Has anyone had any experience with 1

[madddog]

Has anyone had any experience with decreasing the electrical load of a residential HVAC system by using a varible voltage drive to control the speed of the compressor over the heat cycle of a day?

 

[fredb]

Hello madddog,

Go to:

http://www.drivesurvey.com/dcforum/DCForumID8/46.html

for a fairly complete discussion of this idea plus some that you may not have thought of.

 

[madddog]

Thanks for the info Fredb.

Are you familiar with VSDs?

I have to assume that the second person's response comes mainly from the fact that any system used in a residential manner MUST be "user proof" since you are dealing with users typically with no experience in electrical matters.
In this instance the market for this technology might be killed by user if it is not "user proof".

The third response is very interesting and brings up some good points about the reaction of the gas / liquid phase change poblems at lower speeds.

Thanks

 

[ClydeMule]

Maddog:

I have looked at VFD's (VDS's) for capacity control for the systems I build, and don't think it is a good idea. First of all, I think VFD's are great, if applied properly. Unfortunately, it adds a degree of cost and complexity to a residential A/C system that is not justified by the benefits. In addition, there are other technologies that are better suited.

Here are some negatives:
1. Compressors manufactures are really sensistive to you putting anything but line voltage to the compressor. They get real antsy whenit comes down to a warranty situtation.
2. Lubrication- The demand for oil doesn't necessarily decrease proportionally with a decrease in speed. Most res. A/C systems have hermetic compressors and no auxiliary oil pumping system. Your car can't run without oil, nor can a compressor.
3. Scroll compressor technology is overtaking res. A/C. Many are Copeland Compliant scroll. I can't speak for other mfg's, but the compliant sroll uses the centrifugal force of the orbital scroll to maintain the seal to the fixed scroll. If you slow down too much, you lose the seal, and don't pump. This may actually work as an unloading strategy, but is probably not endorsed by Copeland.
4. Some manufacturers (Manuerop/Danfoss) use scrolls with tip seals, and have reciprocatiing compressors that are packaged with VFD's. THe recips are small, and they don't recommend running at less than 40 Hz. I'm not sure that modulating between 40-60 Hz would really add much benefit versus the cost.
5. Someone made a post on that above link about using an external "prime mover;" an open drive compressor. I don't think they currently exist less than 50 HP (For R-22). They would be so much more expensive that it wouldn't be feasible for residential AC even if they made them in the 3-5HP range. Plus the seals leak if not properly maintained. What homeowener will take care of that properly? I probably wouldn't.

If you go to

http://www.copeland-corp.com,

they have some articles about their scroll compressors with built in unloading. They also have a thing called a digital scroll wich unloads the compressor by axially unloading the scrolls (due to the compliance feature I mentioned above). It can modulate faster than a VFD because you don't have to worry about the inertia of the rotor and compressor.

I don't want to be a Luddite. VFD's have many applications in A/C (controlling fan speed to maintain head pressure, works great), varying the rate of chilled water to fan coils, etc.

In short, great concept but in reality, the costs outweigh the benefits.


Clyde the Mule


P.S.- I don't work for Copeland, I just use a ton of their compressors.

 

[quark]

ClydeMule!

I accept your statements except one. The consumption of power reduces by 3.375 times that of full load if you are operating at 40 Hz frequency. (P1/P2 = N1^3/N2^3)

VFDs are proved good with centrifugal chillers.

Regards,

Truth: Even the hardest of the problems will have atleast one simple solution. Mine may not be one.