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Old 07-12-2019, 07:53 AM  
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Default Testing between windings?

Was at a plant a little while ago and someone was testing a motor to see if it was burnt out or needs replacing. What they did though was take a meter and check resistance between the winding leads, like T1 to T2, then T1 to T3 and so on. They said if the resistance if off between them, then it's bad. Most of the readings were something like 7-8 Ohms and one was 4 or 5 and they said it was pooched.
I'm just an apprentice so forgive me, but is this indicative of anything? I thought the only way to test 3 phase motors was
1) checking individual leads to ground
2) voltage between leads
3) ammeter on leads
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Old 07-12-2019, 09:30 AM  
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You can sometimes find what is open or shorted, but usually smoked is smoked when diagnosing. Still changing it out, almost a waste of time.

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Old 07-12-2019, 09:34 AM  
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You can sometimes find what is open or shorted, but usually smoked is smoked when diagnosing. Still changing it out, almost a waste of time.

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Sure but resistance between winding won't tell you this. If it was shorted to ground then between winding would read infinite wouldn't it.
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Old 07-12-2019, 10:43 AM  
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Sure but resistance between winding won't tell you this. If it was shorted to ground then between winding would read infinite wouldn't it.
Think ohms law. The winding's are resistors, and when hooked up for the lower voltage they are hooked up in parallel. So the total resistance of that winding is the value of a resistance divided by two. If one winding opens it goes to the resistance of just one resistor or winding in this case. This is what happens a lot.

Most times as long as the reading are close you are OK for that test. If they do read off and readings were not taken right ate the pecker head open and check connections at pecker head, resistance readings that are off are caused quite often by a bad connection there.

Keep asking questions

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Old 07-12-2019, 11:18 AM  
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Sure but resistance between winding won't tell you this. If it was shorted to ground then between winding would read infinite wouldn't it.
There are multiple readings all depending on the cause. Water in the motor or peckerhead can give you some odd readings.

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Old 07-12-2019, 12:24 PM  
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Measuring resistance across each winding is a quick test on a small motor. The bigger the motor, the harder it is to read ohms. Motor shops sometimes use a high resolution ohm meter that is very accurate.

On big motors measuring resistance will not tell you anything unless the winding is open. There is little resistance in big motor windings. Usually unreadable with a meter.

On small motors measurements across the windings should be the same or very close.
On bigger motors you really need a megger. Or allow the motor shop to check it for you.
A good shop will have a surge tester and everything else needed to determine and repair motors.
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Old 07-12-2019, 01:38 PM  
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When reading winding resistance, you need to know what numbers the windings are......

All 3Ø dual voltage motors have 6 windings and the leads are numbered T1 - T12. If 9 leads are brought out, then T10, T11 and T12 are spliced internally.

If all 12 leads are brought out, the windings are T1 - T4, T2 - T5, T3 - T6, T7 - T10, T8 - T11 and T9 - T12. The resistance on all 6 windings needs to be the same and infinite from any winding to any other winding.

If 9 leads are brought out, it gets tricky because T10, T11 and T12 are spliced internally. The motor can be wound either Y or ∆. The easiest way to tell is look at the 230 volt connection diagram, if it shows T1, T6 and T7 together, it's ∆. If it shows T4, T5 and T6 spliced together, it's a Y. Generally speaking, motors less than about 15HP will be Y. Larger will be ∆ but exceptions abound......

If it's a ∆, the windings are T1 - T4, T1 - T9, T2 - T5, T2 - T7, T3 - T6, T3 - T8. The resistance should be the same on all windings.

If it's Y, then the windings are T1 - T4, T2 - T5, T3 - T6. These should have the same resistance. Next, measure T7 - T8, T8 - T9 and T7 - T9. These readings should be double the other ones because these windings are in series.

Complicated.......yes, it can be......welcome to the wonderful world of motors!

P.S. Using a basic meter can give you a good test but using a megger will give far better results.

Last edited by micromind; 07-12-2019 at 01:42 PM. Reason: Still can't spell worth beans........
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Old 07-12-2019, 01:46 PM  
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In a best case scenario you have 2 identical motors, one known good, where you can make reference measurements on one and compare to the other. Or you have taken these measurements before and you know what they were last time you read them.
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Old 07-12-2019, 03:55 PM  
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Measuring resistance across each winding is a quick test on a small motor. The bigger the motor, the harder it is to read ohms. Motor shops sometimes use a high resolution ohm meter that is very accurate.

On big motors measuring resistance will not tell you anything unless the winding is open. There is little resistance in big motor windings. Usually unreadable with a meter.

On small motors measurements across the windings should be the same or very close.
On bigger motors you really need a megger. Or allow the motor shop to check it for you.
A good shop will have a surge tester and everything else needed to determine and repair motors.
But in this regard, what's "the same or very close." I'm not sure the scale so on an ohm level, 4 is very close to 6 but on kohm, it's not. Also, what the hell is a peckerhead? You didn't mention it but others have and I've never heard the term before.

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Old 07-12-2019, 04:30 PM  
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But in this regard, what's "the same or very close." I'm not sure the scale so on an ohm level, 4 is very close to 6 but on kohm, it's not. Also, what the hell is a peckerhead? You didn't mention it but others have and I've never heard the term before.
Motor termination box. The box on the side of a motor where the supply wires connect to the windings..
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Old 07-12-2019, 05:36 PM  
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Sure but resistance between winding won't tell you this. If it was shorted to ground then between winding would read infinite wouldn't it.
If you measure resistance across the coils, you get resistance across the coils. The ground path is not being measured. Since it is in megaohms and coil resistance is in milliohms, ground resistance does not matter. Even if they are dead shortec therd is only kne path back to the meter on the phase connection. This is why you may get different readings checking phase to ground on the wiring but when testing insulation on a motor, all three phases will be the same. One reading is good enough as long as they're wired together. When in doubt, just short them all together when measuring insulation resistance. Isolate when measuring coil resistance.

Motors have two kinds of insulation. Ground wall insulation prevents connections to ground. It sees the full voltage to ground. Coil insulation insulated individual turns from one another. It is generally much thinner, especially in form wound coils, where voltage is only a few volts turn to turn. If it overheats, coils will short together but at least at the start, still insulated from the motor frame. The connection usually creates a hot spot though and eventually burns out the ground wall insulation too, but it can also short out enough turns to trip the protection system or cause an open circuit without failing the ground wall insulation. Mechanical damage or environmental damage typically damages indiscriminately, even the high voltage ground wall insulation so we first detect it as failed insulation to ground but the coils are intact.

Both need specialized tools. Capacitance masks insulation to ground issues so a high voltage source (500-5,000 V) is used to quickly drain pesky capacitance. That is what a megger does. After temperature correction, 5 megaohms under 1000 V and 100 megaohms above 1000 V is passing for most motors. For coil resistance they are typically in the millliohm range. A standard multimeter won't work. Millliohm or microohm meters are a must. Differences in resistance between coils of even 1-2% indicate a problem. On a standard multimeter with 1-2% as a limit, you can't accurately measure under 10 ohms with a 0.1 ohm accuracy which only works on very small motors. When doing the same test on wiring, starters, breakers, and so on most standards suggest up to 50% differences as an indication of trouble.

The statement about pairs of coils is partly true. Most motors have pairs of poles for goid reason. If it is wired with 2 coils on opposite sides the torque is even so the motor doesn't develop a wobble from the uneven torque. A two pole 3600 RPM motor has 6 sets of coils, wired with two per phase. They can be series or parallel but to minimize voltage across the insulation single voltage motirs are usually series. Dual voltage motors can be series or parallel or possibly wye vs. delta. But we don't need to know this.

Long before a cojl totally burns up it will read more than 1-2% off from the average. This is even more true from transient damage that burns up the first turn or two only. The very thin coil insulation will easily cook.And insulation that is scuffed from a bearing wearing out will probably still read less than 1% coil to coil but the insulation resistance will be far less than 5 megaohms where the ground wall is scuffed. I frequently get readings that are clearly bad (10-25%) on motors that have suffered transient voltage damage but megger very good. This is not even close to 50% of normal readings and easily missed by megger only testing.


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Old 07-13-2019, 12:28 PM  
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Originally Posted by Funkadelicfred View Post
But in this regard, what's "the same or very close." I'm not sure the scale so on an ohm level, 4 is very close to 6 but on kohm, it's not. Also, what the hell is a peckerhead? You didn't mention it but others have and I've never heard the term before.
In your example the difference is big if you are measuring ohms. Big enough to say the motor is bad.
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Old 07-13-2019, 02:58 PM  
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In all honesty history is more important in the field than a cheap meter.

If you find the breaker tripped you have your 5 basic players,

1/motor
2/wire short
3/hardware short
4/breaker
5/controls causing starter to jerk off.

2 and 3 are quick to check with a meter,
1 is easy as long as its toast, smell test also helps
4 and 1 look the same and can be where it becomes complicated if you don't have the proper test equipment sometimes its a wild guess. (helps if you can find a spare breaker nearby for a temporary test, or hook a motor to the disconnect before installing it).
5 will only show when running (sometimes you get a clue by the state of the contractor) but other times its hard to find until it happens.

A damp motor will fail all the tests yet will run if you throw fire to it so knowing when it was last run also helps. What it was doing and where it belongs in the system also help if you have experience.

reading 8ohms, 8ohms, 6ohms using a standard meter may indicate that the motor is the problem but i wouldn't bet my job on that. I would still meg test all the other players before making the call. (mechanics don't forget when they change a motor and it still trips)


forgot to add number 6 which is a motor that will not turn. Sometimes the breaker is faster than the overloads.

Last edited by gpop; 07-13-2019 at 03:02 PM.
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