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VFD on Heat Pump Compressor

01/28/2025 7:37 AM

Will a VFD work on a standard heat pump compressor without damaging the compressor or other components?

This will be used only for the compressor, not any other controls, like reversing valves, fans, etc. although a fan is possible at a future date.

I am considering implementing a VFD and a 3mode (PID) controller (instead of the thermostat) for varying the speed according to demand. I know they make VFD drives for the newer heat pumps but I do not know the extent of the associated changes.

I have a used compressor that can be used as a test but I would like some advice before I start.

Any advice or help on this from an expert will be sincerely appreciated.

Thanks in advance for your time and efforts on this.

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#1

Re: VFD on Heat Pump Compressor

01/28/2025 7:53 AM

What is driving the speed change setpoint input in the <...VFD...>?

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#2
In reply to #1

Re: VFD on Heat Pump Compressor

01/28/2025 10:40 AM

The temperature difference between set point and actual temperature.

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#3
In reply to #2

Re: VFD on Heat Pump Compressor

01/28/2025 11:30 AM

The thermostat is a high gain temperature signal amplifier, in control system terms. Stability of a system is ensured by the slow rate at which temperature varies in most systems in response to various inputs.

There is rarely a need to use an analog signal to control temperature; one exception might be the thermal cracking stills used in oil refineries where temperature overshoot can seriously reduce the working life of the tubes used in the cracking process.

Therefore though it would possibly work, the use of a <...VFD on Heat Pump Compressor...> would appear to be overkill - a solution looking for a problem, perhaps.

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#6
In reply to #3

Re: VFD on Heat Pump Compressor

01/28/2025 4:26 PM
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#15
In reply to #6

Re: VFD on Heat Pump Compressor

01/29/2025 2:55 AM

It sounds as though this issue is already dealt with; no further comment is necessary.

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#14
In reply to #3

Re: VFD on Heat Pump Compressor

01/29/2025 1:56 AM

Industrial control systems are not all slow responding. Flow is very fast and dynamic, while heating a large vessel of liquid is slow. The ideal system for either type, or in between these extremes is a system that will maintain setpoint with minimum variation even during an upset of the system.

A 3 mode control system is required to do this. (PID) Proportional (gain) control responds to the amount of deviation from setpoint but cannot drive the process back to setpoint. There will always be a deviation from setpoint. Integral(reset) responds to the gain of the system and drives the process back to the setpoint, but there is a delay and possible overshoot. Derivative responds to the rate of change of the deviation and adjusts gain accordingly to minimize offset. Most modern 3 mode control systems do it automatically now, but in the past they were adjusted manually, which can sometimes be time consuming.

There were many methods to do this but I always used the momentary upset to adjust gain for a sustained oscillation, and then adjusted integral according to the frequency of the sustained oscillation, and derivative at about 1/8 of the integral.

This always put me in the ball park, only requiring minor tweaking to get a very stable system.

The most efficient heat pump is one that is properly sized and runs all the time and is stable at all temperatures regardless of the outside/inside demands.

Some will disagree with this, but it is true nonetheless.

Moving heat is roughly 3 times more efficient than producing heat. A 10kw heat strip is equivalent to about 30,000 BTU at 3 times the power requirement of a heat pump to move that much heat.

The newer VFD scroll compressors have a proven higher SEER than standard on-off control.

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#4

Re: VFD on Heat Pump Compressor

01/28/2025 12:07 PM

What are you trying to do here? If you are trying to vary the speed of the compressor to match the load, this is already done with the fan speed....If you are trying to reduce the current spike in starting the compressor to save energy, these are already available...

The jury is still out on how well these work at reducing energy and promoting longevity of compressor...theoretically it should work, but as we all know the devil is in the details....

Lastly you don't want the AC unit to match the load as the unit will run all the time which will raise your electric bill and shorten the life of the system..at least in residential units, in commercial application you might have other goals...but ideally the compressor produces more than the heat load calls for so the unit only runs 50% of the time during peak load...

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#5
In reply to #4

Re: VFD on Heat Pump Compressor

01/28/2025 4:20 PM
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#8
In reply to #5

Re: VFD on Heat Pump Compressor

01/28/2025 7:30 PM

Total BS...

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#7

Re: VFD on Heat Pump Compressor

01/28/2025 4:32 PM

Imagine driving a car where you put the pedal to the floor, and then totally release it every mile. Repeat when car slows down by 1 mile per hour.

Compare this to cruise control.

This is a normal heat pump compressor. Simple ON_OFF control. There will always be an offset from setpoint in a on-off control.

A VFD with proper 3(PID) mode tuning will never have an offset from setpoint.

Which do you think is more efficient?

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#9
In reply to #7

Re: VFD on Heat Pump Compressor

01/28/2025 8:35 PM

It's much more important to have a properly sized unit, properly installed, and well insulated space, than a complicated expensive unreliable machine that promises perfection with no guarantees...choose wisely grasspopper...

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#18
In reply to #7

Re: VFD on Heat Pump Compressor

01/30/2025 7:45 PM

Ah…

Imagine driving a car where you put the pedal to the floor, and then totally release it every mile. Repeat when car slows down by 1 mile per hour.”

Nice to see ExMrsDoorman#2 getting some attention in this thread.

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#10

Re: VFD on Heat Pump Compressor

01/28/2025 8:59 PM

Really depends on compressor design, some can't survive the lower (or higher) RPM a VFD can produce because their lubrication relies on splash and/or centrifugal oil "pumping", some will not have that problem, but all will suffer from more heat to first windings due to hard swiching high frequency harmonics, and a simple torroid filter on input does help on this. S.M.

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#13
In reply to #10

Re: VFD on Heat Pump Compressor

01/29/2025 1:24 AM

Thanks for the first really helpful answer to my question.

A lot of things need to be considered to implement a VFD on a compressor that I had not considered before.

Guess I will put that idea on the shelf and when my existing units expire, I will upgrade to a newer technology unit that is purpose-built for VFD compressor and other associated components.

Thanks again!

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#11

Re: VFD on Heat Pump Compressor

01/29/2025 12:32 AM

Other responses have concentrated on the ability of the compressor to cope with VFD.

I will point out another change to the system needed with a variable speed compressor.

Instead of a fixed constriction to get the two pressure levels in a fixed speed system, the constriction needs to be adjustable to suit compressor speed, so that superheat and sub cool are obtained for efficiency of thermal transfer.

A Linear Expansion Valve (LEV) is used for this and manufacturers of Heat Pumps spend a lot of time matching LEV position to compressor speed for a wide variety of loads and temperature differences (outdoor temperature v discharge temperature v space temperature) and still keep the refrigerant within its safe working range of temperatures and pressures.

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#12
In reply to #11

Re: VFD on Heat Pump Compressor

01/29/2025 1:14 AM

Over here, they are called a Thermostatic Expansion Valve(TXV).They respond automatically to temperature and pressure to maintain the correct flow of freon to the evaporator and compressor, which relies on the cool evaporator vapor to cool it, and to prevent "Slugging" of the compressor with liquid on the suction side. There are now Electronic expansion valves that are more efficient and reliable than the liquid filled bulb mechanical type. This type Expansion valve could handle a certain range of speed, but it would of course have limits to how much it could do without a special redesign of the internal orifice.

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#16

Re: VFD on Heat Pump Compressor

01/29/2025 5:41 AM

Another consideration is for the motor cooling. If the compressor offers relatively constant torque (constant pressure from the TXV?) it may overheat at low speed if its cooling is from a centrifugal fan. We see that a lot in conveyor systems. Often, a supplementary blower is added to cool the motor.

This supplementary cooling is rarely required when the load has a variable torque, like driving a fan or a large blower.

In the case of a small AC compressor, the refrigerant usually cools the motor which may be enough. As someone else said, the lubrification may be the issue. You are likely to want to limit the low speed to something like 30% or 50%.

The other thing is that single phase motors use either a run capacitor and/or winding in different configurations. It may not work at all with variable frequency drive.

Good luck.

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#17
In reply to #16

Re: VFD on Heat Pump Compressor

01/29/2025 7:25 AM

You are right on all counts. The idea was just a fleeting one without much forethought. In retrospect, I should have considered all of the variables involved, as you and others mentioned, cooling, lubrication, and the resonant frequency of the run capacitor/winding inductance to maintain a low Q.

I now see why a complete system redesign is required to achieve most efficient results.

Thanks to all for putting another wrinkle in my grey matter!

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#19

Re: VFD on Heat Pump Compressor

01/30/2025 9:34 PM

How about for a simple home electric furnace? Simultaneously ramping up and down heating coil current and fan speed?
I'd like to have one of those.

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