing.

IR Measurement Test Voltage

Recommended minimum insulation resistance values at 40C

Note on Temperature Compensation

The limits above are for windings at a temperature of 40°C. The megohm test results are temperature compensated because the windings usually are not at this temperature when tested.  Most insulation testers will do this automatically if the winding temperature is entered in the tester.  Resistance values must be temperature compensated when IR is tracked over time.  The temperature must also be above the dew point for accurate comparisons of results.

The most common temperature compensating formula states that the insulation resistance drops by a factor of 50% for every 10°C increase in temperature. Therefore, it is clear that the insulation properties drop precipitously as the temperature rises.  IR of 10,000 megohms (10 Giga Ohms) at 20°C (~68°F) drops to 2,500 megohms at 40°C, and to 39 megohms at 100°C.

There are several other temperature compensation formulas.  The above formula is likely the most conservative. Different types of insulation systems in form wound motors have unique temperature characteristics. These are only obtained from the manufacturer of the motor.

The bottom line is temperature has a significant effect on insulation resistance and must be compensated for best results.

Limitations of Interpretation

Question:  How much better is test number 1 than test number 2?

IR Test comparison

Answer:  Who knows? A difference of 0.01µA could be the result of a number of variables.  These variables might include temperature, changes in environmental conditions, electrical noise, or instability in the voltage or current.

The difference in insulation resistance is high due to how resistance is calculated. The only physical change is the current and this change is  extremely small. Some insulation testers display leakage current to the 3rd or even 4th decimal with a resolution as low as 1nA or 1pA.  The instrument calculates and displays IR in Terra-Ohms (TΩ). The accuracy in the last digit(s) is not specified or is poor for good reason. It is too dependent on variables other than the leakage current it is intended to measure.

Other Advice and Tips from IEEE 43-2013 

  • Before starting a test, the winding insulation should be discharged to avoid measurement errors.
  • For motors with a stress-control coating applied to the end-windings, there may be an increase in the surface leakage current and thereby lower megohms than expected.
  • For winding temperatures below the dew point, it is impossible to predict the effect of condensation on the surface. Therefore, a correction to 40 °C for trend analysis introduces significant errors.
  • For directly water-cooled windings, the water should be removed and the internal circuit thoroughly dried. The winding manufacturer may have provided a means of measuring results of the insulation resistance test without need for the coolant water to be drained.
  • A minimum discharge time of four times the voltage application duration is recommended. All Electrom Instruments discharge the motor through a resistor.  For motors with voltage less than 100V, connecting the winding directly to ground with the instrument ground lead or a shorting stick or jumper will complete the discharge immediately.  Any residual absorption charge takes longer to discharge.  Keep motors with absorption charges connected directly to ground if it is handled soon after a test.
  • Absorption discharge takes more than 30 min depending on the insulation type and physical size of the motor.
  • A significant decrease in insulation resistance (increase in measured current) with an increase in applied voltage is an indication of insulation problems in an insulation resistance test.
  • A steady increase in the IR with age indicates decomposition of the bonding of insulation materials especially when they are  thermoplastic.
  • When a low PI occurs at temperatures above 60°C, a second measurement below 40°C and above the dew point is recommended as a check.
  • PI can be used to indicate when the drying process of insulation is complete.  This occurs when the PI exceeds the recommended minimum.
  • If the IR value at 40 °C is greater than 5000 MΩ the PI is ambiguous and is disregarded.