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Are we abusing our master-contactor solenoids?

Hartstoc

Well Known Member
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/ph...5494CBE8862409EA11D8D7C-vi.png?solenoid-specs
 
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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.
 
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......
 
. 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
 
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.

:cool:
 
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- I?ve 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.
 
Point taken-

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 I?ve 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?). I?m installing dual lightspeed EI?s, 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 ?emergency buss?). Still, with the flip of a switch, battery #2 will be available to take over completely for battery #1 if needed.
 
I just have the questions and no answers, but I?m 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.
 
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?

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.
 
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.

:cool:

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?
 
Key to this discussion is an understanding of why the "standard" architecture became the standard. It's been hinted at here... Unlike a car, we have great big swinging scythes of death hanging off the end of the crankshaft of our aircraft engines. Getting that engine to stop in the event a contactor becomes welded closed. If the starter contactor welds closed the only way to stop the prop is to open the master contactor. If one wires direct from battery to starter contactor this additional layer of control no longer exists and in the instance of a stuck starter contactor the prop will spin until the battery depletes or the starter goes up in smoke.

While this sounds totaly improbable, I've had it happen once. It seems like it took forever for me to realize I could stop the prop by switching off the master. Afterward I was darned glad to have had the master there as a fail-safe.
 
I hope I'm not stating the obvious, but be sure that any contactor you're using for a master is rated for continuous duty. Most starter contactors have much higher coil current to drive the contacts together harder and faster (and stronger springs to release faster). But most aren't rated for continuous duty.

Also, most master contactors tie the coil positive to the battery terminal, and the coil negative is switched to ground, which saves one wire run. Starter contactors switch the coil positive (safer control).

Neither of the above are deal killers; just something to know when making a decision on architecture.

On the original question of starter contactor directly to the battery: It's been done in exp a/c. Your plane, your choice; just know all the ramifications.

Hate to sound like a stuck record, but all the above has been hashed out repeatedly on the Aeroelectric List. Much better info there than here.
 
Here is an experiment I would like to try but it will be about 2 weeks until I am able...

While cranking the starter motor shut off the master solenoid and see if it actually removes power from the starter motor. In other words, once power is removed from the master contactor coil, is the spring tension strong enough to overcome the force of the high current draw (don't know the technical name for that force that holds two conductors together during high current flow) and return the contactor to the open position.
 
There is no force that holds the contacts together during high current flow, except the same force that holds them during low current (or no current) flow. Start contactors use stronger coils to make/break *faster*, to minimize contact arcing.

It's not that a master won't open, but it will shorten it's life if you *repeatedly* ask it to make/break during high current (starting) loads. At some point, it likely would either stop making contact (burned contacts) or would weld its contacts together.
 
>There is no force that holds the contacts together during high current flow.

Then how come when I'm removing the positive cable from a battery and the wrench shorts to ground :eek: it feels like it is pulled in that last fraction of an inch, and is harder to break away from ground than one would expect. The same thing occurs when one ( I ) :eek::eek: gets between two conductors of a light switch under load. Electricians call it getting "Hooked Up".
 
Here is an experiment I would like to try but it will be about 2 weeks until I am able...

While cranking the starter motor shut off the master solenoid and see if it actually removes power from the starter motor. In other words, once power is removed from the master contactor coil, is the spring tension strong enough to overcome the force of the high current draw (don't know the technical name for that force that holds two conductors together during high current flow) and return the contactor to the open position.

This is may be a bad idea - but check the exact specs on each contractor first. The master may not be rated high enough for a high current break and could damage the contacts. (+1 for rvcharlie above)

Kudos for asking and investigating, though. We should know the design criteria, validation criteria and an FMEA to be sure a redesign will be better. There is probably some unusual behavior of components from the past that have driven the design. Like a smallish wire to the starter with enough voltage drop that resulted in uncommanded release of the solenoid. And a reengagement . . .

Let us know if something smokes or breaks.
 
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I agree with Charlie. High currents do not hold contacts together, at least in an airplane. High voltages (thousands) are a different story. The force that holds a wrench against ground is called welding. Read about spot welding on Wikipedia. Relay contacts can also be welded if they are not rated for the current and do not open fast enough.
 
I am in the camp of "if it ain't broke don't fix it".
I think it was a good question to ask for general education purposes though.
All this stuff just so we can use a switch.
 
Here is an experiment I would like to try but it will be about 2 weeks until I am able...
While cranking the starter motor shut off the master solenoid and see if it actually removes power from the starter motor.

I strongly recommend that you do NOT conduct this experiment. The master solenoid was not meant to be switched "HOT". Being switched off with that much current flowing will produce a pretty good arc, probably damaging the master switch contacts.
 
Some good comments here and now my own...

It it's not high current but instead high voltage, what is _that_ force called?

The welding explanation would explain why it's hard to remove the wrench but what about it being pulled in? I do like this as a possible/partial explanation however.

One of the reasons it will be a couple of weeks before I conduct the experiment is because that is when my new master relay will be here. Experiment first, replace the old relay (time is service concerns) second.

The fellow that felt the need to comment about my technique should wiki the term self-deprecating humor.

I will conduct the experiment and report back, it seems like a worthwhile thing to do. Besides, the title of the thread is "are we abusing our master-contactor solenoids?".:(:D
 
A wrench does NOT get pulled in. It might seem that it does, but the only force is inertia while the wrench is being turned. Electric motors convert electricity into motion using magnetism. In order for a wrench to be "pulled in", it must be subject to a large magnetic field. And the only way to have a large magnetic field is by having a large electrical current. And there is no large electrical current until AFTER a wrench accidentally shorts positive and negative together. If it could be done SAFELY, conduct an experiment that demonstrates a wrench getting pulled in.
 
The force pulling the wrench in to the battery terminal is the same force that tries to pull you over the edge when you look over a cliff.


;-)

BTW, the force in your example of getting 'hooked in' when grabbing the hot wire in house mains voltage is the same force that your brain uses to make your hands grab something. It's just a few million times stronger.
 
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A wrench does NOT get pulled in. It might seem that it does, but the only force is inertia while the wrench is being turned. Electric motors convert electricity into motion using magnetism. In order for a wrench to be "pulled in", it must be subject to a large magnetic field. And the only way to have a large magnetic field is by having a large electrical current. And there is no large electrical current until AFTER a wrench accidentally shorts positive and negative together. If it could be done SAFELY, conduct an experiment that demonstrates a wrench getting pulled in.


Beautiful!! I love it!

could it be the that the Arc of the electrical connection being made is what creates the sensation that the wrench is being pulled in? And, once the arc has been established then the magnetic field has also been established??

Interesting...another opportunity for an experiment.
 
The force pulling the wrench in to the battery terminal is the same force that tries to pull you over the edge when you look over a cliff.


;-)

BTW, the force in your example of getting 'hooked in' when grabbing the hot wire in house mains voltage is the same force that your brain uses to make your hands grab something. It's just a few million times stronger.

Sorry, I've got thousands of skydives and have looked over many cliffs and have been hooked up a couple of times, but can't make the same connection as you.
 
Different 'forces'. You may not be susceptible, but many of us *feel* pulled toward the edge when we look over a cliff. No real effect unless we step off.

The force when you get 'hooked up'...I don't know how to explain it any more clearly, but that one is real. Just a lot stronger when you grab 120 volts than when you grab a sandwich.
 
Not sure if you are talking psychology or physics. (But assuming the latter since that is what I thought the thread was about...)

>many of us *feel* pulled toward the edge when we look over a cliff. No real effect unless we step off

Care to conduct your own experiment? I'll provide the parachute and, a reasonable level of training to assure you survive.

>but that one is real

But is it stronger when you grab 240 or 480 volts? And is it the voltage or the current that is the dominate factor? I haven't been to that level myself (although I once got "hit" by 277 volts :eek::eek: ) so I can't speak to it.

So many questions, so few answers..
 
The interwebs really do have very effective humor and nuance filters.

The 'edge of a cliff' example was intended to be an analogy. Some of us feel a weird psychological 'pull' when looking over a cliff, but gravity only works down; not at an angle or curve. The analogy is to feeling the wrench 'pulled' toward a terminal where it shouldn't be. Now, if either the wrench or the terminal is highly magnetized (and both are made of ferrous metal), they could be attracted. A starter motor or alternator/generator may well have permanent magnets inside, or even residual magnetism (depending on design). But with a lead battery terminal, you shouldn't have magnetic attraction (but the psychological attraction may be there, for some of us).

On getting 'hooked up': the electrical impulses from your brain are quite weak. 120V is a *lot* stronger. 240V is stronger still, etc.

Honestly, I won't attempt to answer whether it's voltage or current that actually causes the 'grab' action, but it doesn't really matter in practical terms because of Ohm's Law. If you grab a 12 volt line, you probably won't even feel it if your hands are dry and even slightly calloused. There isn't enough 'pressure' (to use the old water flow analogy) in 12V to push current (electron flow) into and through your body to trigger your muscles. Electrical safety codes have traditionally used 70V as the threshold between 'safe' and 'dangerous'; requiring any terminals at >70V potential to be protected from incidental human contact.
 
You are correct,auto starters stopped using separate solenoids in 50s, in auto conversion,
(Subby) ignition switch worked starter flawlessly with out separate solenoids.when was
last time you burned up starter in auto cause stayed engaged ? Tom
 
You are correct,auto starters stopped using separate solenoids in 50s, in auto conversion,
(Subby) ignition switch worked starter flawlessly with out separate solenoids.when was
last time you burned up starter in auto cause stayed engaged ? Tom

The main reason for using 2 separate solenoids on aircraft is to avoid having an "always hot" wire running to the starter. In automobiles the starter terminal is not readily accessible while working under the hood.
On aircraft, we are typically working in close proximity to the starter terminal on a regular basis.

Note that I didn't say what's right or wrong. It's just the reason it's been done that way for so many decades.

And on the discussion about "voltage/current" grabbing you. You are comparing DC to AC. When working with AC, it's the alternating current that tightens you muscles and won't let you turn loose. DC does not do this.
 
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The main reason for using 2 separate solenoids on aircraft is to avoid having an "always hot" wire running to the starter. In automobiles the starter terminal is not readily accessible while working under the hood.
On aircraft, we are typically working in close proximity to the starter terminal on a regular basis.

Note that I didn't say what's right or wrong. It's just the reason it's been done that way for so many decades.

And on the discussion about "voltage/current" grabbing you. You are comparing DC to AC. When working with AC, it's the alternating current that tightens you muscles and won't let you turn loose. DC does not do this.


And of course some careless person may short out the terminals with spanner. :)
 
In response to Mel's post, not only is there the inconvenience of burning a hole in a tool (or yourself) on that hot starter terminal, there's a much bigger danger in the case of a crash, where that wire will be hot and likely arcing to ground around hot fuel and oil. Much less risk with a starter contactor next to the battery, even if it's not routed through a master contactor.

But I'd have to disagree with the comment about AC vs DC & muscle contraction. Ask anyone who's been Tased.

https://en.wikipedia.org/wiki/Electrical_injury

It's really about Ohm's law; not AC vs DC.

http://www.brighthubengineering.com/power-plants/89792-ac-and-dc-shock-comparison/

The 2nd link answers the question about how much current it takes, but it still depends on Ohm's law.
 
But I'd have to disagree with the comment about AC vs DC & muscle contraction. Ask anyone who's been Tased.

A little research will show that a taser IS (for all practical purposes) AC. Typically they alternate at about 19 pulses per second.
 
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A little research will show that a taser IS AC. Typically they alternate at about 19 pulses per second.
And, until current flow stabilizes, for a fraction of a second DC acts like AC.

waveform-chart-dc-insrush-current-spike.jpg


:cool:
 
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https://en.wikipedia.org/wiki/Electroshock_weapon

Pulsed DC is not the same as AC.

But hey, I think I remember being wrong before, so maybe I'm wrong now.

Regardless, DC *will* cause muscle contraction, and that's the real point.
FWIW: AC is any "alternating current" so pulsed DC is actually a type of AC. What most people call AC, and what you are probably referring to, is actually Sinusoidal AC. Easy to confuse Sinusoidal AC with AC.

:cool:
 
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