Like to use meggers?

15289 Views 19 Replies 12 Participants Last post by HighWirey, Mar 8, 2008

5volts

Discussion Starter · Mar 5, 2008 (Edited)

I am sorry but this kinda has bothered me so i guess ill just have to vent a little here. It suprises me so much how often electricians don't use a megger or don't really no how a megger works and they end up getting burned. We had an incident recently at our plant where the electricians Suspected their was a bad motor and the vfd was o.k. Well they either did not read the megger properly or they did not use one, maybe just used their digital meter, and ended up changing the motor we did not have one in stock so our plant machinist had to modify a different motor the shaft/key etc.. Turns out their were bad feeders leaving the drive going to the motor pecker head. Hours And Hours of downtime. I here a lot of times that a megger is a motor tester and they tell me they don't work on motors. Well just for some info i tell them a motors windings could read good but the motor could still be bad. A megger is used to take readings for a break down of insulation, period. Sometimes i also ask the electrician when they read a motor with the megger if it read good or not and they give me this confused look. A megger is used to check if the motor windings are shorted to ground, i prefer to use a simpson 260 to check resistance between motor windings to make sure its balanced, not a megger which i see electricians do. A good rule of thumb that i use for a 480 volt motor is for every volt of electricity i need at least 1000 ohms. So that would be at least 480,000 ohms, ruffly half a meg. However i ve experience some vfds don't like low megs and will not drive the motor however you hook it up across the line and it will run. Anyway these are my methods that i go by and if anyone has any input or comments or ideas i would like to here them. I understand depending on the field of electrical, you work would depend on how often you would need to use a megger. Anyway i just see a lot of tenure journeyman who don't no the ins and outs of the megger. A Quote I always use at work is
"Meggers don't lie, electricians lie."

Thanks

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simmo

· #2 · Mar 5, 2008

I for one, will use a megger if a drive is displaying a constant fault with ramp up. If at first an input to the drive has been made and I'm sure the drive is not at fault through any phase failure, then I'll immediately turn my attention to the motor and it's windings.
Meggers are the only true way of defining the status of the windings and their condition. I agree whole heartly with everything you said above.

micromind

· #3 · Mar 5, 2008

I use a megger alot, and not just for motors. I can't even remember the last time I energized a feeder without meggering it first. Much better to find a fault with limited energy.....

waco

· #4 · Mar 6, 2008

Three things. First, meggers create high enough voltage, they can "break down" insulation. Personally, I think they are often misused.

Second, meggers are exceedingly difficult to calibrate (at least they were when I was a calibration technician) and I bet the ones you have access to are not regularly calibrated.

Finally, measuring winding resistance is probably better than nothing, but I doubt it is much use to determine if only some windings are shorted. Few ohmeters are precise enough to show a slight loss of resistance.

Anybody ever use "growlers" to test the condition of field-windings.

5volts

Discussion Starter · #5 · Mar 6, 2008

I use a megger alot, and not just for motors.

Totally agree.

John

· #6 · Mar 6, 2008 (Edited)

waco said:
Three things. First, meggers create high enough voltage, they can "break down" insulation. Personally, I think they are often misused.

Second, meggers are exceedingly difficult to calibrate (at least they were when I was a calibration technician) and I bet the ones you have access to are not regularly calibrated.

Finally, measuring winding resistance is probably better than nothing, but I doubt it is much use to determine if only some windings are shorted. Few ohmeters are precise enough to show a slight loss of resistance.

Anybody ever use "growlers" to test the condition of field-windings.

Growlers are used to test the armature of motor not the stator of a motor. The most common type of motor that would either be universal motors (Think 120 v drill motors) or DC motors.

Testing a motor with a megger and measuring the resistance of the winding is still the best way to determine the condition or a motor.
The megger reading should be high. A measurement of something less than 5 Meg needs to be investigated further.
The measurement of the motor windings should be consistently low and all about the same 1 to 4-7, 2 to 5-8 and 3 to 6-9

gilbequick

· #7 · Mar 6, 2008

Isn't one of the best ways to utilize a megger for motors to take consistent readings at given intervals (say, 6 months) to see the trend in the megger readings? Once you see a drastic decline you know something is going to have to be serviced in the near future. (Or so from what I've read.)

John

· #8 · Mar 6, 2008

gilbequick said:
Isn't one of the best ways to utilize a megger for motors to take consistent readings at given intervals (say, 6 months) to see the trend in the megger readings? Once you see a drastic decline you know something is going to have to be serviced in the near future. (Or so from what I've read.)

Yes it would be a good practice to check motors on a regular basis. VFD controls are very hard on motors and have a tendency to break down the insulation on the motor windings and just generally destroy a motor. I assume that the OP works in a manufacturing plant and a well run maintenance program should include preventive, proactive and predictive equipment maintenance practices. But in the real world sometimes this does not happen and most of the work is reactive. It also would help a lot if the maintenance personnel were properly trained.:wallbash:

I do service calls all the time and all I get is “The Motor Don’t Work “ or “The Machine Don’t Work”, so come and fix it. PITA if they don’t take care of their machines.:no:

simmo

· #9 · Mar 6, 2008

John said:
I do service calls all the time and all I get is “The Motor Don’t Work “ or “The Machine Don’t Work”, so come and fix it. PITA if they don’t take care of their machines.:no:

And you would find in even very well maintained plants, that more often than not - Accountants run Maintenance Departments and not the actual Maintenance personnel.

Hence, he who holds the purse strings the tightest wins

5volts

Discussion Starter · #10 · Mar 6, 2008

you would find in even very well maintained plants, that more often than not - Accountants run Maintenance Departments and not the actual Maintenance personnel.

Basically production calls the shots in our plant. We run till fail. However when the line goes down we the electricians are under the gun to get it going asap. Thats why it is very important that each electrician is properly trained or will ask for help if he is not sure. This could minimize down time. The company does not care how much money it cost in parts they just want production up and running. I don't beleive in "shot gun troubleshooting", there is no satisfaction in that. I like to know when i replace that overload block, motor, vfd, wires, we are going to be up in running no doubt. We are a 24/7 operation and the production goes down seldomnly for cleaning. I do agree a well run maintenance program should include preventive, proactive and predictive equipment maintenance practices, however in a large plant run by production this is not realistic. We do have state of the art automation controls and machinery but poor pm planning, its just a political corporate score card. They want scheduled work and electricians to charge time to these work orders so production can get their year end bonus.

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simmo

· #11 · Mar 7, 2008

Much the same with the company I'm with p logix,
Production takes priority, but they will only spend when they have to. We have a maintenance program that falls way short of what it's capable of, and we have far too many Chiefs and not enough Indians to make the whole organisation work effectively. Our maintenance dept off load most of there costs onto the production cost centres where they can.

Back on subject - I wonder how many electricians still use the wind up meggers or know how to use them, I still have one, but rarely use it. My choice of megger is a digital type, and has the facility to use an RS 232 connection to download results back to a PC. The results can then be held in a tablet window associated with the plants mimic board of all the drives on our control system for cranes and conveyors, and allows us to see which drives and motors have good or not so good motors, by simply pulling up the window of that particular drive.
I love the Siemens automation systems, they have incorperated so many extras which is so handy in fault finding and diagnosis of automated plants.

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brian john

· #12 · Mar 7, 2008 (Edited)

Three things. First, meggers create high enough voltage, they can "break down" insulation. Personally, I think they are often misused.

Hence why it is called non destructive testing???????

Second, because some electricians do not understand how to properly perform a beneficial test we should not perform this test.

Second, meggers are exceedingly difficult to calibrate (at least they were when I was a calibration technician) and I bet the ones you have access to are not regularly calibrated.

Calibrated to NIST standards yearly, even if out of cal STILL BETTER than a continuity tester.

Finally, measuring winding resistance is probably better than nothing, but I doubt it is much use to determine if only some windings are shorted. Few ohmmeters are precise enough to show a slight loss of resistance.

Meggers In my expierence are not utilized to megger windings to determine resistance of the windings, the purpose of meggering motors to ground is 1. Track resistance deterioration. 2. To locate shorts to ground on windings.

5volts

Discussion Starter · #13 · Mar 7, 2008 (Edited)

Meggers In my expierence are not utilized to megger windings to determine resistance of the windings

I agree, In a AC motor the resistance between the windings is going to be very low, as long as were balanced were depending on the inductive reactance to limit current flow.A megger is used to detect shorts from the windings to ground and insulation breakdown. We do not calibrate our meggers yearly however it works great for the AC 480 Volt wye motors we check regularly and wiring. For the work i do No need to be that precise you just have to know how to use one. I use both the crank handle greenlee, push button analog greenlee, and digital greenlee which i prefer to use the least because of being around so much inductance. By the way where i work i have no use for using a growler.

John Valdes

· #14 · Mar 7, 2008 (Edited)

Not having a megger in an industrial setting is part of what keeps motor shops in business. Meters are useless for checking windings unless the it is a small motor with higher resistance. Larger motors you must have megger as the resistance is so low you cannot read ohms at all. Now, a dead short can be found with a meter, but a VFD will find the insulation breakdown before anything will find it, including the megger. Even in the motor shop I have seen all their equipment pass a winding test to just have the VFD trip when it is started. Usually this situation (motor) ends back up with the manufacturer.
Another thing I have seen is people megging motors while they are still connected to VFD's and soft starts. It's a sure way to take out both even if the motor is good.
A megger in the right hands is a beautiful thing.

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frenchelectrican

· #15 · Mar 7, 2008

John V.,,

I agree with that part and i think we should set up a topic where we can able get more details in depth with the traning and how to use it in proper way.

Myself the megger do come in handy more than one way and it actally save my neck more than once.

kinda little off track a little but it related to here i know most electrician from UK/ and from other area they have to use megger as part of their work to test the system.

Merci, Marc

brian john

· #16 · Mar 7, 2008

If you are energizing 208/120 or 480/277 distribution gear and not meggering the switchboards and feeders you are putting yourself at risk. Not to mention the added cost of gear replacement. Saying meggers are wrong because you or someone else do not understand their use is WRONG. Educate thyself.

I guarantee a megger saves more equipment and lives that they have ever caused damage to equipment. 600 VAC equipment is designed to be tested at the recommended test voltages.

Thomp

· #17 · Mar 7, 2008 (Edited)

I like using a megger on a wye wound motor. If the internal windings (7,8,9) are shorted or welded together you can't tell with a multi meter, but you can with a megger.

randomkiller

· #18 · Mar 7, 2008

brian john said:
If you are energizing 208/120 or 480/277 distribution gear and not meggering the switchboards and feeders you are putting yourself at risk. Not to mention the added cost of gear replacement. Saying meggers are wrong because you or someone else do not understand their use is WRONG. Educate thyself.

I guarantee a megger saves more equipment and lives that they have ever caused damage to equipment. 600 VAC equipment is designed to be tested at the recommended test voltages.

Well stated, I agree whole heartedly. I won't energize any new components / circuitry we have installed without megging them first. I don't want to be the guy that trips a GFI main because of a short in some new equipment I just flipped on blindly.

5volts

Discussion Starter · #19 · Mar 7, 2008

but a VFD will find the insulation breakdown before anything will find it, including the megger. Even in the motor shop I have seen all their equipment pass a winding test to just have the VFD trip when it is started. Usually this situation (motor) ends back up with the manufacturer.

I agree VFD dont like low meg readings, I have also seen people hook up the megger to the t leads still connected to VFD an POOOF!!!

HighWirey

· #20 · Mar 8, 2008

brian john said:
If you are energizing 208/120 or 480/277 distribution gear and not meggering the switchboards and feeders you are putting yourself at risk. Not to mention the added cost of gear replacement. Saying meggers are wrong because you or someone else do not understand their use is WRONG. Educate thyself.

I guarantee a megger saves more equipment and lives that they have ever caused damage to equipment. 600 VAC equipment is designed to be tested at the recommended test voltages.

Amen. Most experienced electricians would not energize a new feeder, motor or gear without first meggering or hypotting, spec or no spec.

BTW Brian, thanks for linking us to the A-Stitch-In-Time manual (several months ago). Been years since I have seen that pub, but it is nice to freshen-up every so often.

http://www.metercenter.com/biddle/A%...0In Time.pdf

Best Wishes Everyone

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