Generator vs AC

[ga80486]

Plugged in to power and generator running. Which one takes over?

If I want to run the generator to power both A/C units, do I need to unplug or can I just start it up then shut it down after we're cooled off?

[MisterT]

generally your generator will trump any shore power, the ATS(AutomaticTransferSwitch) senses the generator and switches over to it...

many times when we enter an rv park during hot weather and have the generator running for the a/c's, we leave it running while I make the hook ups, then shut it down when I re-enter the motor coach - the 50a shore power then takes over automatically.

[Atom Ant]

If you have a power management system, it should tell you the source of power it happens to be on.

You have me curious now as to which one is priority. I'll have to try it out tomorrow.

[Adam Hunter]

I agree with misterT - I would bet generator will always have priority over the 50amp (shore power).

My only add on would be don't short cycle your AC units by shutting off the generator while plugged into shore power.
I select my AC's off then shut the generator off, let the transfer switch kick over to shore, wait 5 minutes and then turn the AC back on.

Correct me if I'm wrong but I think most AC units need 4 or 5 mins between compressor cycles.

[Luv2go]

As others have said the generator has priority.

If you're plugged into a 15A outlet, for example, you can leave it plugged in, start your genny, the ATS will switch the coach power to the genny. Turn on the A/C's to get the coach cooled down and turn them off before you stop the genny. Coach will be back on the 15A outlet at that time.

[vsheetz]

Genny typically has priority over shore power if both are present.

Good practice is to not have large loads such as air conditioning active when the ATS switches. If large loads active, there is arcing occurring between the ATS contacts. Cumulatively, over time, the arcing will likely lead to ATS failure.

[Lug_Nut]

The practice of running on genset on a very hot day, arriving at camp and plugging in, and then shutting the genset down, may damage your generator. Once plugged in, the A/C should be shut off, leaving the genset running for at least several minutes with very light load only. This is required to allow the generator windings to cool down somewhat prior to stopping

[MisterT]

[shutdown procedure?]

people have stated this many times, though I don't understand the questioning of 'shutting down' a/c units prior to shutting off the generator, or switching to shore power...

The AGS(AutomaticGeneratorStart) that most coaches come with provides an automatic power up of the Generator, EVEN with both a/c loads already ON, powering them up as soon as the ATS(AutomaticTransferSwitch) quicks over.
When the AGS then later shuts OFF the generator, both a/c's also are shut off immediately.

There is no 'warm up' or 'cool down' period in either situation, and we have never seen any detrimental actions to when this takes place in our coach in many situations in the past two years.

If ONAN believed that this was a detrimental operation, why would they not build in the 'warm up' and 'cool down' periods?

[nothermark]

Originally Posted by MisterT

[shutdown procedure?]

people have stated this many times, though I don't understand the questioning of 'shutting down' a/c units prior to shutting off the generator, or switching to shore power...

The AGS(AutomaticGeneratorStart) that most coaches come with provides an automatic power up of the Generator, EVEN with both a/c loads already ON, powering them up as soon as the ATS(AutomaticTransferSwitch) quicks over.
When the AGS then later shuts OFF the generator, both a/c's also are shut off immediately.

There is no 'warm up' or 'cool down' period in either situation, and we have never seen any detrimental actions to when this takes place in our coach in many situations in the past two years.

If ONAN believed that this was a detrimental operation, why would they not build in the 'warm up' and 'cool down' periods?

Because Onan does not control the A/C system and really does not care if it fails. They probably also do not sell or maintain the transfer switch that also fails from multiple switches under load.

The advice to shut off is standard practice for the simple reason that folks with the time to watch those things figured it out long ago. Nothing has changed to improve the situation. The damage is cumulative so one has to do life cycle tests to see it. You are into year 2 or 3 of your life cycle. Odds are good you will see failures before your neighbor who shuts off first if you both keep your units for, say, 10 years.

[MisterT]

I hear ya, but ONAN certainly SHOULD care - the AGS is part of THEIR product operation, not the air conditioner manufacturer. The AGS controls either low voltage Charging, or AIR CONDITIONER Power On for temperature control.

[Lug_Nut]

Generally the auto start/stop of the genset will probably shutdown at a far lower load than the all A/C's on, A/C all off. When you arrive into a park on a hot day, most will have all (2, 3 or 4 A/C units operating) Shutting down with this heavy load is not ever recommended. If your unit was running after auto starting due to temperature, it will probably shutdown with only one A/C unit operating. After all, what are the chances of 2 or all thermostats reaching the desired temperature at the same moment.

Bottom line, no generator should ever be shutdown while under heavy load. Any genset maker will confirm this.

So, the auto start/stop may be smarter than we think.

Peter

[Adam Hunter]

Run them however you want - you paid for it so its yours to use as you wish. We are just trying to describe conservative best practice. Below is a cut and paste from Onan's RV Generator Handbook......
I think I'm with Lug_Nut on this one - hopefully it keeps the repair bills down.



Air Conditioner “Short-Cycling”
When an air conditioner runs, the compressor builds refrigerant pressure. When the
compressor is then shut off, either manually or by satisfying thermostat demand,
it must stay off for some period of time, usually 3-4 minutes, to allow pressure in
the system to equalize before safely restarted. Trying to restart the air conditioner
compressor before pressure equalizes is known as “short-cycling”.
A “short-cycled” air conditioner could 1) trip the air conditioner circuit breaker;
2) shut down the generator due to overload; or 3) trip the air conditioner due to
thermal overload. The air conditioner thermal overload will reset itself after the
compressor cools. Typically, you cannot manually reset.
Some, but not all, RV air conditioner thermostats have built-in time delays to
prevent short-cycling. Short-cycling occurs in some RVs when the thermostat is
mounted so that cold air blows directly on it. If this happens, the thermostat shuts
the compressor off before the cabin has cooled, but quickly warms back up and
signals the compressor to restart before refrigerant pressure equalizes. In this case,
direct the flow of air from the air conditioner away from the thermostat or relocate
the thermostat. You can prevent air conditioner short-cycling by always ensuring
the air conditioner rests for 3-4 minutes after the compressor shuts off before trying
to restart it.
---------------------------------

Starting And Stopping Procedures
Your Cummins Onan generator can be started and stopped from the integral
control panel on the generator, or from the optional remote control panel inside the
vehicle if your RV is so equipped. Your Operator’s Manual outlines the simple steps
for starting and stopping. Remember to:
• Before starting the generator, turn off air conditioners and large electrical loads.
• Before starting in cold weather, turn off all appliances for best long-term
performance.
• Prime by holding stop (all Quiet Diesels, Quiet Gas 3600-7000).
• To start - press and hold start at the control panel or at the set.
- Quiet Diesel: auto pre-heat flash, then crank/start
- Don’t over-crank with no start
• Before turning on appliances, let the generator warm up for a few minutes.
• Turn off air conditioners and large electrical loads and allow the generator to run
for 3-5 minutes before stopping, to allow the generator to cool down.

[Romeo]

I don't have an answer to the subject question; but I can provide what happened to me:
It was a hot day and we were running one house A/C on the generator while pulling in and setting up, as others do. I went outside and pluged into the 50 amp port on the pedestal... the transfer switch clicked over and I began to smell hot wires/plastic. I felt the transfer switch plastic housing and it was hot and began to bubble and melt. I quickly ran in and shut down the generator and returned to the pedestal to flip the circuit breaker.

The transfer switch plastic box was still melting and remained hot for some time. I finally was able to open the box and found the the heavy gauge wire to one of the contactors was pretty well toasted as was the contactor.

Needless to say, I had to replace the transfer switch. When installing the new switch I learned that the switch over should happen automatically without a problem, so I assume that the melt down may have been caused by loose clamping screws.

I will probably now make sure to shut down the A/C and generator before plugging into the pedestal, just to make sure it doesn't happen again. But again, the transfer switch has provisions to make the switch over with needing to shut down the generator. There is a built in switch over delay.

[nothermark]

Originally Posted by MisterT

I hear ya, but ONAN certainly SHOULD care - the AGS is part of THEIR product operation, not the air conditioner manufacturer. The AGS controls either low voltage Charging, or AIR CONDITIONER Power On for temperature control.

Off hand how does the AGS know to start? Assuming there is control of the AGS for that function the same controller should do the timing for the cool down cycle then turn off the generator by telling the AGS to shut down. None of that has anything to do with Onan and their connection between the generator and AGS.