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01-21-2025, 09:35 PM
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#1
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Montana Fan
Join Date: Dec 2023
Location: Green Cove Springs
Posts: 156
M.O.C. #33751
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VFD for air conditioners
I'm sure this thread will stir up the group.
I want to change my air conditioner compressors to run on 220 volt, three phase power. I want to use small VFDs (variable frequency drive) to run the air conditioners. The VFD uses 120 volt input then converts the output to 3 phase 220 volt. The VFD in conjunction with the 3 phase motor, replaces the capacitor start type of motor typically used in most 120 volt appliances, and eliminates the need for a separate soft start. The result is much better reliability with built in controlled soft start, and the cost of a 3 phase motor / VFD is slightly less than a typical 120 volt motor and soft start. The 3 phase setup eliminates the capacitor(s) which is usually the failure item in these circuits.
My problem... I can't find 3 phase compressors for these air conditioners.
Has anyone ever done this and if so, where did you get the 3 phase compressors.
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01-22-2025, 05:14 AM
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#2
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Montana Fan
Join Date: Feb 2022
Location: Cynthiana
Posts: 371
M.O.C. #30449
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I like your thinking, but there are a number of obstacles ahead of you.
There are numerous VFDs on the market that have 120 volt input and 240 volt 3 phase output and they're pretty cheap. That's the easy part. As you mentioned, finding a compressor that size will be a challenge. Typical RV air conditioners are 15 KBTU in the rear, and a 13 KBTU in front, which both are an oddball size. I believe you'll find most aftermarket compressors are only available in 0.5 ton increments. A ton of AC is 12,000 BTU. They will normally start at 1.5 ton and come in 0.5 ton increments up to 5 tons. A 1.5 ton is 18 KBTUs. Should you stumble onto a 15 KBTU model, will it even fit into the shroud? Since they normally come pre charged to keep them dry; they might require an EPA license to purchase. Compressors in this size range are hermetically sealed with the motor and compressor both contained in a steel capsule which is welded together, there's no such thing as just replacing the motor. Copeland is by far the largest manufacturer of compressors in this size range with Tecumseh a distant second.
If you should find a compressor, you will have to cut into the refrigerant tubing going to the coil. This will require brazing in service valves, purging the coil and tubing with dry nitrogen to dry any residual condensate, a vacuum pump to evacuate all of the air, and a set of gauges to properly charge the system. I have no idea if you have the proper tools or maybe you could rent or borrow them.
There's also a blower motor in that unit that will still require a capacitor. A true variable speed AC varies the compressor speed, the refrigerant flow with an electronic VRF (variable refrigerant flow) valve and the blower speed all in proportion and usually controlled by some proprietary controls.
Another option would be just install one of the new Furrion Cube Chill ACs. They are made by LG who has a fairly long history of building quality variable speed units. I recently read they have now released a ducted model. You will be ahead in budget and far ahead in comfort.
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01-22-2025, 07:22 AM
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#3
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Montana Master
Join Date: Jan 2018
Location: Granger TX
Posts: 2,881
M.O.C. #21044
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Quote:
Originally Posted by Bourbon County
Another option would be just install one of the new Furrion Cube Chill ACs. They are made by LG who has a fairly long history of building quality variable speed units. I recently read they have now released a ducted model. You will be ahead in budget and far ahead in comfort.
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18k Furrion Cube Chill on Amazon for $925!
Bourbon County’s excellent description of the DIY backyard approach wore me out just reading it. I would have ZERO chance at getting it done by myself…however, with YouTube’s help plus this forum (and my son’s help getting the thing up on the roof and the old one off the roof) - I think I could get it done. It seems like pushing the EASY BUTTON compared to stick-building a new AC.
__________________
MikenDebbie Aggie ‘77 in the sticks near Austin TX
2019 Chevy 3500 High Country DRW
2018 Montana 3921FB
Aussie Gus + Texas Heeler Jimmy
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01-22-2025, 09:11 AM
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#4
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Montana Master
Join Date: Oct 2004
Location: Salem
Posts: 7,783
M.O.C. #2283
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I have a machine shop with 3 CNC machines and many manual machines. It’s all ran off of a 40 HP electric motor and a phase converter. What I can’t understand is why you would want to use a phase converter if you can get by without it.
Lynwood
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01-26-2025, 02:27 PM
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#5
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New Member
Join Date: Jan 2021
Location: Platte City, MO
Posts: 3
M.O.C. #28017
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Quote:
Originally Posted by Mikendebbie
18k Furrion Cube Chill on Amazon for $925!
Bourbon County’s excellent description of the DIY backyard approach wore me out just reading it. I would have ZERO chance at getting it done by myself…however, with YouTube’s help plus this forum (and my son’s help getting the thing up on the roof and the old one off the roof) - I think I could get it done. It seems like pushing the EASY BUTTON compared to stick-building a new AC.
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JMO of course, but Furrion appliances for RV's don't seem to have the best reputations for reliability.
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01-26-2025, 06:10 PM
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#6
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Montana Fan
Join Date: Dec 2023
Location: Green Cove Springs
Posts: 156
M.O.C. #33751
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Using a VFD to drive the refrigerant compressor and the condenser cooling fan will improve reliability because the capacitor, contactor, and centrifugal switch for each motor are eliminated. The 3 phase motor also draws less amperage so the wiring to the motors can be smaller.
As an example, the biggest failure of central AC units in Florida is the capacitor for the condenser cooling fan which uses a capacitor start, single phase fractional HP motor. A service call to replace that capacitor is typically $200. The capacitor costs about $35.
With the progress of current VFD technology we shouldn't be using capacitor start motors anywhere. The cost of a VFD for the cooling fan is less than $100, which is less than the cost of the capacitor and contactor.
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01-26-2025, 09:07 PM
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#7
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Montana Fan
Join Date: Mar 2019
Location: Heber City
Posts: 145
M.O.C. #23676
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Furrion
Quote:
Originally Posted by HoopyFrood
JMO of course, but Furrion appliances for RV's don't seem to have the best reputations for reliability.
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That’s for sure. I’m on my fourth Furrion 12v fridge in 4 months on a brand new 3123RL.
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01-27-2025, 07:12 AM
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#8
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Montana Fan
Join Date: Feb 2022
Location: Cynthiana
Posts: 371
M.O.C. #30449
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Quote:
Originally Posted by jkxtreme
Using a VFD to drive the refrigerant compressor and the condenser cooling fan will improve reliability because the capacitor, contactor, and centrifugal switch for each motor are eliminated. The 3 phase motor also draws less amperage so the wiring to the motors can be smaller.
As an example, the biggest failure of central AC units in Florida is the capacitor for the condenser cooling fan which uses a capacitor start, single phase fractional HP motor. A service call to replace that capacitor is typically $200. The capacitor costs about $35.
With the progress of current VFD technology we shouldn't be using capacitor start motors anywhere. The cost of a VFD for the cooling fan is less than $100, which is less than the cost of the capacitor and contactor.
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Inverter driven residential split system ACs have been on the market for years, mini-splits even longer. The Copeland variable speed scroll compressors use a BPM (brushless permanent magnet) DC motor. The DC motors have a better torque to speed ratio than AC and can produce nearly 100% torque at very low speeds. They use ECM (electronically commutated motor) for the condenser blowers and air handler blowers. They are extremely efficient, especially in heat mode on a heat pump. They are also very reliable; manufacturers offer very long warranties on the compressors. They use communicating controls, the compressor, both blowers, and refrigerant metering valve all have to work in proportion. The variable speeds do an amazing job of dehumidification too; at very low or no cooling loads, they never completely cycle off, just slow to a very low speed and are still removing humidity.
If you can get a capacitor replaced for $200, that's a bargain! If you have any DIY skills, that's the easiest thing to replace on the system other than maybe a filter. Buy a capacitor tester for about $20, check it every spring. If it reads >5% below nameplate rating, replace it.
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01-27-2025, 07:17 AM
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#9
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Montana Fan
Join Date: Feb 2022
Location: Cynthiana
Posts: 371
M.O.C. #30449
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Quote:
Originally Posted by HoopyFrood
JMO of course, but Furrion appliances for RV's don't seem to have the best reputations for reliability.
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The Furrion Chill Cube models are LG variable speed mini-splits in a Furrion housing and badge. Check out the reviews on the LG mini's; they are awesome products. They've only been on the market for about a year and doubt there's a lot of data available on them.
I really hope these work out and other manufacturers go this route. It can possibly be a game changer for one of the most problematic systems on an RV
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