Rising Resistance Readings When Meggering Compressors

[Tayters]

I've seen this myself once or twice where meggering a compressor the reading starts off at a respectable value (say 10M ohms) then gradually rises to a complete open circuit or at least well into Giga ohms.

Can anyone confirm what is going on here please?

Andylogic tells me that there is obviously some resistance which builds up, a bit like a capacitor but I can't see how compressor windings can give that effect. Is it because there is a certain moisture level in the compressor or contaminent in the refrigerant which somehow changes the longer the test lasts?

In the cases I've come across there has been no fault with the compressors and they have run fine afterwards. They were inverter compressors on splits or VRF so just an inverter compressor we all know (and love!).

If the resistance started off at a low level and then rose would this be a cause concern (I think it would but haven't come across this case yet) as opposed to the situations I've encountered?
The megger voltage is on the 500 or 1000V scale.

Thanks chaps!
Andy.

[install monkey]

had 3 ph motors that have blown fuses and whilst meggering shows less than 1 meg then rises slowly as the windings warm up,it clears and runs ok.
usually inverter winding resistance phase to phase is around 2 ohms,but as you test a comp windings to earth-dunno, am i correct as to think you have disconnected the diode module/comp leads and meggering just the compressor terminal
i

[Tayters]

Windings warming up eh? Hadn't thought about that one.

Generally when meggering I've gone for a reading about 12M as acceptable - got that from various compressor testing guides which were more involved in refrigeration plant but hey, a motor has the same principles no matter what it's running. In practice though they seem to either pass well or fail miserably so no grey areas but then I don't get involved in much other than AC compressors nowadays.

Agreed about the phase to phase - many are even lower than an ohm but as long as all 3 are the same (and not obviously 1 a dead short) then fine for me and yes the compressor is disconnected from the inverter module so just checking direct at the compressor terminals or unplugged from the PCB and going down the cables to the comp. That only leaves a thermal jobbie if the comp has one,

Thanks for the reply,
Andy.

[monkey spanners]

I thought it was due to the windings and body acting as the two plates of a capacitor, but don't know for sure!

[Tayters]

So there could be something to Andylogic after all then!

The more I'd read up on it became apparant that there is no set pass or fail value. Factors as oil, contaminants and temperature can set you off on the wrong track but as to the reading rising I can't find a firm answer. Yet :-)

Cheers,
Andy.

[NH3LVR]

Monkey Spanners has it correct.
The initial low reading is caused by the capacitance between the grounded (earth on your side of the pond) frame and the windings. As the charge builds up in what now is acting like a capacitor the reading goes to it true value.

[Grizzly]

An Interesting read this one Andy!
With some knowledgeable answers, I will question your comment about 12mohm being your benchmark Andy?
I am assuming that is on smaller equipment?
I have seen situations where 1 or 2 megohms have been observed, particularly if the motor has been off for a long time.
12 does seem rather high?
Grizzly

[install monkey]

Under 1 megohm will trip a rcd

[Grizzly]

Under 1 megohm will trip a RCD

Logically You are correct IM.
Hence my reference to smaller motors being probably discussed by Andy?
The size I am used to don't normally have RCD's on them.
The instructions I have kept over the years do refer to the need sometimes to "drying out" motors with resistances at the lower end!
Anything below 2 Mohms I would assume could be classed as low.
I have come across motors below that, that have successfully been resurrected.
I suppose I am trying to say that a low Megga reading does not always mean the scrapping of a motor.

My experiences of the above do not include Hermetic comp motors though!
As an aside, my missus decided to clean our electric main oven at the weekend.
So at 1700hrs on Sunday it was discovered that the Oven when switched on.
Would trip the Main RCD, when the control knob was rotated through fan run to the element being energised.
I ended up removing the element powering it up separately, during which it smoked and steamed a bit.

But upon replacing it back in the oven, all worked OK.


Hence my agreeing that low resistance readings will indeed trip an RCD. (Mine read 22 0hms initially, sadly I did not meter it again after cooking it!)

Grizzly

[chilliwilly]

Where the windingleads connect to the terminals on the compressor internally, there is onlymanufactured applied insulation if any at all. Then the obvious shelac on thewindings where they are formed on the stator.

The 500v+ insulation test will measure the dilectric constant between the contents of the system, ie therefrigerant, oil, any residual moisture or other contaminants, and associatedapplied insulation and any poorly insulated metal work. The increase intemperature will cause a positive coeficient (increase) in resistance of the contents (dielectric), and the windings resistance which is a natural occurance.

A firm I worked for back in the eighties would treat a 20-50 meg reading on a service visit (whensystem cold) as imminent trouble and would reccomend a flush and recharge,which would then give a 100 meg+ reading. This reading was always the norm on all sytems newly installed, any lower caused the boss to be nervous and concerned. The 1 meg minimum reading at thattime (15th ed) had no true bearing on periodic test result on a refrigeration compressor due to its application and designation, as itwould on a general motor. Even the 17th edition doesn't mention the trueacceptable test results for refrigeration compressors, it treats all motors asthe same with 1 meg result being acceptable.

Maybe in future ammendments of the regs there might be a section for specific motor applications, as hazardous areas require motors to be flash tested at 3000v on top of the usual insulation test.

An RCD will trip at 0.03 amp and a gfci will trip at 0.006-8 amp, the lowest reading that will allow this current to flow would be 8000 ohms at 240v and 30000 ohms at 240v respectively.

If I ever get involved with the wiring of outdoor circuits, especially for supplying condensing units. I always design a 40 amp circuit to get round the application of using 30 ma RCDs as they cause more nuisance than the high safety they're designed for. Even on circuits rated around 3KW loading unless there is a risk of ingress of water jets or mechanical damage I often ommit the use of RCDs.

But as you would expect I do come up against protest from site electricians that only read and work to the "on site guide" and are general lighting and power installers. They soon come round to my way of thinking though once I show them where to find the ommision of 30 ma RCDs on certain circuits written in the 17th edition wiring regs, reg 314.1 (i), (iv), 411.3.3 (i), (ii), (a), (b).

[install monkey]

disconnect the earth cable and buy her some insulated oven gloves!

over 1 meg will not blow a bs88 fuse
Measure the resistance between terminals using a tester.
(Winding temperature : -20 ~ 40°C)
Normal
U-V
U-W
V-W
0.78 ~ 1.11 ohm
taken from mitsi service manual for a m series inverter split

Logically You are correct IM.
Hence my reference to smaller motors being probably discussed by Andy?
The size I am used to don't normally have RCD's on them.
The instructions I have kept over the years do refer to the need sometimes to "drying out" motors with resistances at the lower end!
Anything below 2 Mohms I would assume could be classed as low.
I have come across motors below that, that have successfully been resurrected.
I suppose I am trying to say that a low Megga reading does not always mean the scrapping of a motor.

My experiences of the above do not include Hermetic comp motors though!
As an aside, my missus decided to clean our electric main oven at the weekend.
So at 1700hrs on Sunday it was discovered that the Oven when switched on.
Would trip the Main RCD, when the control knob was rotated through fan run to the element being energised.
I ended up removing the element powering it up separately, during which it smoked and steamed a bit.

But upon replacing it back in the oven, all worked OK.


Hence my agreeing that low resistance readings will indeed trip an RCD. (Mine read 22 0hms initially, sadly I did not meter it again after cooking it!)

Grizzly

[Grizzly]

Where the windingleads connect to the terminals on the compressor internally, there is onlymanufactured applied insulation if any at all. Then the obvious shelac on thewindings where they are formed on the stator.

The 500v+ insulation test will measure the dilectric constant between the contents of the system, ie therefrigerant, oil, any residual moisture or other contaminants, and associatedapplied insulation and any poorly insulated metal work. The increase intemperature will cause a positive coeficient (increase) in resistance of the contents (dielectric), and the windings resistance which is a natural occurance.

A firm I worked for back in the eighties would treat a 20-50 meg reading on a service visit (whensystem cold) as imminent trouble and would reccomend a flush and recharge,which would then give a 100 meg+ reading. This reading was always the norm on all sytems newly installed, any lower caused the boss to be nervous and concerned. The 1 meg minimum reading at thattime (15th ed) had no true bearing on periodic test result on a refrigeration compressor due to its application and designation, as itwould on a general motor. Even the 17th edition doesn't mention the trueacceptable test results for refrigeration compressors, it treats all motors asthe same with 1 meg result being acceptable.

Maybe in future ammendments of the regs there might be a section for specific motor applications, as hazardous areas require motors to be flash tested at 3000v on top of the usual insulation test.

An RCD will trip at 0.03 ma and a gfci will trip at 0.006-8 ma, the lowest reading that will allow this current to flow would be 8000 ohms at 240v and 30000 ohms at 240v respectively.

If I ever get involved with the wiring of outdoor circuits, especially for supplying condensing units. I always design a 40 amp circuit to get round the application of using 30 ma RCDs as they cause more nuisance than the high safety they're designed for. Even on circuits rated around 3KW loading unless there is a risk of ingress of water jets or mechanical damage I often ommit the use of RCDs.

But as you would expect I do come up against protest from site electricians that only read and work to the "on site guide" and are general lighting and power installers. They soon come round to my way of thinking though once I show them where to find the ommision of 30 ma RCDs on certain circuits written in the 17th edition wiring regs, reg 314.1 (i), (iv), 411.3.3 (i), (ii), (a), (b).

Brilliant answer and thanks for the regs links. Chillwilly.

I M.
Priceless.
Grizzly

[Tayters]

Thank for your replies chaps.

It would appear then the rising resistance is due to a capacitance effect and the reading to go by is the higher reading.

With regard to the actual value, this wasn't the point of the thread but obviously there's no avoiding it!

I go by some info found in the Thermacon testing guide. This is aimed at larger stuff by the content of the manual but in there there's a section about 12M is good, 1-2M then clean out the terminal posts first and recheck. Most of my stuff is AC so this doesn't specifically apply so I base my results around this but keep an open mind if a low reading is found.
So far the ones I've come across are a dead short or well into the M ohms so no argument there.

Incidently I came across some LG Multi V checking info today. There it stated that with a 500V megger and no charge in the system then reading should be over 1M, if there was charge reading should be 10M (I think, somewhere around there anyway).

Cheers,
Andy.

[Grizzly]

Reason: Will dish the points out when allowed! dish the points out when allowed!

Ditto!
Grizzly

[install monkey]

you shouldnt megger a comp with no gas in!

Thank for your replies chaps.

It would appear then the rising resistance is due to a capacitance effect and the reading to go by is the higher reading.

With regard to the actual value, this wasn't the point of the thread but obviously there's no avoiding it!

I go by some info found in the Thermacon testing guide. This is aimed at larger stuff by the content of the manual but in there there's a section about 12M is good, 1-2M then clean out the terminal posts first and recheck. Most of my stuff is AC so this doesn't specifically apply so I base my results around this but keep an open mind if a low reading is found.
So far the ones I've come across are a dead short or well into the M ohms so no argument there.

Incidently I came across some LG Multi V checking info today. There it stated that with a 500V megger and no charge in the system then reading should be over 1M, if there was charge reading should be 10M (I think, somewhere around there anyway).

Cheers,
Andy.

[Grizzly]

you shouldn't megger a comp with no gas in!

Not exactly correct my friend.
Technically no gas could be at atmospheric pressure?
Basically you should not megga when the system is in a vacuum, splitting hairs I know but there is a big difference all the same.
Particularly as you can have a system without gas, but to have one in a vacuum take human / mechanical intervention!

Just correcting! No offence meant!
Grizzly

[install monkey]

no offence taken- usually its my job to question- cant give any reps

[Tayters]

you shouldnt megger a comp with no gas in!

You shouldn't sell an AC system with poor software, components and copper but that never stopped LG!