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Home / Technical Articles / How to measure insulation resistance of a motor

Winding insulation resistance

If the motor is not put into operation immediately upon arrival, it is important to protect it against external factors like moisture, high temperature and impurities in order to avoid damage to the insulation. Before the motor is put into operation after a long period of storage, you have to measure the winding insulation resistance.

How to measure insulation resistance of a motor
How to measure insulation resistance of a motor (photo credit: elecls.cc.oita-u.ac.jp)

If the motor is kept in a place with high humidity, a periodical inspection is necessary.

It is practically impossible to determine rules for the actual minimum insulation resistance value of a motor because resistance varies according to method of construction, condition of insulation material used, rated voltage, size and type. In fact, it takes many years of experience to determine whether a motor is ready for operation or not.

A general rule-of-thumb is 10 Megohm or more.

 Insulation resistance value Insulation level
 2 Megohm or less Bad
 2-5 Megohm Critical
 5-10 Megohm Abnormal
 10-50 Megohm Good
 50-100 Megohm Very good
 100 Megohm or more Excellent

The measurement of insulation resistance is carried out by means of a megohmmeter – high resistance range ohmmeter. This is how the test works: DC voltage of 500 or 1000 V is applied between the windings and the ground of the motor.

Ground insulation test of a motor
Ground insulation test of a motor

During the measurement and immediately afterwards, some of the terminals carry dangerous voltages and MUST NOT BE TOUCHED.

Now, three points are worth mentioning in this connection: Insulation resistance, Measurement and Checking.


1. Insulation resistance

  • The minimum insulation resistance of new, cleaned or repaired windings with respect to ground is 10 Megohm or more.
  • The minimum insulation resistance, R, is calculated by multiplying the rated voltage Un, with the constant factor 0.5 Megohm/kV.

    For example: If the rated voltage is 690 V = 0.69 kV, the minimum insulation resistance is: 0.69 kV x 0.5 Megohm/kV = 0.35 Megohm

2. Measurement

  • Minimum insulation resistance of the winding to ground is measured with 500 V DC. The winding temperature should be 25°C ± 15°C.
  • Maximum insulation resistance should be measured with 500 V DC with the windings at a operating temperature of 80 – 120°C depending on the motor type and efficiency.

3. Checking

  • If the insulation resistance of a new, cleaned or repaired motor that has been stored for some time is less then 10 Mohm, the reason might be that the windings are humid and need to be dried.
  • If the motor has been operating for a long period of time, the minimum insulation resistance may drop to a critical level. As long as the measured value does not fall below the calculated value of minimum insulation resistance, the motor can continue to run.

    However, if it drops below this limit, the motor has to be stopped immediately, in order to avoid that people get hurt due to the high leakage voltage.

Reference: Grudfos – Motor Book

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Edvard Csanyi

Electrical engineer, programmer and founder of EEP. Highly specialized for design of LV/MV switchgears and LV high power busbar trunking (<6300A) in power substations, commercial buildings and industry facilities. Professional in AutoCAD programming.

49 Comments


  1. Hocine Meng Fodil
    Sep 19, 2018

    normally when it is constructed the insulation is subject to certaines electrical tests and if i remember well the test are hight tension test, short circuit test, blocked rotor test.
    thank you


  2. Joel aquino
    Sep 12, 2018

    What is your recommendation if the motor has low or bad insulation?


    • NAJAT
      Sep 18, 2018

      If the motor has a bas insulation, there is 3 cases:
      1. The winding enclose humidity (it’s may be stored in a humid place, or the heating resistance is not running,..etc): This is the most common case, in this case the motor must be dismantled and heated to absorbe all the moisture)
      2. The insulation of the motor is too weak (the motor exeed its life, or running in bad conditions of température or charge): in this case, i think we have to rewind the engine.
      3.Court circuit or flash, here, when we dismantel the motor we can see the damage inside (rotor or stator) : in this case, automaticaly, the only solution is rewinding


      • CONCORDIO TANEDO
        Oct 19, 2020

        How bad is the insulation? can we talk about figures in Mega Ohms for example and the system voltage? Is anyone familiar with this 1 kV = 1Mega Ohms + 1? do you believe this formulae?


    • Samurdhi
      Sep 05, 2019

      The motor should dismantle and the stator must be washed by a water gun by a recomended cleaning method. the stator then must keep out for well drying. this can be done by an oven. All the water particles and moisture must be gone. then apply motor shellack to the stator and armature.Then re check the insulation. If its still bad the motor must be replaced.


      • Concordio Tanedo
        Oct 19, 2020

        Washing with distilled water is possible but very dangerous if we use ordinary water which has minerals that can add more harm on the core, core will become corroded. I myself use a motor winding cleaner chemical it removes oils and dirt, it is a non conductive chemical, i use it with a spray gun with less than 20psi pressure.


    • CONCORDIO TANEDO
      Oct 19, 2020

      Replace the motor, and disassemble the motor clean the winding and let it dry using heaters and monitor the insulation continue drying and heating until a good insulation can be obtain. and re apply varnish let it dry again, let it cool and re assemble again, once completely reassembled, check the insulation again.


  3. DHANEESH RAJEEV
    Sep 04, 2018

    hello we have a motor that pumps Acetone to cylinder which is highly flammable and we dont know the pump/motor is fire rated/proofed how we know or check if it is fire rated?


    • NAJAT
      Sep 18, 2018

      Hello,
      To know if your motor/pump is rated, you should verify the nameplate. First, the it must be Atex (the symbol is EX inside a triangle or hexagone) group IIA


    • Concordio Tanedo
      Oct 19, 2020

      Contact the manufacturer of the motor.


  4. mikegen
    Feb 14, 2017

    This article is really helpful to me and solved half of my worries concerning resistance insulation. But there is something that is not clear to me or maybe I don’t understand it. Well, its base on resistance in ohms. please, tell me the general rules guiding winding resistance in ohms.

    I will be very glad if you can solve this worries of mine.

    Thanks.


  5. Jerry
    Oct 20, 2016

    My candit option is the article have been so explicit, however, you have said the IR, varies, depend on the type of material used, the size… Is it in kW? Pl use another example, because, .35mohms/kV, is not clear to me. I want to know how to calculate the minimum IR, of an electric motor.


  6. Tsvetan
    Mar 08, 2016

    Is it common to test the resistance between the windings as wells as the one between winding and ground?


  7. ketan
    Oct 12, 2015

    Nice ,articles provides good practical knowledge


  8. Emeka
    Sep 21, 2015

    75HP (55KW) motor has 0.1M ohms L1 – L2 – L3 conductors and 0.0M ohm conductors to GND after three idle years. It was cleaned, dried by 1000W lamp heat and re-vanished to get 2.2G ohms conductors to GND. Thereafter, motor draws currents 22A on NO-Load and 149A when coupled to 75HP Fire water pump which caused over heat to 250Kw and 350Kw power generator. What is the fault of the above procedure and possible solution?


    • CONCORDIO TANEDO
      Oct 19, 2020

      The problem is on your driven pump, (at no load the motor draws 22A and when it’s coupled with pump load current becomes 149?) What is the Full Load current of your motor? and what is the overload relay setting?


  9. Ernest Anderson
    Jul 11, 2015

    Tank u for de help pls we need more information about magger


  10. murat baykal
    Mar 10, 2015

    Very useful artciles are take place here. As an unexperienced engineer those articles are very educating and informative.


  11. yphamngoc
    Nov 16, 2014

    How about the ratio one min per 15s and 10mins per 1 min?
    Thanks


  12. Michael okikiri
    Nov 06, 2014

    Am a bit confused on the illustrations on insulation resistance test. If 2 megaohm is considered bad base on the thumb rule and you calculated the minimum insulation resistance in the example to be 0.35megaohm is this not below the 2megaohms considered bad? Pls clarify

    Michael okikiri


    • tv
      Nov 16, 2014

      In general, it is considered as 2 M ohms. But for critical operation this can be lowered to 0.35 M Ohms as stated. Once the emergency situation is out, the cause of low insulation should be attended.


  13. farai benie
    Oct 30, 2014

    I want to learn more about electrical engineerinf department


  14. awny
    Oct 01, 2014

    how could i download the pdf of this article?please help me…


  15. Mustafa Rabiul Hassan
    Sep 18, 2014

    how could i download the pdf of this article?please help me…

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