There are a number of options for motor speed control, it all depends on how big the motor needs to be and how cheap you want it.
Simplest is probably a dc motor from a treadmill, scooter or wheelchair.
You already mentioned washing machine motors which give several speeds and can be found as scrap.
Also for a shaft drive compressor you could just use a 3 phase motor and inverter.
For sealed compressors you need some sort of VFD either single or three phase.
Somewhere i have an old (Probably 15 years old) 12v to 240 inverter, i once took it apart to see what was inside ans found a potentiometer that controlled the output frequency. I remember trying it with a single phase motor of some kind and it did control the speed.
I would guess that modern inverters are microprocessor controlled and therefore have no easy way of altering the frequency.
Another option could be a frequency converter, these were often used to generate high frequencies for motors before inverters became economically viable. We used to use one to create a 300hz supply for hand held die grinders. They consist of a motor driving a generator which has far more poles(6x in this case). We only required a fixed high frequency but the drive motor could be either dc or 3phase with a VFD to create a variable frequency for a sealed compressor.
One more very wacky idea which may yield similar results in a heatpump or refrigeration application, no idea on the efficiency implications though.
From measuring power consumption vs heat output from an ASHP, the power consumption is reasonably relative to the amount of heat being moved.
Therefore by leaking a variable amount of heat from the output to the input of the compressor you could reduce the output and power consumption which i guess is what your after. I've never tried this but see no reason why it wouldn't work to some extent.
Will see if i can look up some actual measurements on this from my past data logging.
Steve
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