Lowest power factor for a motor

[mull982]

What is the lowest power factor that a motor can have when unloaded or lightly loaded. I have a 480V 3P 150hp motor which is lightly loaded and when measuring with a power meter I see a power factor of .179. This power factor seems extremely low. I know that a lightly loaded motor will have a low power factor, but this value seems very low. Does this power factor of .175 seem too low?

The full load power factor for this motor is about .80. I have double checked power meter connections and everything seems correct. We are running about 50A when measuring and according to datasheet this 50A corrosponds to about 1/4 loading on the motor. For 1/4 loading on the motor the datasheet lists a power factor of about .60. So I'm wondering if I am doing something wrong in my measurments or can a motor really have such a lower power factor?

 

[winnie]

If the motor were 100% efficient, then at no load the only current flow would be magnetizing current...and thus the power factor would be _zero_. Of course, real motors are not 100% efficient, so you have losses which show up as real power consumption. With the motor you describe, I would not be surprised by a PF of 0.05 or lower with no mechanical load.

-Jon

 

[Ranch]

If the motor were 100% efficient, then at no load the only current flow would be magnetizing current...and thus the power factor would be _zero_. Of course, real motors are not 100% efficient, so you have losses which show up as real power consumption. With the motor you describe, I would not be surprised by a PF of 0.05 or lower with no mechanical load.

-Jon

Well stated - zero is the answer for ideal analysis. Consider the similarity to those harmonic measurements on a non linear driven AC induction motor that are so much higher at light loads. I don’t find the OP’s measurements unusual whatsoever.

 

[Besoeker]

What is the lowest power factor that a motor can have when unloaded or lightly loaded. I have a 480V 3P 150hp motor which is lightly loaded and when measuring with a power meter I see a power factor of .179. This power factor seems extremely low. I know that a lightly loaded motor will have a low power factor, but this value seems very low. Does this power factor of .175 seem too low?

The full load power factor for this motor is about .80. I have double checked power meter connections and everything seems correct. We are running about 50A when measuring and according to datasheet this 50A corrosponds to about 1/4 loading on the motor. For 1/4 loading on the motor the datasheet lists a power factor of about .60. So I'm wondering if I am doing something wrong in my measurments or can a motor really have such a lower power factor?

The 50A may be about 1/4 of the motor full load current but you cannot directly correlate current to motor mechanical load. It's not uncommon for a motor to take about 30% of FLC with no mechanical load on the shaft and this would typically correspond to a pf of 0.1 or lower.
As I said in another thread, I regularly have to calculate motor performance so I looked back at a couple of previous calculations and no-load pf was around 0.05.

For your particular case, 50A is about 42 kVA and the no-load losses might be about 3% or 3.4kW. Corresponding no-load pf would be around 0.08.

 

[weressl]

What is the lowest power factor that a motor can have when unloaded or lightly loaded. I have a 480V 3P 150hp motor which is lightly loaded and when measuring with a power meter I see a power factor of .179. This power factor seems extremely low. I know that a lightly loaded motor will have a low power factor, but this value seems very low. Does this power factor of .175 seem too low?

The full load power factor for this motor is about .80. I have double checked power meter connections and everything seems correct. We are running about 50A when measuring and according to datasheet this 50A corrosponds to about 1/4 loading on the motor. For 1/4 loading on the motor the datasheet lists a power factor of about .60. So I'm wondering if I am doing something wrong in my measurments or can a motor really have such a lower power factor?

Relaince power factor data on a 1785RPM motor:
100% load: 85.3% 175A
25% load: 56% 71A
no load: 8.07% 55.6A (Mind you in this case there is nothing connected to the shaft)

 

[Besoeker]

Relaince power factor data on a 1785RPM motor:
100% load: 85.3% 175A
25% load: 56% 71A
no load: 8.07% 55.6A (Mind you in this case there is nothing connected to the shaft)

Your 0.0807 is not hugely different from my 0.08.
Sort of thing I ought to be able to calculate to a reasonable level of accuracy.
Anyway, if we are both there or thereabouts it implies that Mull's motor was at light load rather than no load.

 

[weressl]

Your 0.0807 is not hugely different from my 0.08.
Sort of thing I ought to be able to calculate to a reasonable level of accuracy.
Anyway, if we are both there or thereabouts it implies that Mull's motor was at light load rather than no load.

Yes and no.

Yes the pf is close enough, but

No, he indicated that his 50A reading was close to the 25% motor loading data, whereas my data for both was higher at that point.

Yes AND no, because there is a variance in motor models, motor manufacturers and actual data from typical data that without knowing the test data of the specific motor, it is not possible to detemine of the data is false or not. As we discussed earlier in another post pf deterioration on repaired motors can also be a factor.

 

[Besoeker]

Yes and no.

Yes the pf is close enough, but

No, he indicated that his 50A reading was close to the 25% motor loading data, whereas my data for both was higher at that point.

Yes AND no, because there is a variance in motor models, motor manufacturers and actual data from typical data that without knowing the test data of the specific motor, it is not possible to detemine of the data is false or not. As we discussed earlier in another post pf deterioration on repaired motors can also be a factor.

See post #4.

 

[mull982]

Attached is the motor datasheet for this motor showing motor performance values from test results.

Like I mentioned in my OP when we ran this motor is was coupled to the load equipment, however there was no material load. This is a bucket elevator that was coupled to the motor but no material in the conveyor.

When we ran the equipment we were seeing about 50A. From the attached datasheet 50A is somewhere between No load and 1/4 load for the motor. The 1/4 load p.f. is 64.6 and the no load p.f. is 4.23. So I guess then when we ran this motor our power factor could have been anywhere between 4.23 and 64.6. I would have expected it to be closer to 64.6 since this 50A is very close to 1/4 load, however I guess it is possible for the power factor of .179 to be true.

Maybe the steep expoenential rise of the P.F. curve changes the power factor that drastically between the 50A and 57.5A load points?

 

[mull982]

Sorry forgot attachment

 

[mull982]

One more time!

 

[mivey]

...I would have expected it to be closer to 64.6 since this 50A is very close to 1/4 load, however I guess it is possible for the power factor of .179 to be true...

How close is 50A to the no load current?

add: nevermind, I see the attachment now.

 

[mivey]

Your load factor is 17.9 or 22.6% of the way from the no load value to the 1/4 load value.

Your amp draw is 50 or 50.3% of the way from the no load value to the 1/4 load value.

It is not a straight line curve but even so:
0.179 (22.6%) would yield 45.82 amps.
50 (50.3%) would yield a pf of 0.346.

Looking at the curve, you will actually see the amp graph curves upward but is reasonably linear somewhere past the 25% load point. The pf graph rises sharply and curves downward to level out with a knee at about 30% load.

 

[mivey]

Maybe the steep expoenential rise of the P.F. curve changes the power factor that drastically between the 50A and 57.5A load points?

Plotting pf and % load with amps on the x-axis shows a steep curve so that is reasonable. At any rate, there is a big difference in the pf curve vs the load or amp curve.

 

[Besoeker]

One more time!

FWIW I have taken your motor data and worked backwards to get equivalent circuit values for the motor model. It gives a fairly good match to the published data.
On that basis, the loading at 50A is about 8% of full load torque and also about 8% of full load power since the speed doesn't change much.

 

[Besoeker]

Revised the above and got about 6.5% with a better fit to the data

 

[mull982]

FWIW I have taken your motor data and worked backwards to get equivalent circuit values for the motor model. It gives a fairly good match to the published data.
On that basis, the loading at 50A is about 8% of full load torque and also about 8% of full load power since the speed doesn't change much.

I would be very interested in seeing these calculations if they are avaliable