Generator Switch Box

[pgchin]

(03-02-2013 11:48)Arcticdude Wrote:  Pete,

My 96 will run all 3 on the genny. I've got to be a little careful, as the middle and rear are on leg 2, along with a bunch of other stuff. It's really easy to imbalance the two legs. But other than that, I have no issues with the A/C's.

Thank you Articdude. You and David confirmed my suspicions. I have mine wired like yours and believe the middle AC is bad and always has been. I need to change them all anyways so when I do, I guess they will all run on the genny. Truth be told I never run 3 at anytime.too cold for me! Living in Florida thins out your blood! Maybe I need more "antifreeze / alcohol" in my system! hahahahahah

[Arcticdude]

I'm good to -30 with a "premium multiblend antifreeze"!!

[Darrell Snell]

(03-02-2013 11:27)davidmbrady Wrote:  Darrell,

The installation manual specs locked compressor inrush at 60A and locked fan motor at 6A. I don't think there's any delay built in to delay compressor startup. I believe this cause there are addons to dometics to insert delays. What are your leg voltages during compressor startup? If the genny is a weak voltage source then voltage will fall more than normal and inrush current will increase above those spec-ed in the manual. I think max inrush would occur only for the first half second or so and quickly disappear and be over within 3s (exponential decay). I have had no problems running three AC's on the genny. The manual specs a minimum generator capacity of 3.5KW per AC unit, so our 15KW units are easily adequate if operating normally.

http://www.dometic.com/1f24ead8-6316-41b...6bad.fodoc

Dave
Had a look at AC voltage today with shore power and genny.
SP has 125v no load, and 116~v when 1 compressor kicks in.
Genny shows 120v and 119v when 1 compressor kicks in.

I have an extension cord 100' long with #8 wire so I think this is part of the problem with voltage drop. It hasn't concerned me since I won't be using this arrangement once my permanent AC connection is in place in a couple of weeks (before air conditioning season).

But the problem with high starting current still occurs with genny power.
If I turn on the forward AC switch and create a demand at the thermostat all will work well. The fan will come on and a short time later the compressor will pull in with a quick spike. But if I should turn off the forward AC switch while there is a demand and turn it on again there is a prolonged spike of about 10 secs + that is kinda scary. This happens on all units.
I plan on contacting Dometic to see what gives with delays as it seems they have one for one condition but not one for an equally important condition.
D

[davidbrady]

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

[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

[pgchin]

David,
Outstanding root cause, thank you!!!! Darrell, its been decades since the "walk through" with my coach but now that David has posted your root cause, I now remember them telling me to not use the dash switches, use the thermostats, Dometics "don't like" it. I never did and never knew why, even though some people use the switches without issue. Now I know! Thanks again David!

[pgchin]

(03-05-2013 12:30)Ms. Bee Wrote:  Great explanation, David!!! And thanks Pete for your add-on

So as I'm the one with the hot flashes, and then freezing a minute later , let me see if I understand what you just said.

Not touching THAT line even with a 10 foot pole!

Leave the toggle switches in the on position? Always???
And then as the up and down person, I should turn the A/C on by the thermostat? So we don't get the spike?

Yes, that's what Donna does for the middle and back AC's. I have 3 thermostats, old configuration, so I use the front one by the pilot to turn it on, we usually only run with the front and coach/ engine AC on and it is cool enough for me..... when Donna gripes about it being too hot, I tell her to snap on the middle one on her way to the head, the dogs and I have to wait until we stop at a rest area...... we are not ready for diapers yet again! We also close off the aft cabin with the kitchen door so the 2 acs cool less space, then when we get to where we are going, I'll snap on the aft cabin one and in 10 minutes the whole bus is freezing me out so I go outside and the dogs stay in with Donna and desert me! hummm must be a plot ! We are always running with the genny on anyway in warm weather, I need to keep my stocked wine fridge cool and my outside mancave beer fridge cool!!!!hahahahah


Yes or no?

Ms. Bee

PS. Oh and as I don't think Dee knows about the reputation reward system yet, I'm giving you one on his behalf. Will splain it all to him later

[davidbrady]

Thanks everyone, glad to serve!

That's right Ms. Bee (and thanks for the rep! I appreciate it! ), as best you can, control the AC's via the Duo Therm CCC thermostat.

Now, those switches up front were put there for a reason, and I think the best use of them is to turn off the AC's to prevent overload of the inverters.

My bus has two AC's wired to one inverter. I have to actively manage that cause the inverter can only handle one. So, if you find yourself pulling into a dusty area and you want to turn off the generator (and fan to keep things clean), use the switches up front to conveniently and quickly turn off the AC's. Another scenario is plugged in at a campground. Before I remove the shore power I often reach inside and flip the AC switches off. Using them as a means of temperature control; i.e., turning them on when hot and off when cold is probably a no-no. Use the CCC for this.

It also occurs to me that this may be why BB poo-pooed the idea of running an AC on an inverter. They wired them that way, but in time BB came to recommend to not run AC's via an inverter. I never quite got this cause the inverter and engine mounted 50DN alternator can easily run an AC and I've done it many times. One time my genny broke down in Texas and the only AC we had for the house was an AC running on an inverter. But, you'll hear it time and time again, "don't run AC's on the inverters". Well I'm thinking that the AC switches up front might be the cause of this folklore. Imagine a lowly 3600W inverter (modified sinewave) attempting to deal with the kinds of compressor starts and amperage draw that Darrell has seen. That'll definitely bring the inverter and the AC's to their knees, and probably shorten their lives.

The other benefit of letting the CCC brains do it's job is that by inserting a 2 minute delay, the high side and low side pressures equalize a bit allowing for an easier compressor start.

[Hisham Amaral]

(03-05-2013 12:52)davidmbrady Wrote:  Thanks everyone, glad to serve!

That's right Ms. Bee (and thanks for the rep! I appreciate it! ), as best you can, control the AC's via the Duo Therm CCC thermostat.

Now, those switches up front were put there for a reason, and I think the best use of them is to turn off the AC's to prevent overload of the inverters.

My bus has two AC's wired to one inverter. I have to actively manage that cause the inverter can only handle one. So, if you find yourself pulling into a dusty area and you want to turn off the generator (and fan to keep things clean), use the switches up front to conveniently and quickly turn off the AC's. Another scenario is plugged in at a campground. Before I remove the shore power I often reach inside and flip the AC switches off. Using them as a means of temperature control; i.e., turning them on when hot and off when cold is probably a no-no. Use the CCC for this.

It also occurs to me that this may be why BB poo-pooed the idea of running an AC on an inverter. They wired them that way, but in time BB came to recommend to not run AC's via an inverter. I never quite got this cause the inverter and engine mounted 50DN alternator can easily run an AC and I've done it many times. One time my genny broke down in Texas and the only AC we had for the house was an AC running on an inverter. But, you'll hear it time and time again, "don't run AC's on the inverters". Well I'm thinking that the AC switches up front might be the cause of this folklore. Imagine a lowly 3600W inverter (modified sinewave) attempting to deal with the kinds of compressor starts and amperage draw that Darrell has seen. That'll definitely bring the inverter and the AC's to their knees, and probably shorten their lives.

The other benefit of letting the CCC brains do it's job is that by inserting a 2 minute delay, the high side and low side pressures equalize a bit allowing for an easier compressor start.

David
The M450 does run two A/C's on the inverters and the alternators. Both the living room and the badroom A/C's run off the invertor while on the road. The smart energy management control the overload and shots down components as needed to keep the use in line with available power i.e 30 amp on each leg on the smart controller. The alternator load does show about 100 to 150 amps when running the these A/C's.
I know that the 97 was wired to run the bedroom A/C on the inverter while on the road with engine running, that did put big load on the alternator. I tried running the BR A/C with engine running on the 97 the alternator out was will in the 250 Amps, I quite doing it.

[davidbrady]

Hish,

It's interesting the way BB did it over the years. On my LXi both the bedroom and the living room AC's are wired to just one of the inverters, so I get to choose which one I'd like to run, but I can't run both. It makes sense cause it prevents overload of the 50DN alternator (which is good for 300A as you know, and it's rated to output that power continuously). Dometic recommends 3500W of power per air conditioner.

I'd like to hear more about your Energy Management System. For instance, what brand is it? Does it also work when on shore power? Does it have a watchdog that starts the generator in the event of shore power loss? Seems we all can learn a lot from the wonderful technology built into the M450. A Balmer alternator smart regulator is already on my list!