VFD on Compressor motors ???
Can VFD's be used on small (residential) compressor motors (Air conditioning) ?
Going down this road to possibly save a standby genset from being upsized.
It is certainly theoretically possible to do so; VFD fed compressors are on the market for larger scale (industrial and commercial) cooling, eg. http://turbocor.com/ (these are 75 ton compressors)
However I don't know if anyone actually makes such systems for the residential market. In theory, you would not only get nice soft-start features, but you could dynamically change the pump speed to optimize efficiency for given loading.
I'd definately not slow down the condensing coil fan, and only put the VFD on the compressor. Just keep in mind that the compressor relies on the refrigerant for cooling, so if you get your compressor slowed down too much, there won't be enough "leftover cold" in the refrigerant from the evaporator to properly cool the compressor.
Residential air conditioning compressors are positive displacement compressors. Slowing them down is not going to do much. Part of the inefficiency is leakage past valves and piston seals, and you are going to have the same amount of time for leaks, so I suspect that efficiency will go down.
You don't want to slow down the blowers because you need lots of air to get good heat exchange.
Large industrial compressors are centrifugal compressors where the pressure depends on the square of the speed and slowing them down can be used to adjust the operating point when there is less demand.
Your best bet if you have multiple compressors as some large residential systems do is to lock one out when you are on the generator, or shut off other demands when the compressor is running.
Manual or automated load control is probably your best choice to control demand on the generator.
I don't have a VFD example, but have used a solid start starter on a air compressor. The issue was motor inrush that lowered the utility voltage to the point where neighbors complained to local utiltiy.
What I did is replace the FVNR starter with a solid state soft starter. I also added an unloader to keep the compressor from trying to compress until after the soft start was completed and the motor was running at speed.
Worked like a charm. Neighbors have not complained to utiltiy anymore!
It was cool to see it work mostly because the local air compressor sales/application engineer had never seen it done before.
What is an unloader? Is it just a time delay relay?
I think an unloader works to open the compression cylinder to atmosphere so that it would not do any work.
COMPRESSOR UNLOADING VALVES The Unloader valve uses an electric solenoid valve to by-pass a portion of the discharge gas into the suction chamber. Since both the suction and discharge pressures on the unloaded cylinder are approximately the same, the piston and cylinder do no work other than pumping vapor through the by-pass circuit. This prevents compressor from cycling off and on during fluctuating load conditions. A compressor supplied with an Unloader valve will always start in an unloaded stage, lowering the starting amps. A customer will see increased energy efficiency on a chiller system with compressor unloading. The Unloader valve is controlled by the electronic thermostat.
You may be out of luck if the motor is single phase. Some of the boys here bought a single phase motor driven pump, and after playing with it, decided that variable speed control was needed. No one sold a VFD with a single phase output.
You can get single phase in, but they all generate a 3-phase out.
We hit Google et. al. pretty hard looking because it was a spendy motor.
You should also be sure that the windings can handle the HV spikes that come from PWM. Older motors and motors not designed for VFD use tend to reduce cost by not being as beefy in the insulation department.
What BobNH said is the primary problem with existing HVAC residential compressors, they are positive displacement and therefor not suitable for variable speed.
There is also a problem which what is called "slugging" when you start messing with refrigerant compressor speed, regardless of the type of compressor. If the refrigerant cannot absorb enough heat from the coils, the fluid fails to vaporize and gets back to the compressor as a liquid. Since liquids can't compress, the remaining small amounts of vapor get severely over compressed in the compressor and the extreme pressures start to exceed the capabilities of the seals, valve etc., destroying the compressor. Anyone considering using variable speed on a refrigeration compressor had better know at LOT about this issue before attempting it. Experiment failures are extremely expensive.
For everyone's edification, there are VFDs designed to work with single phase motors, but they only can work with 2 specific types, PSC (Permanent Split Capacitor) and Shaded Pole. Unfortunately, the vast majority of installed residential HVAC compressors are Cap-Start / Cap-Run type single phase motors, one of the types that you cannot use a VFD on.
New residential HVAC systems designed to meet the upcoming efficiency requirements are being designed to use small scroll compressors, the most efficient design that works like a centrifugal pump and therefore more amenable to having variable speed applied. Some of them are being built with small 3 phase motors and VFDs that convert the residential 1 phase power to 3 phase just for the motor. That will likely be the norm some time in the future, but that doesn't help with anything already installed.