Can you put a VFD on a regular air conditioner or refrigerator? - Page 1
Forum section: electronics / projects

author: CopperCone
author_rank: Super Contributor
timestamp: July 03, 2018, 07:30:54 pm
content: Rather then relying on a thermostat or having to use peltier coolers, can you put a VFD on the standard consumer compressor pump?

author: schmitt trigger
author_rank: Super Contributor
timestamp: July 03, 2018, 07:43:19 pm
content: Plain vanilla AC compressors are always capacitor start/capacitor run single phase units.The capacitor's capacitance, is tailored to a single specific frequency, to provide a specific reactance value.Adding a variable frequency will modify this reactance, which in turn will change the magnetic vectors required to turn the rotor.

author: Miyuki
author_rank: Frequent Contributor
timestamp: July 03, 2018, 07:49:21 pm
content: Quote from: schmitt trigger on July 03, 2018, 07:43:19 pmThen you can get rid of capacitor and use "single" (two) phase VFD to drive it

author: CopperCone
author_rank: Super Contributor
timestamp: July 03, 2018, 07:49:54 pm
content: I thought you can start and switch with a capacitor motor if the torque is high.

author: CopperCone
author_rank: Super Contributor
timestamp: July 03, 2018, 07:57:08 pm
content: Does the compressor do anything funny being run slow?

author: NiHaoMike
author_rank: Super Contributor
timestamp: July 03, 2018, 07:58:24 pm
content: It has been done before, works very well if done right.

author: james_s
author_rank: Super Contributor
timestamp: July 03, 2018, 07:59:06 pm
content: VFD driven AC compressors are becoming quite common, though I think most of the consumer ones are actually BLDC motors. They still use a thermostat, but they do modulate the compressor speed in order to run as continuously as possible as it's more efficient than cycling off and on which takes several minutes to really get going each time.There's no technical reason why you couldn't add a VFD to a conventional compressor, especially the 3 phase type common in commercial units although I recall you do have to be careful about high voltage spikes causing arcing. They make special inverter grade motors with improved insulation.

author: CopperCone
author_rank: Super Contributor
timestamp: July 03, 2018, 08:38:23 pm
content: Hmm he says its usable from ~25-70 Hz. Do you think you can put some kind of thicker grease in there to shift that frequency range down? Any ideas? So it can run like 5-30Hz. Would it kill just peak power or efficiency too?

author: NiHaoMike
author_rank: Super Contributor
timestamp: July 03, 2018, 08:47:55 pm
content: I think it's a limitation of the oil pump design, which is usually centrifugal. Almost a 3:1 speed dynamic range is not bad at all for a HVAC application. Some designs like what Gree uses can do more, although I'm not sure the actual speed dynamic range. (The "power dynamic range" is often quoted because that gives very impressive looking numbers.)

author: CopperCone
author_rank: Super Contributor
timestamp: July 03, 2018, 08:56:39 pm
content: I thought they just dip into an oil pool on the bottom?

author: Miyuki
author_rank: Frequent Contributor
timestamp: July 03, 2018, 09:02:10 pm
content: All compressors have efficiency curve which drop on high speed and low speed, with best performance somewhere in midleThere is many types some sit in oil, some uses refrigerant oil mixture circulating trought system, some have pressire oilDatasheet will say more

author: schmitt trigger
author_rank: Super Contributor
timestamp: July 03, 2018, 09:15:24 pm
content: Quote from: james_s on July 03, 2018, 07:59:06 pmExactly correct.3 phaseinduction motors do not require start and or run capacitors. As such there is no frequency dependent reactance.It is true that a two phase VFD could be used. However, the auxiliary (2nd phase winding) has a different voltage rating than the main winding. The reason being that the capacitor's reactance will cause the applied winding voltage to be different to the line voltage. The VFD would have to compensate for that.As NihaoMike mentioned, it will work, if done correctly.

author: james_s
author_rank: Super Contributor
timestamp: July 03, 2018, 09:41:11 pm
content: The oiling system typically used in refrigeration compressors is pretty clever. There's a spiral groove around the bottom end of the shaft which normally sits in an oil sump. That lifts oil up through the hollow shaft where it is sprayed out by the spinning shaft and rains down on the moving parts. Oil also circulates with the refrigerant and that keeps the inside of the compressor lubricated. I'm sure there is a speed range below which the oiling system would fail to work adequately but that is likely to be well outside the reasonable RPM range you'd want to run anyway. You don't need it to go from 0-100%, even a range from 50-100% can result in significant energy savings.This is especially true of heat pumps where the ideally sized system is significantly larger for heating than for cooling. At least one manufacture made a dual-displacement compressor for that application, IIRC it used a single speed motor that ran one direction to drive the main piston while the secondary crank freewheeled, then reversed to engage both pistons.

author: NiHaoMike
author_rank: Super Contributor
timestamp: July 04, 2018, 12:24:09 am
content: Quote from: james_s on July 03, 2018, 09:41:11 pmThat's true for most areas in the US, but some places near the south like Houston would need a lot more cooling than heating.

author: james_s
author_rank: Super Contributor
timestamp: July 04, 2018, 01:52:59 am
content: Well in that case the variable speed or variable displacement compressor is still beneficial, just in the opposite direction. What I should have said is that heating and cooling needs are rarely balanced.

author: tecman
author_rank: Frequent Contributor
timestamp: July 05, 2018, 12:45:13 am
content: Be careful. Some compressors, like scrolls, are designed that the running speed and centrifugal forces are part of the design for sealing forces. Reduced speed can effect efficiency and may lead to premature failure.paul

author: james_s
author_rank: Super Contributor
timestamp: July 05, 2018, 01:22:34 am
content: In most cases they should still work fine over a significant useful range. There are certainly scroll compressors designed for inverter drive, and it's probably not going to make sense to run one below about 50% of the rated speed anyway.