RV AC 521 compressor saver

I've been researching this circuit for a couple weeks now and, the following is what I've learned. The 521 3-wire circuit uses the usual boost run cap but has the addition of a relay that reads the back emf of the compressor motor. When the motor reaches speed and the back emf is sufficient to open the relay, the boost cap is disconnected from the motor - stated as the safest method. Other circuits such as 521 2-wire or hard start booster PTC/cap don't work consistently or pose extra risk of failure. The 521 3-wire mod reduces start current to half or better which allows much smaller inverter-generators or more additional loads for the big guys. This mod has been used for decades in HVAC and from what I've read, was included by the AC manufacturers many years ago. There is a question about which type of AC it can be used in safely. Since I haven't seen a 521 mod listed for RV's, it may not be suitable. I believe the Supco hard start kit is a PTC/capaitor. From the tests I've seen, this mod doesn't lower the start current. It works by getting the motor up to speed faster giving a shorter duration current pulse. It also appears to be temperature sensitive and may not not work under certain conditions. If anyone finds any error in this, please let me know - I'm still climbing the learning curve.