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?

 

[rbulsara]

Corkey:

Firstly, you are welcome here.

There is no set formula for no-load current of induction motor. For small motor it’s even worse and it’s all over the place, so what appear like different opinions were in fact the experience of various people. There is no one answer to your question and that is why you did not get one answer. Plus you had non-coherent statements in your original post, which were only clarified when questioned, several posts later. We only read what you write; we have no way of knowing other circumstances of the matter.

At end of it all, I would bet there is nothing wrong with your motor. But if you still doubt it, replace it. There is no medicine for suspicion.

No hard feelings here, but onus here was on you to be respectful of the opinions you were getting (even if they were to be wrong, which they were not). Only that the answers were just not what you were hoping for. No opinion was wrong; you just failed to comprehend that. After all, it was you that came in here for assistance and all of who responded only tried to help to best of their ability and they were all good. Do not forget that all of this was for free!!!

 

[Corkey8]

Rbulsara:

Please believe that I certainly do appreciate the advice and respect the hospitality. It started out well before posting here with what I saw as contradictory information. Obviously my experience nil compared to the others here.

For the first time just now you state "There is no set formula for no-load current of induction motor. For small motor it’s even worse and it’s all over the place. . ." That is new news to me. And interesting too that it's all over the place. But why?

For instance, does that mean if I tested two brand new identical motors, at identical voltage, I might get two very different readings for their no-load amp draw? What would explain that?

 

[davidbeach]

Two motors per the same design should give similar results, but there are many different designs. Those different designs, intended to have different performance characteristics for different types of loads, can have very different operating performance.

 

[Corkey8]

Fair enough, but are their really any significant design differences for a general purpose single-phase motor of the type I previously described? What I mean is that for this garden variety motor, could the normal no-load amp draw be all that different from one to another?

I've not seen no-load amps stated for any particular manufacturer's specifc motor of this type. Do you know where I can find that kind of information?

 

[oftenlost]

Motor manufacturing is a highly competetive business. The onus on the engineers is to produce a motor which will deliver rated horsepower to the load at a given rotational speed, start loads within a specified range, not burn up due to inrush, acceleration time or continuous running, be cheap enough to produce and, most importantly, be capable of being manufactured at a profit. The aim is to produce a motor which will do the job, not be so sorry as to tarnish the company name, be cheap enough to manufacture so that the combination of name recognition and price is enough to enable the sale of these units.
At no point in the above was any mention of no load current. No load current values on a given design is often related to limiting inrush. Manufacturers know what these values are and can provide them but there is no specific mention of them and no cataloging of them and no contrasting them against one another. Motors of a specific, off the shelf design will have no load currents very close to the motors manufactured with the same stampings, rotor and design specifications so you will not normally see wide variations in motors of the identical design. However, you can see large differences between different manufacturers offering products that do the same WORK. It is a machine designed to do work and there is not a lot of attention paid to the unit when it is not producing work.
This subject is most often discussed in motors which have been misapplied, have had problems after installation, had the load (pump, fan) repaired improperly. The other scenario is where a blown motor comes into a repair facility, is repaired and on final test, it is noted that the current is higher than expected. At that point, no load current is often the subject of discussion as it might divulge a screw up in the repair. That is the reason that rules of thumb such as 30 to 50 percent of FLA come into play. I have seen motors with unloaded currents between 15% and 120% which performed normally and there is seldom a mention on the motor specification sheet.

 

[Corkey8]

That's quite enlightening. Thanks for the insights about the motor biz. I see what you mean about no load amps being unimportant. You said those percent of FLA rules of thumb may come into play when in situations where a repair may be faulty.

That said, it would seem to me that a shop would not be justified in judging whether a motor is good or bad based solely on the observed no-load amps. I presented the motor and told them of the intermittent breaker trips, that's it. It was bench run no load, and showed 100% of FLA. One guy said that's normal and the motor was good, the other chap said that's much too high and concluded motor was going bad.

 

[jraef]

As a rule, the statements about No Load Amps being lower are true when referring to industrial 3 phase motors, biult to NEMA standards. But when looking at small 1 phase motors, the issues raised by oftenlost are probably the norm rather than the exception. Your "other chap" may have had more experience with larger 3 phase motors, whereas the first one was more experienced with commercial / appliance 1 phase motors.

 

[edison123]

It is mostly the temperature that kills the windings. If your motor temperature on load is within the limits for its class, then it should work fine.

As a rewinder of small and large motors, I have seen wide variation in no-load currents even with the motors from the same manufacturer. I agree with oftenlost that often the clients come with a notion of 1/3rd no-load current and it falls on me to explain them about no-load current.

In single phase motors, the rule seems to be anything goes.

*Why does a man think he outrun a chasing dog when it has twice as many legs ?*

 

[oftenlost]

I fail to see why you are trying to hold your apparatus repair desk sales representative to a higher standard than your doctor who usually has much more formal education, makes a lot more money and screws up often enough to require a very large insurance policy.
You brought your unit in for an opinion which, I would guess, was willingly given and possibly free. You got what you paid for.

 

[rbulsara]

Corkey:

I belive davidbeach and others have adequately answered your last query.