Auto transfer switch

> Your signature says "Hughes" which means I have no direct experience. However, just as a point of reference the GMC upfit coach runs both ACs
> anytime - on generator or shore power with a OEM 50 amp cord. My Royale runs only the center AC on a 30 amp shore power cord, but will run both on
> the generator (with an additional from Coachmen fuse box). So would recommend you keep discovering your existing situation or have an expert
> decipher it. If you look on BDUBs gmc site for manuals, you can get the Coachmen electric wiring diagrams and see if they tell you anything
> helpful.
>
> Yet another solution I have seen is leave the main AC wired in a typical fashion, and the second (usually rear) air hard wired to a 20 amp plug
> that is plugged into a shore power connection when both a 30 amp and a 20 amp are available on the same pole. Someone else can comment if that
> actually provides two sources of power, but I'm just saying it has been done that way.

Yah i have Coachmen and Avion schematics which help but not the Hughes..... so i just try to piece it all together. I think both a/c units will work
off the 30amp source (if i go that route) though npt that i think id run them both at the same time....lol.

--
Rich Mondor,

Brockville, ON

77 Hughes 2600
 
The 30 second (overkill) delay in some of the auto transfer switches is a low cost way to keep from having the AC essentially look at itself until the
magnetic field in transformers and/or motors dies down. If the peak waveform of one source sees the peak of the other it may cause enough current to
drop a breaker or open a fuse. (In high power setups it certainly will). It probably isn't necessary in the GMC unless the A/C is running and
doesn't reset the compressor timer. Half a second is plenty. If you have a simple relay and the A/C breaker pops when you swap, that's why. Ask me
how I know thism :)

--johnny
--
Foolish Carriage, 76 26' Eleganza(?) with beaucoup mods and add - ons.
Braselton, Ga.
I forgive them all, save those who hurt the dogs. They must answer to me in hell
 
I don’t agree your comments on Transfer switches and RV AC units.

I have had transfer switches on all three of the coaches that I own or have owned. I installed 2 myself and one was already installed on the stretch coach. The transfer switches only have the time delay when they go from shore power to generator power. The 30 second delay is to allow the generator to come up to speed. There is no delay when switching from back to shore power. When I shut my generator off the switch to shore power is almost instantaneous, the power will have a small blip. If your generator is running and your shore power comes back the generator still powers the coach, it doesn’t switch back to shore power.

Now lets discuss AC units. All RV AC units have a time delay relay, typically a 2 minute delay. This is to allow the pressures to equalize in the freon side of the system so the compressor does not start under load and possibly cause the AC breaker to open. It has nothing to do with the transfer switch. If by chance you have a power blip at your campsite and the power just goes off and then back on then the AC unit will not start cooling until the time delay relay times out, only the fan runs. Now if you turn on the AC unit the compressor will start or if you have the AC in Heat ( if it has a heat strip ) or just in FAN mode, it will start with no time delay.

In a sense the transfer switch and AC time delay relay work together to help both the generator and AC unit.

As alway comments are welcome.

J.R. Wright
GMC Great Laker MHC
GMCGL Tech Editor
GMC Eastern States Charter Member
GMCMI
78 GMC Buskirk 29.5’ Stretch
75 GMC Avion (Under Reconstruction)
Michigan

>
> The 30 second (overkill) delay in some of the auto transfer switches is a low cost way to keep from having the AC essentially look at itself until the
> magnetic field in transformers and/or motors dies down. If the peak waveform of one source sees the peak of the other it may cause enough current to
> drop a breaker or open a fuse. (In high power setups it certainly will). It probably isn't necessary in the GMC unless the A/C is running and
> doesn't reset the compressor timer. Half a second is plenty. If you have a simple relay and the A/C breaker pops when you swap, that's why. Ask me
> how I know thism :)
>
> --johnny
 
We differ on a couple of points. What's the minimum disruption your A/C will recognize as a power drop? What's the minimum time for your source
relay to pull in? If the second is lower than the first and if the two sources are out of phase, you'll take a breaker or fuse down. Realize, transfer
is not a problem, in that you're leaving a dead source. Retransfer however can bite you since you're switching between two live sources and don't
know the phase relationship between them.
Back in The Day the gensets had 'parallelling' lights, simply lights connected across the A phase of both sources. Throw the switch when the lights
go out. To automate this we hung a relay across the lights and let it inhibit the transfer switch motor when it was energized. This got the transfer
to within 30 degrees or so of equal which didn't hurt anything. The option was/is a trip to the plant, usually in a storm.

--johnny
--
Foolish Carriage, 76 26' Eleganza(?) with beaucoup mods and add - ons.
Braselton, Ga.
I forgive them all, save those who hurt the dogs. They must answer to me in hell
 
But that isn’t the reason for the 30-second delay. The transfer switch
doesn’t turn shore power off, delay for 30 seconds, and then connect to the
genset. It waits 30 seconds, and then switches the relays/contactors
directly from shore power to genset, just as it switches the reverse
directly when the genset is turned off.

If the relay switching time isn’t enough to handle the phase relationship,
oh well. It’s wise to shut off the air conditioner before switching between
two live loads.

The delay is there to let the engine on the genset stabilize before
imposing a load.

Rick “fulfilling a different requirement” Denney

On Wed, Sep 18, 2019 at 2:04 PM Johnny Bridges via Gmclist <

> We differ on a couple of points. What's the minimum disruption your A/C
> will recognize as a power drop? What's the minimum time for your source
> relay to pull in? If the second is lower than the first and if the two
> sources are out of phase, you'll take a breaker or fuse down. Realize,
> transfer
> is not a problem, in that you're leaving a dead source. Retransfer
> however can bite you since you're switching between two live sources and
> don't
> know the phase relationship between them.
> Back in The Day the gensets had 'parallelling' lights, simply lights
> connected across the A phase of both sources. Throw the switch when the
> lights
> go out. To automate this we hung a relay across the lights and let it
> inhibit the transfer switch motor when it was energized. This got the
> transfer
> to within 30 degrees or so of equal which didn't hurt anything. The
> option was/is a trip to the plant, usually in a storm.
>
--
'73 X-Glacier 230 "Jaws"
Northern Virginia
Offlist email: rick at rickdenney dot com
 
I understand the delay for the set to spool up.. we got hospital spec because most transmitters will restart in ten seconds without going through the
start sequence again. If it misses, you might take half a minute to get it cooking again. 30 seconds of dead air in a top ten market was death,
research showed the average listener would tolerate ten to twenty five seconds of silence maximum.
Any more, with transistor radios, the start sequence is much shorter so it isn't the problem it could be. All you want is to have the sources in
phase when you swap back from hot to hot. Given the modern switches we had towards the last, it wasn't a problem and alls you got was a hell of a
BANG and the lights blinked slightly. Hook it out of phase and the breaker on the transmitter and probably the AC point at the floor instantly.
That being said, the dynamics of the profession have changed along with the changes in the Industry. It's the same on a smaller scale here. If you
turn the coach A/C off, as someone noted, you pretty much preclude the problem. I could take the time to turn it off, or I can take the same time to
swap an air gap switch. Cheapskate that I am, I'd do the latter.
My perspective is admittedly somewhat different.

--johnny

--
Foolish Carriage, 76 26' Eleganza(?) with beaucoup mods and add - ons.
Braselton, Ga.
I forgive them all, save those who hurt the dogs. They must answer to me in hell
 
I'm slightly baffled by this thread's discussion. I've had the transfer
relay in my X-Birchaven since at least 1999. I doubt that it's switched
between two live sources 3 times during those 20+ years.
Ken H.
Americus, GA
'76 X-Birchaven w/Cad500/Howell EFI & EBL,
Manny Brakes & 1-Ton, Troy-Bilt APU, etc., etc., etc.
www.gmcwipersetc.com

On Wed, Sep 18, 2019 at 6:21 PM Johnny Bridges via Gmclist <

> I understand the delay for the set to spool up.. we got hospital spec
> because most transmitters will restart in ten seconds without going through
> the
> start sequence again. If it misses, you might take half a minute to get
> it cooking again. 30 seconds of dead air in a top ten market was death,
> research showed the average listener would tolerate ten to twenty five
> seconds of silence maximum.
> Any more, with transistor radios, the start sequence is much shorter so it
> isn't the problem it could be. All you want is to have the sources in
> phase when you swap back from hot to hot. Given the modern switches we
> had towards the last, it wasn't a problem and alls you got was a hell of a
> BANG and the lights blinked slightly. Hook it out of phase and the
> breaker on the transmitter and probably the AC point at the floor
> instantly.
> That being said, the dynamics of the profession have changed along with
> the changes in the Industry. It's the same on a smaller scale here. If you
> turn the coach A/C off, as someone noted, you pretty much preclude the
> problem. I could take the time to turn it off, or I can take the same time
> to
> swap an air gap switch. Cheapskate that I am, I'd do the latter.
> My perspective is admittedly somewhat different.
>
> --johnny
>
> --
> Foolish Carriage, 76 26' Eleganza(?) with beaucoup mods and add - ons.
> Braselton, Ga.
> I forgive them all, save those who hurt the dogs. They must answer to me
> in hell
>
>
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>
 
> I'm slightly baffled by this thread's discussion. I've had the transfer
> relay in my X-Birchaven since at least 1999. I doubt that it's switched
> between two live sources 3 times during those 20+ years.
> Ken H.
> Americus, GA

That's my thinking as well. I really can't imagine a scenario where it's really important to do an instantaneous switchover. Yes, you might have to
wait for the A/C units to spool back up, but in the five or so years I've been using my coach, it's never, ever been an issue to disconnect
momentarily when changing from shore to generator (or back).

--
Mark Hickey
Mesa, AZ
1978 Royale Center Kitchen
 
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