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Originally Posted by jeff5may
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.
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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.
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The mfr's are trending towards oversizing (sometimes massively) the outdoor heat exchanger to minimize mainly compression ratio.
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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.