Determing Power Factor From Amp Reading Only
Determing Power Factor From Amp Reading Only
(OP)
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!
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!





RE: Determing Power Factor From Amp Reading Only
"Venditori de oleum-vipera non vigere excordis populi"
RE: Determing Power Factor From Amp Reading Only
1HP=0.746kW
Other way of looking at it is that at full load the motor kW is 250HP*0.746=186.5. The kVA at full load and rated voltage and rated current is 1.732*4.16kV*34A =245.
So the power factor at full load should be 186.5kW/245kVA=0.76. I am not sure where does 83.9 pf comes from, unless there are power factor correction capacitors installed.
From above you also can see what you need to calculate the power factor.
RE: Determing Power Factor From Amp Reading Only
Rbulsara forgot the efficiency.
kWin = HP*.746/EFF = 250*.746/.9073 = 205.555 kW
kVA = 1.732*kV*Amps = 244.98
P.F. = kWin/kVA = 205.555/244.98 = 0.839
Answering your question: No you can not get the PF unless you measure the electric kWin and the kVA. An alternative is to have the phase angle between the current and voltage.
To interpolate ask for the performance curves from Toshiba, so you could estimate the parameters of operation based on the demand of amperes.
RE: Determing Power Factor From Amp Reading Only
Thanks for pointing out the omission. However, 90% efficiency would be a very low value, making it a pretty inefficient motor, imho. I think efficiency for a standard (not high efficiency) 250HP indcution motor should be closer to 94%.
RE: Determing Power Factor From Amp Reading Only
Alternately, for a bit more involved approach how to measure the power factor, visit
http://www.designnotes.com/CIRCUITS/pwrfact.htm
RE: Determing Power Factor From Amp Reading Only
Served from CTs/VTs on the motor feeder, the Multilin box likely has integral metering functions. Seems like if real and apparent/reactive power are displayed, PF could be easily calculated.
RE: Determing Power Factor From Amp Reading Only
RE: Determing Power Factor From Amp Reading Only
Blame Toshiba for that low efficiency not me.
Jbartos:
You stated "one shunt resistor for measuring line current may suffice to find out the power factor."
Original post:
"The motor is drawing between 24 and 30 amps at 4160V."
For safety, the high voltage must be handled with PT's and TC's.
What is the difference with the current he has measured actually?
What he needs to measure is the phase angle (phi) between Voltage and Current. PF=cos(phi)
RE: Determing Power Factor From Amp Reading Only
Clam down. I was not blaming you for any thing.
RE: Determing Power Factor From Amp Reading Only
RE: Determing Power Factor From Amp Reading Only
RE: Determing Power Factor From Amp Reading Only
In your case, since the motor is not running at rated load (24 to 30 A as against rated 34 A), having both PT's and CT's is the only way you can measure the motor p.f. at that particular load. Then, you can determine the efficiency of the motor at that load based on the formula by aolalde.
btw, why do you need to know the p.f. (for p.f. correction ?) ?
and is your load current hunting between 24 to 30 A ?
RE: Determing Power Factor From Amp Reading Only
Jbartos:
You stated "one shunt resistor for measuring line current may suffice to find out the power factor."
Original post:
"The motor is drawing between 24 and 30 amps at 4160V."
For safety, the high voltage must be handled with PT's and TC's.
///Generally, yes. However, the shunt in line would be producing relatively low voltage. The voltmeter attached to shunt should be isolated from other phases and ground. (Don't birds sit on 500kV transmission lines?)\\\
What is the difference with the current he has measured actually?
///CT adds a current shift angle. The shunt resistor over voltage will not. So that the power factor angle can be measured on the oscilloscope screen as an angle between PT voltage received on one channel and voltage obtained from the shunt over voltage insulation probes.\\\
What he needs to measure is the phase angle (phi) between Voltage and Current. PF=cos(phi)
///Or between voltage and another voltage proportional to the current.\\\
RE: Determing Power Factor From Amp Reading Only
Generally, the CT shift is very negligable unless you need PF accuracies beyond 0.1%.
RE: Determing Power Factor From Amp Reading Only
Also, the shunt will actually have some phase shift associated with it as well due to the inductive properties of the shunt (depends on the type, wire wound, etc).
RE: Determing Power Factor From Amp Reading Only
There a number of 'High Voltage' probes available from the likes of Tektronix which, on paper, have suitable voltage rating for the duty. These probes do not like the switching and fault transients which exist on the MV system because these transients often exceed the rating of the probe. Additionally these probes are not normally designed to contain a high energy fault safely, potentially resulting in MV reaching the 'scope (irritating) and the user (fatal).
I don't know of anyone who uses shunts for current monitoring on an MV utility supply. The method is inherently less safe than contactless alternatives such as the CT, Hall Effect transducer, or Rogowski coil. High energy faults are terrifying, as anyone who has witnessed one or its aftermath will testify. Why take the risk? Why tell someone else to take the risk? Armchair speculation about the theoretical possibility of directly connecting transducers to high energy sources is ok as long it is recognised as such.
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If we learn from our mistakes,
I'm getting a great education!
RE: Determing Power Factor From Amp Reading Only
Please keep in mind that these fora are studied by a very broad audience. That includes seasoned and experienced engineers, newbies and also people that have no idea about electricity.
It is potential murder to even mention measuring current with a shunt to the "lower half" of that audience. It suffices that just one of all these readers actually tries something as foolish as that to have a major accident happening. And this goes for ordinary low voltage installations in the 100 - 400 V range. At medium voltage levels, like the one discussed here, there is not only a risk, the accident is certain to happen if someone less experienced (=having to ask questions about measuring) tries it.
With regard to phase shift introduced by CTs: You seldom need more than 1 percent accuracy in these measurements and the phase shift introduced by a CT does not mean a lot. If, for instance, the phase shift is 30 degrees then one degree of error in the CT translates into a 0.008 change in PF. Most people would find this insignificant in the context discussed here.
Also, shunts do introduce phase shift. And more so the "thicker" they are. A 60 mV shunt at 3000 A has a 20 micro-ohms resistance. And the inductive part of its impedance is roughly 0.1 micro-henry. This means that the reactance is about 2 x PI x 60 x 0,1 = 38 micro-henries, which translates in a 62 degrees phase shift. So the phase shift introduced by this shunt is greater than the phase shift of the motor. A 50 A shunt (which would have been selected in the example discussed) would probably have something like 1 or 2 degrees, but still more than most CTs have. Of course, you can use co-axial shunts and such things, but why not go for the safe standard solutions like CTs, clamps or Rogowski coils?
RE: Determing Power Factor From Amp Reading Only
One other item about the scope measurements that should be mentioned is most scopes are not isolated (if they plug into 120), so an isolation transformer had better be used as well or the second you hook one end of the probe to anything above the rating of the probe, as referenced to the system ground, you will get some nice sparks flying and ruin your scope (and maybe you). Isolation transformers, in my opinion, should be standard with every scope. Maybe the new ones on the market are but I am doubting it.
RE: Determing Power Factor From Amp Reading Only
RE: Determing Power Factor From Amp Reading Only
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.
RE: Determing Power Factor From Amp Reading Only
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.
RE: Determing Power Factor From Amp Reading Only
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
RE: Determing Power Factor From Amp Reading Only
RE: Determing Power Factor From Amp Reading Only
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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If we learn from our mistakes,
I'm getting a great education!