Fluke 757

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jjwitty

Member
Location
Indianapolis
Occupation
Automation
Preface: I’m an automation professional by training (degree), but I’ve been unofficially designated as the plant “specialist.” That role puts me in charge of all things electrical—which inevitably includes about 150 motors.

Last year, I requested a megger, but Megger Inc. quoted me $18,000 for a ≤600-volt device. My director scoffed at the price but approved a Fluke 757 instead. It’s not a real megger, though some people seem comfortable calling it one. Personally, I refer to it as an insulation tester. I assume those who casually use “megger” have never actually seen a real megger quote—but I digress.

Since getting the 757, I’ve yet to find a single bad motor, even though maintenance keeps pulling “bad” ones off the floor. I don’t want to be that guy who tells them they don’t know what they’re talking about and then points at the 757 results—but the truth is, I have no real baseline for what a “bad” motor even looks like on this tester.

Typically, the motors they claim are “shorted” are ones where someone didn’t secure the wiring properly, and a wire nut fell off and grounded out. Sparks and molten insulation are enough for them to assume it’s toast. But when I test it with the 757, it reads fine. Just to be sure I always do a basic ohms test phase to phase also. So, instead of scrapping them, I clean up the wiring and put the motors back on the shelf.

Now, I’m here to make sure I’m testing these motors correctly. All of them are 480V. The Fluke 757 came with three leads—two standard ones, and a third with the test button for resistance testing. I connect that to the designated spot, move the black lead under the insulation tester port, set the voltage to 1000V, and test between phases. Every time, I get a reading of exactly 2 MΩ. I also test to casing.

Here’s the weird part: if I press the test button without the leads connected to anything, I still get the same result—2 MΩ. The Fluke instructions are horrible, and their YouTube videos somehow manage to be worse.

So—can anyone confirm if I’m doing this right? If possible point me to a useful training video or material?
 
Sounds like you're using the meter correctly, I do believe that the model number referenced is incorrect. We had the same Fluke megger where I previously worked, and found it effective for predicting issues, mostly with large submersible pumps.

Try searching for a Stich in Time by megger. It will give you all the information you want on the subject.

From my limited understanding, motor insulation testing is something that needs to be trended overtime to gauge overall motor health. Most motor issues I've found could be troubleshot with a multimeter and a clamp on.
 
You should buy your own tools, and take ownership of proper use. Never believed in buying cars for my kids.

Broke used insulation testers each year, until eBay had this BM80/2 in 2009 for $114

Broke leads, stripped battery screws, cleaned battery acid, and this thing keeps going.

Find appliance defects @ 100V that wont show on Ohm meters, but double check motors @ line voltage.

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I used a best in class Fluke model 1587 combination VOM / Merger for over ten gears before retiring. Like to have a $1 every time it fell from a 6' step.ladder. We were taught that the test voltage should be motor voltage times two so you should use a,1,000 volt megger to pick up grounded motors. We had over 500 VFD'S & hundreds of starters to PM. If motor was not running we meggered it. Liked to have at least 10 megohms for motors on VFD'S. A five minute class would be enough to teach anybody how to ring out continuity on the usually six motor windings on a dual voltage three phase motor and how to.properly megger. Hopefully Fluke still has a 6 to 10 page how to use a megger on their web site. For my first 35 years as a sparky used a 500 volt bullet proof hand cranked Biddle company megger. One good thing about them if a winding was grounded handle would have more turning resistance. Have to remember that the larger the motor the lower the winding resistance. While measuring the six windings on a 200 HP motor windings to new people might ampere to be shorter because each large winding might be less then 0.5 ohms. Sorry but I forget the horrible reading that you get if you blow the 1,000 volt rated glass fuse in the Fluke 1587 meter. I blow that fuse a few times rushing to get an important drive back on line. I own a Simpson 5,000 volt battery operated megger that I used to test 4,160 volt motors for grounded windings and grounded wire. Might want to give people applying for a electrician job a meter and a three phase motor and have them check & record motor winding resistance and then megger it. While in a great Go Tech school they had us ring out approximately 50 wires going thru a pipe that wires were was cemented. Had wires going to 50 screen on each side of pipe with #'s on each screw. Some wires were taped off, others doubled back to same side & others were spliced together and one grounded to.pipe so when you used the ohmeter would cover continuity between 6 or 8 screws. Only two if us got a perfect score. While working in a 65 man maintenance shop they had job applicants screw on hex nuts on 8 pieces of different diameter rods. Two were left hand threads and over halve failed that test. They asked me if I know how to wire in a basic start stop pushbutton.While writing a little drawing told them where the three wires went where. Told me over halve of the applicants did not know how. PS I'm old school and tape every motor wire nut especially if on vibrating equipment. Be a friend while tapping wire nuts a please leave a flag. Makes it easier to remove especially if oil gets on tape.
 
Jjwitty, your right. A megohm setting doesn't mean megger. But I also know most faults, maybe
99%, can be found with most any meter. Even a cheapy. But there are times where a fault isn't
shown because of some fluke.
I've had a couple meggers. I now have a Biddle. I used them to log resistances in compressors
over time. Occasionally I've used them to troubleshoot also but most faults show up on my VOM.
Sounds like your crew needs some training.
I've been told a high percent of compressors sent back for warranty are OK.
 
ptonsparky said:
I finally managed a look at the specs which for insulation, is up to 2G.
I would expect that with open leads.
Click to expand...
I have received 2 Gigli ohm measurements while meggering 480 volt motors while performing approximately 75 PM'S a month and false earth ground ( ground fault ) on Danfoss drives. You can only get 2G ohm readings while on the 1,000 volt test range. Think the seldom used 500 volt test range could only produce 550 meg ohms if my old memory served me right. Another reason I loved this bullet proof tuff meter it replaced two meters.
 
garbo said:
I have received 2 Gigli ohm measurements while meggering 480 volt motors while performing approximately 75 PM'S a month and false earth ground ( ground fault ) on Danfoss drives. You can only get 2G ohm readings while on the 1,000 volt test range. Think the seldom used 500 volt test range could only produce 550 meg ohms if my old memory served me right. Another reason I loved this bullet proof tuff meter it replaced two meters.
Click to expand...
I was looking at specs for the meter. 2G is max for it.
 
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