Simple question for measuring true power with a true power meter

Discussion in 'General Electronics Chat' started by chunkmartinez, Aug 3, 2014.

1. chunkmartinez Thread Starter Senior Member

Jan 6, 2007
180
1
Guys I bought a true power meter a while back and I used it for my first time he other day but I used it on a 3 phase motor.

My meter is not listed for use with 3ph equipment. I measured a 30hp motor with a nameplate rating of 43A(or 34 cannot recall there were two similar motors with slightly diff ratings). I got around 10kw on one of the two motors I measured, well a little under. This Measuremnt was from one phase to ground(voltage probes while clamp was on the phase) and from a phase to another with the voltage probes I got a out half(5kw or so).

Well my calculation from the name plate was a out double(around 20kw however this is a rough recollection so don't hold me to the sime math it is irrelevant to my question.

Oh and I forgot to mention the voltage was 480v(460 on the nameplate).

So, I know the basics to 3 phase such as li e voltage vs phase voltage but I don't understand everything.

Do I need a "3 phase power meter"to measure 3ph power? Because I do not know if there is such a meter and what the difference is? Does the meter clamps all 3 phases? And is it possible to calculate 3ph true power from using a single phase meter like mine and how do I do it? And why is the power about double from ground to a phase then it is from phase to phase?

After reading a little I picked up that 3ph motor already have a high power factor meaning that a simple V*A calculation should be close to a true power meter reading right? And if this is true then why do 3ph motors have a high power factor, I mean why?

Also, last question: I saw on the motor plates of these motors say PF 83.6. Is this not power factor because I know that power factor is always under 1. Is it possible it means .83 or so?

2. KLillie Active Member

May 31, 2014
126
14
Power factor
Also given on the nameplate as "P.F." or PF," power factor is the ratio of the active power (W) to the apparent power (VA) expressed as a percentage. It is numerically equal to the cosine of the angle of lag of the input current with respect to its voltage, multiplied by 100. For an induction motor, power factor also varies with load. The nameplate provides the power factor for the motor at full load. Active power is the power that does work; apparent power has a reactive component. This reactive component is undesirable - the utility company must supply it, but it does no work. A power factor close to unity (100%) is most desirable. Because there are tradeoffs when designing an induction motor for improved efficiency or other performance parameters, power factor sometimes suffers. It can be improved by adding capacitors.
http://www.elongo.com/pdfs/MotorNameplate990519.pdf

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3. AlphaDesign888 New Member

Jul 27, 2014
189
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Simple? You need to change the title of this thread to 'Complex Discussion'

4. inwo Well-Known Member

Nov 7, 2013
2,416
314

Most likely motor current will be balanced.
Online calculator.
Haven't found an online calculator to enter unbalanced load.
You'll have to do the math.
http://www.rapidtables.com/calc/electric/Amp_to_kW_Calculator.htm

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5. chunkmartinez Thread Starter Senior Member

Jan 6, 2007
180
1
I understand the fundamentals to complex impedance, appearent, reactive power/impedance etc...I learned it in the e books here. As far as power factor goes, I forgot that it can be expressed as an actual factor(under 1) or a percentage. So 80% is the same as a power factor of .8 . This is a little embarrassing lol.

Okay so back to my original post. Today I read the FLA current rating on a motor and multiplied it by 470v and got about 1500VA. So I guess either my buddy who did the calculation didnt do it right or be read the FLA of another motor. We have to service about 50 30hp motors is why. Well I am realizing now that there are different motors. Some have FLA ratings of 43A, some of about 33. This, and a power factor of say 83% and It appears that things are not as off as I Had thought.

6. inwo Well-Known Member

Nov 7, 2013
2,416
314
You can use the simple voltage and current measurements and factor in the PF, or you can use the reading from your true power meter.

True power reading has taken the PF into consideration. So don't do both.

If all phases are equal multiply true power (one phase) by square root of three.

Edit:

I haven't looked it up, but power factor listed on motor must be at a certain load. Full load I would assume.
So that means you have to measure PF or true power for your calculations.

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