Correct meggar techniqiue

[theapprentice]

Due to working at a large factory that has its own on-site electricians I have been getting very little electrical experience at work. Im starting to get more and more but one thing I am not confident with is using a meggar to check compressors and motors. What is the correct process, as in leads go to what places. I no to always test with double the supply voltage. As an example could someone explain the process to meggar a simple hermetic compressor.
Thanks very much,
chris.

[cool_tech]

hey mate hows the apple isle,
first you check your known eatrh on the ohms scale. should be o ohms between eatrhs. then black lead to earth plug and red to each terminal all should read above 1M ohm. when testing insulation on motors with thermistors correct me someone if im wrong, you twist wires together of thermistor and test between each phase.

[modvalve]

hey mate hows the apple isle,
first you check your known eatrh on the ohms scale. should be o ohms between eatrhs. then black lead to earth plug and red to each terminal all should read above 1M ohm. when testing insulation on motors with thermistors correct me someone if im wrong, you twist wires together of thermistor and test between each phase.

first you must disconnect/isolate wiring to compressor/ motor then connect the black lead to the compressor body making sure there is a good connection ie no paint. Next connect the live lead of the megger to a terminal on the compressor and check the reading. As long as it is above .5m ohm your resistance to earth is ok but in reality it should be infinity. Hope this helps

[Yuri B.]

With megger we test quality of winding's electrical insulation from "earth" (body of the motor), and from other windings.

If there is only three leads on the motor, you only do 1 measurement - between any of the leads (no matter which) and the earth.

If there are 6 leads, you should determine with an ohmeter, or by a look at the start schematic, how many windings to test are there.

If the start is star/ star (1 or 2 contactors), there are 2 windings, and you do altogether 3 tests: winding - winding (1 test), and each winding - earth (2 tests).

If motor starts star /delta (3 contactors), there are 3 windings and you do altogether 6 tests (each winding to an other - 3 tests, and each winding and the earth - 3 tests too).

[Yuri B.]

If there is only 3 leads, there is only possible 1 winding test.

[theapprentice]

so by three leads you mean something like a simple dome compressor where you have a start common and run pin and 6 leads is something like a three phase motor where you have the 6 pins where you connect up in delta or star.

[Yuri B.]

It is like this, as it is irrelevant what (actual) windings are there, how many pairs of poles in each phase (1....12), how many turns in each winding, etc. A WINDING (is there 2, or 3, or 6, no matter actual windings) taken by ohmeter as an entire winding is subject to the high pot test. In this respect 1 phase motor is like 3 phase motor (when into the latter's contact box comes only 3 leads from its windings) - they are tested similarly.
Remember always never not to megger a comp winding when comp is in vacuum - such test most sertainly will lead to insulation rupture somewhere.

[theapprentice]

ok it is all slowly starting to make sense now. I think with bit of hands on work I will be alot more confident with it.

[Yuri B.]

Be careful with your hands. As windings meggered may get charged - they have some capacitance - dont forget to discharge after test with a shunt (short circuit all windings between themselves and every winding to earth).

[Coolie]

Remember always never not to megger a comp winding when comp is in vacuum - such test most sertainly will lead to insulation rupture somewhere.

Never even have the power to the comp when it is in vac (ie when doing repairs) Some mercury chillers have a constant 415v to the scroll comp, EVEN WHEN COMP NOT RUNNING. So when vaccing, isolate and pull the fuses. Or you blow the comp up:eek:

[Eeram]

Please do not test thermister circuits on compressors or any motors with a megger! You will destroy them!

[nike123]

http://www.refrigeration-engineer.com/forums/showpost.php?p=94054&postcount=1

[theapprentice]

that link seems pretty good information from first galnce. will have to read bit further

[iftekhar]

With megger we test quality of winding's electrical insulation from "earth" (body of the motor), and from other windings.

If there is only three leads on the motor, you only do 1 measurement - between any of the leads (no matter which) and the earth.

If there are 6 leads, you should determine with an ohmeter, or by a look at the start schematic, how many windings to test are there.

If the start is star/ star (1 or 2 contactors), there are 2 windings, and you do altogether 3 tests: winding - winding (1 test), and each winding - earth (2 tests).

If motor starts star /delta (3 contactors), there are 3 windings and you do altogether 6 tests (each winding to an other - 3 tests, and each winding and the earth - 3 tests too).

my dear i have small but critical problem i have working in york ycas series screw compressor and 3 contactors means star-delta , so i need to megger this compressor i heartly request u to please give me some solutions.

[nike123]

my dear i have small but critical problem i have working in york ycas series screw compressor and 3 contactors means star-delta , so i need to megger this compressor i heartly request u to please give me some solutions.
Why don't you follow link which I provided in my post?

[Eugenio]

As Yuri point out, be carefully with existing voltage. Prior to any measurement, check voltage with Voltage function of your megger.
As Eeram says, analyzes if parts supports voltage you go to apply.
I use for R isolation, the RCD sensibility criteria.
Un/Isens = Risolation min
i.e. RCD with 30 mA sensibility. V = 240 VAC
240 V / 30 mA = 8 Mohms, over this value RCD does not blow, and people and equipment are safe. Obviously O.L. (infinity) is better.
I come from electronic side, so, please be patient with my electric jargon.

[trieuphu]

Meggering a motor/ compressor is very simple. Meggering is a process to measure the insulation break down of a motor. Etheir 3 phases or single phase, measuring technique is the same. DO NOT WORRY about 9 leads or 6 leads, most of motors are internal connected or may already be connected before all 3 T leads stick out. For single plase, connect 1 lead of the megger to motor frame or ground (make sure a good ground) scrape the paint of the motor frame or panel housing and the other lead of megger connects to one of the winding ( either start or run does not make any different). The resistance is should be a minimum of 1M ohm. On the 3 phases motor, connect the other lead to any T lead and expect to be more than 1M ohm. There are many different megger models in the market some of them you must push green button down to megger and minimum time for megger to be effective is 30 seconds. Safety: Do not touch any leads while measuring, 500-1000vdc is injectecd into the system.

[Improver]

hi apprentice, there is a video on you tube of an engineer meggering compressors 1phase and 3phase. It helps when you see the test being done rather then reading how to do it. Type in at the youtube search HVAC ohming compressor windings and there will be tutorials from a cheesey yank called dr zarkloff.

[jimbo82]

Remember single phase motors can have a start winding and a run winding and if present both are used all the time. Best place to check is to disconnect the windings from terminals just before they enter the motor casing ensuring to jot down which wire went where - don't try and remember, its not fun if you get distracted and come back to it later! Run winding is _sometimes_ thicker gauge if you forget. Make sure capacitors are NOT connected, you can damage them and/or kill yourself with the stored voltage. You are testing between earth and each of the windings, and if there are 2 or more windings, between the windings themselves as well. The point of the megger test is to check for a loss of insulating ability of the windings insulation. This test will not show up a short between coils of the same winding unless it is also leaking to earth and this will be seen. An ideal figure (AS3000 compliant) is above 1Mohm, less can be seen especially in a DC motor with lots of brush carbon present and the motor is still OK. I am an electrician who has checked large industrial motors and a couple of small single phase ones, if there is more to add to this please pipe up!

[johnirvin]

Insulation Resistance (IR) test... In the megger instrument there is a voltages 250V, 500V, 1000V.. set your megger selector to 500V (ideal voltage). The black lead connect to the earth(screw area), the red lead connect to any of the windings and press the button and read the result (IR).