Unbalanced motor 1

[Jk1996]

How out of balanced can a motor be I tested a brand new motor which read I 34.4ohms on two of the windings and 38ohms on the third. This was installed by someone else that didn’t check the motor. The motor is not tripping the overload but how bad is this?

 

[waross]

Not bad. Check the voltage balance and the current balance. Balance issues almost always originate with supply unbalances rather than motor problem.
On AC, the effective impedance is more important than the resistance. The inductive reactance is many times greater than the resistance and given the quadrature relationship between resistance and inductive reactance the difference to the impedance will be negligible.
An estimate for small voltage unbalances is the square rule.
The current unbalance is the square of the voltage unbalance.
eg: 2% voltage unbalance = 4% current unbalance. 4% voltage unbalance = 16% current unbalance.
This is not accurate and is not scientific.
It is more useful to illustrate than a small voltage unbalance will cause a much greater current unbalance.

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Ohm's law
Not just a good idea;
It's the LAW!

 

[electricpete]

more than 10% resistive variation? That sounds a little high to me.

I wouldn't necessarily be worried about it causing unbalanced current, I'd more wonder what exactly is causing that high resistive unbalance. For example maybe there's a high resistance internal connection in that one high reading. Or far less likely shorted turns causing one coil to have low resistance which shows up in two different phase to phase readings if motor is wound wye (unlikely because typically shorted turns cause escalating damage leading to trip pretty darned quick)

For a critical motor in an industrial plant (one whose failure could have big consequences), I'd want to investigate further to understand. Was the motor done directly at the motor terminals (good) or was it done through cables which can add imbalance. Was the measurement done immediately after an insulation resistance test? (sometimes high residual voltage on the winding capacitance can interfere with low voltage measurements). Is the meter sufficient precision and are the results repeatable. Are there previous records of resistive balance test and was the imbalance present then? Are there any other test results available or that can be easily done to help evaluate motor condition. If I could not get a higher comfort level from looking into these questions for a critical motor, then I'd probably ask to have it replaced.

But if motor is not particularly critical and it's running fine now... then let it ride.

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(2B)+(2B)' ?

 

[edison123]

34 to 38 ohms? That's more of a heater than a motor. Are your measurements right?

Do a Baker surge comparison test to check for turn shorts, wrong wound coils etc.

Muthu

http://www.edison.co.in

 

[waross]

Are you sure that you are reading Ohms and not megOhms?
If that is really Ohms, the motor is so small that it will be quicker and cheaper to change the motor than to even waste time talking about it.

--------------------
Ohm's law
Not just a good idea;
It's the LAW!

 

[zlatkodo]

If 34.4 and 38 ohms are the resistances of the phase windings, it is about very small (fractional HP) motor and difference is too high.
Or did you measure something else?

ACWindings

 

[Jk1996]

It’s a 0.55kw motor

 

[waross]

One of the leads may be dirty.

--------------------
Ohm's law
Not just a good idea;
It's the LAW!

 

[zlatkodo]

I assume the motor is running in a Wye (Y) connection.
Try to measure all 3 currents at full load (not with cheap clamp-meter).
If the values are less than the rated current there will be no problem.
It is very likely that the currents will be almost identical.

Follow us

 

[waross]

If the currents are different, roll the leads and see if the odd current stays with the same incoming phase or follows the motor lead.

--------------------
Ohm's law
Not just a good idea;
It's the LAW!

 

[electricpete]

The question in my mind is whether the resistive unbalance is unexpected… a condition that leads us to question the quality of the motor, for example high resistance internal connections which can be a reliability concern irrespective of whether or not they cause unbalanced current during operation.

I looked at EPRI 3002008052 – low voltage motor repair/rewind spec. The scope (section 1.2) is random wound ac squirrel cage induction motors up to 600v (there is no exclusion for small motors in this spec although I’ll bet it is typically used for larger motors since motors as small as op's 0.55kw aren’t often rewound). Section 9.2.2.13 gives a spec for post-rewind winding resistance balance: 2.0% for lap windings and 3% for concentric windings. They don’t define unbalance, but a reasonable / generous interpretation is max deviation from average, expressed as a fraction of average. For op’s motor, average(34.4, 34.4, 38) = 35.6. And unbalance = (max-avg)/avg = (38-35.6)/35.6 = 6.7%. So the unbalance in op’s motor is more than twice as high as allowed by EPRI rewind spec.

I have no doubt this would be an anomaly of concern (worthy of investigation) for a larger motor let’s say 50hp. For op’s fractional motor it still seems a concern by the spec I referenced, whose scope does not exclude small motors. I guess it's possible that the spec is intended to apply only to larger motors and they just forgot to mention it in their scope and larger unbalances are expected/tolerable on small motors, I can't say for sure. I still land the same place, if it’s critical than investigate further (some suggestions above), if not and it's running fine, then let it ride.


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(2B)+(2B)' ?

 

[Gr8blu]

electricpete: There is no difference between a small (under 200 HP), medium (200-1000 HP), or large (1000+ HP) machine. The allowable resistive unbalance limit is the same, for much the same reason. An unbalance in the winding circuit resistance results in unbalanced current flow (for the same source voltage level), which in turn yields unbalanced thermal stress on portions of the winding and eventually premature winding failure. How quickly the failure occurs is directly proportional to the unbalance value: a higher resistive unbalance mean higher current unbalance, means faster time to failure.

The limit - coming out of a reputable OEM or repair facility - is a maximum 2 (or 3, for concentric/random windings) percent resistive unbalance. The assumption built into that limit is there is likely to also be some reactive unbalance, which would tend to further modify the current through that circuit to where the current unbalance becomes something closer to 3-4%. That being said, how you defined the unbalance (a percentage difference from the arithmetic mean) is correct.

JK1996: I did not see where your measurements were taken. As with all machines, a resistive or impedance measurement should be taken as close to the machine winding terminations as practicable to limit the effect of external cable routing and connectors. If you have a significant (longer than perhaps 10 feet) of lead external to the motor, the impedance of the lead can become an appreciable factor in the circuit measurement. Likewise, the connectors used also become part of the circuit, yielding additional potential trouble spots (e.g. high resistance joints, poorly-crimped lugs/contacts, etc.).

Bottom line - two readings of 34.4 ohms and one of 38.0 ohms is a cause for concern when measuring actual circuit resistance (or impedance) in ohms, regardless whether the winding is connected as a delta or wye. The same readings (in M-ohms) for INSULATION RESISTANCE between a given phase circuit and ground is NOT cause for alarm, although having all measurements under 100 M-ohms on an ostensibly "new" winding raises some questions on the reliability of the OEM and/or the transportation company and/or the storage facility where the motor was stored before you received it.

Converting energy to motion for more than half a century

 

[electricpete]

The allowable resistive unbalance limit is the same

Thanks for your input.
For me it's easier to find specs, trend data and other info for large motors, I don't have as much available for tiny motors, and it may be a different world in terms of what is normal for all I know. The concentric winding is an example of a design feature common in small motors that leads to higher resistive imbalance. I don't see concentric windings in the large induction motors I work with.

... , for much the same reason. An unbalance in the winding circuit resistance results in unbalanced current flow (for the same source voltage level), which in turn yields unbalanced thermal stress on portions of the winding and eventually premature winding failure. How quickly the failure occurs is directly proportional to the unbalance value: a higher resistive unbalance mean higher current unbalance, means faster time to failure.

I'll have to disagree with you on that. Sure, a high resistance internal connection can cause a current unbalance, but that doesn't mean the motor is necessarily going to fail from the heating caused by unbalanced current rather than failing from the high resistance connection burning open. As I said above "... high resistance internal connections which can be a reliability concern irrespective of whether or not they cause unbalanced current during operation."

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(2B)+(2B)' ?

 

[LionelHutz]

The only thing that really matters is the current imbalance in conjunction with the motor load. The closer to rated load, the less the current imbalance can be. I don't know of any direct specification for allowable current imbalances. NEMA MG-1 has details about voltage imbalances and derating. It allows up to about a 0.75% voltage imbalance without derating. Then, it says that voltage imbalance can cause a current imbalance that is 6-10x higher. So, a 0.75% voltage imbalance equates to a 4.5% to 7.5% current imbalance, again this is what is allowed without derating the motor.

What's the above mean? Not too much really. Just set the overload properly and quit worrying about it.

 

[electricpete]

The only thing that really matters is the current imbalance in conjunction with the motor load.

If that is response to me, I believe you're missing my point. It depends what's causing the current unbalance. If you have a high resistance connection inside the motor, that connection can overheat and burn open, irrespective of the level of current unbalance and the motor's ability to withstand that current unbalance.

Let's say we postulate a high-resistance internal connection right where the T-leads attach to the winding. It is effectively in series with the winding, indistinguishably electrically from an external high resistance connection. You guys aren't saying that this scenario would always lead to motor failing by thermal effects from the resulting current unbalance before the connection burns open... are you? Because if you are saying that, then I'll show you a high-resistance motor lug that burned open during service on a motor that was healthy both before and afterwards, and I'll ask you why the motor winding didn't go first (why didn't the motor winding act like a fuse to protect the lug!).

I have great respect for both of you guys... Lionel the long-time eng-tips power guru and Gr8blu the new guy here who already has given a lot of great input on this forum (I've learned some things). I think we're just looking at it in different ways....

You're asking what amount of resistive unbalance will cause a current unbalance that the motor cannot tolerate. Sure that's a question we can ask but it's not the only question. Another key question we can ask is what's causing that resistive unbalance (some causes are more concern than others). But we don't know the answer to what's causing it without inspecting it closer, so we fall back on a related but easier-to-answer question "is this resistance imbalance within normal variability for this type of motor". If it's within normal variability we can attribute it to normal manufacturing variations (T-lead lengths differences, endwinding pole jumper configurations, coil endwinding length variations, maybe minor internal connection resistance variations, measurement accuracy), but if its outside of normal variability then we have to consider other explanations (high resistance connection, or maybe coil miswiring). We can see the evidence of that approach in the EPRI document that I quoted 13 Sep 21 14:13... they have higher resistance unbalance limit for concentric wound motors than for lap wound wound motors. What is the basis for that difference? It's NOT because concentric wound motors can tolerate thermal effects from current unbalance due to resistive imbalance better! It's because the endwinding geometry of concentric windings can create an additional phase resistive unbalance higher than lap wound motors. In other words they're using the approach of checking whether the measurement falls outside the range of at normal variability. And if it falls outside the range of normal variability, then it raises the question of something unusual going on inside the motor like possibly a high resistance internal connection. So to recap, the standard approach used by the EPRI specification quoted above for evaluating resistive unbalances in induction motors is NOT based on the resulting current unbalance and the motor's ability to tolerate it.... It's based on deviation from normal expected variability.

Again there are lots of ways to look at it. Maybe you don't think postulating a particular condition like high resistance connection inside the motor is fair.... I wouldn't necessarily disagree, I'd say that's a whole 'nother discussion to be had. As we all know it depends upon the criticality of the motor, the history of the motor (was it just rewound, do we trust those rewinders, how long has the motor survived already with this resistive unbalance, is the unbalance trending up, etc)...



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(2B)+(2B)' ?

 

[LionelHutz]

No, the response was not towards you. Yes, you are correct that if there is a bad internal the motor will shortly fail. But, it's a 0.55kW motor so it's not worth pulling apart to look at, so use it and warranty it if it fails.