Determing Power Factor From Amp Reading Only 5

[mhuckaby]

Is it possible to determine the power factor of a motor when you only have the amp reading off of the motor? Is there a way to interpolate the approximate power factor from the amps reading only?

We have a motor on an air compressor with a GE Multilin meter. The meter has CTs, but PTs.

The motor is drawing between 24 and 30 amps at 4160V.

Toshiba has told me that the power factor is 83.9 when the motor is at full load.


Here is the motor nameplate data:
Toshiba World Energy Series
2300/4160V
60/34A
250HP
1.15SF
1775RPM
40C Continuous

Thanks!

 

[Skogsgurra]

jb,

You seem to forget that when you measure the HV in a TV set (which is closer to 30 kV today) the measurement is referenced to chassis/ground and that there is very little power behind it. I started out as a TV service technician in 1957 (that is when we had 12 - 18 kV acceleration voltage) and got myself some shocks from the HV connector. Unpleasant, but not lethal.

A shunt in a live conductor is something else (and I think that you understand that, but for some obscure reason you do not admit it). Both signals are at virtually the same potential - around 4 kV in this case - so an ordinary HV probe cannot be used. You need a differential probe or an isolation probe/amplifier that can stand 5 kV at least. You do not find them easily. Most of the isolation amplifiers or differential probes I have used are limited to 600 V Cat.III, possibly 1 kV.

A flash-over from an inadequately insulated instrument to a live conductor or earth will produce severe arcing and burn the person that is close to it. I have worked together with people that have been badly burnt and they have either been killed or had sight or hands/arms destroyed. It is really nothing that you should take lightly. It is a matter of life or death.

 

[Skogsgurra]

Explanation

The above posting is an answer to a jb posting that seems to have disappeared. It may therefore seem a little "over the top". But what is said is valid - although not neccessary any longer to make the point clear.

 

[TomG33]

Mhuckaby,
By this stage it would be hard not to say something that has not been covered by previous posts, and indeed there has been some good points raised.
Your original post seemed to miss a word and I took the posting to say that "It has CT's but NO PT's" feeding the Multilin relay. What is the model of Multilin you have?. It may already have the capacity to display Power factor with the simple addition of a PT. This would be by far the most simple and safe solution, even on a temporary basis.

Model "269" has no provision for PF Monitoring
Model "269 Plus" has an optional power Factor meter
Model "369 and 469" Both have a power factor monitoring function

Remember the motor basic equation mentioned by AOLALDE has many unknown variables.
The nameplate gives you a set of values and you can calculate the efficency but unfortunately this is at the rated power only. From my experience both powerfactor and Efficiency vary in a nonlinear fashion between no load and full load.
You simply have too many variables to make Power factor calculations from line voltage and current alone.

As suggested, You could get some operating curves from Toshiba, but putting on a temporary PT may be the easiest way out

Tom

 

[johnnyw]

Assuming the voltage and current are sinusoidal, wouldn't the power factor be the cosine of the angle between phase voltage and phase current? If line voltage and phase current are observed on a scope, wouldn't you expect to see a 30 degree phase shift for a unity power factor since the line voltage is displace 30 degrees from phase voltage? If the waveforms are not sinusoidal, then the angle between voltage and current zero crossings won't yield accurate power factor measurements.

 

[ScottyUK]

Hi johnnyw

You are correct in theory, but line voltage is 4160V and there is no VT available at this location. I know you weren't considering connecting the 'scope to the MV line voltage like a previous poster 'suggested' - were you? - so there is no means of obtaining the voltage signal to use in your method. The MV network is not a place for lab instruments and test equipment - the otherwise excellent HV differential probes from the likes of Tektronix are not rated for direct connection to very high energy systems (according to Tektronix UK at least), and their probes are typical of this type of equipment so the advice is probably valid for other similar diff. probes.

The initial problem posed by mhuckaby was how to measure pf without the availability of a voltage signal. The only primary transducers available are CTs on the MV supply to the motor.




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