Measuring Phase angle to detrmine capacitor size

[ATOZHVAC]

Hi

I saw the quoted post below at hvac-talk.com and no one can answer my questions and im curious if anyone here can help me to better understand the process of determining the phase change angle and PF.
Reference is made to a meter that measures PF. What type of measurement is that? Frequency? Phase?
Are the measurements amps,voltage,back emf??? are the readings on the COMMON, START and/or RUN leads of the comprerssor??
Any info on how to measure phase angles and PF is appreciated.



Quote:
Originally Posted by hvacrmedic


I found an article recently on Google, back door type of thing, apparently it's a pay site, so I won't provide the link. It's instructions were to provide the capacitance that will push the phase offset between the windings to a 90 deg angle. According to the article the inductive component would generally provide a lag of about 50 deg, and thus a 40 lead was required by the capacitor. The two, pushing phase in opposite directions, add together to get total phase angle. Equations were provided to caclute required capacitance, but were followed with "bench testing would still be required, becuase of the unknown inductance value". If you have a meter that measures PF, then you should be able to work it out. My meter does have this capability, and you can bet I'm going to check a few motors out. Last one I checked (factory install) had an 87.5 deg phase shift between run and start, so that seems to pan out.

 

[ScottyUK]

Already posted in thread403-228896. Probably not the best place for it, but don't create a fragmented discussion with multiple posts. Follow the link...


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If we learn from our mistakes I'm getting a great education!

 

[waross]

Are you trying to correct the power factor and why?
Are you trying to get exactly 90 degrees between the start windings and the run windings and why?

Bill
--------------------
"Why not the best?"
Jimmy Carter

 

[ATOZHVAC]

Hi

My mission here is to understand how hvacmedic got the readings (measure what and where) to determine the 90 degree or close to 90 degree phase angle of an A/C compressor.

The whole topic came from a thread where guys were trying to determine the proper capacitor size for a compressor in the field when the original capacitor information is not available.

As far as power factor all I know is that according to hvacmedic if you have a meter that measures PF then you should be able to figure out the degrees of phase shift.

hvacmedic nor anyone else at the hvac-talk site will respond or offer me any help.

Thanks

 

[waross]

The power factor is the cosine of the phase angle. An angle of 90 degrees corresponds to a power factor of zero.
For work in the real world, draw a power triangle.
The base is the real power or the watts.
The altitude is the VARs.
The hypotenuse is the VA.
The angle between the base and the hypotenuse is the phase angle. The power factor is the cosine of this angle and/or the ratio of the base (watts) over the hypotenuse (VA).


Bill
--------------------
"Why not the best?"
Jimmy Carter

 

[ScottyUK]

As already posted in the other thread, PF is the cosine of the phase angle between current and voltage. The phase angle between the currents in the two windings of an a/c compressor isn't a power factor, it's just a phase angle. The phase angle between windings in a 1-phase moltor is ideally 90[°] because that gives the maximum torque from the motor. In view of the limited detail posted by hvacmedic I'm not entirely convinced he really knows what he is talking about but it would be interesting to read the quote in the original context. Meters that can accurately measure phase angle between two currents are fairly unusual - certainly not the common multimeters - and fairly expensive. I posted some examples in the other thread.


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If we learn from our mistakes I'm getting a great education!

 

[waross]

I agree with you Scotty. Thanks.

Bill
--------------------
"Why not the best?"
Jimmy Carter

 

[ScottyUK]

Bill,

I know that you know that I know that you know...


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If we learn from our mistakes I'm getting a great education!

 

[7anoter4]

In the old good time we did the "vector diagram" measuring the kW by means of monophasic wattmeter very simply. If we had the voltage and the current P= U*I*cosFi
So PF=P/U/I accurately enough. But we had to do for each phase separately.
A power recorder, a voltage recorder and current recorder would make a better job.
So, what's the problem?

 

[ScottyUK]

Not really a problem other than it is the phase relationship between the currents in the two windings of a single phase motor the OP is comparing, not a voltage and a current, which makes discussion of power and power factor meaningless in this context. It's simply the phase angle between two currents which is being measured, and from there the intent is to optimise the phase angle between those currents to 90[°] by judicious choice of capacitance in series with one winding.


----------------------------------

If we learn from our mistakes I'm getting a great education!

 

[waross]

Hi Scotty;
PF as a percentage is quite common in North America. I think that the only time that I have used the phase angle is in the classroom, either studying or teaching.
I have been doing PF calculations and PF corrections for many years and have never known the exact angle nor had occasion to look it up.
I suppose that the expression of PF as a percentage is partly due to the method of determining PF. Historically, power factor measurements were done in the field decades before sophisticated instruments that reported phase angles were readily available.
We used watt meters volt meters and ammeters, or kWHrs over kVARHrs from the utility bill. There is also a method of using a temporarily connected capacitor and an ammeter. The meter readings were analysed either with the law of cosines or by scaling on a drafting table.
It always came down to W/VA and was often expressed as a percentage.

Bill
--------------------
"Why not the best?"
Jimmy Carter

 

[7anoter4]

Thank you, ScottyUK.
So, it is a single-phase induction motor with capacitor. If it is for air-conditioning compressor may be provided with 2 capacitors-one for start and one for run.
Start capacitor may be calculated measuring the resistance [d.c. bridge] and the impedance [a.c. voltage divided by current].
If the main winding will be Zmain=Rm+jXm then Xm=sqrt(Zmain^2-Rm^2)
The same for the auxiliary winding: Zaux=Ra+jXa and Xa=sqrt(Zaux^2-Ra^2)
Now, we have to add a capacitor to the auxiliary winding in such a way to get a 90 degrees lag with respect to the main.
The main angle is Fim=arctang(Xm/Rm)
The aux. angle will be Fia=arctang[(Xa+Xc)/Ra]
But Fia=Fim-90[or pi/2]
Xc=TAN(Fia)*Ra
C=1/w/Xc where w=2*pi*frq
BTW you are right waross, if efficacy is usually noted as % why PF does not also?
Best Regards

 

[7anoter4]

Sorry, Xc= TAN(Fia)*Ra-Xa

 

[7anoter4]

I think I exaggerated neglecting rotor resistance [measuring by means of dc bridge only the armature resistance].
Better it is to measure the kW by means of a single-phase wattmeter, as I said. The total resistance will be Rm= Zm*cosFim or Ra=Za*cosFia [Also the Ia to Im lag angle will be Fia-Fim].
The voltage has to be minimum required for an accurate measurement so that the current will not overload the winding above rated.