A.C. Motor Current Draw

[Corkey8]

Can't seem to get a consistent answer on what I believe is a basic question, and would appreciate some feedback. It concerns a used A.C. induction motor of the type used for boat lifts, and drives a v-belt. This motor operates in a sea air environment, and is used infrequently. For an unknown time, it was operated at sub-standard voltage and at times would trip a 20A thermal-type breaker but only after starting and running for a couple minutes under load. The voltage deficiency was an electric utility fault and was repaired. I'm trying to determine to what extent, if any, the motor may have been compromised.

The motor can be wired for either 115V or 230V. It's 3/4 HP, 1725 RPM, capacitor start, 115V@10.8 Amps/230V@5.4 Amps, 60Hz, reversible, SF=?.

On a no-load bench test @115V, the motor draws approx 11AMPS, essentially the labeled rating. Two motor shop guys assert that modern motors draw close to their rated Amps no-load and that doesn't change much under load if the motor is good. The motor in question does draw around the same Amps, load or no-load, with no more than a 1Amp difference. Another motor shop guy asserts that this motor should draw at most 6-7Amps, no-load, and feels the motor is bad. In other words, he says the labeled Amp draw is under-load, while the no-load Amp draw should be much lower.

Who's right?

 

[sreid]

I don't buy the "draws almost full load current at no load." The 6-7 amps no load would be correct.

 

[jraef]

Unless it's a gear motor, especially one with a very high gear ratio. They tend to have very little difference between loaded and unloaded power draw. If it was a winch motor, that is highly likely. If it is, a problem might exist in the gear box as well. Generally they do not draw close to motor FLA like that.

As to the likelihood of your getting a definitive answer on whether or not the motor is "compromised", I highly doubt it. Running a motor at low voltage does increase the heat and excess heat in the windings cuts the motor life because the insulation will break down prematurely. But how much life has been lost is impossible to determine, especially if you have no idea as to the voltage level, run time, duty cycle etc. etc. etc.

 

[Corkey8]

This is not a gear motor or anything special. It's just a garden variety general-purpose 3/4HP with 5/8" shaft that turns 1725RPM.

I wasn't hoping for a definitive answer on the motor's condition. However, the contradictory information I mentioned in my original posting indicates that the motor's likely to be either good or bad.

Even answers I received so far in this forum have the same contradiction. Only one is correct. Jraef's appears the one, but I hope others will confirm.

 

[SteveKW]

I repair these motors all the time and would agree that a standard motor should draw around 7 amps no load.

The only easy answer for you is to reinstall it and run it under load!
If it stays at nameplate amps... Just keep using it!

 

[waross]

I feel that the high amp draw is unusual but not impossible.
As to whether the motor has been compromised: If the motor has been damaged to the point that the no-load amps have doubled, total failure may be expected within an hour or so. Leave it running no-load for several hours and see if it survives.
Alternatively try it on the winch on a calm day. In the event of failure you don't want your boat stuck halfway with the waves picking up.
respectfully

 

[rbulsara]

What does it draw on load?? You said it trips a 20A breaker. Did you ever measure to see that it indeed draws 20A or more underload??

If so either your motor had incorrect nameplate or it may be wired for wrong voltage or it may indeed be bad.

On small motors they draw signinfican currnet close to full load current on no-load but it is mostly reactive current. So when its loaded the net current does not "Appear" to be much different but its power factor is lot higher.

Try to find the right answers based on the facts and not what you want to hear. I get a feeling that you want to hear what you think is right.

 

[Corkey8]

Rbulsara. Thanks for your weighing in. As far as you're questions, I believe you'll find all the info stated in my original posting. Your "feeling" about me is totally and completely wrong -- it's bizarre. My question calls for a pure, matter of fact, answer -- and if you take the time to read my post and even the answers here, you can acquaint yourself with the only reason I asked. I GOT CONTRADICTORY INFORMATION! Got it?

 

[sreid]

I don't know how helpful this is but if the motor is a standard NEMA Frame, a 3/4 HP motor is fairly inexpensive to replace.

 

[oftenlost]

Motor nameplates are statements of a motor's characteristics doing work of some sort. If the motor got abused, there is little doubt that the insulation was compromised in some fashion. However, looking at no load current will not tell you if you have a problem unless you know what the no load current was before the "event". If the motor provides advertised load at the advertised current and does not appear to be burning up, use it.
One note on "high temperature", Do not trust your fingers. I have had customers with motors running for years at a given surface temperature and one person questioning the heat and all convince each other that the motor is running hotter than normal.

 

[Skogsgurra]

Why do we at all care about a motor that should be scrapped anyhow? Is this really an engineering question?

And, Corkey, your attitude is not very productive. We are not an institution. We do as good as we can (and that is quite good, actually), but there's no law saying that we shall give you a perfectly valid answer that also suits you and your purposes.

Gunnar Englund

http://www.gke.org

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100 % recycled posting: Electrons, ideas, finger-tips have been used over and over again...

 

[rbulsara]

Corkey:

Gunnar expressed my views much more effciently. Hopefully you get the point.

More to the point you said motor draws 11 Amps and the diff between load and no load is 1 amp. So max amp may be 12A. So why does a 20A breaker trip on 12A?

If the motor draws about 12A when listed for 10.8A, it may just be fine. Your voltage may not be exactly 115V.

 

[edison123]

Corkey - you said
"The motor in question does draw around the same Amps, load or no-load, with no more than a 1Amp difference."

If load current is 12 amps, why is your 20 amps breaker tripping ? Either you measured the load current wrong or your breaker is not working properly. Check both.

If the load current is more than 20 amps (causing the breaker to trip), then the motor winding is wrongly configured.

*Nothing exists except atoms and empty space; everything else is just an opinion*

 

[Corkey8]

Gunner:
If you will read my original posting, I just asked a question. Then it was suggested I have motives -- which is totally ridiculous. I don't think your attitude is productive either, and I see you didn't venture to offer an answer. I don't care a wit whether you think the motor should be scrapped -- completely irrelevant.

Rbulsara:
Now, on to further clarification. The 20A intermittant breaker trip no longer happens. It stopped after the electric utility company linemen repaired badly corroded street connections that were causing sub-standard line voltage. It appears that the low supplied voltage drove up the amps drawn by the motor enough to trip that breaker. This no longer happens. As you might guess, before the utility problem was determined, a faulty motor was initially suspected. This led to bench testing the motor. It does function now that it gets the proper voltage, but I'm concerned that it could fail at an inopportune time. The reason for the concern is the no-load amp draw. It's a lot of work to change this motor because of its location on a piling out in the water.

In sum, the load and no-load current draw are essentially the same, at approx 11A -- the label rating is 10.8A. Based on that, some say there's no indication of a problem, as that's normal. Others say, the no-load amps should be much less (6-7A), which tells me the motor is on its way out. All I'm trying to understand is which is normal for this motor.

Thanks to all.

 

[Skogsgurra]

I do not usually enter into fruitless and meaningless discussions. But, this is an exception:

Yes, I did read your first question. And I read the answers to it. Quite carefully.

The reason is that you were complaining about contradicting answers. And that is after having read two members' answers (more people hadn't answered at that time).

And, when you started flame-throwing at one member after he said what I was thinking already, although he said it in a somewhat milder way. I felt i had to say it my way.

Now to the motor and the question. Induction motors have a rather strange behaviour. No-load current can actually be higher than full load current. This is because of the power factor being very low when idling and closer to one when loaded. This is described by the Ossanna diagram aka circle diagram. See thread237-126286 for a similar discussion. There are also links to such a diagram.

In conclusion. We pride ourself to be knowledgable, gentle and understanding in these fora. And we mostly are. But we expect others to be at least two of those things, too. In your case, it is only natural that you do not know about the innards of an electric motor. But then, it would have been better if you had shown some of the two other qualities.

I hope that you understand. And you are welcome back with more questions. Any time.

Gunnar Englund

http://www.gke.org

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100 % recycled posting: Electrons, ideas, finger-tips have been used over and over again...

 

[itsmoked]

Corkey8;
A couple of points.

1) You should work on your attitude. You don't like an answer just shut up about it. Continue to ask questions, work for a clarification of your answer until you're satisfied. Don't waste your time sniping back.

2) Why do you wait until your fourth post to tell a large part the story?

3) You should be getting the point that a bunch of experienced professionals can't clearly come to a solid consensus on the specific current(I) situation with the motor. Hence, you are not going to be convincing a power company to replace it. If it smoked during the wiring problem you'd have a pretty good case. Now? It still runs? Never!

4) Your last part of the story is really key. These motors only cost about 200 bucks. If you spend more than that on it you're on the losing end. If it takes a crane, and hazardous effort, a crazy ladder, a perfect tide, etc. ,you get the point. Just replace it.

Good luck.

Keith Cress
Flamin Systems, Inc.-

http://www.flaminsystems.com

 

[Skogsgurra]

"It's a lot of work to change this motor because of its location on a piling out in the water."

But it wasn't more work than you thought that it was worth taking it down for a bench test? "On a no-load bench test @115V, the motor draws approx 11AMPS, essentially the labeled rating."

And put it back again?

Gunnar Englund

http://www.gke.org

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100 % recycled posting: Electrons, ideas, finger-tips have been used over and over again...

 

[Corkey8]

Gunnar:

You stated "Induction motors have a rather strange behaviour. No-load current can actually be higher than full load current. This is because of the power factor being very low when idling and closer to one when loaded." That I think answers my question, along with Rbulsara's reply. Thank you.

As noted in my original posting, two out of three motor techs agreed with you, and that's partly why I didn't replace the motor. At that time, the low voltage utility problem was undiscovered. Not to make the story too complicated, but there are two such motors in the system and it's advisable to replace the pair. And wiring may need to be replaced or spliced in an underwater conduit to use the preferred replacement motor. There's also radio controlled remote control for the system that enters the picture.

Counting all votes at this point, I think the answer is rather clear. I thought I asked a simple question that had an answer that was not a matter of opinion. I guess not, and I got flamed to boot.

 

[Skogsgurra]

As I said, welcome back! Any time.

Gunnar Englund

http://www.gke.org

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100 % recycled posting: Electrons, ideas, finger-tips have been used over and over again...

 

[waross]

Has a megger test been done? It only takes seconds and will probably show up impending failures.
I think that there have been some misunderstandings and some regretable overreactions by more than one party in this thread.
Welcome back any time.