Which type of voltmeter is best for testing fields in a 3 phase motor?

[cosmopedro]

I have a WEG 5HP 3PH 200V 3440RPM motor I cannot test run because I don’t have 3PH power available, and was hoping to get some feedback on how to test the fields so I can decide whether to buy a VFD and keep it in service on my compressor or buy a single phase motor...



I have a digital multimeter and an old analog multimeter, and an electrician co-worker said to measure across the three pairs of wires as indicated in the photo (T1-T6/T2-T4/T3-T5) and should get very low - and even - resistance on each...

Problem is, T1-T6 aren’t both W,U or V... same obviously goes for other two pairs.

Help?


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[captaindiode]

I believe you need to measure between L1 & L2, L2 & L3 , and L1 & L3. I would use the digital on the lowest ohms scale, but the analog will work as well, if you know how to read the scales. Low ohms would indicate the windings are not open. Infinite ohms, a "1" or "OL", on the digital meter would indicate an open winding. Touch the leads on the mter together first and be sure it goes to zero.

 

[kschauwe]

 

[wyliesdiesels]

Testing motor leads with an ohm meter will NOT tell you if the insulation is breaking down which can cause a ground fault when turned on.

Only way to determine this is by energizing the leads.

So the best way to test motor windings is by megging them.

https://forums.mikeholt.com/threads/meggering-a-motor-phase-to-phase.92310/

this is the megger I have which is expensive for one use DIY scenario. There are some cheaper meggers out there though....

https://www.amazon.com/dp/B017OVC2UI/?tag=atomicindus08-20

 

[cosmopedro]

Can you please explain diagram? Not much on understanding motor wiring... can use multimeter once I know what to test and what to look for!


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[cvairwerks]

For compressor use, you are almost at parity between buying a correctly sized VFD and replacing the motor with a single phase unit.

 

[cosmopedro]

For compressor use, you are almost at parity between buying a correctly sized VFD and replacing the motor with a single phase unit.

I get that, and would prefer to keep original motor/pulley arrangement, but if the original motor isn’t working then my hand is forced... hence the request for assistance on testing it!


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[American Locomotive]

Just measure across L1-L2, L2-L3, and L1-L3. You should have the same resistance on all three pairs. Then measure L1, L2 and L3 to ground. They should be open circuit.

Not sure what UVW has to do with anything. Those are just labels, just the same as L1, L2, L3.

 

[Norcal]

That is a 200V motor intended for use on a 208V supply it’s going to be better to buy a proper single phase motor then screw around with a VFD.

 

[cosmopedro]

That is a 200V motor intended for use on a 208V supply it’s going to be better to buy a proper single phase motor then screw around with a VFD.

OK - but isn’t part of the beauty of a VFD is it’s ability to drive these strange (to residential electric) combos, like using single phase 220V to drive 3 phase 200V?

Not challenging anyone, I just thought that was kind of the point...


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[Norcal]

OK - but isn’t part of the beauty of a VFD is it’s ability to drive these strange (to residential electric) combos, like using single phase 220V to drive 3 phase 200V?

Not challenging anyone, I just thought that was kind of the point...


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220V is a obsolete voltage but if it did exist it would be in the upper range allowed for a 200V motor but a 10 -15 HP VFD would be required because of derating and code requires the circuit for the drive to be sized at 125% of the rated input of the drive, after 3 HP most VFD’s are 3 phase input & some allow the phase loss protection to be disabled & some do not plus at least 50% derate or more is required making the economics doubtful over just buying a new motor.

 

[cvairwerks]

A VFD that will run that motor on 220/240 single phase needs a 60 amp circuit and runs over $1000, even for an inexpensive one. A 5hp Leeson single phase motor is about $450 retail....

 

[wyliesdiesels]

OK - but isn’t part of the beauty of a VFD is it’s ability to drive these strange (to residential electric) combos, like using single phase 220V to drive 3 phase 200V?

Not challenging anyone, I just thought that was kind of the point...


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sure if you have a fat wallet and like wasting money

 

[Robbie B]

I have a WEG 5HP 3PH 200V 3440RPM motor I cannot test run because I don’t have 3PH power available, and was hoping to get some feedback on how to test the fields so I can decide whether to buy a VFD and keep it in service on my compressor or buy a single phase motor...



I have a digital multimeter and an old analog multimeter, and an electrician co-worker said to measure across the three pairs of wires as indicated in the photo (T1-T6/T2-T4/T3-T5) and should get very low - and even - resistance on each...

Problem is, T1-T6 aren’t both W,U or V... same obviously goes for other two pairs.

Help?


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You want to test t-1 to t-6, t-2 to t-4 and then t3-to t5. Should have some resistance but very low, that tests the individual windings separately. Then test L-1 to L-2, L-1 to L-3, then L-2 to L-3. I’m assuming then W,U and V are the way it’s labeled if the wires are already connected internally but I’m not sure where that is coming from if you have 3 lines coming out of the motor instead of 6 then you’ll just test all three to each other. And as mentioned above this isn’t the best way to test a motor because it can test good here and break down under load. You’d want a megger to test that but most people don’t just have those laying around. And you still have A 200v motor that is a good brand but obsolete. You can just go get a good 5hp, 240v motor at the correct rpm and run it. It’s all depending on how important it is to you to keep it original. I’d just pop a new motor in and roll with it.


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[alfredeneuman]

A VFD that will run that motor on 220/240 single phase needs a 60 amp circuit and runs over $1000, even for an inexpensive one. A 5hp Leeson single phase motor is about $450 retail....

https://www.driveswarehouse.com/ode-3-320153-1042
$499

 

[wyliesdiesels]

https://www.driveswarehouse.com/ode-3-320153-1042
$499

have to derate the drive so that wouldnt work. See norcals comment below

220V is a obsolete voltage but if it did exist it would be in the upper range allowed for a 200V motor but a 10 -15 HP VFD would be required because of derating and code requires the circuit for the drive to be sized at 125% of the rated input of the drive, after 3 HP most VFD’s are 3 phase input & some allow the phase loss protection to be disabled & some do not plus at least 50% derate or more is required making the economics doubtful over just buying a new motor.

 

[Norcal]

According to the link it is a single phase input drive, still think a new motor is the most economical choice though.

 

[alfredeneuman]

According to the link it is a single phase input drive

Which is already rated for the HP

 

[American Locomotive]

Basically it's going to be a wash.

You could either spend $500 on a new budget replacement motor, or $500 on a new budget single-phase input drive.

 

[cosmopedro]

I appreciate everyone’s inputs on this, and even though I sourced a 10HP VFD that claimed to be able to run it it seems to me that running a single phase 240V 5HP motor would use less energy than an oversized VFD running a 3-phase motor at the lower limits of the VFD’s output voltage range...

So I’m getting a new motor and H-bushing do I can re-use the old pulley - and while I’m at it I’ll get a frame-conversion plate so it’s still bolt-up. Plus, w/the mounting plate I’ll get the benefit of an adjustment system for belt tension!

Thanks again, everyone! Truly appreciate it!!


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[American Locomotive]

The efficiency difference would be negligible either way, IMO.

Why are you getting a frame adapter and a pulley bushing? Just buy another 5HP 3500 RPM 184T Frame motor and it will drop right in...

 

[cosmopedro]

The efficiency difference would be negligible either way, IMO.

Why are you getting a frame adapter and a pulley bushing? Just buy another 5HP 3500 RPM 184T Frame motor and it will drop right in...

Because...

https://compressor-source.com/produ...otor-56hz-frame-3440-rpm-single-phase-weg-new @ $219.95

+

https://compressor-source.com/produ...adjustable-slide-plate-universal-56c-mounting @ $19.95

<

https://compressor-source.com/produ...motor-w182-4y-frame-3465-rpm-single-phase-weg @ $299.95

And I get belt tension adjustment to boot.



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[cosmopedro]

The efficiency difference would be negligible either way, IMO.

Why are you getting a frame adapter and a pulley bushing? Just buy another 5HP 3500 RPM 184T Frame motor and it will drop right in...

HOWEVER... if y’all know of a 5HP 3500 RPM 184T Frame motor - with a 1-1/8” shaft - for under $259.85 I’m all ears!


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[theoldwizard1]

Most DMM do NOT do a good job of measuring small resistances (<10Ω), however you can still use them to COMPARE resistances of each winding. The should all be close.