Variable Frequency (Inverter) Drive

[randyguthrie]

I cant seem to find any useful references to variable frequency compressors on this site, which is strange given the other lengths that people seem to be going to with their refrigeration units.

One post from two years ago talked about building a variable frequency drive from scratch, which I think is silly given that you can buy one on Ebay for around $100. (Search under Inverter Drive.)

Phase change coolers would seem to be an ideal application for variable speed compressors given the unique demands of CPU cooling. The main issue is that the compressor cannot be turned off while the CPU is running, even if the CPU is idling. This problem leads to higher electric bills and wear on the compressor. A variable speed drive can slow down the compressor so that it still keeps the CPU cold but while running at a lower power level.

Another interesting use of a VFD is for running the compressor faster than 60Hz. Some compressors might be burned out by the extra load but others might be ok at 80Hz or even 100Hz. People will get much better results from running the compressor faster than by adding extra fans and stuff.

I have a Mach II GT and its stock compressor's specs are as follows:
http://de.refrignet.danfoss.com/TechnicalInfo/Literature/Manuals/06/NF9FX_R134a_115V_60Hz_07-03_Cd43r822.pdf

Danfoss also makes the identical unit in a variable speed configuration. The specs are as follows:
http://de.refrignet.danfoss.com/TechnicalInfo/Literature/Manuals/06/nlv84f_r134a_115v_50-60hz_08-2007_ed300c222.pdf

Given that the variable speed unit is factory spec'ed at 4000RPM that would seem to indicate that this compressor can be safely operated at at least 70Hz, or 4200RPM.

I am hoping that maybe someone who builds or modifies phase change coolers professionally will try adding a VFD and share the performance figures with us. I dont want to do the prototyping work for just one unit since I am not in that business.

Adding a VFD is very easy. You just place it in the AC line between the compressor and the controller. You can then adjust the frequency manually. Figuring out how to make the drive slow down and speed up when you want it to is a bit more difficult.

[[XC] gomeler]

I have considered this idea in autocascade and cascade situations but I was never able to find a single-phase inverter that was cheap enough to warrant the experiment. My goal was to use the inverter to manipulate the compressor and therefore adjust the massflow. If I want the evap to warm up, slow it down, and vice versa. I want to do something like this as it doesn't involve fiddling with valves but simply a single interface that could then be compared with a look-up chart of performance at various frequencies.

Just searching through ebay, most of those inverters are meant for 3 phase to 3phase or 1 phase to 3 phase. I've never even looked into 3 phase power so I wouldn't know where to start :shrug:

[randyguthrie]

You are right there are not any single phase units right now, but they do come up if you keep looking. I found a new one at

http://www.galco.com/scripts/cgiip.exe/wa/wcat/itemdtl.r?listtype=Catalog&pnum=MVX001A0-1-CHGP

Obviously that is not cheap, but for people looking to save money it should be easy to get one out of a junked A/C unit or fridge.

[sjg0]

Hey gomeler,

I dont think this would work because if you speed up the frequency then the rpms increase, but if the compressor is not powerful enough (in terms of hp) when running faster then the compressor may not be able to compress the gas at these faster speeds, which could cause the compressor to "hiccup" or stop. That sounds pretty bad to me especially if you are running R290. Do you get what Im saying? What do you think?

[randyguthrie]

That is why you have to be careful about how fast you drive the compressor. 70Hz should be no problem though.

I found a cheap 110V single phase drive unit, but it is only 1/2HP. My compressor is 3/4HP.

http://cgi.ebay.com/Allen-Bradley-PowerFlex-4-AC-Drive-22A-V2P3N104_W0QQitemZ280312937225

[sjg0]

That is why you have to be careful about how fast you drive the compressor. 70Hz should be no problem though.

I found a cheap 110V single phase drive unit, but it is only 1/2HP. My compressor is 3/4HP.

http://cgi.ebay.com/Allen-Bradley-PowerFlex-4-AC-Drive-22A-V2P3N104_W0QQitemZ280312937225

70Hz? Oh. I thought you would be trying much higher frequencies. I dont really see a 10Hz frequency increase justifying the trouble of making such a setup. Would be interesting to see results though, and how far exactly the frequency could be pushed before you reach the compressors limit.

Buy 2 and put them in parallel lol :). Only thing is you have to make sure the frequencies are synced almost exactly i think. I dont really know anything about VFD's, but shouldn't they have automatic frequency drift correctors and since they both take the same 60Hz from the wall.....

[Stewie007]

VFDs aren't designed to make a 60Hz motor operate at 70Hz. They are designed to slowly increase the frequency at start up and to match the demands of the system. Motors are rated for either 50 or 60Hz depending on the power grid to which they will be accessing. In the US, it is typically 60Hz. If you think you can run the compressor a greater speeds then you might as well crack some eggs on that one. VFDs are for control, not Tim the Tool Man Taylor rigings to "Haurgh Haurgh Huargh" beef up your system.

[aenigma]

I guess something like this would also be good to get around a coldbugged CPU?

[Moc]

With the correct compressor there is no problem: Here a link to a video made by August123 (who buildt the massive 4-stage cascade) :
http://august123.sp3.speed-hoster.eu/Video023.MP4
He let the compressor run with 200Hz :D !

[randyguthrie]

VFD's can be used for whatever you want to do. Some of them can go up to 400Hz or even more. Dont try to run a 60Hz motor at 400Hz though. Just look at my original two links. In one of them you can see a 60Hz motor being run at 4000RPM, or 67Hz.

Variable speed also helps to transfer the maximum amount of power to the fluid. It is like having a variable pitch prop on a boat. For high flow applications it is better to run a pump at lower speed while for high pressure applications it is better to run the pump at a higher speed. I dont know how this principal applies to a refrigeration system, but it stands to reason that people who are using custom freon might benefit from adjusting their compressor speeds since the custom freon might have a different viscosity from the stock freon.

[[XC] gomeler]

Hey gomeler,

I dont think this would work because if you speed up the frequency then the rpms increase, but if the compressor is not powerful enough (in terms of hp) when running faster then the compressor may not be able to compress the gas at these faster speeds, which could cause the compressor to "hiccup" or stop. That sounds pretty bad to me especially if you are running R290. Do you get what Im saying? What do you think?

My goal wouldn't be to increase capacity, it would be to decrease capacity in idle and low-power situations. I can't imagine there being a problem with running the compressor at 40Hz or lower.

[Stewie007]

True, but you have to buy multiphase compressors..... I guess I should have known though.... You folks probably don't care about spending a ton of money on this stuff... :-p hehe

[SexyMF]

To slow the compressor down you can just run at the lower frequency. This works fine as long as you don't go to slow (the compressor gets really hot).

But really, a hot gas bypass is easier for load control.

Stewie007, inverters are cheaper than compressors, and they are not just designed for soft-starting.

[sjg0]

gomeler;3654048']My goal wouldn't be to increase capacity, it would be to decrease capacity in idle and low-power situations. I can't imagine there being a problem with running the compressor at 40Hz or lower.

..... Umm do you mean making a non-custom phase unit? So one that would run on a 25W Atom and a 350W i7 without any retuning, simply decrease frequency?
.... Sounds like an interesting idea....

[Stewie007]

To slow the compressor down you can just run at the lower frequency. This works fine as long as you don't go to slow (the compressor gets really hot).

But really, a hot gas bypass is easier for load control.

Stewie007, inverters are cheaper than compressors, and they are not just designed for soft-starting.

Yes, but how many 3 phase compressors are 115V? So if you don't have 230/208 power you're gonna need it. That generally means an electrician is gonna have to run a line to your basement, because most houses have appliances that use such power. I wouldn't recommend splicing it yourself. You can run any compressor at increased frequency; the question is for how long. I could pep up my car to 350 HP with modifications, but it doesn't mean that my transmission is gonna handle it well for long.

All of the extra costs that go into your power bill, and chiller cost is prohibitive, and that's why you don't see people doing it. That is not to say that it won't work. I'm sure that it will, and I agree that I really can't argue that it will not. I simply am saying that the cost-benefit to doing such a system is obviously deemed not work it by most folks here.

[randyguthrie]

There are a few single phase inverters out there. There seem to be even more with single phase input and three phase output. It should be possible to only use one out of the three phases on the output side. You just have to use an inverter that is three times larger than you need since the rating is for 3x per phase.

Running the compressor slow is what inverters are used for in energy efficient fridges and A/C units. I have them and I really like the A/C not blowing warm and then too cold. With an inverter it blows cool all the time. These units for the home are almost all single phase and generally 110V hence a good source for parts.

Once again here is a new 110V single phase inverter drive unit http://www.galco.com/scripts/cgiip.e...VX001A0-1-CHGP

[[XC] Lead Head]

You can actually build your own VFD using a stereo amplifier and a transformer/variac. I've seen it done before, and they work quite well provided you can find a large enough audio amplifier.

Alls you have to do is feed the amplifier/receiver a sine wave at the frequency you want

[randyguthrie]

You would have to get a custom transformer made so as to have the correct output voltage since I dont think that stereo amps come with 170V PP rails.

[[XC] gomeler]

..... Umm do you mean making a non-custom phase unit? So one that would run on a 25W Atom and a 350W i7 without any retuning, simply decrease frequency?
.... Sounds like an interesting idea....

No. I mean to make an autocascade that can be dialed back to save on power when it is idling or if I'm dancing around a chip's coldbug temp. A lot of my time benching is spent tweaking with down-time between each change. It would be nice to slow the compressor down by 30% or 50% and then ramp it back up once I need full cooling capacity.

[Stewie007]

Running the compressor slow is what inverters are used for in energy efficient fridges and A/C units. I have them and I really like the A/C not blowing warm and then too cold. With an inverter it blows cool all the time. These units for the home are almost all single phase and generally 110V hence a good source for parts.

Once again here is a new 110V single phase inverter drive unit http://www.galco.com/scripts/cgiip.e...VX001A0-1-CHGP

That's what I was saying... slow start up, variable frequency to ramp up to full load.

However, I don't know of many home units that use 110V... Window A/C units do, but when you're talking 3 Tons, which is typical for a home, its 230V.

[sjg0]

gomeler;3654743']No. I mean to make an autocascade that can be dialed back to save on power when it is idling or if I'm dancing around a chip's coldbug temp. A lot of my time benching is spent tweaking with down-time between each change. It would be nice to slow the compressor down by 30% or 50% and then ramp it back up once I need full cooling capacity.
Ohhhh... ok. That would be a pretty cool feature :up:

I just saw on craigslist 2 VFD's for $50 the other day. They were for I think 4HP and could be wired for 1phase(120V) in and 3phase out OR 1phase(120V) in and 3phase out. :)
Maybe I should have picked one up... Cant find the ad anymore.... :shakes:

[NotSoCoolJ]

You could get a compressor for a small car and use a brushless motor and controller to vary the speed at will. Sounds like a good idea to me. Many RC motors are capable of well over 1hp. The only problem here is that RC controllers use a DC supply. If solar panels where cheaper I would go that route and use a battery system and some solar panels. Green PC cooling at its best. Too bad you would need to spend $4000 in panels to have enough power.

Another option is a DC compressor. You could easily construct a controller for variable speed. Uncle Jimbo has designed a TEC controller that would do the job with a few modifications.

[randyguthrie]

People dont like changing brushes in DC motors.

[randyguthrie]

Ever since inverter drives became cheap about ten years ago AC motors have been replacing DC motors for variable speed applications.

[SexyMF]

The specs of a motor are rated at a given frequency but that is only a point on a graph. A 60Hz built works at 50Hz. You just get different performance characteristics.

Also inverter drives don't just come in voltage in = voltage out range. A lot of them are made to take phase voltage in and line voltage out (ie 230 in, 415 out)

[randyguthrie]

What is the difference between phase voltage and line voltage? Are you talking about phase to phase vs ground to phase?

[[XC] Lead Head]

You would have to get a custom transformer made so as to have the correct output voltage since I dont think that stereo amps come with 170V PP rails.


Some actually do, but that is why I said use a Variac, or measure the output voltage of amplifier and find a suitable transformer. Most 120v things will run just fine +/- 10 volts

[SexyMF]

Line voltage is line-line (415V/208V etc)
Phase is line-neutral (230V/115V)

Voltages talked about are nominal. +/- %10 variation is expected.


You could run 3 small compressors of a VFD with phase voltage out.