Issues in Measuring Power Factor in Energy Meters

The word <...inaccurately...> requires more meaning, and some numbers would help.

Please describe the outcome of the telephone discussions that have taken place with the equipment manufacturer <...i.e. Schneider...> on this topic.

I assume that your metering and the utility billing meter (Schneider 8650) don't agree during your motor starting tests. This normal for two reasons; first, DOL starting is a highly transient situation, second, unless you are using exactly the same meter as the utility, the sampling rate differences will affect the real-time calculation of the PF. This will be especially noticeable if one meter is digital and the other is analog.

Hire an experienced electrical engineer to install a modern digital Power Quality Meter to record what's really happening during these transients.

As a novice in the subject of analog and digital meter design I am curious how the PF is actually measured or calculated or displayed. I can guess but I don't know.

But I would have thought the results would be the same if using meters made by reputable manufacturers.

Or put another way, if I bought a PF meter I would expect it to give the correct reading.....whatever make or design concept used.

.....but judging from the posts it seems not.....

Knowing the correct PF is not on my list of things that matter to me at the moment.... Just curious.

Don't judge by the posts, just get a copy of the ANSI C12 series or IEC 62503-21 standards, they describe all the testing and accuracy criteria for revenue metering accuracy.

" Don't judge by the posts, just get a copy of the ANSI C12 series or IEC 62503-21 standards, they describe all the testing and accuracy criteria for revenue metering accuracy."

Thank you RAM for this, but I doubt if a copy of these standards will answer the question about the different results the OP is faced with.

If these standards have any force, then manufacturers would be obliged to comply with them. In which case it would be reasonable for the user to expect the same results. Which is not the case here it seems.

If meters are the 'same' then it must come down to methodology - ie, how and where is the meter connected to measure the PF.

eg. if the meter is connected to measure the PF in the final leg of the motor circuit, and then compared to a reading from a meter connected at the incoming mains, probable with a time delay between readings, it is perhaps not surprising there is a difference.

Just a thought.

Suggest you read post 15 below. Until OP has proved that both the utility and his metering read exactly the same under identical conditions (which is the test methodology in the standards), then he is doomed to failure. Steady state measurements will usually be close, but transients and nonlinear loads (both of which occur during motor starting) can cause great differences in readings.

When I started doing power testing everything was analog. Along came digital metering and my the testing personnel started reporting results that differed from the previous tests. All it took was some concurrent testing (current inputs in series, voltage inputs in parallel), coupled with oscilloscopes to monitor the various waveforms and a whole bunch of metering theory research, to convince them why the readings varied so drastically. nb- all our instrumentation was traceable to the National Bureau of Standards and calibrated periodically.

Mr Ankit Jain,

DO NOT ASSUME that standard company "SCHNIEDER" does not mean the algorithm used to compute power factor is standard. There are different definitions of power factor and different methods of measurement. So you may find differences between meters as each may be using internally different techniques. It is particularly so , if the wave forms are distorted. Don't ask engineers and college students- they only know textbook definitions, but not real life definitions..

Quite the opposite, utilities purchase meters from various sources and test them to international standards to ensure that they all read the same statistically, despite internal differences in circuitry, algorithms, etc.

Consumers aren't held to the same standard so that it is more probable that the error is on their side. The only way that the OP should have an issue is if he is using exactly the meter type, CTs and PTs, as the utility does and the readings are different then the standards would allow.

Could it be that one meter gives the displacement PF while tho other gives the overall PF (displacement and distortion)? With a perfectly sinusoidal current and voltage, they are both the same but they start diverging when distortion is present.

Thanks for all the responses but let me first explain the situation in detail.

As per the diagram when i am supplying the power to submersible pumpset the Energy Meter installed at the power panel for Single phase is showing correct current values and Voltage values as verified with the Clamp on meter. But the power factor sensed is 0.99 due to which the efficiency of the Pump Set decreased like anything. Aain a pf of .99 is highly impossible scenario as the Pump Manufacturer himself claims pf of 0.8 as the max. This one issue similar issues are observed at other power supplies as well but this one needs to sorted out first for us.

"As per the diagram when i am supplying the power to submersible pumpset the Energy Meter installed at the power panel for Single phase is showing correct current values and Voltage values as verified with the Clamp on meter. But the power factor sensed is 0.99 due to which the efficiency of the Pump Set decreased like anything." --But are you testing a single phase pump motor when looking at the single phase reading?

Usually the fight in the marketplace takes place over apples being compared to oranges. Although another usually results when one person's apple is another persons applesauce.

by the way, labels are for soup cans, placards are for motors.

Yes Sir. We are very looking at the single phase reading while testing a single phase motor. Anyways thanks a lot for your valuable inputs.

Are you sure this is the maximum power factor, because 0.8 pf sounds more like a power factor at maximum LOAD? When a motor (including single phase) is unloaded and just spinning, I think you can rest assured the power factor is much nearer 1.0 than it is 0.9.

"... a motor (including single phase) is unloaded........ the power factor is much nearer 1.0 than it is 0.9...."

Are you sure about this ? I think you will find the PF is very low for motors running off-load.

Yes the pf for motor at No Load should be quite low. But herein the motor is working at rated load as the submersible pump set is submerged in the water tank and throwing pumping the water. Thus it is very much loaded. But still a pf of 0.99 is not possible.

Can improper grounding be a reason for the same or something be acting as capacitive load which leads to high power factor.

Can anybody please highlight on the same.

Until you verify that your metering setup is identical to the utility's you're wasting your time looking elsewhere.

OK, I had it all backwards: PF = Re(EI)/Total (EI)

Apparent power = Total power (both real and imaginary components). At no load, there is apparent power without much real power at all, and PF could approach 0, (not necessarily = 0).

At full load, the PF is higher by quite a lot, but not as high as 0.99, unless over compensated by some capacitance, i.e. another no load motor/generator with over excitation, or some other stray capacitance.

Fine. To my mind's eye I 'see' power factor as vectors of the phase angle between peak current and peak voltage. With a motor being highly magnetic reactive the current always lagging the voltage - more on light load than on high load.