VFD's

launboy · 08-05-12, 02:57 AM

Variable Frequency Drives. Could installing one of these on the compressor and possibly fans of a standard air conditioner give one Inverter drive efficiencies? It would have to be a TXV system I'm guessing, to keep correct metering with the slowed compressor speeds, but I don't see why this couldn't work?

Also, sidenote here that didn't seem worth posting to its own thread, as far as converting a mini split to air-water, couldn't you simply replace the coil with a HX, replace the fan with a variable speed pump, and move the sensors to corresponding places? I.e air out temp sensor on water out of exchanger, etc, and just let the unit follow the programming code its got build in?

Food for thought from a boy with big dreams but no cash to support them right now.

P.s. picked up 2 Dehumidifiers from the trash last week. One was flat on charge, other cooled well. dunked the evap coil in the condensate bucket, moved the condensor coil so the unit is a smaller package, and I have myself a small chiller. The condenser coil from the flat dehu fits perfectly on a small fan I had. Even used an old A/C thermostat that I've set to maintain chiller water temp at 35*. Works pretty well.

Adam

launboy · 08-08-12, 04:42 PM

No one thinks its a good idea? lol

MN Renovator · 08-08-12, 06:42 PM

The idea around inverter drives being used in air conditioners for higher efficiency has two parts:
1. They use a BLDC motor and that brushless DC motor(HVAC industry usually calls these ECM motors) is more efficient than an AC motor, which has a fair amount of slip built in which reduces efficiency. AFAIK a BLDC motor is about as efficient you can get on the motor side but it moves some efficiency losses to the inverter but it's less loss than you get if you go with the traditional PSC motor that's in most central air conditioning systems today.
2. The inverter drive allows for you to modulate capacity. This is a great way to improve efficiency because the larger the surface area of the condenser and evaporator coils in your system, the more efficient that the heat transfer is and also less amperage from the compressor. When you modulate the capacity you essentially are making the ratio of coil size to capacity and making a bigger coil in relation to the capacity.

What does this mean for you? You will need to very carefully control the speed of the blower against the evaporator coil. If you lower the speed/capacity of the compressor and don't lower the air flow you lose the temperature drop over the evaporator and lose dehumidification as that air isn't getting pushed to an appropriate temperature to bring it to a dew point that causes the air to sweat up the coil and dehumidify. If you lower the air flow too much, you will lose sensible efficiency and the temperature of the room won't drop as much but you also stand to freeze up the coil and then the refrigerant might still be a liquid when it gets back to the compressor and that liquid could slug the compressor and kill it.

The other thing to consider is that since you are using the compressor the unit originally had and using a VFD, the VFD is adding switching losses and you are still using a PSC compressor that is slippy and might also not be as efficient at a different speed than it was designed for.

So in my opinion, it could work but it'll be tricky to get it right and the effort involved might be a bit much for what might be little gain unless you can get it all perfect.

It's up to you but I don't have any suggestions as to how to do what you are trying to do other than aim for air handler air flow that creates a 20 degree drop over the evaporator for the best efficiency(unless you are going the chiller or dehumification route) and make sure the coil doesn't freeze.

launboy · 08-08-12, 07:33 PM

Very interesting, thanks MN. It would be interesting to get an ammeter and see what kind of an efficiency difference you could actually get. Can you subcool the liquid too much? What happens if you do?

Anyone have any input on the mini split idea?

randen · 08-08-12, 08:38 PM

launboy

VFD s are made for the three phase motors and not single phase (either 110 VAC or 240 VAC) they are very cool however for smaller systems of less then 5 hp. I don't see much of an advantage. As mentioned by MN Renovator other considerations are required for the all the speeds of the fans & pumps in GSHPs and these motors should be the BLDC and speeds and capacities programmed with 100s of hours of development time. Building your own GSHP with nice large HX s for excellent performance is do-able. Spend the 100s of hours in insulating and other envelope performance endeavers. How air tight can you get your home?? What about building a air exchange unit with a HP heat exchange unit.

Randen

08-05-12, 02:57 AM	#1
launboy Apprentice EcoRenovator Join Date: Mar 2011 Location: SE Wisconin Posts: 107 Thanks: 3 Thanked 11 Times in 9 Posts	VFD's Variable Frequency Drives. Could installing one of these on the compressor and possibly fans of a standard air conditioner give one Inverter drive efficiencies? It would have to be a TXV system I'm guessing, to keep correct metering with the slowed compressor speeds, but I don't see why this couldn't work? Also, sidenote here that didn't seem worth posting to its own thread, as far as converting a mini split to air-water, couldn't you simply replace the coil with a HX, replace the fan with a variable speed pump, and move the sensors to corresponding places? I.e air out temp sensor on water out of exchanger, etc, and just let the unit follow the programming code its got build in? Food for thought from a boy with big dreams but no cash to support them right now. P.s. picked up 2 Dehumidifiers from the trash last week. One was flat on charge, other cooled well. dunked the evap coil in the condensate bucket, moved the condensor coil so the unit is a smaller package, and I have myself a small chiller. The condenser coil from the flat dehu fits perfectly on a small fan I had. Even used an old A/C thermostat that I've set to maintain chiller water temp at 35*. Works pretty well. Adam

08-08-12, 04:42 PM	#2
launboy Apprentice EcoRenovator Join Date: Mar 2011 Location: SE Wisconin Posts: 107 Thanks: 3 Thanked 11 Times in 9 Posts	No one thinks its a good idea? lol

08-08-12, 06:42 PM	#3
MN Renovator Less usage=Cheaper bills Join Date: Nov 2010 Location: Minneapolis, MN Posts: 939 Thanks: 41 Thanked 116 Times in 90 Posts	The idea around inverter drives being used in air conditioners for higher efficiency has two parts: 1. They use a BLDC motor and that brushless DC motor(HVAC industry usually calls these ECM motors) is more efficient than an AC motor, which has a fair amount of slip built in which reduces efficiency. AFAIK a BLDC motor is about as efficient you can get on the motor side but it moves some efficiency losses to the inverter but it's less loss than you get if you go with the traditional PSC motor that's in most central air conditioning systems today. 2. The inverter drive allows for you to modulate capacity. This is a great way to improve efficiency because the larger the surface area of the condenser and evaporator coils in your system, the more efficient that the heat transfer is and also less amperage from the compressor. When you modulate the capacity you essentially are making the ratio of coil size to capacity and making a bigger coil in relation to the capacity. What does this mean for you? You will need to very carefully control the speed of the blower against the evaporator coil. If you lower the speed/capacity of the compressor and don't lower the air flow you lose the temperature drop over the evaporator and lose dehumidification as that air isn't getting pushed to an appropriate temperature to bring it to a dew point that causes the air to sweat up the coil and dehumidify. If you lower the air flow too much, you will lose sensible efficiency and the temperature of the room won't drop as much but you also stand to freeze up the coil and then the refrigerant might still be a liquid when it gets back to the compressor and that liquid could slug the compressor and kill it. The other thing to consider is that since you are using the compressor the unit originally had and using a VFD, the VFD is adding switching losses and you are still using a PSC compressor that is slippy and might also not be as efficient at a different speed than it was designed for. So in my opinion, it could work but it'll be tricky to get it right and the effort involved might be a bit much for what might be little gain unless you can get it all perfect. It's up to you but I don't have any suggestions as to how to do what you are trying to do other than aim for air handler air flow that creates a 20 degree drop over the evaporator for the best efficiency(unless you are going the chiller or dehumification route) and make sure the coil doesn't freeze.

08-08-12, 07:33 PM	#4
launboy Apprentice EcoRenovator Join Date: Mar 2011 Location: SE Wisconin Posts: 107 Thanks: 3 Thanked 11 Times in 9 Posts	Very interesting, thanks MN. It would be interesting to get an ammeter and see what kind of an efficiency difference you could actually get. Can you subcool the liquid too much? What happens if you do? Anyone have any input on the mini split idea?

08-08-12, 08:38 PM	#5
randen Uber EcoRenovator Join Date: Apr 2011 Location: Strathroy Ontario Canada Posts: 657 Thanks: 9 Thanked 191 Times in 129 Posts	launboy VFD s are made for the three phase motors and not single phase (either 110 VAC or 240 VAC) they are very cool however for smaller systems of less then 5 hp. I don't see much of an advantage. As mentioned by MN Renovator other considerations are required for the all the speeds of the fans & pumps in GSHPs and these motors should be the BLDC and speeds and capacities programmed with 100s of hours of development time. Building your own GSHP with nice large HX s for excellent performance is do-able. Spend the 100s of hours in insulating and other envelope performance endeavers. How air tight can you get your home?? What about building a air exchange unit with a HP heat exchange unit. Randen