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Old 01-11-14, 06:34 PM   #1
doug30293
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Default Bristol 3PH reciprocating with VFD

Surplus City Liquidators has a 2.75 ton 3 phase compressor (their #171546) for $153. Has anyone tried running a conventional 3PH compressor with a VFD?

I was wondering what frequency range might be considered sane for a compressor that probably wasn't intended for inverter duty.

Doug

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Old 01-11-14, 06:38 PM   #2
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Old 01-11-14, 08:15 PM   #3
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The builder in the video said his system ran 2400 watts at 60Hz and around 900 watts at 30Hz. I wonder what the practical relationship is between frequency and compressor output. It would be interesting to see a plot of COP vs frequency.

$153 is a good price for the compressor though I expect the shipping to be another $60. I already have a few VFD's.
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Old 01-12-14, 01:19 PM   #4
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Ok, Doug, I'm going to try to address some of your questions to help you decide whether or not to embark on this adventure. I'm not a refrigeration engineer (far from it), so my answers will be practical in nature.

Conventional 3-phase compressors are not built the same as the new(er) variable speed compressors. The OEM's have come a long way with advancing the state of the art in the last 5 years. You can run a conventional 3-phase compressor at between 50-110% of its rated speed without running into major problems with motor overheating and oil control. As you might imagine, these are the areas in which the mfr's are optimizing their variable speed cxr's. The units specifically designed for speed control have better heat-sinking ability and improved oil pumps that work at lower minimum speeds.

The effect of COP vs. frequency doesn't rely so much on the compressor efficency. In most all applications, power draw is almost linear in relation to refrigerant mass flow. As far as the compressor is concerned, power draw is a function of intake pressure and compression ratio. This relationship is more dependent on flow control and heat exchanger efficiency. The mfr's are trending towards oversizing (sometimes massively) the outdoor heat exchanger to minimize mainly compression ratio.

When speed control is invoked, it is used to match input power with unit capacity. Different mfr's take different approaches to satisfy demand. Some mfr's have decided to maximize COP at the cost of raw BTU output, where others have chosen to sacrifice COP when it gets really hot or cold outside to satisfy demand. During "normal" season, the compressor is throttled back as the unit gets closer to its setpoint, minimizing short cycling, providing better temperature leveling, and reducing input power as a result of higher intake pressure at a certain compression ratio.

For example, Mitsubishi (and many clone brands) have invented a "turbo heat" function that over-revs the compressor in low ambient conditions to maintain BTU output. This function also messes with the indoor fan speed to raise the condenser temperature. Average customers are highly satisfied with this feature, especially those who have little or no backup heat sources. Unlike traditional heat pump units, the turbo units appear to put out high-temp air when the others put out merely medium-temp air.

Last edited by jeff5may; 01-12-14 at 01:36 PM.. Reason: more words
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Old 01-12-14, 04:08 PM   #5
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Quote:
Originally Posted by jeff5may View Post
Conventional 3-phase compressors are not built the same as the new(er) variable speed compressors. The OEM's have come a long way with advancing the state of the art in the last 5 years. You can run a conventional 3-phase compressor at between 50-110% of its rated speed without running into major problems with motor overheating and oil control. As you might imagine, these are the areas in which the mfr's are optimizing their variable speed cxr's. The units specifically designed for speed control have better heat-sinking ability and improved oil pumps that work at lower minimum speeds.
I suspected the latest technology was something more than just a 3 phase motor and VFD. The volume and competitiveness of this market justifies optimal design. BLDC motors perhaps?

50-110% is sufficient for my needs. Short cycling any hard starting motor with a contactor leads to early death. My experience with VFD's at work convinced me that they are probably the best motor protection devices available. The only motors we don't blow up are those on inverters.

I am considering R290 so oil control shouldn't be a problem. The project is primarily for hydronic heating and DHW. If I can come up with a reversing valve I might also use it for cooling my shop by running the water thru a truck radiator.

Quote:
The mfr's are trending towards oversizing (sometimes massively) the outdoor heat exchanger to minimize mainly compression ratio.
I have a perfect example of this. The outdoor coil in my new 2 ton 15 SEER Goodman is about 15% larger than the older 5 ton coil I scrounged for the project.

The 2.75 ton compressor is probably more than I need. The price is right and a VFD gives me the option of fine tuning. Even if I end up at a fixed speed I will still have a better system.

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