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Join Date: Mar 2010
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Megger Value of motor

02/24/2011 3:49 PM

Dear All,

Kindly tell me the normal megger value of insulation of motor? Does it depend on voltages of the system or KW rating of the motor?

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Guru
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#1

Re: Megger Value of motor

02/24/2011 3:53 PM

Is it giving several types of earth fault - a clockwise earth fault and an anticlockwise earth fault, for example?

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#2
In reply to #1

Re: Megger Value of motor

02/24/2011 4:01 PM

it is giving slack fault indeed

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Guru
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#3
In reply to #2

Re: Megger Value of motor

02/24/2011 4:05 PM

Would that be the directional fault that answers the earlier question?

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#5
In reply to #3

Re: Megger Value of motor

02/24/2011 4:12 PM

Dear PW Slack,

We are using 7sj602 relay for HT Incoming and Out Goings at 11 KV. This relay gives us i> , i>> and i>>> Also IE>, IE>> and IE>>>. For IE, it represents the earth fault current. When we consult the manual of this relay, there are two distinguished types of fault currents. These are Directional and non-directional earth fault. My previous question was about this matter. Now this question is about Megger Value calculation. We are using the pet formula of 2 M Ohms for insulation at 400 V. While for 11 KV, we take the insulation level of 12 M Ohms and greater. I have to verify and discuss with the experts here.

Regards,

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Guru
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#7
In reply to #5

Re: Megger Value of motor

02/24/2011 6:17 PM

What is the earth fault protective device set to trip at?

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#4

Re: Megger Value of motor

02/24/2011 4:07 PM

As a rule from practice: minimum acceptable insulation value is 1 kOhm per volt, example - 440 v supply voltage - 440 kOhm minimum insulation resistance (if higher, better).

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Power-User

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#6

Re: Megger Value of motor

02/24/2011 5:10 PM

For motors rated more than 600v minimum acceptable insulation resistance is 30 M ohm at 40 degree C (cold), when stopped after running (hot) 10 mega ohm is the minimum.

You have to apply temperature correction if temperature is other than 40 deg C.

Dear AA in your previous post I think you have created confusion by using term "directional" but rather I try to clear the matter right there.

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#8

Re: Megger Value of motor

02/24/2011 8:38 PM

IR result must be in unit of MEGA OHM..Not depend rating of motor

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Guru

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#9

Re: Megger Value of motor

02/24/2011 9:49 PM

The minimum insulation required depends on voltage, not the KW (or HP) of the motor.

Minimum insulation (Mega-ohm) required is KV + 1.

See these links:

http://cr4.globalspec.com/thread/56906/Insulation-of-a-Motor

http://www.megger.com/common/documents/Above1kV_UG_en_V04.pdf (page 25)

- MS

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Commentator

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#10
In reply to #9

Re: Megger Value of motor

02/24/2011 11:15 PM

AC Motors, 75°C 120-volt - 1.12 megaohms

208-volt - 1.20 megaohms

240-volt - 1.25 megaohms

480-volt - 1.50 megaohms

DC Motors, 75°C All voltages - 1.00 megaohms

High-Voltage Circuit Breakers, 20°C 10,000 megaohms

High-Voltage Bushings, 20°C 10,000 megaohms

Insulated Cable - Varies as a function of type and thickness of insulation; see ICEA Standards

Transformers, Oil-Filled, 20°C

Where:

E = Voltage rating of winding under test

kVA = Rated capacity of the winding under test

Transformers, Dry type, 20°C, where E and kVA are as defined above

Other Equipment - Refer to Manufacturer's Data

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Commentator

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#11
In reply to #10

Re: Megger Value of motor

02/24/2011 11:26 PM

sorry for missing the formula

1.5E/SQRT(KVA) for oil filled

30E/SQRT(KVA) for dry type

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Anonymous Poster
#12

Re: Megger Value of motor

02/25/2011 1:38 AM

above1000 Mohm, it doesn't depend rating of motor only depend warnishing and enamal coating of wire.

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Guru

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#13

Re: Megger Value of motor

02/26/2011 1:43 AM

Refer to IE Rule No. 48 ans IS 732 if you are from India.

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Users who posted comments:

AA (2); Anonymous Poster (2); electricalexpert65 (1); loadshare (1); mountk2 (1); msamad (1); PWSlack (3); vijaymail_electrical (2)

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