Duh, I should have known that already. Now that you mention it, I have read posts you made about it elsewhere in the forum. Please forgive me, I'm an idiot.
Since you are considering the Emerson pots for your heat pump, here's a curveball for you to catch. They have two main types of variable-capacity scroll compressors: the ultra and the digital. Neither one needs a vfd to do its job.
The ultra scroll has two stages of capacity: high and low. The compression scroll has two suction ports and one discharge port inside the shell. One suction port is located at the outside edge of the scroll, and is open all the time. The second port is located between the first suction port and the discharge port. It has a "swing plate" that normally uncovers the port. When activated, it covers the port, which enables the whole scroll and increases pumping capacity. These models follow the ZPS*K5 naming, where the * is the high speed rating.
The digital scroll is continuously variable between 10 and 100 percent output. This is accomplished mechanically by allowing the top scroll to float slightly away from the bottom scroll. When this happens, the seal between the scrolls is lost, pumping stops immediately, and the motor is unloaded. Capacity is varied by duty cycle, by energizing an internal solenoid valve. When energized, the solenoid valve uses the compressor's own suction pressure to disengage the valves from each other. When released, the scroll goes right back to full output.These models follow the ZPD*KSE naming.
Both of these lines have been out since the 90's, so there should be some out there somewhere. I imagine they do what they should pretty well. Much too expensive for my modest systems, though.
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