glock19 Show full post »
Peddler
We have not seen a problem with scroll compressors, the return to utility is somewhere around a 100-140 microsecond break.  I'm not saying it couldn't happen but we have not seen it.  We have several switches capable of in phase transfer but they have their trouble too.  Many times they time out and transfer not inphase because they are unable to sync with the utility.
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Cobranut
78buckshot wrote:
Well, as long as we're on the subject of generators and A/C I can add and expound on JayH's comment, I have seen more than once when the A/C is running and utility power returns that the splitt-second interruption in power can result in the compressor reversing rotation. With a scroll compressor running backwards it cannot compress the gas and even though the compressor and condenser fan are running there will be no refrigeration. I can't say I have seen a reciprocating compressor run backwards but it would still compress and move refrigerant but electrically probably would overheat. Just my 2 cents.


That's another reason I like the in-phase transfer feature in my Thomson transfer switch. 
If I'm watching for it, I can barely detect the flicker of the lights, and I can't hear any interruption of the A/C or other compressors when it transfers.
On most occasions I don't even know the utility is back on until I open a back door and hear that the genset is no longer running.
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Cobranut
glock19 wrote:


Any solution to prevent that?  Will the compressor eventually shut down and then revert to the correct rotation on the next cooling cycle?


If it can't cool the house, that cooling cycle will never end.  If you're not home it could run that way indefinitely, which I imagine would eventually damage the compressor, as the system isn't going to be returning oil back to the compressor.

Edit: Thanks Goofy for reminding me about the thermal cutout.  I agree though, that anytime the compressor gets hot enough to trip that, it's hot enough to have sustained some damage.
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78buckshot
The compressor has an internal overload switch embedded in the motor windings - heat sensitive. It can trip on heat generated from high amp draw and/or a motor running hot due to reduced cooling. Refrigeration compressors are cooled by the refrigerant gas returning to the compressor, if the compressor is running in reverse then no cooling effect is taking place anywhere in the system and the compressor will eventually stop on the internal overload. When the switch closes and energizes the windings it will start in the right direction. If however you are aware of it running and not cooling then shut it off at the thermostat.
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woodspliter
Hello Glock 19.  There is a lot of information on this post and one thing I might add.  Before the new TS had HVAC load shed I installed a 12VDC control relay in the TS to hold off the AC compressor during standby.  It wires off the 194/23 connections.  The relay receives 12VDC from the controller as the controller trips the TR in the TS.  I used the closed contact and opens on transfer.  This is a simple way to drop the load out permanently during standby.  The closed contact is wired in line to break a low voltage control wire to the compressor.  Usually Yellow.  (you will have to run one from the relay contact to the AC contactor)
Also on the AC quick start issue, meaning when the power blinks on the AC and it tries to restart in seconds after the blink .  It is rough on the unit.  The pressures in the compressor have not equalized and the pressure on the high side are at the peak.  Go to a HVAC supplier or on line and purchase a time delay relay made for that purpose.  It is adjustable in minute settings and wires in line with the low voltage contactor of the compressor.  After the power drops to the AC compressor, on restart voltage to the AC the timer will start counting and hold off the AC contactor for minutes until the pressure is equalized then allow it to start.  I have had these timers on my ACs for 25 years just to protect the high pressure start.  Before I ever installed a generator.  Good luck, sounds like you are sneaking up on all the issues. 
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glock19
woodspliter wrote:
Hello Glock 19.  There is a lot of information on this post and one thing I might add.  Before the new TS had HVAC load shed I installed a 12VDC control relay in the TS to hold off the AC compressor during standby.  It wires off the 194/23 connections.  The relay receives 12VDC from the controller as the controller trips the TR in the TS.  I used the closed contact and opens on transfer.  This is a simple way to drop the load out permanently during standby.  The closed contact is wired in line to break a low voltage control wire to the compressor.  Usually Yellow.  (you will have to run one from the relay contact to the AC contactor)
Also on the AC quick start issue, meaning when the power blinks on the AC and it tries to restart in seconds after the blink .  It is rough on the unit.  The pressures in the compressor have not equalized and the pressure on the high side are at the peak.  Go to a HVAC supplier or on line and purchase a time delay relay made for that purpose.  It is adjustable in minute settings and wires in line with the low voltage contactor of the compressor.  After the power drops to the AC compressor, on restart voltage to the AC the timer will start counting and hold off the AC contactor for minutes until the pressure is equalized then allow it to start.  I have had these timers on my ACs for 25 years just to protect the high pressure start.  Before I ever installed a generator.  Good luck, sounds like you are sneaking up on all the issues. 


Woodsplitter,

Even with HVAC load shed I think your idea of a relay that would completely eliminate the A/C while on gen power is a good idea.  Taking the A/C compressor out of the circuit would be an inexpensive way to prevent the heavy load and lugging on the gen on A/C cycles.  However, here in Texas, the summers get a little warm and if one is at home they sure want that cooling to take place.  If away from home it could definitely work.definitely be used.

In my situation, it appears the installation of a soft start solution would also provide a time delay feature.

I was under the impression that most modern A/C units had a time delay relay built into the unit.  Am I wrong??

Also, on the time delay relay are we talking "milliseconds" of no AC power and restart to activate the relay and disable the compressor?
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Goofy4TheWorld
glock19, the SureStart soft starts DO NOT provide a time delay under normal operating conditions.  The only time a SureStart module delays a restart is if the SureStart itself detects an error and "pulls the plug" on the compressor for a set delay of 5 to 30 minutes, depending on the particular fault.  Those fault errors include compressor running backwards, compressor drawing excessive amps on startup, and low input voltage faults.  The simple failure and then return of utility power (or generator power) does not in-and-of-itself trigger a delay from the SureStart, it requires some kind of fault to occur first.

Also, it is not uncommon for even brand new HVACs to come with no built-in delays.  Some do, with my LIMITED experience being that straight air conditioners (with either no heat at all, or only gas heat) do not usually have a delay, where a unit that is also a Heat Pump will have a better chance of having a delay built into the circuit board that controls the slightly-smarter heat pump system.  Many manufacturers are assuming the thermostats in your home includes a delay, which may or may not be the case.

Even if you add a delay module directly to your compressor, VERY FEW (if any) delay modules will actually "break" when there is only millisecond drop in power.  So depending on a compressor delay for interrupting the compressor circuit during a change in the power source won't likely work.  It's pretty hard to address "millisenond" loss of power when it's just an ugly blink of utility power, almost nothing will detect that drop.  The only way to drop the compressor for the millisecond loss of power when going from generator power to utility power is by use of a load shedding module.
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