alternator test during runup

GeorgeC

GeorgeC

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During runup, after loading up the electrical bus (pitot heat, etc), do you:

a) flip the alternator field switch off and back on to observe a discharge followed by a charge, or

b) leave the field on and observe that no discharge occurs?

I ask because we used to do (a) but then were recently asked to do (b), since apparently method (a) was frying alternators.

It's probably relevant to mention that the workload of the training fleet in question involves many short flights every day.
 
Turn on the landing light or pitot heat and look for the needle to move?
 
Either fly a Piper or put a shunt on the alternator output wire and indirectly observe amps produced. Cessna's choice is rather poor.
 
Greg Bockelman said:
That is how I do it.
Click to expand...


It helps to turn off your rotating beacon so you can more easily see the various loads come up. My beacon pulls enough amps that the ammeter moves up and down with the turn of the beacon masking the subtle changes of incremental loads like the Pitot Heat.
 
If your amp meter is showing a discharge before start and a charge rate after start why play with switches?
 
Shutting off the alternator produces a voltage spike from the alternator's field coil in the rotor. That spike can take out radios and other expensive stuff. Turn on landing lights and any other available loads and see that the ammeter doesn't show a discharge.

Dan
 
I start my plane with the alternator off. It is my understanding that, after doing so, if one observes a charge indication, the starter is hung. Once I verify that there is NOT a charge (actually a very small discharge, with the flicker showing that the beacon is flashing, as was observed upon reaching in the cabin and turning on the master during pre-flight), I turn on the alternator master and verify a charge indication (the low voltage light also goes off). This is part of my flow, and does not add a step at run-up. Also, the JPI will alert you pretty soon if you forget to turn on the alternator.

Wells
 
W. Stewart said:
I start my plane with the alternator off. It is my understanding that, after doing so, if one observes a charge indication, the starter is hung. Once I verify that there is NOT a charge (actually a very small discharge, with the flicker showing that the beacon is flashing, as was observed upon reaching in the cabin and turning on the master during pre-flight), I turn on the alternator master and verify a charge indication (the low voltage light also goes off). This is part of my flow, and does not add a step at run-up. Also, the JPI will alert you pretty soon if you forget to turn on the alternator.

Wells
Click to expand...

I can't see that the ammeter would show a charge indication if the alternator was off and the starter hung. The ammeter is between the bus and battery; any charge indication is caused by the alternator causing electron flow in a reversed direction through the battery. If the alternator is off the reversed flow can't happen.

But starting with the alternator off is still a good idea. The alternator sucks a bit of horsepower from the engine when it's energized, and that will load the starter just a bit more than necessary. Could make the difference on a cold day.

Dan
 
Dan Thomas said:
I can't see that the ammeter would show a charge indication if the alternator was off and the starter hung. The ammeter is between the bus and battery; any charge indication is caused by the alternator causing electron flow in a reversed direction through the battery. If the alternator is off the reversed flow can't happen.

But starting with the alternator off is still a good idea. The alternator sucks a bit of horsepower from the engine when it's energized, and that will load the starter just a bit more than necessary. Could make the difference on a cold day.

Dan
Click to expand...

A starter with a permanent magnet will be become a generator when spun by the engine if the starter does not disengage. The ammeter would show that as a charge flowing into the battery regardless of what the alternator is doing.

As for turning the field coil on/off with the engine turning, there is mounting evidence that this is not conducive to long life of the voltage regulator circuits. Some people swear by it, some swear at it, some just do what they are told to do because they don't want to think about it. Let me pose this question to you - in your car you can't turn off the field for start even if you wanted to - and statistics will show you that the number of voltage regulator failures on automobile alternators (per operating hour) are much lower than aircraft.

As Paul Harvey used to say - "Just what, not why..."

I keep my field on all the time, haven't had a failure yet.
 
airguy said:
A starter with a permanent magnet will be become a generator when spun by the engine if the starter does not disengage. The ammeter would show that as a charge flowing into the battery regardless of what the alternator is doing.
Click to expand...

All starters have a one-way clutch to prevent the engine spinning the starter.

Dan
 
Dan Thomas said:
All starters have a one-way clutch to prevent the engine spinning the starter.
Click to expand...

That can and do fail...

Cessna Pilot's Association recommends the Alt field off and check the Ammeter technique.

Protection of Avionics from the resulting spike should be done by leaving the Avionics Master switch off until the start and check for a hung starter procedure is completed.
 
Capt. Geoffrey Thorpe said:
:hairraise:
You are kidding, right?
Click to expand...

Flipping the alternator on and off is what I was taught. I don't like it either for the reasons mentioned above but hey, it's not my rental plane or alternator.

AFAIK there haven't been any failures due to the practice but I think that is more due to diligence on behalf of the engineers that designed the power filters in the radios/GPS than to the people that designed the alternator check procedure. :D
 
How much effort does it take to notice that the Ampmeter is discharging when you crank the engine, and is charging after the start?
 
airguy said:
A starter with a permanent magnet will be become a generator when spun by the engine if the starter does not disengage. The ammeter would show that as a charge flowing into the battery regardless of what the alternator is doing.
Click to expand...
You're correct WRT a PM starter acting as a generator but since the path from the starter to the battery doesn't include the ammeter, the reading on the ammeter will not directly show any current from the starter being dumped into the battery. A voltmeter should show an increase and if any of the loads on the main bus increase significantly as a result of the voltage rise, the ammeter would actually show a discharge (or less charge if the alternator is producing current).
 
Tom-D said:
How much effort does it take to notice that the Ampmeter is discharging when you crank the engine, and is charging after the start?
Click to expand...

Exactly. I have an ammeter and voltmeter. I look at voltage and current with lights & radios on.

via Tapatalk
 
Dan Thomas said:
I can't see that the ammeter would show a charge indication if the alternator was off and the starter hung. The ammeter is between the bus and battery; any charge indication is caused by the alternator causing electron flow in a reversed direction through the battery. If the alternator is off the reversed flow can't happen.

But starting with the alternator off is still a good idea. The alternator sucks a bit of horsepower from the engine when it's energized, and that will load the starter just a bit more than necessary. Could make the difference on a cold day.

Dan
Click to expand...
An alternator doesn't "suck" any more horsepower from the engine during cranking when it's energized than when it's not. Until the RPM reaches about 600 RPM the voltage generated is less than the bus voltage and no current flows through the diodes. The field does draw an amp or two from the battery though but compared to 200+ amps of the starter there's not much of an issue there either.
 
gismo said:
An alternator doesn't "suck" any more horsepower from the engine during cranking when it's energized than when it's not. Until the RPM reaches about 600 RPM the voltage generated is less than the bus voltage and no current flows through the diodes. The field does draw an amp or two from the battery though but compared to 200+ amps of the starter there's not much of an issue there either.
Click to expand...

I have a board with a basic aircraft electrical circuit on it and an alternator driven by an AC motor. I take the alternator belt off, energize the alternator circuit, and show the students the drag that's created just as soon as the field goes to work. The force required to turn it by hand is not large, but it's a lot more than when the field is off.

Dan
 
For those who think there are starter motors that have a permanent magnet armature that can be run as an alternator best take a few apart and show me one.
 
Tom-D said:
For those who think there are starter motors that have a permanent magnet armature that can be run as an alternator best take a few apart and show me one.
Click to expand...

Any permanent-magnet brush-type motor will generate electricity if it is spun. It's a generator, not an alternator. I used a tiny wind-driven one on my homebuilt. The problem would be that the bendix would have to seize for an engine to drive a starter. Never seen that, and if it did the starter would overspeed so enormously that its armature would explode.

Dan
 
Tom-D said:
They are not used as starters in GA.
Click to expand...

http://www.lamartech.com/body_lamarnews.htm

Quote: Lamar Technologies has received PMA approval for both the 12Volt and 24Volt starters for Teledyne Continental 360, 470, 520 and 550 engine applications. These new starters utilize the same patented permanent magnet design for reliability, efficiency and reduced weight. See Press Releases for more details.


http://www.kellyaerospace.com/starters.html

Quote:

E-Drive Starter Features:

  • PMA - FAA Approved
  • Weight 9.5 lbs.
  • Immune to Kick-Backs - No Shear Pin costs here!
  • 2 Year Unlimited Warranty
  • No Bendix - Proprietary Solid-state Solenoid
  • Aviation Designed & Developed -
    No Automotive Aftermarket Parts Used!
  • Low Current Draw
  • Environmentally Tested - Under the most extreme conditions
  • 12 volt or 24 volt
  • Superior Duty Cycle
  • Powerful Permanent Magnet
    Motor     -     High Torque
  • TBO Starter - Capable of providing Maintenance Free service to your engine's TBO... and beyond.

http://www.skytecair.com/Lycoming.htm

Quote:
[FONT=Arial, Arial, Helvetica]The Original Flyweight™ is also the lightest toughweight in the business. Newly improved strontium-fortified alloy casting. The best value in Lycoming replacement starters. Hi-torque, permanent magnet motors. Ultra light weight. High Performance. Save up to 10 pounds under OEM heavyweight starters. Twice as fast as old, heavyweight OEM starters. Faster spin = easier engine starts. Especially improves hot engine start performance.

End of quotes.

These permanent-magnet starters have been around for ten years or more. We have them on every airplane in the fleet, and they give far less trouble than the old Prestolite wound-field starters and are considerably lighter. That's not to say that they don't have their problems; some of them have had the usual teething troubles. The early Lamar wouldn't crank very well and some had gears break. My preference is the Skytec, and it has been the best of a wide range of PM starters.

Dan

[/FONT]
 
Dan Thomas said:
http://www.lamartech.com/body_lamarnews.htm

Quote: Lamar Technologies has received PMA approval for both the 12Volt and 24Volt starters for Teledyne Continental 360, 470, 520 and 550 engine applications. These new starters utilize the same patented permanent magnet design for reliability, efficiency and reduced weight. See Press Releases for more details.


http://www.kellyaerospace.com/starters.html

Quote:

E-Drive Starter Features:

  • PMA - FAA Approved
  • Weight 9.5 lbs.
  • Immune to Kick-Backs - No Shear Pin costs here!
  • 2 Year Unlimited Warranty
  • No Bendix - Proprietary Solid-state Solenoid
  • Aviation Designed & Developed -
    No Automotive Aftermarket Parts Used!
  • Low Current Draw
  • Environmentally Tested - Under the most extreme conditions
  • 12 volt or 24 volt
  • Superior Duty Cycle
  • Powerful Permanent Magnet
    Motor     -     High Torque
  • TBO Starter - Capable of providing Maintenance Free service to your engine's TBO... and beyond.

http://www.skytecair.com/Lycoming.htm

Quote:
[FONT=Arial, Arial, Helvetica]The Original Flyweight™ is also the lightest toughweight in the business. Newly improved strontium-fortified alloy casting. The best value in Lycoming replacement starters. Hi-torque, permanent magnet motors. Ultra light weight. High Performance. Save up to 10 pounds under OEM heavyweight starters. Twice as fast as old, heavyweight OEM starters. Faster spin = easier engine starts. Especially improves hot engine start performance.

End of quotes.

These permanent-magnet starters have been around for ten years or more. We have them on every airplane in the fleet, and they give far less trouble than the old Prestolite wound-field starters and are considerably lighter. That's not to say that they don't have their problems; some of them have had the usual teething troubles. The early Lamar wouldn't crank very well and some had gears break. My preference is the Skytec, and it has been the best of a wide range of PM starters.

Dan

[/FONT]
Click to expand...

So, how long have you been flying these and teaching the test procedure, and how do these relate to the OP question?

this thread was about how to check the alternator after start.
 
Tom-D said:
So, how long have you been flying these and teaching the test procedure, and how do these relate to the OP question?

this thread was about how to check the alternator after start.
Click to expand...
Yes the issue of a starter becoming a generator represents some thread drift but it was you who claimed that PM starters aren't used on GA airplanes. I have two in my Baron and I'm dead certain they'd act as generators if they were turned by the engine. Fortunately there are two clutches on each engine that would prevent that. So, at least in my case you are partially correct in that should a starter drive remain engaged after the engine is running, this won't result in the starter acting as a generator unless both clutches (which are of a completely different design) failing at the same time.

And IMO, an even bigger issue exists with a "stuck" drive regardless of the type of starter motor. Given the significant gear reduction from the starter to the engine, there's a good chance any starter driven by the engine will turn fast enough to physically explode if the engine RPM is increased sufficiently.

Perhaps one could look at this self destruction as a "feature" that limits the amount of excess current/voltage produced by an overrunning starter.:D
 
Last edited:
gismo said:
Yes the issue of a starter becoming a generator represents some thread drift but it was you who claimed that PM starters aren't used on GA airplanes. I have two in my Baron and I'm dead certain they'd act as generators if they were turned by the engine. Fortunately there are two clutches on each engine that would prevent that. So, at least in my case you are partially correct in that should a starter drive remain engaged after the engine is running, this won't result in the starter acting as a generator unless both clutches (which are of a completely different design) failing at the same time.

And IMO, an even bigger issue exists with a "stuck" drive regardless of the type of starter motor. Given the significant gear reduction from the starter to the engine, there's a good chance any starter driven by the engine will turn fast enough to physically explode if the engine RPM is increased sufficiently.

Perhaps one could look at this self destruction as a "feature" that limits the amount of excess current/voltage produced by an overrunning starter.:D
Click to expand...

Yes,,,,,,, I wasn't thinking of the new light weight starters when I made that statement, because they relatively new tech.

but the point of them being a hazard is so remote it isn't worth the thread drift.
 
OBTW.... a magnet running in a field coil is a alternator, not a generator,

http://en.wikipedia.org/wiki/Alternator#Principle_of_operation

The new starters don't have a solid magnet armature, they have a field wound armature and a commutator and brush set. and operate using the same principles as the old starters do. the big difference is they use a smaller motor and a gear drive which allows the smaller motor to run at a higher speed and draw less current. Same as the old 50s Chrysler starters.

So there really aren't any solid magnet starters used in Aviation.
 
Tom-D said:
OBTW.... a magnet running in a field coil is a alternator, not a generator,

http://en.wikipedia.org/wiki/Alternator#Principle_of_operation

The new starters don't have a solid magnet armature, they have a field wound armature and a commutator and brush set. and operate using the same principles as the old starters do. the big difference is they use a smaller motor and a gear drive which allows the smaller motor to run at a higher speed and draw less current. Same as the old 50s Chrysler starters.

So there really aren't any solid magnet starters used in Aviation.
Click to expand...

They are a permanent-magnet field motor, same as any other permanent-magnet motor. No permanent magnets in the armature. And they will generate DC, not alternate, due to the commutator and brush arrangement. The rotating-permanent=magnet alternators are found as bicycle light units or in some portable gasoline-driven powerplants.

The difference between the modern PM starter and old PM starters is the supermagnet technology. Niobuim, columbium and other rare earths are used instead of iron or ferrite and their power is astounding.

Dan
 
+1 @ Dan.

I worked in the GM Research Lab when they developed Magnequench and used it in starter motors as a first application.

The magnetizing machine was awesome. I think it could have turned a quarter into a black hole. :D
 
When i'm flying the 172, i hit the flap switch down then up real quick to see if the ammeter moves
 
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