Measure Phase Angle and PF on A/C compressor

[ATOZHVAC]

Hi

I saw the quoted post below at hvac-talk.com and no one can answer my qustion 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]

Unusual for a standard multimeter to have PF measuring capability. A power analyser will do it as a matter of course.

Power factor is defined as the cosine of the phase angle between voltage and current. Thus the measurement is one of phase angle between current and voltage, and therefore the instrument needs to measure both simultaneously in order to display it. Normally you would measure line voltage and line current supplying a piece of equipment, i.e. at the power inlet. PF doesn't apply in the normal sense to the internal connections of a split phase motor, which I think the item that hvacmedic is busy confusing himself about.

A dual-channel oscilloscope is one way of measuring phase, although if you are doing the measurements on the mains supply you will need isolating probes for both current and voltage to do it safely. There are all sorts of dangerous tricks used by reckless and stupid people in order to avoid buying the right tools, such as disconnecting the 'scope chassis from mains earth, but there really is no excuse for this given the capabilities of affordable modern instruments.

The other option is to buy a power analyser from (e.g.) Fluke, Voltech, or Yokogawa. These instruments will tell you more than you ever wanted to know about your mains supply.

By the way, 87.5[°] between currents in a start and run winding seems very optimistic.

The wiki entry isn't a bad starting point:

http://en.wikipedia.org/wiki/Power_factor

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

 

[waross]

90 degrees between the start and run windings may be the otimum.
On the other hand, a power factor of 87.5% is a little low, but not that bad.

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

 

[ScottyUK]

Hi Bill,

Cos 87.5[°] = 0.0436, not the most outstanding PF I've ever seen!

Anyway who started the trend of quoting PF as a percentage? Is it a US thing?


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

 

[burnt2x]

Scotty,
PF = 1 means I is in phase with V(I wrt V) or 100% of I is in-phase with V. Another way of explaining what PF means!

Bill,
"87.5 deg phase shift between run and start" - I think what it means is that the starting phase shift (or PF) was very low (almost 90 deg from V) compared to running PF, which could be 0.999, cos(90-87.5).

I think this values are only possible with motors with PFC and measured at the input lines.
My guess only.

 

[ScottyUK]

Yeah, I know the definition of PF pretty well, but over here in Europe I don't see anyone quoting cosines as percentage values. Cosines by their definition range from -1 to +1 over 2[Π] radians or rotation. Seems like a worthless exercise to introduce percentages. Just my opinion!

Anyway, the phase angle between the currents in the start and run windings isn't the power factor, it's just a phase angle.


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