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Switches on Alternators?

rockitdoc

rockitdoc

Well Known Member
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I intended to use breakers on the field leads for my alternators (60A Plane Power and 40A B&C). But, looking at the wiring diagrams for both products I see switches in the circuit between the breakers and the alternators.

What is the downside to not using these switches??
 
rockitdoc said:
I intended to use breakers on the field leads for my alternators (60A Plane Power and 40A B&C). But, looking at the wiring diagrams for both products I see switches in the circuit between the breakers and the alternators.

What is the downside to not using these switches??
Click to expand...

You'll need to pull the breaker to turn off field current. Not a great solution as CB's aren't designed to be switches...that's why we have switches :)
 
bjdecker said:
You'll need to pull the breaker to turn off field current. Not a great solution as CB's aren't designed to be switches...that's why we have switches :)
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I only have a breaker on my alternator. I see no need to turn in off in normal operation. If you plan to turn it off routinely yes put a switch.
 
bjdecker said:
You'll need to pull the breaker to turn off field current. Not a great solution as CB's aren't designed to be switches...that's why we have switches :)
Click to expand...

Why would one want to turn off the field current? In other words, what is the downside to not having switches?
 
rockitdoc said:
Why would one want to turn off the field current? In other words, what is the downside to not having switches?
Click to expand...

This debate comes up about once or twice a year, and probably should be in the “never-ending” category, because honestly, you can do what you want. I personally don’t use a switch anymore - the alternator comes up with the engine, just like on my cars, trucks, tractors, etc. If I have a problem and need to to turn the alternator off, I can pull the field breaker….but that is only if I have a failure. Normally, it is just “on”.

Other like to turn the alternator on after the engine is running. To me, that’s just one more switch to throw, and one more item to fail…but that’s my take on it.

Paul
 
Great minds think alike.

Ironflight said:
This debate comes up about once or twice a year, and probably should be in the “never-ending” category, because honestly, you can do what you want. I personally don’t use a switch anymore - the alternator comes up with the engine, just like on my cars, trucks, tractors, etc. If I have a problem and need to to turn the alternator off, I can pull the field breaker….but that is only if I have a failure. Normally, it is just “on”.

Other like to turn the alternator on after the engine is running. To me, that’s just one more switch to throw, and one more item to fail…but that’s my take on it.

Paul
Click to expand...

My thoughts exactly. Thanks Paul
 
Alt switch

My ACM has a provision for alternator control and breaker so I added the switch. It actually controls the regulator. Regulator controls the alternator.
 
I have been starting my engine with the alternator field on. Unfortunately, my alternator failed after ~343 hours. I don’t know if there was a relation, but I found this on the Plane Power website:

“You need to follow the aircraft manufacturer’s recommendations for start-up/shut-down procedures if published. However, if not published, the field switch should always be off when the engine is started. There is no reason to subject the electrical system, alternator, and regulator to the power fluctuations during the starting/shut-down process. Once the engine is started, and has stabilized, turn on the field switch and add bus load.”

I plan to follow this guidance in hopes it makes a difference. YMMV
 
Hmmm

MED said:
I have been starting my engine with the alternator field on. Unfortunately, my alternator failed after ~343 hours. I don’t know if there was a relation, but I found this on the Plane Power website:

“You need to follow the aircraft manufacturer’s recommendations for start-up/shut-down procedures if published. However, if not published, the field switch should always be off when the engine is started. There is no reason to subject the electrical system, alternator, and regulator to the power fluctuations during the starting/shut-down process. Once the engine is started, and has stabilized, turn on the field switch and add bus load.”

I plan to follow this guidance in hopes it makes a difference. YMMV
Click to expand...

Food for thought. I wonder if others have had similar experience
 
MED said:
I have been starting my engine with the alternator field on. Unfortunately, my alternator failed after ~343 hours. I don’t know if there was a relation, but I found this on the Plane Power website:
“You need to follow the aircraft manufacturer’s recommendations for start-up/shut-down procedures if published. However, if not published, the field switch should always be off when the engine is started. There is no reason to subject the electrical system, alternator, and regulator to the power fluctuations during the starting/shut-down process. Once the engine is started, and has stabilized, turn on the field switch and add bus load.”
I plan to follow this guidance in hopes it makes a difference. YMMV
Click to expand...

My experience was just the opposite. When I built my -6 in the late '80s I had a switch and always started with alternator off. Aft replacing the alternator several times, I removed the switch and left it on with the master. After that the alternator lasted another 25 years.
 
****. The debate goes on.

Mel said:
My experience was just the opposite. When I built my -6 in the late '80s I had a switch and always started with alternator off. Aft replacing the alternator several times, I removed the switch and left it on with the master. After that the alternator lasted another 25 years.
Click to expand...

Oh well. Debate resumes.
 
Mel said:
My experience was just the opposite. When I built my -6 in the late '80s I had a switch and always started with alternator off. Aft replacing the alternator several times, I removed the switch and left it on with the master. After that the alternator lasted another 25 years.
Click to expand...

My experience also. Internally regulated type.
However, internally regulated or external and how the external regulator is wired may dictate different operation.
 
Mel said:
My experience was just the opposite. When I built my -6 in the late '80s I had a switch and always started with alternator off. Aft replacing the alternator several times, I removed the switch and left it on with the master. After that the alternator lasted another 25 years.
Click to expand...

Exactly my experience using internally-regulated alternators. Automotive parts - automotive operations….
 
I have a 3 position SW: OFF/MASTER ON/MASTER+ALT ON
I'm thinking I should start in the +ALT position and still be able to kill the power to the field if need be.
 
MED said:
...I found this on the Plane Power website:

“You need to follow the aircraft manufacturer’s recommendations for start-up/shut-down procedures if published. However, if not published, the field switch should always be off when the engine is started. There is no reason to subject the electrical system, alternator, and regulator to the power fluctuations during the starting/shut-down process. Once the engine is started, and has stabilized, turn on the field switch and add bus load.”

I plan to follow this guidance in hopes it makes a difference. YMMV
Click to expand...

Off/Master/Master+Alt is what I did - the PP guy during an Oshkosh presentation in 2019 I believe said what is written above. Seemed to make sense and he knows a lot more about alternators than I do.
 
here's a reason for a switch, or not . you tell me cuz i don't know. this occurred to me a couple days ago.
my ovm opens a relay in the b lead if there is an over voltage. i have heard that disconnecting the b lead from a load can ruin the alternator.
so if my engine is shutdown by turning off the master this opens the field and at the same time opens the relay in the b lead. is there still current in the b lead to damage it? is the miniscule lag in the b lead relay opening enough to protect it?
 
B&C

rockitdoc said:
B&C is external regulated. I’ll call them to see if switch is required.
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I asked B&C this question 5 years ago. They told me it does not make a difference starting switch on or off. For me I leave on but do test monthly in flight to insure my caution and warning alarms are set up correctly and working. Since I have a diode bridged primary bus to engine bus want to insure that is performing as designed and helps having switches installed to test the system.
 
dmattmul said:
I asked B&C this question 5 years ago. They told me it does not make a difference starting switch on or off. (Snip)
Click to expand...

That’s funny how even the manufacturer has differing opinions. I was told just the opposite a couple of years ago. They told me it was much healthier for the alternator to turn it off/on via a field wire switch before/after starting the engine.
 
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When you start with the field on it gives the alt time to ramp up the output slowly avoiding a large current surge, this is especially true after start when the battery will be accepting an initial large charge current.
Bottom line, start with the alt ‘on’.
 
Walt said:
When you start with the field on it gives the alt time to ramp up the output slowly avoiding a large current surge, this is especially true after start when the battery will be accepting an initial large charge current.
Bottom line, start with the alt ‘on’.
Click to expand...

To play devil's advocate, if the alternator is on when starting, the regulator sense voltage is zero so the field current will be at its maximum value. Having full field current available will cause the starter to work harder to overcome the alternator drag and turn the engine, essentially requiring more current from the battery.

With the switch off, there is no field current or additional drag from the alternator, which allows the starter to require less current to start the engine.

Once again, I think people truly overthink this stuff; It works either way.

If you really want to know what to do, follow you alternator's manufacturer recommendations. Why? They are the ones responsible for the warranty should something happen. The same goes for engine break in.
 
Alternator switching

I have dual alternators and have a switch that is on/on. And it is powered up before start.
In other words the alternator field power is flowing to the switch and up is 60 Amp primary and down is 40 Amp B&C backup
My luck varies Fixit
 
rocketman1988 said:
To play devil's advocate, if the alternator is on when starting, the regulator sense voltage is zero so the field current will be at its maximum value. Having full field current available will cause the starter to work harder to overcome the alternator drag and turn the engine, essentially requiring more current from the battery.

With the switch off, there is no field current or additional drag from the alternator, which allows the starter to require less current to start the engine.
Click to expand...

Field current is generally less than 5 amps, almost nothing compared to the amp draw of the starter.
As far as the drag, we’ll never measured that, with no output I would imagine the drag to be close to zero though.
 
Walt said:
Field current is generally less than 5 amps, almost nothing compared to the amp draw of the starter.
As far as the drag, we’ll never measured that, with no output I would imagine the :Ddrag to be close to zero though.
Click to expand...

You can imagine it, but that’s not the case. The drag is measurable and not zero. Will it make a difference? Probably not but that’s not the point.

My point is that either way, on or off, works. People are seriously overthinking it.

Anyway, like everything else, it’s up to the individual to decide. Now maybe we should start up the boost pump discussion, again…:D
 
The alternator on my Rocket is from a Toyota. It does not know its on an airplane and I dont treat it any differently. No switch for me.
 
bobnoffs said:
here's a reason for a switch, or not . you tell me cuz i don't know. this occurred to me a couple days ago.
my ovm opens a relay in the b lead if there is an over voltage. i have heard that disconnecting the b lead from a load can ruin the alternator.
so if my engine is shutdown by turning off the master this opens the field and at the same time opens the relay in the b lead. is there still current in the b lead to damage it? is the miniscule lag in the b lead relay opening enough to protect it?
Click to expand...

I believe the experts like Bob Nuckolls and the PP guys say that you don't want to disconnect the B-lead while the alternator is under load for exactly this reason - it takes a few milliseconds for the alternator to stop generating power, and that power has to go somewhere. Some alternators can handle this sudden voltage increase, some can't.

I'm sure that someone has done the research on this using a scope and can give the facts, but I don't have that handy. As others have said, we sure spend a lot of time on this topic, and have yet to come to a solid, clear set of best practices, besides the frequently heard "just buy a B&C".
 
I think the majority of us learned to fly in a Cessna. If I remember correctly the standard ops in the 172 was to start with the Alt off and turn on after the start.
I built my plane with a two position main switch however I always start with the alt on.
Go figure.
 
MarkW said:
I think the majority of us learned to fly in a Cessna. If I remember correctly the standard ops in the 172 was to start with the Alt off and turn on after the start.
I built my plane with a two position main switch however I always start with the alt on.
Go figure.
Click to expand...

I just looked at the POH for a 182Q. I think the 182 has the same double rocker master/alt as the 172. No mention of turning on the Alt after start. It just says 'Master on'. My recollection is that the double rocker including the master and the alternator was switched on at the same time.
 
This is What B&C Had to Say

I asked:

Hi,
I have your BC410-H and regulator on my RV-14A as a backup to my Plane Power 60A unit up front. I am in the process of wiring everything. Your wiring diagram indicates a switch between the +12v bus and the field wire. If I do not use a switch, the alternator will be powered up on engine start. What problems might this cause, if any? I ask because I would like to limit the number of switches on my panel.
Thanks,
Scott

This is the response:

Scott,

Thank you for contacting us with that question. There should not be a problem with that arrangement, though it could add an immediate 3A load to your bus in certain conditions (perhaps not desirable if you have a low battery). This could always be mitigated by pulling the 5A field circuit breaker associated with the backup alternator, however – so it could be addressed as a matter of pre-flight procedure. Note: this assumes you have a conventional bus with circuit breakers & bus bar rather than a VPX bus management system.

I hope the above makes sense and is helpful. If you have any other questions, please just let me know.

Kind Regards,

“TJ” Jenkins
B&C Specialty Products
 
Question: If you have both the primary and secondary (back-up?) alternators on-line at the same time, how would you know if either one failed, essentially leaving you without a back-up? :confused:
 
MED said:
Question: If you have both the primary and secondary (back-up?) alternators on-line at the same time, how would you know if either one failed, essentially leaving you without a back-up? :confused:
Click to expand...

Idiot light on panel tells me if primary fails. PFD tells me. Currrent from battery is monitored. If lots of current flowing out of battery, neither alt is working.
 
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