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Author Topic: Auto Transfer Switch Failure  (Read 404 times)

AJLease

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Auto Transfer Switch Failure
« on: July 08, 2018, 12:05:28 PM »
So a few weeks ago i started up my generator without realizing that the power switch for my fridge was on still. The generator tripped and a small amount of smoke came out near my inverter/converter and fuse panel. I replaced both of those and power still was not coming from my generator. So after some troubleshooting and finding out that my generator was in fact producing power, I landed on a fault in my auto transfer switch. I just located the switch, and it is attached to the back of the housing of my inverter/converter and fuse panel and I can't even open it. Has anybody else run into this problem? and what did you have to do to fix it?

Utclmjmpr

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Re: Auto Transfer Switch Failure
« Reply #1 on: July 09, 2018, 11:15:09 AM »
  You can probably only replace it.>>>Dan
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youracman

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Re: Auto Transfer Switch Failure
« Reply #2 on: July 09, 2018, 09:40:01 PM »
I don't have an ATS, but was considering adding one.  I don't understand why it is necessary to disconnect a fridge (or any load, save maybe the A/C or other "large" load) prior to starting the generator.  Just my WAG: Maybe this is because the transfer occurs before the generator is up-to-speed and "settled down" to 60 cps?

I am  looking at a Progressive Dynamics ATS (30 Amp) which has a time delay to allow the genny to come up to speed prior to transfer; but  if I have to make sure every load is "off" before starting the genny, I don't think I am interested.  Perhaps not all ATS's have the delay feature ........ especially older ones?





Ed Sievers/Denver, CO
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youracman

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Re: Auto Transfer Switch Failure
« Reply #3 on: July 09, 2018, 10:56:35 PM »
Wow! No wonder so many ATS's and/or appliances connected to it get smoked.  Here I thought the ATS (FIRST) disconnected the RV from the pedestal via its number 1 (NC) relay/contactor and then a few milliseconds later connected the RV to the generator via the second relay/contactor in the ATS......the whole sequence being initiated by starting the genny.

Live and learn, I reckon.

Thanks for the quick response......and safe travels.  ed s
Ed Sievers/Denver, CO
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Lou Schneider

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Re: Auto Transfer Switch Failure
« Reply #4 on: July 09, 2018, 10:58:36 PM »
I don't have an ATS, but was considering adding one.  I don't understand why it is necessary to disconnect a fridge (or any load, save maybe the A/C or other "large" load) prior to starting the generator.  Just my WAG: Maybe this is because the transfer occurs before the generator is up-to-speed and "settled down" to 60 cps?

I am  looking at a Progressive Dynamics ATS (30 Amp) which has a time delay to allow the genny to come up to speed prior to transfer; but  if I have to make sure every load is "off" before starting the genny, I don't think I am interested.  Perhaps not all ATS's have the delay feature ........ especially older ones?

It has to do with the fact that AC means Alternating Current ... the current (and voltage) changes from + to - and back 60 times per second.  Two sources could be exactly at 60 Hz but not be in phase with each other, i.e at any given moment one could be at +120 volts while the other is at -120 volts, or vice-versa.+

Large, industrial transfer switches have sophisticated circuits that compare the phasing between the two sources and only switch when they are in sync with each other .... i.e both sources are producing the same polarity voltage at the time of the switch.  The best ones also time the transfer to start as the waveforms from both sources are crossing zero voltage.

RV transfer switches aren't nearly that sophisticated.  They just see voltage from the new source and make the jump.  This means if there's any current flowing through the circuit, the switchover can create a large surge that can arc over and burn the switch contact.

The surge also doesn't do any good to any devices drawing current at that time.  Motors are particularly susceptible as their magnetic and physical inertia wants to follow the old phasing and the jump creates damaging internal current surges.

Lou Schneider

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Re: Auto Transfer Switch Failure
« Reply #5 on: July 09, 2018, 11:02:41 PM »
Wow! No wonder so many ATS's and/or appliances connected to it get smoked.  Here I thought the ATS (FIRST) disconnected the RV from the pedestal via its number 1 (NC) relay/contactor and then a few milliseconds later connected the RV to the generator via the second relay/contactor in the ATS......the whole sequence being initiated by starting the genny.

It does, but generally the changeover takes less than one cycle.  60 Hz is 16.66 milliseconds, and most relays can change over within one or two cycles.  Unless the new source is in phase with the old one, you'll get voltage and current surges from the inductive elements of the load.

Commercial transfer switches take the phasing of the sources into account before it initiates a changeover, RV transfer switches just crash and go.
« Last Edit: July 09, 2018, 11:08:32 PM by Lou Schneider »

youracman

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Re: Auto Transfer Switch Failure
« Reply #6 on: July 09, 2018, 11:12:57 PM »
It does, but generally the changeover takes less than one cycle.  60 Hz is 16.66 milliseconds, and most relays can change over within one or two cycles.  Unless the new source is in phase with where the old one will be apon re-connection, you'll get a voltage and current surge from the inductive elements of the load.

OK............got it (I think).  So for inductive loads (maybe especially rotating machinery) they can get a "kick" they don't really like.  Kinda like unplugging your home vacuum sweeper and plugging it back in before the motor has stopped.  Not likely the replug will be in phase, for sure.  Vacuum will survive just fine of course, but the replug probably made it a little mad.  :^)

Thanks again, Lou      ed s
Ed Sievers/Denver, CO
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Sun2Retire

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Re: Auto Transfer Switch Failure
« Reply #7 on: July 10, 2018, 08:02:17 AM »
When I had to replace my transfer switch I spoke at length with the techs at the replacement switch company (name escapes me, well known). My concern primarily was the health of the switch and whether they were designed to switch under load. He indicated that technically the answer was yes, but due to the possible arcing that Lou mentioned it was desirable to remove heavy loads before switching. I make it a habit to turn off the ACs before switching any direction, then turn them back on. Plus the builtin couple minute startup delay allows the genset a little warmup before hitting it with a full load.
Scott
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John From Detroit

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Re: Auto Transfer Switch Failure
« Reply #8 on: July 10, 2018, 09:53:47 AM »
A lot of what I'm reading by way of explanation.... Does not apply.

The switches are Break before Make so for a Fridge (Resistive load) Water heater (Restive load) or space heater.. You guessed it resistive load) a sudden phase change .. Who cares.

BUT for anyting with a coil of wire NOT a resistor. Like a Motor or a transformer...

Well have you ever heard the pipes rattle when you quickly shut off water?    What is happening is the water does not want to stop flowing so the pressure at the tap you shut off can go from say 60 PSI to over 100.. THen the water flows back the otyher way and the pressure drops to say 30. then it rushes back.. and forth  (In electronics we call that Ringing by the way).

Same thing exactly happens in that big coil of motor wire.. Plus we have a added KICK from the motor turning into a generator when power is lost while it is turning.

So the voltage the Transfer switch sees can go from 120 volts to 1200 volts in 1 60th of a second, Jump the gap and WELD the contacts.

Mostly happens with Inductive loads (Motors and transformers) BUT... Any load there is a tiny spark.

So the more you turn off. The less damage.  Note it is worse on BREAK than on MAKE.

Oh what is a Break before Make.. Think ON Generator---OFF----ON Shore power.
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Gary RV_Wizard

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Re: Auto Transfer Switch Failure
« Reply #9 on: July 10, 2018, 10:02:39 AM »
Trying to stay more basic, the fridge being on is not a problem for an RV-type auto transfer switch. It's only a modest load (about 300W in most fridges) and entirely resistive in nature.  Sure, the ideal is to have zero amps passing through the contacts when they move, but anything short of a large inductive load (an a/c unit) should be within the capability of the ATS.  Repeated arcing will eventually damage the contacts, but ought not to hurt the genset or the appliances.

Some shoreline ATS have a built-in delay to allow the generator to stabilize and prevent an a/c compressor from stuttering from a momentary sag in voltage or frequency.  [This is NOT true of the fast transfer switches built into some inverters, but that's a different topic.] So do some powerline monitors (aka "surge protectors").

The short answer to AJLease's question is that the transfer switch module is generally replaced.  It may have been a component of the power center, or merely an accessory mounted adjacent to it, but most times it is just replaced.
« Last Edit: July 10, 2018, 10:06:59 AM by Gary RV_Wizard »
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