Electric Motor running full amps under no load

[mkbug]

I have a motor (for wood lathe) at the repair shop and the tech is explaining that the motor is pulling a full load of amps while it has no load. The motor is from 1996 Leeson 3 phase. And use a VFD to run it.

They say its old and should be replaced, but it still works, but may stop.
I'm waiting on a quote for a replacement motor to decide my options.

My questions - what would cause it to pull full load? Would you replace it?

 

[strutaeng]

That doesn't sound right. If it is at full load under no load, then when you load it is "overloaded." Perhaps the load is very small that it doesn't matter? Or, you are for sure into the safety factor range. Do you have any overload protection on it?

I was checking amp draw with this air compressor I cobbled up years ago. It is a 7.5 HP single phase. The compressor head has a constant-run unloader valves and when it unloaded, the motor would draw hardly any amps. I can't remember exactly...basically just enough power to keep the momentum of the belt and crank and flywheel going, but around 2-3A, if I recall.

I hardly use it anymore, but will start it on the weekend to make sure it still runs. I'll check amp draw.

Maybe the bearings are starting to go out, causing a lot of friction, but you could tell spinning it by hand. Shorted windings? I don't know much of the internals of electric motors.

 

[930dreamer]

The repair shop should have the amps recorded by each phase during testing 1,2,3 some will very close numbers wise others not. They might just want to sell you a new motor? I did this every day for 6 months at a shop.

 

[wyliesdiesels]

if its running at FLA with no load and you put a load on it the current draw will go up even more and could overheat the windings.

bearings could be going out.

Did you measure the current yourself?

can you post a pic of the nameplate?

does the VFD have overload protection setup?

 

[mm08822]

Is this motor attached to a gearbox with unloaded output shaft or just motor only?

Also check the speed at 60HZ. It should be at nameplate value.

 

[Dingleburry]

Windings shorted or grounded
Bad bearings
Wrong voltage
VFD setup wrong
But he should make sure this is all correct.

Had a 3phase 60hp motor at work that kept tripping breaker immediately
With a normal DMM (fluke 87v) winding ohms were even
All wiring and contactors/disconmects were good, no ground fault
etc etc everything checked out fine.
They brought in a special test instrument dont remember name (wasnt a megger) but apparently it was like 30,000$ or something
That showed a problem.

Found out A balance weight fell off and knicked the windings while it was running
Motor had to be rewound

 

[SGKent]

if it is showing full amperage with no load it likely has a shorted winding. That acts as a load.

 

[mkbug]

if its running at FLA with no load and you put a load on it the current draw will go up even more and could overheat the windings.

bearings could be going out.

Did you measure the current yourself?

can you post a pic of the nameplate?

does the VFD have overload protection setup?

Here is what I have - https://martinturns.com/gallery

thanks to all for the comments and advice.

 

[fitter30]

There is a good possibility your vfd ate the motor. Was your vfd bad? Can you see the winding through the end bells or take one of them off to inspect the windings for dark winding.

 

[930dreamer]

The winding should be checked for a short before anything else is done, if their bad no point going any further. The bell ends should have been removed, rotor out and the three leads checked for short, stator checked for burned areas etc.

 

[TRWham]

There is a good possibility your vfd ate the motor. Was your vfd bad? Can you see the winding through the end bells or take one of them off to inspect the windings for dark winding.

Yeah, VFDs can be hard on motors. We used to have Magnetech build special inverter duty motors for variable speed open drive compressor applications (20 to 50 hp was typical). There are now cataloged motors available that are specified for VFD duty.

 

[walta]

Before I replaced the motor I would program the VFD to supply the motor with the motors min rated voltage 208 volts and set the max current to 105% of the FLA 5.98 only if the VFD starts giving you error codes would I replace the motor.


Walta

 

[wyliesdiesels]

Why would you set the vfd for min rated voltage? You want the motor to run hotter?

 

[Citation]

Wait! Don't assume the motor is bad!

What type of motor is this? Odds are it's an induction motor. Those will show near fill current draw regardless of load. [edit: Correction, smaller motors will show this per a comment I previously found here https://www.quora.com/What-is-the-no-load-current-of-an-induction-motor ]

Not long ago I asked a similar question when trying to understand why my AC fan motor was drawing the same current with and without the fan belt.

Ac induction motors basically have a current draw that lags voltage. So with a pure resistive load like a light bulb the peak current going through the motor aligns with the peak voltage of the AC power going in. With an induction motor things are more complex. The peak current lags the voltage by some phase/angle. This is "power factor".

My description isn't very good but if you are dealing with an induction motor it basically will draw the same current over almost all loads but the phase lag changes which in turn adjusts the true power consumption of the motor. Inconveniently you can't figure out the pet consumption of your motor just by taking voltage*current.

 

[wyliesdiesels]

What is your source for that?

Ive metered motors with and without load and the current draw does change.

 

[rlitman]

What is your source for that?

Ive metered motors with and without load and the current draw does change.

Correct. To the previous poster’s point, the no load current will also not be zero. However if you were to measure watts (with the right meter) you’d see that from full load to no load, watts drop off a lot more than amps. Still, the magnitude of current does vary with load, not merely the phase angle.

 

[Citation]

What is your source for that?

Ive metered motors with and without load and the current draw does change.

My explanation is very simplified and I haven't studied this in detail so I'm sure some of my details are wrong. However, I can say that with my 1/3hp blower motor the difference between no belt and belt was no more than 5% change in current. However when I connected my power meter (a Kill-A-Watt type device that can account for power factor) I clearly saw a change in power consumption from the motor. I do recall finding a forum post that said smaller induction motors see little change in current but motors over a certain size (perhaps 1hp?) started to show some change.

In searching for some graphs on the subject most are motor speed vs current. We want motor torque with the assumption that the motor starts at speed but with little load.
These articles show it a bithttps://www.globalspec.com/reference/10797/179909/chapter-3-ac-and-dc-motors-ac-motors-protection-and-ratings
(graph 3-35)

http://electricalacademia.com/induction-motor/three-phase-induction-motor-performance/

If what I read last time was correct then these are basically larger AC motors and the current change is more significant vs what I saw. These graphs show what looks like 50% current at 0 load.

I edited the original post and added a link to my source from a while back.

 

[MattT]

However, I can say that with my 1/3hp blower motor the difference between no belt and belt was no more than 5% change in current. However when I connected my power meter (a Kill-A-Watt type device that can account for power factor) I clearly saw a change in power consumption from the motor.

So you're talking about a single phase PSC motor? The OPs motor is 3 phase and shouldn't be drawing anywhere near FLA with no load.

 

[SGKent]

a motor like that spins because the magnetic fields are constantly changing making the armature both chase and run from the magnetic field - just like putting two magnets together - they attract or repel. When a winding is shorted, whether one turn or a hundred, that turn siphons off some of the magnetism and acts like a transformer. The problem is that since it is shorted to itself, the shorted turns act just like shorting a transformer - the current goes way up in that winding. That acts as a load on the motor and attempts to slow it. So when one sees a motor iike that pulling way too high of current, it is probably a shorted winding. This isn't a small bathroom fan aluminum rotor motor. A piece of metal filing - the varnish breaking down - many things can cause it to fail. Used to be most motors you could take to a local motor shop who could test and rewind it for you cheaper than a new motor, but today only the most expensive motors get rewound. I remember back in High School in the 1960's that GE used to provide motor kits to the shop classes and we'd compete by building a motor. Each would be judged to make sure it ran, wasn't shorted, the windings were neat and tied properly etc..

 

[Citation]

So you're talking about a single phase PSC motor? The OPs motor is 3 phase and shouldn't be drawing anywhere near FLA with no load.

I was talking in general terms and I don't know the size or specifics of the OPs motor. Since this hadn't been mentioned I pointed it out.

 

[MattT]

I was talking in general terms and I don't know the size or specifics of the OPs motor. Since this hadn't been mentioned I pointed it out.

You were talking in specific, fractional PSC, terms and tried to incorrectly apply them generally. And the OP mentioned 3 phase in his opening post and also posted a link to a picture of the motor nameplate later in the thread.

 

[Citation]

You were talking in specific, fractional PSC, terms and tried to incorrectly apply them generally. And the OP mentioned 3 phase in his opening post and also posted a link to a picture of the motor nameplate later in the thread.

Is this one of those condescending "I'm smarter than you because you were so dumb to post X" sort of posts? Sadly people seem happy to post such things around here.

So to answer your questions, I missed the link to the pictures when I read this on my phone (slow data doesn't help). So the original forum discussion I linked to said motors up to ~1.5kw my show near FLA even at very low loads. That was the case with my furnace blower. The OP's motor may be 3 phase but at 1.5hp it's in the same ~1.5kw range that was originally mentioned in the discussion I found (depending if the 1.5kw is input or output power).

Yes, the generalized graphs I found which I hoped would help others understand the same thing I had to look up, showed about 50% current at 0% load but those graphs are generalized. Since everyone was jumping on the "must be broken" bandwagon I figured it was at least worth verifying that the shop wasn't confused by the same property that initially confused me when I went to service my furnace.

If you notice, when I started my post I didn't claim I was right, I only cautioned that it's worth verifying there is something wrong before spending money to get the motor serviced.

 

[wyliesdiesels]

Apples to oranges

 

[rlitman]

...However, I can say that with my 1/3hp blower motor the difference between no belt and belt was no more than 5% change in current...

These graphs show what looks like 50% current at 0 load...

There's a big difference between 5% and 50%. What sort of meter do you have? Me thinks it's not true RMS.

 

[Citation]

There's a big difference between 5% and 50%. What sort of meter do you have? Me thinks it's not true RMS.

My Fluke 187, my Greenlee DM-820a... Both trms...check. But that reading was taken with a Uni-T 210e. It's also trms. And yes, there is a big difference between 5 and 50%.
Still, it seems my recall was a bit off. Like the forum post suggested, I saw 100% current at zero load
https://www.garagejournal.com/forum/showpost.php?p=7479969&postcount=32
Yes, I'm very familiar with electrical instrumentation both for hobby use and professional work.
When I get my compressor back together I'll repeat these measurements with a 15A, 120V motor and see if the results are similar or closer to the 50% end of the scale.

 

[Citation]

The repair shop should have the amps recorded by each phase during testing 1,2,3 some will very close numbers wise others not. They might just want to sell you a new motor? I did this every day for 6 months at a shop.

I think this is a very important question.
In looking into the question more I think there is a reasonable chance this motor's NLA is closer enough to the FLA that the shop assumed it was bad. Sure, I may be wrong but OP, was the lathe doing something that would make you think the motor was bad or are they suggesting this as preventative?

Here are a few things to consider, earlier I posted a link to a comment that someone made that the NLA on a motor would be close to FLA if the motor was under 1.5kW which this motor is right around.

Now consider this article
https://www.easa.com/sites/files/accreditation_program/NoLoadCurrentBasics_0205.pdf
It notes that several factors govern the expected NLA. The OP's motor is a 6 pole based on the plate RPM. Per the article more poles, more NLA. The article also talks about mfg tolerances etc. Those tolerances are why the NLA is higher on a smaller motor. Gap sizes generally don't scale up as the motor gets bigger. Note that one of the exceptions mentioned in the intro was that NLA/FLA goes up (closer to 100%) as the motor gets smaller.

With all that together it wouldn't surprise me if there is nothing wrong with the motor (assuming the motor wasn't showing signs of problems)