Difference kWh value when measured at main DB and near a motor

[JeffWong]

Dear All,

I have a problems on measuring the kWh consumption of a 3 phase motor with full load ampere of 5A since I measured difference kWh value with a Fluke 41B meter when I measure the reading at the main Distribution Board and near the motor.

The setup is as follows:
The main distribution board (for the whole floor of a building) is only supply electricity to a motor (since turn off all the other fuses in the main DB which provide to other equipments) which is running in unload condition Therefore, the power factor of it is just around 0.17. We take the reading at the main Distribution Board and near the motor with two Fluke 41B meter at the same time.

The result is as follows:
At Main DB:
Supply voltage: 386V
A rms: 2.85A
1.91kVA
0.32kW
1.87kVar
Power factor: 0.17

Sub DB near the motor:
Supply voltage: 386V
A rms: 2.75A
1.84kVA
0.34kW
1.80kVar
Power factor: 0.18

I would like to know why they have different power factor, KWh, kVAh and I rms between taking data on the main DB and the sub-DB?
I think that the utilities will count our consumption of electricity in the method of kVAh x site power factor. And if I put a sub meter for counting the power consumption , the kWh meter will calculate the kWh consumption in kVAh x p.f. of the motor? And some engineers told me that it is more accurate to count the consumption of the motor with using a kVAh meter? Moreover, I would like to buy a mechanical kVAh meter without other functions. Can you tell me where I can buy it?

Hope you can provide some opinions to me?
Thank you !

 

[electricpete]

Did you measure this more than once to confirm the repeatability of the data? Did you measure the same phase both times?

If I pay attention to the 3rd significant figure I see that there is slightly more kw (0.01kw) flowing into the motor than flowing at the distribution board. You have some power generating device connected between? Highly unlikely I think.... probably just measurment error for some reason.

A bigger difference is that the kvar decreases 0.07kvar at the motor compared to the panel This would seem to indicate some small inductive load connected between? Seems unlikely you would be unaware of that.

Once again it seems like measurement error of some sort would be the key thing to look at.

 

[busbar]

Please understand that it’s difficult to troubleshoot problems not being there, but consider these comments.

i. Study the meter’s published accuracy figures. The reading variations are likely within stated limits. Expectations of exactness in digital readings of analog signals are often flawed.

ii. Has the instrument a recent calibration check? (The reported very low PF seems somewhat unusual.)

iii. Is the conductor position through the clipon CT core centered in both cases? Makes a difference, to a greater degree on low values.

iv. Have you determined instrument connections/measurement points the same at both ends of the circuit?

v. Review basic textbook wattmeter measurements. The 41 “fudges” with one potential and one current measurement, violating Blondel’s theorem. It is qualified as accurate only for balanced 3ø 3-wire circuits with sinusoidal waveforms. As a check, record per-phase currents and ø-ø currents and manually calculate VA readings.

vi. Are there any controls connected to the circuit upstream of the motor [between measurement nodes] like a magnetic-starter coil?

 

[IRstuff]

Since you used two different meters and clamps, did you try swapping both?

TTFN

 

[jbartos]

Suggestion: Visit

http://www.thomasregister.com

and type Meters: Watt Hour under Product or Service which will return 49 companies to inquire about the sought meter.

 

[JeffWong]

Dear All,

Thank you for all of you! I confirm it, the unreasonable data is not due to the error on meter or due to wrong methods to measure it. Moreover, I have also tried to use different meters or just exchange the meters which near the main DB and near the motors.

Hope you can provide more opinions to me!

Thank you!

 

[d23]

JeffWong

Are you using a motor starter with a holding coil on this motor or just switching on and off?

David