Alternator on or off during engine start and stop?

[Dan Thomas]

Holy necropost!

this has been happening a lot lately.

Slow news days........

 

[Jon Wilder]

I know it's an old post but it still comes up in Google searches. I'd noticed several posts eluding to the alternator drawing from the battery and felt the need to clear up the fact that the ALT master isn't doing what some apparently think it does (i.e. disconnects the alternator from the master/battery bus, which it does not). It simply turns the alternator on or off through its field winding.

 

[Dan Thomas]

I know it's an old post but it still comes up in Google searches. I'd noticed several posts eluding to the alternator drawing from the battery and felt the need to clear up the fact that the ALT master isn't doing what some apparently think it does (i.e. disconnects the alternator from the master/battery bus, which it does not). It simply turns the alternator on or off through its field winding.

In the '96-on Cessna singles, there is an alternator relay that actually cuts the alternator's output off from the bus. It's a completely different setup from older airplanes.



All those contactors are in a box on the firewall. With its cover off you see this:



Ignore the 9V battery connectors. Those are shown as temporary parts of a system test.

 

[Rgbeard]

I know it's an old post but it still comes up in Google searches. I'd noticed several posts eluding to the alternator drawing from the battery and felt the need to clear up the fact that the ALT master isn't doing what some apparently think it does (i.e. disconnects the alternator from the master/battery bus, which it does not). It simply turns the alternator on or off through its field winding.

thanks so much for your service to the world.

 

[Jim Kickland]

I like Dans comment about not forgetting to turn it on. I've done it before. I must need to be in a group home.

 

[flyingron]

It's important to realize what's actually happening here. The alternator isn't turned "on or off"... it runs whenever the engine runs as it's driven by prop shaft (usually a belt just behind the prop). The switch controls if any electricity from your alternator makes its way to the main electrical bus or not.

Umm, no. Not in my plane or most of the other ones I fly. The alternator switch turns on and off the alternator field. The alternator is always connected to the bus. While it turns with the engine, it doesn't make juice until the field is energized by the alt switch.

The battery switch works a contactor that connects the battery to the bus, but the alternator just switches the low current field wiring.

 

[Dan Thomas]

Umm, no. Not in my plane or most of the other ones I fly. The alternator switch turns on and off the alternator field. The alternator is always connected to the bus. While it turns with the engine, it doesn't make juice until the field is energized by the alt switch.

The battery switch works a contactor that connects the battery to the bus, but the alternator just switches the low current field wiring.

More correctly, the ALT switch turns the regulator on, and the regulator then energizes the field. The field current comes from the regulator's A terminal, which is connected to the alternator's output terminal or the bus (they're both at the same potential), and that A terminal feeds both the voltage sensing circuity and the field.

 

[Jon Wilder]

@flyingron and @Dan Thomas ...nailed it.

Typically the field current originates from the bus through the regulator. Although they are at the same potential when the alternator is generating, it is ultimately the battery that supplies the initial field current (the alternator couldn't generate if this were not the case).

In the case of the newer Cessnas, it appears that the "ALT FIELD" breaker is now also sourcing current for an alternator relay that does, in fact, disconnect it from the bus, while also sourcing the field current to the Alternator Control Unit. A bit different setup from the traditional system and is definitely not the "standard" on most aircraft.

 

[wanttaja]

Replaced the generator on my Fly Baby (Continental C85) with an B&C Alternator this year.

When I start the engine the alternator doesn't charge. If I cycle the alternator switch, it starts working.

So I eliminated the middle-man and now start with the alternator off. Works every time....

Ron Wanttaja

 

[Dan Thomas]

Replaced the generator on my Fly Baby (Continental C85) with an B&C Alternator this year.

When I start the engine the alternator doesn't charge. If I cycle the alternator switch, it starts working.

So I eliminated the middle-man and now start with the alternator off. Works every time....

Ron Wanttaja

Need to see the wiring diagram. That issue was a problem in the Aerostars (Ted Smith/Piper); one or both alternators wouldn't start charging some of the time at startup, and there was a hassle getting them going. There was an SB to fix it, involving, IIRC, a diode between a couple of points in the circuitry. I only fixed one and don't remember now how it went (ten years ago) but that system, I think, used a bootstrap circuit, feeding the regulator input off the alternator output, and an alternator field firing wire from the starter to get it started. The short period between starter disconnect and alternator speedup left the alternator with no field current. I remember helping another mechanic try to figure out why the stupid alternators wouldn't catch every time, taking voltage readings as he cranked, studied the wiring diagrams, and found that flaw in the circuit. I figured there must be an SB on it, and sure enough there was when we went looking for it. Fixed it.

 

[RyanB]

Is it just me, or does anyone else think it’s weird that airplanes have independent switches to turn the alternator on and off. I haven’t yet discovered a reason why I would choose not to run it while the engine is running.

 

[wanttaja]

When I start the engine the alternator doesn't charge. If I cycle the alternator switch, it starts working.

So I eliminated the middle-man and now start with the alternator off. Works every time....

Need to see the wiring diagram.

The regulator itself is B&C's Automatic Voltage Controller, which combines regulation with overvoltage protection. The checkmarks indicate where changes were made from the previous generator system.

Ron Wanttaja

 

[PaulS]

I follow the checklist.

 

[wanttaja]

Is it just me, or does anyone else think it’s weird that airplanes have independent switches to turn the alternator on and off. I haven’t yet discovered a reason why I would choose not to run it while the engine is running.

If the alternator develops a short and starts sucking down juice instead of creating it, you can switch it offline and fly to the nearest airport under battery power.

Ron Wanttaja

 

[Dan Thomas]

The regulator itself is B&C's Automatic Voltage Controller, which combines regulation with overvoltage protection. The checkmarks indicate where changes were made from the previous generator system.

What is this thing here, circled in yellow? Looks like a buzzer, but it's in a curious spot.




The cranking of the starter will cause a big voltage drop at the bus (the red spot) and perhaps the ACU (regulator) or that little "buzzer" thing, is cutting the system out due to the voltage drop, requiring recycling of the ALT switch.

 

[Rgbeard]

Is it just me, or does anyone else think it’s weird that airplanes have independent switches to turn the alternator on and off. I haven’t yet discovered a reason why I would choose not to run it while the engine is running.

Let's read a POH for an airplane with a split-switch.

I found one for a Cherokee 6-260 on the Interwebs.

One of the instructions in the POH is that, in case of no output indicated on the ammeter, (again, you need to understand ammeters, load meters and volt meters), turn the switch for 30 seconds, to reset the overvoltage relay, and re-start the otherwise offline alternator. In a car you could simply pull over and turn the car off and back on again.

Having a switch on the alternator field simply gives you a degree of control and/or troubleshooting that you would not have without it.

 

[flyingron]

What is this thing here, circled in yellow? Looks like a buzzer, but it's in a curious spot.

View attachment 102613

.

Those are circuit breakers.

 

[Mahneuvers]

Is it just me, or does anyone else think it’s weird that airplanes have independent switches to turn the alternator on and off. I haven’t yet discovered a reason why I would choose not to run it while the engine is running.

My club does not want the alternator on during start. The reason stated is it adds load to the system. If having trouble starting on a cold day and the battery is getting along in age, it might make the difference.

 

[Palmpilot]

I like Dans comment about not forgetting to turn it on. I've done it before. I must need to be in a group home.

One of the Citabrias I rent had a separate alternator switch added a while ago. The checklist that my instructor made now has an item to turn the alternator switch on after engine start. Now I just have to make sure that I don't overlook any items on the checklist.

 

[Palmpilot]

I'm guessing that the current drawn by the alternator's field windings is not significant enough to be an issue, but based on conservation of energy, I would assume that the mechanical power required to turn the alternator would be less when the alternator is not producing electric power. Whether that would be enough to make a difference when there is slow cranking due to low temperature, weak battery, etc., I don't know.

 

[AKBill]

On when starting, alt off and master off when evidence of electrical problems/fire. Follow your POH

 

[wanttaja]

What is this thing here, circled in yellow? Looks like a buzzer, but it's in a curious spot.

Circuit breaker...should be in a circle. Klixon type.

You'll see similar associated with the avionics, though these are automotive blade fuses.

I do have a piezo buzzer in the circuit for the inverter powering the ejection seat, but don't have that circuit in the schematic. It's only on when the seat is running, though, and that's after the engine shuts down.

Ron Wanttaja

 

[Dan Thomas]

Circuit breaker...should be in a circle. Klixon type.

SO that's not the problem. Likely the ACU is dropping offline when the system voltage falls below 10 or so, which it will during start. The battery's limited amperage capacity, and its internal resistance, will do that during large current draws.

That looks like a PM alternator, with no field input. Is that right? If so, the ACU might cut off the alternator's output when the battery voltage falls so low so that the alternator doesn't overcurrent itself.

I do have a piezo buzzer in the circuit for the inverter powering the ejection seat, but don't have that circuit in the schematic. It's only on when the seat is running, though, and that's after the engine shuts down.

 

[Dan Thomas]

My club does not want the alternator on during start. The reason stated is it adds load to the system. If having trouble starting on a cold day and the battery is getting along in age, it might make the difference.

It's true, but it's somewhat nitpicky. That starter is pulling 250 amps or more; the alternator field will never draw more than three or four amps (60-amp alternator) or perhaps double that for a really big alternator. The alternator, with its field on, does add to the torque load required to turn the engine, and on a marginal start it could make difference.

 

[Dan Thomas]

Let's read a POH for an airplane with a split-switch.

I found one for a Cherokee 6-260 on the Interwebs.

One of the instructions in the POH is that, in case of no output indicated on the ammeter, (again, you need to understand ammeters, load meters and volt meters), turn the switch for 30 seconds, to reset the overvoltage relay, and re-start the otherwise offline alternator. In a car you could simply pull over and turn the car off and back on again.

Having a switch on the alternator field simply gives you a degree of control and/or troubleshooting that you would not have without it.

That switch also allows you to shut off a rogue alternator. If the voltage regulator fails in full-field mode, the system voltage will get far too high and start making the magic smoke appear. And if the alternator fails and is dead, you're now running on the battery and there's no use losing another few amps to the alternator's field.

Magneto switches are similarly for more than checking the mags in the runup. That switch is a troubleshooting tool when the engine starts running really rough. If a magneto has suffered distributor gear failure, the sparks are now going to cylinders at times that the cylinders don't want them. You end up with stuff igniting way too soon, like on the intake stroke, that ends up setting off all the fuel/air mix in the manifold, or very early on the compression stroke. Bad stuff either way. So you check right/left and see if it smartens up on one of them, and leave it there and land when practicable.

 

[Mahneuvers]

...So you check right/left and see if it smartens up on one of them, and leave it there...

On a flight not too long ago I experienced ~10 seconds of engine roughness. I applied carb heat w/o benefit. I didn't even think about isolating the magneto. Thanks for the tip.

 

[Jon Wilder]

Alternator stators cannot draw current from the battery!!! There are rectifier diodes inside the alternator that convert the alternator output from AC to DC. They also prevent backfeeding through the stator.

Sure, the field draws from the battery but no more than 5 amps. This is negligible when compared to the 75 amp draw (150 amp for a 14V aircraft) of the starter. Compared to the starter this is an extremely light load. If anything the starter steals current from the alternator field as it is clearly the easier path. Not the other way around.

Not to mention a Cessna Nav III aircraft checklist specifically states MASTER Switch (ALT and BAT) - On on the Before Start Checklist. If it created starting issues to have the alternator alive during starting, they would not say to have both ALT and BAT on before starting.

Some alternators need to "wake up". Does the alternator begin charging when revving it to something above 1,000 RPM?

 

[Dan Thomas]

Alternator stators cannot draw current from the battery!!! There are rectifier diodes inside the alternator that convert the alternator output from AC to DC. They also prevent backfeeding through the stator.

We know that. It's not an issue, normally, but Cessna now has a contactor to disconnect the alternator for that reason. Diodes can fail.

Sure, the field draws from the battery but no more than 5 amps. This is negligible when compared to the 75 amp draw (150 amp for a 14V aircraft) of the starter. Compared to the starter this is an extremely light load. If anything the starter steals current from the alternator field as it is clearly the easier path. Not the other way around.

What starter draws only 75 amps? Some of the newer PM starters need 125 amps. The older series-wound starters could draw 250 amps.

Alternators don't need to "wake up" at 1000 RPM. The generators did that. Old-school stuff, different technology.