Generator Switch Box

[Darrell Snell]

(03-05-2013 00:45)davidmbrady Wrote:  Hey Darrell,

I'm no expert in this but after some digging here's what I've come up with.

When running from the dometic duo therm comfort control center (CCC) thermostat, there is a delay between calling for AC and actually starting the compressor. I think it's open loop, the CCC simply waits 2 minutes (or whatever) before re-starting the compressor motor. This delay allows a little device in the start capacitor and winding circuitry to cool down. The device is called the positive temperature coefficient resistor (PTCR) - it's basically a thermistor. The job of the PTCR is to take the start capacitor and winding out of the motor circuitry after the compressor motor has started. When the CCC calls for AC it inserts it's delay; the delay allows the PTCR to cool down. After the delay, the CCC starts the compressor motor. Because the PTCR is cool it looks like a short (low resistance) and power is applied to the start capacitor and winding resulting in lots of torque and low current to start the motor. As the PTCR heats up the resistance builds and the start capacitor is taken out of the circuitry allowing the motor to run on the run capacitor. If the CCC fails to insert a cool down period before attempting to re-start the compressor motor, then there's effectively no start capacitor and winding in the motor circuitry and all the power is applied to the run capacitor resulting in very low torque ( insufficient phase shift) and lots of inrush current and a long start time. I think when you have the thermostat calling for AC and the compressor signaled to run, and you toggle the AC switches in the driver area, you're attempting to restart the compressor without a cool down delay; consequently, there's no start capacitor and winding in the motor circuitry which results in low torque, excessive inrush current going to the run capacitor and winding circuitry, and a prolonged start period.

Two things you can do, if I'm right:

(1) stop doing that! I mean don't toggle the AC switches up front and instead let the CCC do it's thing or wait 2 minutes between toggling the AC switches, and/or
(2) insert a compressor hard start kit which essentially inserts a cooling delay: http://www.adventurerv.net/dometic-duoth...20207.html

Wow Dave
You are good!!!
Great explanation
I first noticed this problem last summer. We would be going down the road with the dash air on and it would get hot enough to require the front air. I would flip the switch and see this condition.
Very good Dave, thanks.
D