I'm keen to hear the input on this thread. I could've used a megohmeter on 3 separate occasions this week, anybody on here got thoughts on brand, price functions etc?
How many people here actually have a megger in their trucks and how often is it being used. i have a family member that says he uses it all the time in his jobs (of course hes a electrician) is it really worth buying one. or is ohming out a motor just as easy?
thanks in advance
Akelesis
Get er Done!
Do what has to be done
when it has to be done
as well as it has to be done
And doing it all the time.
I'm keen to hear the input on this thread. I could've used a megohmeter on 3 separate occasions this week, anybody on here got thoughts on brand, price functions etc?
You cannot cheat an honest man. But that doesn't stop people trying!
A good megger test can give you a heads up about a potential future problem. A good multi-meter is great for checking motor windings, bad connections, caps ect... I have seen a compressor fail a megger test and run for years.
ohm'n out a motor is a different animal the megging out one.if you held the lead on a megger and pushed that red but it would take you off you feet,meggers test the insulation on the windings and the reading is the leakage of that charge in testing.big tonnage OEMs design their chiller for so many starts when the equipment sees 10X the rated amperages on start.the windings actually move and rub together...number of starts/hours run are a major log input with building engineers...100,000 ohms to ground is mint... solid...tight...new windings on any motor.as that number drops over the years 75,000>50,000 a burn out situation is in the future..and there is nothing a tech can do to fix that reading it being useage run time,to many restarts..add air within the system.. burn out clean up...water into chiller and that drops that numer also...
"when in doubt...jump it out" http://www.youtube.com/watch?v=U1qEZHhJubY
Maxster, any input as to a good "megger" for starting out? I work on everything up to 50tonnes at the moment.
You cannot cheat an honest man. But that doesn't stop people trying!
Grainger item # 3T243 is an industry standard that all the manufacturer tech have for the big tonnage stuff...how important are the readings always remeber calling the factory engineers witha start problem or tripping...you better have that megger reading written on you hand or they will blow you off as a clown...very important.if you have windngs breaking down it will show up on that contactor especially if your doing a PWS the start will take the lock rotor starting amps.this item is a company suplied item.go cheaper if your doing the buying it will give you good results i only used Amprobes for years.that 5ZK90 is a good price and it is Amprobe also there.rooftop companies rarely use them but if you do any chillers it is critical with the possibility of water within the lines..getting on those windings...i had a hellical screw air cooled chiller in the dead of the winter and the CCH took a crap the compressor read a dead short building engineer pushed the start contactor in and ba boooom...started a fire in the panel.replaced the CCH the compressor read 100K to ground good to go...cold frames as with hellicals or actual cold oil sumps..can also give you a dead shorted compressors heat the oil up and your back up in that 80K range to ground....great for large fan motors.WARNING never ever meg with a compressor in a vacuum...you'll short out the termistor winding sensors...that is a bad call to your SM...
"when in doubt...jump it out" http://www.youtube.com/watch?v=U1qEZHhJubY
I use 1 daily. You have to learnwhat the readings are, for example a wet fam motor or pump motor may read bad, but its not.
some construction sites especially basements are so humid you can read a dead short but is the soaked air in and around the motor that your reading.i had to go out side of a site to read fuses with just a OHM meter cause they were reading good but were blown with the humidity within the place.another Megger tip bigtime is if you have a burn out install a new compressor the acid tripping back to the oil sump and collecting can cause a wierd megger reading 30K to ground..then as you clean it up just flipping oil changes...the reading will climb up into that 75K acceptable area...
"when in doubt...jump it out" http://www.youtube.com/watch?v=U1qEZHhJubY
Hello guys Quanchai sent me to ARP he said that you are a very good group. My personal favorite Fluke 1587, It will test insulation from 250v to 1000vdc. It is important to know the range that a motor it would be consider bad, for example I saw that a costumer tagged a 50 hp motor grounded, I megged it it had 1 ohm to ground reading on all legs, there was no burned smell. I decided to brake out the heat gun and warm up the windings, after 30 minutes the motor readings past 100 mega ohms to ground all legs.That is a good motor ,the moisture in the motor made it read grounded. There is heaters that can be added to maintain temperature in the windings and trickle voltage if the motor has a vfd. Don't meg a motor under deep vacuum or with scr's connected.
If your job doesn't demand the use of a megger, then you most likely don't need one. If your intent is to use it like an ohmmeter, then just use an ohmmeter.
BTW, yes I have one, but only use it on electrical jobs. I'm an hvac tech/electrician. I don't use it on compressors.
I have 2 on the truck, old and new, and I use it regularly. Part of PM checklist on most industrial accounts. Most industrial clients would rather spend the money to replace a motor before it fails then wait and replace a motor or comp when it fails and shuts down production. Keeping track of starts, runs, and megging will let you know when it's time to start budgeting for that motor during the next main shut down.
i have a personal one and the company owns one (it's a little nicer!). maxster...i think that your numbers are low. 100k is mint? my Amprobes smallest measurement is 0.1 Megohms (100,000). I use for air cooled motors, >100M (100,000,000 ohms) is ok, under 100M then you should really start paying attention. for refrigerant cooled compressors, the number is closer to 5M because the oil will mess with the numbers.
below the above numbers does NOT mean that you need to replace the motor as others have said...it means that you need to start paying attention. the generally accepted 'GO/NO GO' numbers are: Voltage times 1000....on a 460 VAC motor, the GO/NO GO number is 460 x 1000 = 460,000 ohms or about 0.5 Megohms...below this number, I would not even think about starting the motor.
i use a megger on ALL motor PM's...it is very easy and tells so much.
"Right" is not the same as "Wise".
Don't step on my favorite part of the Constitution just to point out your favorite part.
Just because you can measure it, doesn't mean it is important. Just because you can't measure it, doesn't mean it isn't important.
I use mine any time the need arises, such as annuals, motor issues, checking out wire runs in conduit and the presence of moisture to any of the above. On annuals it goes along with an oil sample and vibration analysis. This way you can trend a system, compressor or motor.-GEO
Once in a while everything falls into place and I am able to move forward, most of the time it just falls all over the place and I can't go anywhere-GEO
We have an oldish Biddle Megohmmeter, runs on 110v and produces up to 5,000vdc. Used it today on a big centrifugal motor, comes in handy when you want to know the condition of the insulation. It may even help make you look like you know what your doing.
...Ron
Roof Rat
I recently used mine to find a bad condenser motor that read o k with a meter.I will admit that i don't use it often,but in this case it saved the day!
On a carrier 50tc there are 4 condenser fans.They are fed by 15 amp fuses The fuses had blown several times and other guys would just replace the fuses and it would work for a day or two.I decided to find and correct the problem so I isolated each motor and megged the motors.1 motor gave odd readings compared to the other 3.We ordered a new motor and replaced it The unit has been working fine for a couple months.Like i said ,I don't do many start-ups so when I break out my megger I am looking for something unusual.By the way, the motor that was bad had a lot of oil inside as if the bearings were bad.
Repeated over amping and overheating of the windings could break down the insulation enough to show a difference in leakage current between that and the other identical motors. I wouldn't have thought of testing for bad bearings like that, but in hindsight the idea seems to have some merit despite its lack of conclusiveness. Thanks for sharing that.
I wasn't looking for bad bearings, that was just something that I did notice about the motor.As far as being conclusive the unit is working fine ,that's good enough for me.There are times when you're good and other times when you're lucky.I bought my megger at a pawn shop for around $40.00,but in this case I wouldn't take a gold quarter for it.When I started out I debated whether or not to get one but I couldn't pass this up.There haven't been many times that I have had to resort to this but I don't like to borrow things and it got the job done.Hope you have a great week!
if the motors are 460v they have a higher rate of going nuke then 208V ones.might want to take a megger reading on a new one just to see if the others change over a season or on spring start up after a blizzard or two melts into the windings and those top bearings..
"when in doubt...jump it out" http://www.youtube.com/watch?v=U1qEZHhJubY