Motor No Load Currents 1

[NCTHAI]

Carrying out the no load run of various motors as part of project commissioning. It is noticed that for small motors, the no -load current is near to the full load current. However, as motor rating increases, this becomes smaller as compared to full load current. For mid range motors, noticed that motor no load current is higher than specified by OEM data sheet. See below chart.

KW FLA NLA-Datasheet NLA - Recorded
7.5 13.8 4.5 5.58
18.5 32 9.25 10.8
30 55.8 25 28.8
55 93.5 28 33.6
90 146 43 26.4

All motors are rated for 415V AC, 50 HZ - Power factor of motor was quite low (in the range of 0.28 during no load run against rated power factor of 0.83)
While no load run test terminal voltage was 450 V AC, 50 Hz (Voltage is higher but can not do anything about it).

The question is we cannot explain the relationship between voltage, current and no load power. Up to certain capacity (30 KW), it is varying with voltage. but after this, even at higher voltage, no load current is much lower than specified by OEM.

Can you please help to explain this phenomenon?

Thanks in advance!!

NC

 

[zlatkodo]

Searching for general rules for no-load current is futile.
- Tha ratio NLA / FLA depends on motor design and type of choosen windings and differs from one manufacturer to another (for same power).
- Also, you forgot to notice the number of poles of the tested motors. NLA strongly depends on the number of poles.
The bigger number of poles - the bigger the ratio NLA / FLA.
Why? Because the low-speed winding has a significantly higher differential leakage coefficient then high-speed. Read more on this topic here:

http://winding.wixsite.com/design/blank-c7lq

- Another reason for high NLA could be when the duty cycle of the motor is not S1 (ED100%) but it is some kind of short term cycle.

Despite the fact that the NLA is very important information, the manufacturers are not obliged to place this information on the rating plate.
The only real solution is to adopt new regulations and force the producers to enroll this information on the nameplate. It could raise the quality of maintenance and repair.

BTW, we have developed an extra tool for the calculation of the NLA but only in case when some additional info are known (see the picture below):

Winding Repair and Design

 

[electricpete]

The question is we cannot explain the relationship between voltage, current and no load power. Up to certain capacity (30 KW), it is varying with voltage. but after this, even at higher voltage, no load current is much lower than specified by OEM

I would expect no load current for a given motor to increase as voltage increases, period. This appears to be the case for all your motors not only up to 30 but also for the 55kw. The only one listed that doesn't follow that behavior is the 90kw motor.

Why do I expect no-load current to increase with voltage? Because the no-load current is dominated by the current through the magnetizing branch. If we consider it a constant impedance branch, then current would increase directly proportional to voltage (Inl~V^1). If we consider that the higher voltage may push the core further into saturation, then current should increase even more than proportional with voltage.

But for the 90 kw motor when voltage increased by 450/415, it went completely the other way (the current decreased by 26.4/43). IF voltage were the only variable changing, then the effective impedance must have somehow increased by a factor 450/415 * 43/26.4 ~1.75, when we generally expect effective magnetizing impedance to decrease as voltage increases above nameplate. I can't think of any logical mechanism to explain this. Therefore I think there must be some other variable at work for the 90kw motor besides voltage - perhaps an error or methodology difference between the OEM data or in your test data. One example: measuring only one phase of current in presence of inevitable voltage unbalance... even a small voltage unbalance will cause a significantly larger current imbalance during no-load test. Another example - errors/differences in characterizing the magnitude of a potentially non-sinusoidal (distorted) current waveform. Or something simple like clamp-on did not close all the way.


=====================================
(2B)+(2B)' ?

 

[jraef]

And what is the value of trying to predict the no-load current? Knowing it empirically has value for long range observations/trending purposes, but only at identical voltages. Other than that, it's a waste of time in my opinion. I have never seen two motors that were exactly the same, even two identical motors side by side on the same feed.


"You measure the size of the accomplishment by the obstacles you had to overcome to reach your goals" -- Booker T. Washington

 

[waross]

Motors with a lower power factor may have a higher no-load current, but power factor is not the only factor.
Most motors will show a slight drop in Full Load Amps with a small increase in voltage.
In the case where a motor has significant no-load losses, such as a motor driving an unloaded conveyor belt, the drop in the real component of the current may be enough to override the reactive current and cause a drop in the net current.
More likely is Pete's suggestion that the current was measured on a phase with a slightly different voltage than the other phases.

Bill
--------------------
"Why not the best?"
Jimmy Carter

 

[NCTHAI]

@ZLATKODO - Thanks for sharing the information.

@ELECTRICPETE - Great explanation,Thanks!! I will collect the speed (no of poles) and power factor information and will share it here. I am not sure how OEM has calculated data but motor data sheets were supplied by OEM and we are using those data sheets to compare the test results.

@WAROSS - The currents measures are average of three phase currents. Currents are measured using the MM6 relay - even the average currents are calculated by relay. So i believe they are accurate enough to rely on.

Thank you all for sharing the knowledge and information.

Regards,

NC

 

[iceworm]

Based on one data point from 40 years ago (~1976)jraef, pete and Bill are right on target.

I was involved in acceptance testing for two 460V, 60Hz, 50Hp motors for use on a nuc sub. Vibration, Full load current, N-L current, current imbalance at full load and N-L. The motors didn't pass. Full Load current - high but acceptable. Full load current imbalance - high. N-L current imbalance - high.

I got sent down with direction to check voltage imbalance. As I recall, the voltage was 460V (nominal), 2% imbalance. Had them set the phase voltages dead on (they could adjust voltage by phase - N-L tap changes). Current imbalance went way down, N-L current still high. Had them adjust voltage down. Magnetizing current dropped quickly until the voltage got down to ~420V. We kept going down to 380V and magnetizing current continued to drop just not as fast.

My conclusion - the motors were wound marginally thin on copper and iron - they were getting into saturation at 460V. **

NC - If you really want the N-L currents down, call whoever has the key to the tap-changer and get the voltage down.

**Nope, I have no clue why a vendor would cheap a motor when they are getting probably 10X the fair price for the same grade motor with an industrial nameplate.

iceworm

Harmless flakes working together can unleash an avalanche of destruction

 

[ScottyUK]

"NC - If you really want the N-L currents down, call whoever has the key to the tap-changer and get the voltage down."

A remarkably bad plan! if you do this, all the properly-built motors will draw an increased current as they try to maintain output power with reduced terminal voltage. To a first approximation, and over a small range of voltage, an induction motor behaves as a constant power load. If the motor is a piece of crap like the ones described above then either reject it before delivery if you can, or replace it if you can't. You don't upset an entire distribution system just because a couple of motors weren't built properly.

 

[iceworm]

A remarkably bad plan! ...
You don't upset an entire distribution system ....

From the OP:
Motors are rated at 415V. Plant voltage is 450V

The op plant voltage is right at +10%. You think turning the plant voltage down closer to nominal is a "bad plan"? You think turning down the plant voltage closer to nominal will "upset an entire distribution system"? Completely lost me on these two. The only thing I can think of is perhaps if I had said, "NC - If you really want the N-L currents down, call whoever has the key to the tap-changer and get the voltage down closer to nominal. Although I would be surprised if you had thought I meant anything other than that.

I have no clue if the OP motors are substandard. I have no idea how thin IEC motors are designed/built in general. In this case, thin/heavy makes no difference - they are still being hit by +10% voltage. My suggestion is still to do all he can to get the system voltage closer to nominal.

Aside: The example I gave is one where I happened to be right there while the tests were being done. Way different than just reading an IEEE paper. It was just given as an extreme example of what happens when motors are subject to overvoltage/voltage imbalance - one I got to personally witness. Anecdote does not have anything to do with what I think the OP motors are like.

... all the properly-built motors will draw an increased current as they try to maintain output power with reduced terminal voltage ...

I'm going to assume you meant if the terminal voltage is reduced below nominal - cause otherwise that is a boneheaded statement. And you don't seem boneheaded. However, again - reminding you the OP system voltage is +10%.

14.30.2 Effects of Variation in Voltage on Temperature
With a 10 percent increase or decrease in voltage from that given on the nameplate, the heating at
rated horsepower load may increase. Such operation for extended periods of time may accelerate the
deterioration of the insulation system.

14.30.3 Effect of Variation in Voltage on Power Factor
In a motor of normal characteristics at full rated horsepower load, a 10 percent increase of
voltage above that given on the nameplate would usually result in a decided lowering in power
factor. A 10 percent decrease of voltage below that given on the nameplate would usually give an
increase in power factor.

One might deduce that with the voltage at +10%, the magnetizing current is up, thus, the N-L current is up.
Different issue if the system voltage is low - but it is not

And the worm is stuck (And yes, the voltage is only 8.4% high - not 10%)

Harmless flakes working together can unleash an avalanche of destruction

 

[ScottyUK]

ice...

In the post I replied to you made the unqualified statement "If you really want the N-L currents down, call whoever has the key to the tap-changer and get the voltage down." It is a valid response to a specific set of conditions, but not universal.

Put in the context of an abnormally high voltage then certainly I agree that getting the busbar voltage down to around the 420V mark would make sense, and my comment about the motors may be unfair. IEC motors are generally designed with a lot less margin than a NEMA motor of the same rating. They're also usually (the equivalent of) a frame size smaller and considerably cheaper than the NEMA equivalent.

 

[controlsdude2]

If the NLA current is close to FLA then the motor is undersized. I am not following the other explanations.

 

[iceworm]

In the post I replied to you made the unqualified statement ...
It is a valid response to a specific set of conditions, but not universal ...

So, my post specifically responding to the OP, does not fit universal, unqualified systems?
A. No shit
B. You are really stretching to find reasons why what I said could be taken wrong.
C. Amazingly, my response was intended to be taken in the context of the OP.
d. That this surprises you - surprises me - again.

my comment about the motors may be unfair. ...

No, not unfair. Out of context, demeaning, arrogant maybe. Your two posts have the appearance of intent to demean a new poster. I'm at a loss this is considered useful or sport.

Generally, I have found your posts to be professional, informative, entertaining. So, you could tell me, provided it was without the appearance of equivocation, prevarication, or psuedology, that your intent was not to demean - and I would believe you.

ice

Harmless flakes working together can unleash an avalanche of destruction

 

[iceworm]

If the NLA current is close to FLA then the motor is undersized. I am not following the other explanations.

Disclaimer: Motor design is not an area of my expertise.

Defining "Undersized" is shaft output at FLC is < the load requires.

Short answer - No. That won't make the N-L current go up. But I suspect that isn't what you meant.

For 4 pole standard NEMA Stuff, N-L current is ~33% - 40% of the motor F-L current. Reference: NoLoadCurrentBasics_0205.pdf from EASA. A large part of the N-L current is the magnetizing current. This current is not in phase with the input voltage - the load is inductive.

So what would make the N-L current high? - Generally, the magnetizing current went up. Some causes are:
[ul]
[li]System voltage is high. Just raising the system voltage will increase the magnetizing current - linear with applied voltage until saturation.[/li]
[li]The mfg built the motor on the cheap - as in on the short end of copper and core iron, the iron can get pushed into saturation and the currents rise much faster.[/li]
[li]The MFG designs the motor light on copper and iron because they are constrained by the package size. The buyer is saying, "Make it small or I'm not buying it."[/li]
[li]Smaller motors have a higher percentage (N-L)/(F-L).[/li]
[li]Lower speed motor have a higher percentage (N-L)/(F-L).[/li][/ul]

Reference: MG-1
​

In the case of the OP, I suspect his motors were fine. Although, I don't know what would be a normal percentage for (N-L)/(F-L) for IEC motors. The system voltage was running ~8.4% high.. So, how much of the increase to the N-L current was due to increase voltage causing a linear increase to the magnetizing current (iron not saturated) or, was the iron starting into saturation. I suspect just linear increase, BUT, I don't know and, likely doesn't matter.

For a NEMA motor, it wouldn't matter. It is supposed to run fine at +10%. Of course fine is defined as, runs hot, likely have to be derated to .95, and it isn't going to last as long. (paraphrased from NEMA MG-1)

Check the ESEA reference. Should be able to find it with an internet search. As I recall, it was not one of their locked up papers.

ice


Harmless flakes working together can unleash an avalanche of destruction

 

[waross]

Were the motors check when they were unloaded or uncoupled?

Bill
--------------------
"Why not the best?"
Jimmy Carter