# Easier access to motor leads for megger testing



## olfactory (Oct 17, 2020)

I'm in the process of beefing up the PM scheme at a manufacturing facility, including regular megging of motors on production equipment. In the past at other sites, this has been as simple as setting up a schedule and then getting to work: open up the terminal box, disconnect leads, start megging. However at this site, a large number of critical motors are not at all easily accessible (buried deep inside machines, mounted between drive belts, etc.), at least not on a regular, PM-type basis to get into the terminal boxes and disconnect the leads to meg each winding.

I'm wondering if anyone has come up with any creative solutions to make it easier to access all 9 leads. I'm thinking of just running a 10-conductor cable from the CP down to the motor and wiring it straight through (+ground), making the winding connections in-panel by with jumpers on terminal blocks, so I can remove them and meg right there in the panel. Of course I know that this will technically mean I'm megging the motor AND the 10 conductor cable, but at least if nothing is wrong, I can move on without having to pull the motor. Also, 10 conductor cable is not exactly readily accessible in a variety of gauges. So has anyone come up with a clean, convenient solution to this problem?

Thanks


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## em158 (Jul 7, 2016)

olfactory said:


> I'm in the process of beefing up the PM scheme at a manufacturing facility, including regular megging of motors on production equipment. In the past at other sites, this has been as simple as setting up a schedule and then getting to work: open up the terminal box, disconnect leads, start megging. However at this site, a large number of critical motors are not at all easily accessible (buried deep inside machines, mounted between drive belts, etc.), at least not on a regular, PM-type basis to get into the terminal boxes and disconnect the leads to meg each winding.
> 
> I'm wondering if anyone has come up with any creative solutions to make it easier to access all 9 leads. I'm thinking of just running a 10-conductor cable from the CP down to the motor and wiring it straight through (+ground), making the winding connections in-panel by with jumpers on terminal blocks, so I can remove them and meg right there in the panel. Of course I know that this will technically mean I'm megging the motor AND the 10 conductor cable, but at least if nothing is wrong, I can move on without having to pull the motor. Also, 10 conductor cable is not exactly readily accessible in a variety of gauges. So has anyone come up with a clean, convenient solution to this problem?
> 
> Thanks


Why would you want to access all 9 leads to the motor for megging? Unless you are talking about an extremely large and expensive motor, simply meg and test winding resistance from the starter including the feeder cables. If a suspicious reading is seen then you disconnect at the motor to isolate the problem. It doesn't need to become a "Science Fair" project.


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## 460 Delta (May 9, 2018)

em158 said:


> Why would you want to access all 9 leads to the motor for megging? Unless you are talking about an extremely large and expensive motor, simply meg and test winding resistance from the starter including the feeder cables. If a suspicious reading is seen then you disconnect at the motor to isolate the problem. It doesn't need to become a "Science Fair" project.


Like @em158 said, just open the door on the MCC bucket or combo unit and megger at the starter lugs. This tests the motor, wire, and starter insulation in one whack. There is no need to test the windings individually, as they are all interconnected to run anyway. If only one tested bad when it was all disconnected you wouldn't put it back in service with the ones testing good would you?
There is more to insulation testing than just the motor, the wire to it is also prone to fail. All of us have heard or hear wire rattle in conduits when motors start up, think of this as a super slow speed grinding action on the wire insulation as the wires rub each other and the conduit wall. It's worthy of being tested the same as the motor.


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## paulengr (Oct 8, 2017)

Things to watch out for:

1. Megger on soft starts andVFDs...must disconnect leads first or you are just measuring the drive. Typically 1-2 megaohms.
2. Grounded wye connection. Mostly common in larger motors over 500 HP. Shows dead shorted every time. Disconnecting the jumper to ground is enough.
3. Capacitors and surge arresters. Again you end up Meggering the wrong thing.
4. Do it by the book, no short cuts for PMs. Take temperature. Megger 1 minute, watch the readings. Use the correct voltage.

On a DIAGNOSTIC Megger I’m looking for pass/fail. If it’s under 5 megaohms (under 1000 V) or 100 Megaohms (over 1000 V) after it stabilizes enough it will obviously “never get there” or is well above these numbers (15+ or 300+), stop the test. We are done. Motor fails or passes. Only go to 60 seconds if it falls close to the line. Most of the time the test is done in seconds. On a PM test we want to measure the trend so that is a much different animal. Repeatability is very important.

If access is really bad or the motors are very critical, install either the Megalert system or the Bender relay system. I’ve used both. Each has their advantages. These are monitor relays installed at the starter/drive that automatically test and you get a display in megaohms, idiot lights, and alarm contacts. Be aware these testers may not run a “true” insulation resistance test so read all the fine print.

As to meggering “all 9 leads”, why? A motor is either delta or wye connected. It has under 1 ohm resistance in the coils and you should be measuring millions of ohms from phase to frame, so effectively all coils are shorted together. If I’m faced with just open leads then I just take a bolt and jumper them all together. If you have just lead wires you can clamp them all In vice grips. Even though it has some resistance it doesn’t matter. I use alligator clip jumpers on a wye delta or two speed starter. You can get a dozen for $10 on Amazon. One test, that’s it.

One big reason to ALWAYS make sure everythjng is tied together is this. If you run a test, stop, then run again, what happens to the readings? They go up. There goes your “PM”. When you do any high voltage test accurately the rule of thumb is to short the leads for 3 times longer than the test ran. This is forcing the insulation molecules to depolarize which is a pretty slow process. So on 9 leads measuring one lead at a time for 1 minute and shorting for 3 minutes plus 1 minute to swap leads around takes 45 minutes to run the test if done correctly. My way takes 1 minute to hook the leads together if needed and 1 minute to run the test. 

And here is why temperature matters. If you are doing this for PM purposes you need an accurate result. This means doing it consistently by the book EVERY time. This means take a temperature. Megger for one minute (60 seconds) with a stop watch. Use a digital (not hand crank) Megger. Watch the readings as the test runs. Stop early only if it dead shorts. Pay attention to what it does.

Going from winter to summer on a cold (room temperature) motor Megger readings will go up by 3 times. This is why the temperature correction is so important. Sure if it reads 1000 megaohms we don’t care but if it reads 10 megaohms that’s the difference between pass and fail. If you do this then your PM means something. If you notice a downward trend you know well before it fails something is happening (contamination, heaters not working, or motor failing).

Second watch the trend. Are the readings bouncing all over the place? That’s moisture. Does it go up steadily for the entire minute or shoot up in seconds if at all then flat line for the whole minute? That indicates leaking to ground no matter what the number says...better be looking for why. On motors with brushes it’s an indication it’s past due for cleaning out the carbon.


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## em158 (Jul 7, 2016)

A one minute megger is fine up to a point. Small motors will stabilize at 1 minute, larger motors require a 10 minute PI test to get a trendable reading. Your experience must very different from mine. I test a lot of large MV motors and I can't remember testing a Grounded Wye motor.


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## gpop (May 14, 2018)

At my last job we had meggers and fancy meters that could diagnose all sorts of winding problems (all 480v). Just about every motor had a disconnect so testing was as simple as opening the disconnect and running the test on the load side. To be honest it didn't really save us anything. It was a useful go, no go test on motors that had been down for a while but it wasn't worth shutting down a motor to test it. 

On a 3hp motor that you can change in under 30 minutes with a back up system if it megs slightly low do you change it (not worth rebuilding) or do you run it till failure. On paper changing the motor 3 months before it fails doesn't work out financially viable. Even the time spent on labor megging the motor is hard to justify. On the larger motors 50hp and above there was a cost saving and of course on critical equipment cost doesn't matter.

Our biggest bang for the buck came on vibration analyzes. Changing bearings was something that could be done on site and most of our motor failures were bearing related before we started the PM schedule.

When i worked with 4160 its was a different story and the time invested was worth the effort. 

The job im doing now is mainly 240/480v submerged pumps all classed as critical so we have auto meggers on everything.


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## Cow (Jan 16, 2008)

Like the others said, I test at the starter/vfd/etc first before I start opening up peckerheads and break motor connections apart.


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## JRaef (Mar 23, 2009)

If you test at the starter, and it tests out fine, you are done. The only time you need to go deeper is if it does not test out fine. 99% of the time it will test out fine, so you save yourself from the extra work 99% of the time.

Side note: If you have a VFD or Soft Starter, disconnect the motor leads from it before using a megger.


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## just the cowboy (Sep 4, 2013)

JRaef said:


> Side note: If you have a VFD or Soft Starter, disconnect the motor leads from it before using a megger.


As a side side note also watch out for small European motors, I had some that were single phase input on a three phase motor. They installed a cap in the peckerhead to simulate the third phase.


Cowboy


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