No load current on a motor 1

[rockman7892]

I have a 480V 1800rpm 200hp motor with a full load of 233A and a P.f. of 84.5.

We have recently started these motors unloaded, and I noticed a no-load current of 90A. This seems pretty high to me. I would expect something like 50-60A such as I have seen on other similar motors. I am trying to track down the datasheet but in the mean time am wondering if this sounds high?

Is there a rule of thumb for estimating no-load current on a motor?

 

[electricpete]

If I put the ratio X1/Xm to 0.043, the no-load current changes to 90 amps and the agreement is even far better than above (I noticed I had 2% fractional error on the X1/XM). I will post those results.

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[electricpete]

Here is the new model:
R_NL = 41.19167808 ohms Resistor simulate portion of No-Load losses - connected direct in parallel with the source
R_1 = 0.012288321 ohms Stator Resistance
X_1 = 0.122340455 ohms Stator Reactance
R_2 = 0.012288333 ohms Rotor Resistance refd to stat - Should be ROUGHLY FullLoadSlip* 3*VLN^2/FullLoadWatts
X_2 = 0.122339986 ohms Rotor reactance refd to stator
X_M = 2.845122833 ohms magnetizing reactance

Here is the new performance which matches all targets to within fractional error of 5E-6 or better.
FullLoadAmps = 233.0000919 Amps Target= 233 FractError= 3.9432E-07 weight= 1
FullLoadEff = 0.945530443 none Target= 0.945530445 FractError= -2.14925E-09 weight= 10
FullLoadPF = 0.844999947 none Target= 0.845 FractError= -6.24187E-08 weight= 10
X2overX1 = 0.999996168 none Target= 1 FractError= -3.83158E-06 weight= 0.01
R2overR1 = 1.00000095 none Target= 1 FractError= 9.50268E-07 weight= 0.01
X1overXm = 0.043000061 none Target= 0.043 FractError= 1.41176E-06 weight= 0.1

The high level of agreement to the X2/X1 and R2/R1 thumbrules is always easily attained over a variety of scenario's.... again not critical.... somehwat artificial. The high level of agreement X1/XM is tougher to attain (higher error) and the error was dramatically reduced when we decreased X2/X1 down from 0.05 to X1/XM=0.043.

With X1/XM=0.043 what do we get for? 89.8A NLA (including R_NL) which is exactly what was measured. It is very good agreement on all parameters...certainly makes the model look more exact than it really is. I don't think the model is that good... maybe just more lucky than good. I'll admit I watched the no-load amps number as I nudged the X1/XM and stopped when it got close to 90..... I haven't carefully evaluated whether this is the exact best match or not I'm not sure but it is certainly better than the previoiusly-posted one with X1/XM= 0.05 because it moved the error associated with X1/XM way down.


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[electricpete]

Clarification in bold:

By the way I assumed a full-load slip of 0.01.... it is not a critical assumption because...

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[electricpete]

I feel bad because I "cheated" on the last scenario and stopped varying X1/XM when I got a value of NLA that matched what I was looking for.

So I repeated the scenario posted directly above varying the target X1/XM. (The other 5 constraints remained constant: FLA, FLPF, FLEFF, R2/R1, X2/X1).

The resulting
TX1/Xm SWSFE No-LoadAmps
0.04 1.24083E-13 94.41606374
0.041 1.01816E-13 92.97093277
0.042 1.14655E-13 91.42529287
0.043 5.49642E-13 89.75549295
0.044 1.72738E-12 87.9269906
where
TX1/XM is "target" X1/XM.
SWSFE is sum of weighted squared fractional errors. It describes the degree of "fit" of the model to the specified parameters.

The best fit occurs where the minimum SWSFE occurs which is at target X1/XM=0.41. That corresponds to a calculated no-load amps of 93.

In summary: 93A is the best estimate of no-load amps from this model (and using these thumbrules about target ratio R1/R2=1 and X1/X2=1 with assumed slip = 0.1).

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[electricpete]

typo correction:
"..with assumed slip = 0.01"

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[electricpete]

another similar typo correction in bold:
"The best fit occurs where the minimum SWSFE occurs which is at target X1/XM=0.041"

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[Marke]

The no load current is primarily magnetizing current and while there are no rules, there are trends.
Large high speed motors tend to have lower magnetizing currents than small or low speed motors.
Motors that are water cooled (submersible pumps) typically have a magnetizing current that is significantly higher than air cooled motors.
In addition to the good information in the previous posts, the magnetizing current is dependent on the flux in the iron.
If the motor is operated on a voltage or frequency other than it's design condition, there can be a dramatic change in the magnetizing current.
If these motors are designed and optimized for 440 volt 60Hz and operated on 480 volts 60Hz, then the magnetizing current would be considerably higher. The increase in magnetizing current with supply voltage is not linear!
Likewise operating at a lower frequency without reducing the voltage will increase the magnetizing current.

If you supply voltage is higher than the "test" or "design" voltage, expect a higher no load current due to a higher magnetizing current. Full load current will be lower because the load current will reduce with increasing voltage.

Best regards,
Mark

Mark Empson
L M Photonics Ltd

 

[electricpete]

One slightly strange thing is that specifying the targets does not give a unique excel solver solution (of the parameters R_NL, R_1, X_1 etc). The solver solution depends not only on the targets, but also to a small extent upon the initial starting point (initial values of those parameters) before running solver. So telling you the targets without telling you the resulting solved parameters is not completely precise, and if you repeat the experiment I described above (varying X1/XL) based only on the targets, you may get slightly different results. So the more precise way to communicate the results is to include the calculated parameters

Here is the description of the parameters and results of my final best-fit model that gives no-load amps of 93:

R_NL = 4.13E+01
R_1 = 0.012408
X_1 = 0.112810
R_2 = 0.012408
X_2 = 0.112810
X_M = 2.751469

The performance against specified requirements is as follows:
FullLoadAmps = 232.9999965 Amps target= 233 FE= -1.507E-08 WT= 1 WSFE= 2.27106E-16
FullLoadEff = 0.94553045 none target= 0.945530445 FE= 4.61964E-09 WT= 10 WSFE= 2.13411E-16
FullLoadPF = 0.845000064 none target= 0.845 FE= 7.53792E-08 WT= 10 WSFE= 5.68203E-14
X2overX1 = 0.999998473 none target= 1 FE= -1.52744E-06 WT= 0.01 WSFE= 2.33307E-14
R2overR1 = 1.00000091 none target= 1 FE= 9.09733E-07 WT= 0.01 WSFE= 8.27613E-15
X1overXm = 0.040999985 none target= 0.041 FE= -3.59819E-07 WT= 0.1 WSFE= 1.2947E-14
SWSFE = 1.01815E-13
where
FE = fractional error
WT = weighting factor
WSFE = weighted squared fractional error
SWSFE = sum of weighted squared fractional error.
Speadsheet showing this solution is attached.

I think I'm done now unless anyone has questions.

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[electricpete]

Mark - all the factors you mentioned (speed, submersible, flux density) affect the full load power factor, correct?

The full load power factor was given in the original post.

To my way of thinking, coverting that full load power factor into a no-load estimate does not require knowledge of the speed of the machine. I'm not sure how I would alter my analysis if you told me the speed. (although I could do a better fit if more data was provided such as full load slip and half-load efficiency and power factor).

Am I missing something? How would the speed of the motor influence your conversion of the original post data into an estimate of no-load current?

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[electricpete]

I apologize Mark. I can see now that your comments were very responsive to this part of the original post: "Is there a rule of thumb for estimating no-load current on a motor?"

rockman, FWIW, here are my general estimates for ratio of NLA/FLA for large motors:
Poles NLA/FLA
2 0.20
4 0.25
6 0.30
8 0.32
22 0.50
This would not include submersibles.



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[sraza]

please see its bearings are fine or not. because if its bearings are giving more friction to rotor then your motor draws more current, another factor is that the windings are when old the draw more current because of winding losses.

 

[zlatkodo]

Marke wrote:
“The no load current is primarily magnetizing current and while there are no rules, there are trends.
Large high speed motors tend to have lower magnetizing currents than small or low speed motors.
In addition to the good information in the previous posts, the magnetizing current is dependent on the flux in the iron.“

I think its true. If we are talking about two similar large motors that are properly connected to the proper voltage and frequency, and have different no-load currents, we can ask: why?
One motor can have a no-load current 30% of the nominal current and the other 60%.
That's because the manufacturer wants to make a motor with greater power than the iron core allows. The reason is commercial in nature.

How can this be achieved?
Manufacturer enters the calculation with the maximum allowed (sometimes even higher) values for the air-gap flux density, which can cause excessive flux density in teeth and/or in back-iron.
Then tries to compensate this by increasing the insulation class, with increased ventilation,( high efficiency) etc, but sometimes these attempts are not entirely successful.
If we make the calculation of turns / coil depending on the dimensions of stator-iron for this motor with common values for the air-gap flux density, I am sure that the result would be higher (less power) , than the number of turn / coil which we have already in the this motor. This means that the manufacturer tried to get more power than we would normally expect. Often, such a motor is not designed for continuous operation at full load.

That's why I think it is desirable to keep the no-load current in normal limits.

A rough but satisfactory estimate, for large motors gave Electricpete:
“here are my general estimates for ratio of NLA/FLA for large motors:
Poles NLA/FLA
2 0.20
4 0.25
6 0.30
8 0.32
22 0.50”
Zlatkodo

 

[electricpete]

I just realized, a few years ago we changed out motor with the exact same horsepower rating (200), voltage rating (460) and speed (1800 rpm) as the one in the original post.

I went and dug up that documentation and attached it.

Page 5 is motor summary data
Page 7 includes current, pf, efficiency curves
page 8 is nameplate photo. By the way, ours in an ODP motor.

Page 5 and 7 allow comparision to your motor This motor of ours has FLA = 235A, FL PF = 0.84.... so it continues to look very similar to the motor in the original post (which had FLA=233A, PF = 0.845).

But Page 7 also plots current all the way down to zero load... from which we can read a no-load current of around 60A! (I have to admit I was surprised it was that low).

A small summary of results of review so far:
* considering constant vars (neglecting leakage reactance), we predicted no-load current of 233*SQRT(1-0.845^2)=125A
* my model-fitting excercize with some rather arbitrary thumbrules came up with no load amps of 93A. The ratio of leakage to magnetizing reactance was not too far from similar size example motors shown in Krause' motor analysis book. But NLA changes pretty dramatically with small changes in that ratio.
* a very similar motor in the attachment had no-load current of 60A
* the thumbrule 25% FLA would predict 58A.

Some conclusions we might draw:
* This shows that neglecting leakage reactance (assuming constant vars from full load to no load) gives a high estimate of NLA... up to a factor of 2 (125/60) too high.
* It makes me wonder a little more how realistic my 93A was... either it was unrealistically high, or else there can there be substantial variability in NLA (93/60>150%) among motors that have the same FLA and same full load power factor.

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[electricpete]

By the way, for me it is an academic excerize to try to figure out the no load current. I would NEVER be inclined to condemn a motor based on NLA magnitude unless I knew for a fact it was way out of line.... which we don't... the only valid comparision would be manufacturer data or previous meausrement of same/identical motor at same voltage. Also, there are certainly not many faults that would cause the no-load current to change (in absence of abnromal voltage) without affecting other parameters or tripping the motor. And there are more relevant parameters to look at (current balance under load, off-line testing etc).

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[electricpete]

If you do find the motor is drawing unusual no-load amps compared to OEM data, then you might check the connections (in addition to voltage). But again, I am not used to investigating no-load current magnitude.

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[Skogsgurra]

I thought it could be interesting to compare Pete's data with a RL data sheet. But you have to be careful when doing so. Have a look at this sheet

http://motor.hyosung.co.kr/lsp/LSP5447.pdf

I am not so sure if I would buy an induction motor from Mr. KIM or Mr. LEE

Gunnar Englund

http://www.gke.org

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[ScottyUK]

Very flat torque curve, efficiency is down and PF is low. Slightly unusual for a standard motor.


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[Skogsgurra]

And NLA is 0 A. Also very unusual for an induction motor.

Gunnar Englund

http://www.gke.org

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[ScottyUK]

At zero slip there's zero torque and zero magnetising current... must be a special purpose motor.


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[electricpete]

Reliance allows you to look up motor data by characteristics

http://www.reliance.com/cgi-bin/mtrquery.pl#modelchar

Here are some more 200hp, 1800rpm, 460 vac motors.

http://www.reliance.com/pdf/pdf/aced/W06432-A-A001.pdf

FL PF = 0.883, NLA = 53.5

http://www.reliance.com/pdf/pdf/aced/E08055-D-N003.pdf

FL PF = 0.873, NLA = 64.1A

http://www.reliance.com/pdf/pdf/aced/L1642A-V.pdf

(designed for variable speed)
FL PF = 0.867, NLA = 70.5


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