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]

The last linked motor above has equivalent circuit parameters listed, which are in the same ballpark as the ones I had computed. On the left are the values I posted 8 Jan 10 16:58, on the right are the values from

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

R_1 = 0.011574312 ohms 0.0208
R_2 = 0.01157434 ohms 0.0154
X_1 = 0.179466053 ohms 0.148
X_2 = 0.177699823 ohms 0.153
X_M = 3.66270788 ohms 3.75
R_NL = 40.27231006 ohms N/A

We see X1 ~ X2 and X1/XM ~ 0.05. I think we all initially suspected based on the relatively small values of X1 and X2, that the vars consumed in X1 and X2 at full load would be relatively small. But on the contrary, in round numbers, the var consumed at full load by X1+X2 turned out to be approximately equal to the vars consumed in XM (since the reactive current is approx 120A at full load and approx 60A at no-load).

The reason is Q = 0.5*I^2 *X.

While the sum of X1 and X2 is only about 10% of Xm, the current through X1 and X2 at full load is on the order of 3x the current through Xm, so the square of current through the leakage reactances at full load is approx 3^2~10x the square of current through magnetizing reactance. One tenth the reactance times 10 times the [current-squared] gives approximately same Q consumed in leakage reactances at full load as is consumed by the magnetizing reactance (very round numbers... just for illustration of how the small leakage reactances can end up playing a large role).

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[Marke]

To determine the magnetizing current for power factor correction, I calculate as above, but instead of using the full load figures, I use the half load figures. (these are usually given in the data sheet) This reduces the errors due to the leakage reactance.
One can use the full load figures and the half load figures to determine what the leakage reactance is and separate out the magnetizing current only.

I have the impression that the new higher efficiency motors tend to have a higher leakage reactance than the older motors, but could be wrong there.

Best regards,
Mark.

Mark Empson
L M Photonics Ltd

 

[elmotor]

For your question "Is there a rule of thumb for estimating no-load current on a motor?", Zlatkodo's genearl estimates are reasonable for "standard" enclosed motors. In case of open type motors as IP23, induction is often designed to be higher and thus the saturation of iron is a lot higher. In this case 90A could be roughly ok.

 

[Skogsgurra]

Pete, you know that I sometimes find your math excercises a bit trying. But, this time, you have shown that there is more to induction motors than I thought. The fact that leakage reactance consumes a significant part of the full load vars is a valuable insight. Thanks for that!

Gunnar Englund

http://www.gke.org

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

 

[parlitu]

Usualy the no load current can get to 30% of nominal current. So 90A seams a little high; First step is to check the bearing health. One cause that can get to a higher current is if you just put vaseline to you bearings; in this case at the beginning the current will be a little high but if you measure the current after 1 or 2 hours, you'll see that you current will decrease. Another cause can be old vaseline to bearing that have been strenghten in time, but if is a new motor we can eliminate this cause, and anyway this problem can rise you current just with 1 or 2 amps. And the last, but the moust probably cause is a bed bearing. You can have a ball bearing that have been damaged, or maybe the bearings was not install corect. You can check this by vibration measurements.

 

[electricpete]

Thanks Gunnar.

Good point Mark – the stuff we're discussing should be built into power factor correction calcs. I googled and found this Siemens document about selecting power factor capacitors.

https://w3.energy.siemens.com/cms/u...plicationNotes/Documents/TechTopics20Rev0.pdf

The leakage reactance current is relatively small, so that the total reactive current is relatively constant (compared to the kW variation) over the range of motor no-load to motor full-load. For a range of medium voltage machines sampled, the ratio between full-load reactive current and no-load reactive current varied from 140-260% (depending on machine design, speed, and voltage). For perspective, the ratio between full-load kW and no-load kW is of the order of 4000%!

Because the variation in reactive current is relatively low over the load range of the machine, a capacitor sized to compensate to a desired power factor level at full load, will maintain the power factor in the near vicinity of the desired level over the entire load range. Typically, a capacitor sized to correct full-load power factor to 95% will maintain power factor in the 95-98% area over the full range from no-load to full-load.

....
Over-correction (and self-excitation):
It is important not to over-correct when sizing capacitors that are connected in parallel with the motor. The motor requires reactive power (kVARs) to create the magnetic flux. The power factor correction capacitor can supply the kVARs required by the motor when the motor is switched off. At the instant that the motor is switched off, the motor and the driven load are at full speed. When the motor is switched off, the motor and load inertia will continue to drive the motor. If the magnetizing current required by the motor is available from the charged capacitor, the motor will operate as a voltage generator, and maintain the voltage on the motor.
In the preferred situation, the power factor correction capacitors are sized at or below 90% of the no-load kVAR requirement of the motor . If the capacitors are too large, the motor can be subjected to self-exciation, which will result in excessive voltages applied to the capacitors and motor. The capacitors are sized based on 90% of the no-load kVAR requirement because the manufacturing tolerance of the capacitors is –0%, +15%.

The 2nd sentence of the quote describes a conclusion very similar to ours: the ratio of reactive current at full-load over reactive current at no-load is 1.4 – 2.6..... average is 2. i.e. no-load reactive current is 50% of full load reactive current..... the same number we came up with as a ballpark. (although they mentioned medium voltage motors, we were working with 200hp low voltage motor example)

However the way it is described in the first sentence is odd "the total reactive current is relatively constant... over the range of motor no-load to motor full-load". They justify this statement by comparing it to the change in real power using a ratio comparision that is a little unnatural imo..... and disguises the point that there is substantial variation in reactive current (that factor of 2).

They suggest correcting FL power factor to 0.95 and imply this will keep PF from 0.95 to 0.98 from full load down to no-load.... seems like a dubious claim to me. In attached spreadsheet I have used the simplifying factor 50% (no-load motor vars = 50% of motor full load vars) instead of the range. By my calculations in attached spreadsheet, if I start with 94% full-load power factor and correct to 95% full load, then I'll end up at 99% PF at no-load. If I start with 83% full-load power factor and correct to 95% full load power factor, then I end up with 100% no-load power factor. If I start below 83% and correct to 95% at full load, then I end up overcorrected at no-load

Later in the section on over-correction, they suggest to use 90% no-load reactive vars... quite a different approach which results in far less correction than correcting to 95% at full load. I assume this would apply to caps switched with the load because of the cited potential for overvoltage during coastdown after switch-off.


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[electricpete]

Later in the section on over-correction, they suggest to use 90% no-load reactive vars... quite a different approach which results in far less correction than correcting to 95% at full load. I assume this would apply to caps switched with the load because of the cited potential for overvoltage during coastdown after switch-off.

I should clarify the 90% number is a fraction of no-load vars, the 95% number is a target PF at full-load.

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[electricpete]

Later in the section on over-correction, they suggest to use 90% no-load reactive vars... quite a different approach which results in far less correction than correcting to 95% at full load. I assume this would apply to caps switched with the load because of the cited potential for overvoltage during coastdown after switch-off.

Not always far less... depends on initial power factor.

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