two roof top AC start coordinator/inhibitor current load minimizer SEAN?

[JohnEd]

Se why my search comes up empty?  How can you say that?

In the 90's, when I bought he Winnie I couldn't start both AC units at the same time.  I never tried cause starting one from scratch almost brought the 6.5 Onan to its knees and I had a lot more than a hunch that starting two would stop it dead in its tracks.  It is wired so that the genny will only run the rear AC unit.  Don't bother telling me that 6.5 is enuf to operate two without issues....it doesn't work.

There was a device I heard about that would prevent starting two compressors simultaneously.  It held off the power to one till the other came up to speed and the current load had fallen to spec.  This allowed a 5.5 KW genny to run two roof tops at the same time.  The max instantaneous load was 8.5 amps plus 17 amps, or just under 26 and your shore line would do that and so would a 4 KW genny.  "As I recall", from 10 years ago....please note that part.  I can't locate anyone that has even heard of that product although everyone says it would be "GREAT". Avoiding a large current surge would be a good idea in itself.  I have a call in to Dick Wright and I think that should conclude the matter either way.

Anyone know what this is and where?  Camping World and other RV outlets have not a clue.

Thanks,

John

[letz4wheel]

Would a load management system work? Progressive Dynamics is one brand that comes to mind. Remember...all this is off the top of my head...it has been known to be unstable 

[Len Silva]

You could easily design a system to accomplish that but I don't think you need it.

When you first fire up the generator, the air conditioners should be off to begin with.  If you then start them one at a time, I think it would be pretty rare for both to come on at exactly the same time as they cycle.

[Sean]

I am aware of two aftermarket load management systems, as suggested earlier, by Progressive Dynamics and Intellitec.  However, these either supply or remove power to the entire A/C unit wholesale -- there is no interaction with starting circuits.

It is possible that Dometic or Coleman-Mach makes a multi-unit thermostat or control box that can coordinate the timing of compressor starts, but I am not familiar enough with the details of their product lines to say for sure.  Perhaps Nick can chime in with an answer.

Frankly, though, a brand-specific control made for externally-controlled units is the only way I could see something like this working.  That's because opening up the guts of an RV air conditioner to splice something in to the thermostat circuit would do two unacceptable things: First, it would void the manufacturer's warranty on the air conditioner.  While many bus nuts might be OK with this, the average RV buyer will not, and no one can afford to build a mass-produced product just for bus nuts.  Second, and this is the killer, any internal modification to an electrical appliance will render the "listing" of that appliance (such as by UL, CSA, or other testing agency) null and void.  Since the National Electrical Code requires all such appliances to be listed, such a modification would create an untenable legal situation for the manufacturer.

I suspect if there was ever a company making such a device that required the installer to open up and modify an air conditioner, is has since been removed from the market for exactly this reason.

All that said, I agree with Len.  Make sure you start them separately, and it is unlikely the compressors will ever try to cycle at exactly the same time.  You can almost guarantee it by turning the thermostat to the coldest setting, which will usually keep the compressor from cycling off at all.  Perhaps once in a blue moon you will get a nuisance trip of the breaker.

If you have external access to the thermostat control, such as can be found on external-thermostat units, then making such a coordinator is straightforward, although a bit complex.  An electronic solution would be best, but it can be done with a handful of relays and a delay timer as well.

-Sean
http://OurOdyssey.BlogSpot.com

[JohnEd]

Well, that explains that.  Thanks Sean.

And this is the legal device that does it.  Turns off one AC unit if you ever exceed the 30 amp limit and keeps it off till the load drops.  Would work with 30 amps when you were using other stuff and would keep you from blowing the breaker.  The "load sensing coil" would have to be bought but the rest seems doable and if Sean has a solid state equivalent circuit I sure would go for that.  Even pay some for those plans.

http://intellitec.com/PDF/5300221.100.pdf

John

[James77MCI8]

Could be accomplished with a pico controller.

[Sean]

...  The "load sensing coil" would have to be bought but the rest seems doable and if Sean has a solid state equivalent circuit I sure would go for that.  Even pay some for those plans.

Sorry, I don't have a circuit.  No need to reinvent the wheel; the Intellitec unit works fine.  Circuit's already built in, and it comes with the current transformer that you mention.

-Sean
http://OurOdyssey.BlogSpot.com

[TomC]

At one time, the A/C manufacturers made a hard start kit, that had condensers and such to eliminate the big surge at the start of the compressor cycle.  Don't know if it is still made. Good Luck, TomC

[James77MCI8]

Hard start kits were an option on some early roof top models. I am not sure if they are standard equipment now or not. They basically were a large capacitor paralleled with the existing start capacitor. The kit had a relay that would drop the larger capacitance out of the circuit once the device was running. The are available from supco and are sized based on horsepower.

[Bob Belter]

Ahoy, JohEd,
I have four (4 ea) 120 vac compressors which I start --  not all at once.  I do this by using a PLC – Programable Logic Controller, and a program to set in time delays. 
I have two independent  A/C systems, so I start the two compressors of a unit just five seconds apart, so that there is not time for a pressure build-up in the system, but a moment for genset recovery --  works just fine.  My other pair is delayed for 30 seconds, plus each has a three minute start delay from last operation, so that pressures can deplete.  That is, the programming is such that whichever compressor pair starts, the other delays for 30 seconds, plus there is always the three minute delay from last operation.
My PLC is an Automation Direct device  ~~ a bit more than $100.00, and it controls four individual 120vac relays for the four compressors.  The PLC and relays probably cost less than somebody's device.  You will have to program it -  Windows program.
If there is any interest, I will see if I still have the program, and will publish the file.

Enjoy   /s/   Bob

[James77MCI8]

http://www.bb-elec.com/product_multi_family.asp?MultiFamilyId=39

This is a link for the small programmable controller

[JohnEd]

Thank you both verymuch.  Great information.

Bob,

I don't think I will stall my compressor if I turn my Dometic of and then back on quickly.  I "think" I have done that without any misshap.  Do mine have some sort of "unloader" in the freon compressor, do you think?  I'll call Dometic tomorrow.

You are both very kind.

John

[James77MCI8]

You probably have one of the units with a factory installed hard start kit

[Barn Owl]

Don't bother telling me that 6.5 is enuf to operate two without issues....it doesn't work.

I realize it is not working for you, but my question is why? I have that setup and have never had a problem running two A/C units plus many extras. It might be better to spend the time and money to get your 6.5 working as it should. Currently the way my 6.5 NH is wired to the bus I can only draw 6k (breaker box wiring limits) and it still does the job effortlessly without stalling or tripping breakers. Something is wrong.

[JohnEd]

Barn Owl,

You are so very correct......the gen has problems.  That's on the list.  Good call and it surprises me that I only got called on that by one Knut....thanks.

The subject interests me. The first time was when I wanted to size a inverter for my needs.  My needs are "run two 13KBTU units and TV".  Each of those draw 9 amps as I recall.  So to keep two running I would need 18 amps and that is a 2.2KW.  To run one and start the other I would need 18 amps start-up plus 9 amps to run or 27 amps from the inverter.  That translates to 3240KW for the inverter capacity.  Given that the inverters will handle a 100% overload for a short period, the 2.2KW inverter could put out 4.4KW briefly so, in theory, a 2.2 KW inverter could run this system with capacity to spare.  Now who ever heard that being advocated....buy a 2.2 or 2.5 KW inverter to run two Rooftop AC units?  What am I missing?

John