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  #1  
Old 12-25-2017, 10:34 AM
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Hartstoc Hartstoc is offline
 
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Default Are we abusing our master-contactor solenoids?

OK, this IS one of those questions that flies in the face of conventional wisdom, so answers like “we all do it and there are no problems” and “if it ain’t broke...” will immediately come to mind, but I have to ask.

Nearly all road vehicles have the battery wired directly to the starter solenoid, and most modern cars also have a plethora of built-in relays that power-up various sub-systems when the ignition switch is in various positions.

Nearly all RV’s instead have the intermittent starter solenoid wired in series with the continuous-duty master-bus solenoid, which in turn IS wired directly to the
Battery. Great idea, right? If the starter solenoid gets stuck you can kill the master! Is this a no-brainer or a bad idea?

I hope the url below will take you to an album with two images showing specs for the master and starter solenoids commomly used in our airplanes. As you will see, the following statements are true of the starter solenoid:
- It is twice as heavy as the master.
- Its coil draws 3amps vs just 1amp for the master.
- It is rated for 900 in-rush Amps vs. 150 for the master.
- It can handle several-hundred amp loads vs 80 amps max for the master.

That big fat wire conecting solenoid to starter is there for a reason- your starter DOES draw hundreds of amps from a well charged battery of appropriate size. The starter solenoid is heavy because it is designed to handle that for the time required to start the engine. Its coil draws higer amperage to insure that it slams the contactors together quickly and firmly to transfer all those amps with minimal arcing and resistance, and to overcome the force of a stronger return spring.

But what about that little guy feeding the master bus- what sort of experience is it having during engine start? True- it is in full contact mode already, so arcing should not be an issue, but those contactors and internal conductors are much smaller than those in the starter solenoid. Also, their face-contact pressures, powered by that little 1-amp coil, are far lower than those in the starter solenoid. Is the master solenoid acting as a resistor during starts? When you are cranking away during a problem hot-start with fuel injection, what sort of internal temperatures is the master solenoid seeing? Is it possible that micro-arcing is ocurring over portions of the contactor faces that are not in hard contact because of the huge current draw? Which of these contactors is really more likely to weld itself shut?

I don’t know if stuck starter solenoids Are really much of a threat. It could be that putting two of THEM in series would be a more proper way to prevent it but ouch, those suckers weight 24 oz each!

Sorry to say, I just have the questions and no answers, but I’m hoping some of you can provide those. One little test worth doing might be to temporarily make a SOLID connection(not with jumper cables!) to the starter solenoid, and observe any difference in cranking vigor. My guess is that any of you with larger, higher compression engines would see a significant improvement

https://images49.fotki.com/v1658/pho...solenoid-specs
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Last edited by Hartstoc : 12-25-2017 at 02:37 PM.
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  #2  
Old 12-25-2017, 11:13 AM
rvbuilder2002 rvbuilder2002 is offline
 
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Not that "Because we have always done it that way" is ever a good answer, but this configuration is not something that is specific to RV's.
It has been the standard on certificated aircraft at least back to the 1950's, so there is some history to support doing it the way it is. I.E., doesn't seem to be detrimental to the master relay (or alternatives would likely have been developed long ago), etc.
In more recent times there have been other configurations adopted to provide one (sometimes more) emergency buss's, and sometimes back-up batteries, but even in those cases it seems to be common practice to have a master contractor.
Other than the slight extra weight, and the additional failure point (rather rare, but does happen) I don't see the benefits of not having one, out weighing the benefits of having one.
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  #3  
Old 12-25-2017, 11:15 AM
gasman gasman is offline
 
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I think the problem people have had in the past is causing the lug to slightly rotate during nut tightening by not having a wrench on the inner nut. This causes the paddle contact to rotate even slightly and not make good contact with the disc.

Properly installed, I think the solenoid has proven itself. But you could install a manual switch for for the starter that would not need to pass through the master therefore eliminating the master for that safety task. Eliminating the start load from the master, you could now install a manual master and eliminate all coil loads.

To make the switches as reliable as possible, they should controlled by push pull rods that pass through to the panel. Remember the starter button on the floor of your jeep or car? You used your foot for a reason......
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  #4  
Old 12-25-2017, 11:19 AM
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Carl Froehlich Carl Froehlich is online now
 
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Quote:
Originally Posted by rvbuilder2002 View Post
. SNIP I don't see the benefits of not having one, out weighing the benefits of having one.
Or two, one for each battery. Add to that smaller solenoids with separate feeds to the panel and you have a robust arrangement to isolate a fault but still keep the panel up.

The last two RVs I used Blue Sea Solenoids for each master relay. These have significant contact ratings as well as a relatively low holding current draw.

Carl
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Old 12-25-2017, 11:37 AM
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GalinHdz GalinHdz is online now
 
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Quote:
Originally Posted by Hartstoc View Post
True- it is in full contact mode already, so arcing should not be an issue, but those contactors and internal conductors are much smaller than those in the starter solenoid.
You answered your own question. Contactor size is used to minimize the effects of the surge when the contactors first engage. After that initial contact surge is over, a much smaller contact can be used.

But, this is the beauty of experimental aviation. You can experiment and do it different than everyone else. Go for it if you want. That way you reap the benefits and pitfalls of your experiment.

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  #6  
Old 12-25-2017, 02:39 PM
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Hartstoc Hartstoc is offline
 
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Quote:
Originally Posted by Carl Froehlich View Post
Or two, one for each battery. Add to that smaller solenoids with separate feeds to the panel and you have a robust arrangement to isolate a fault but still keep the panel up.

The last two RVs I used Blue Sea Solenoids for each master relay. These have significant contact ratings as well as a relatively low holding current draw.

Carl
Carl- thanks for the tip on the blue Sea solenoids- Iíve added specs for one that looks well suited. As you say , it is rated for ample current and draws almost nothing except when changing state.
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  #7  
Old 12-25-2017, 02:57 PM
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Hartstoc Hartstoc is offline
 
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Default Point taken-

Quote:
Originally Posted by rvbuilder2002 View Post
Not that "Because we have always done it that way" is ever a good answer, but this configuration is not something that is specific to RV's.
It has been the standard on certificated aircraft at least back to the 1950's, so there is some history to support doing it the way it is. I.E., doesn't seem to be detrimental to the master relay (or alternatives would likely have been developed long ago), etc.
In more recent times there have been other configurations adopted to provide one (sometimes more) emergency buss's, and sometimes back-up batteries, but even in those cases it seems to be common practice to have a master contractor.
Other than the slight extra weight, and the additional failure point (rather rare, but does happen) I don't see the benefits of not having one, out weighing the benefits of having one.
I do appreciate this point of view. I do wonder if resistance through the master contactor is a factor the several installations Iíve witnessed that require quite a bit of solenoid clicking to turn over if a jug happens to be right at compression in spite of a good strong battery.

My own installation will be pretty immune to this issue(or perhaps non-issue?). Iím installing dual lightspeed EIís, and will be replacing the single battery with a twin pair each rated at about 70% of the AH of the original. There will be two master contactors feeding the main buss, and during starts both will be on, so the master contactors will each carry about 1/2 of the full starter load. Once running, master #2 will be turned off, but a shotky diode will provide ample recharging. In that mode the only load on battery #2 will be a single lightspeed fed from a ring terminal right on the battery post(my ďemergency bussĒ). Still, with the flip of a switch, battery #2 will be available to take over completely for battery #1 if needed.
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  #8  
Old 12-25-2017, 03:52 PM
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Quote:
Originally Posted by Hartstoc View Post
I just have the questions and no answers, but Iím hoping some of you can provide those.
I also shared your concerns, and chose not to run the starter current through my Master relay. The obvious risk of doing that, of course, is that if the starter solenoid sticks, the starter will continue cranking until the battery dies. I chose to accept that risk and mitigated it as much as possible by installing MOVs across the starter contacts to (hopefully) prevent any arcing in there.

For my Master relay, I used a Bosch 75A automotive relay with pre-contacts. It's rated for hundreds of thousands of cycles.

Disclaimer: This is only what I did and I don't recommend my method to anybody else. I chose to accept the risk of a stuck starter relay, but your mileage may vary.
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Old 12-25-2017, 04:21 PM
Aluminum Aluminum is offline
 
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Quote:
Originally Posted by Hartstoc View Post
Nearly all RVís instead have the intermittent starter solenoid wired in series with the continuous-duty master-bus solenoid, which in turn IS wired directly to the
Battery. Great idea, right? If the starter solenoid gets stuck you can kill the master! Is this a no-brainer or a bad idea?
You are not alone in this sentiment. I too was baffled by the "standard practice", so on my first build I replaced the master relay with a marine battery switch. Many years later I still find this configuration to be far superior and will repeat it on the 14A.
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  #10  
Old 12-25-2017, 05:17 PM
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az_gila az_gila is offline
 
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Quote:
Originally Posted by GalinHdz View Post
You answered your own question. Contactor size is used to minimize the effects of the surge when the contactors first engage. After that initial contact surge is over, a much smaller contact can be used.

But, this is the beauty of experimental aviation. You can experiment and do it different than everyone else. Go for it if you want. That way you reap the benefits and pitfalls of your experiment.

Even better, the continuous duty Cole-Hersey contactors actually have two ratings...

24059-08 UL listed
Same as 24059, but UL and CE rated.
Continuous Rating: 65A at 12V DC. Intermittent rating:
750A make, 100A break. 10 sec On, 30 min Off. Circuit G1.


So not only can it pass high currents in the ON state, it can actually switch them and carry them (starter loads) for short periods of time.

Aren't our starter currents in the 200 to 300 Amp range?
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