Cessna style Master Switch

[uk_figs]

I have the standard cessna style dual master switch with the alternator on one side. I have always switched both on to start the engine. In a recent conversation the subject came up of starting with the alternator switched OFF. I had not heard this and am curious if this is the correct procedure and the rationale behind it.
Figs

[rocketman1988]

If you think about how the voltage regulator works, you can find a reason...there may be others.

When you start cranking the engine, the bus voltage drops. The regulator sees this and allows more current to the alternator field to try and recover the voltage. Problem is, the alternator isn't producing sufficient power to recover the bus voltage. As more current is allowed to the field, the alternator becomes MUCH harder to turn, thus dragging the bus voltage even lower.

With the alternator switched OFF, there is no drag on cranking the engine. Once the engine starts, the alternator is switched ON, the regulator sees a low bus voltage, increases the field current and recovers the loss. The bus voltage increases to the set limit, which is above battery voltage, so the battery gets charged...

[N941WR]

On the flip side, pun intended, when I was building, I lent my alternator to a friend who's had failed.

After installing mine, it too failed.

Turns out that after engine start, as part of his preflight, he would turn off the alt side if the master. Apparently this toasted my alternator, and his before it.

The guy who rebuilt both alternators strongly suggested we not do that.

I can tell from my gauges if the alternator is charging or not, so no need for this test.

If you get in the habit of starting on one side, you will also forget to flip it on at some point.

Also, none of the Cessna POH's I have recommend starting on one side.

[Canadian_JOY]

This has been discussed / debated at length here in previous threads.

In short, several good anecdotes were told that supported the theory that alternators don't like to be turned on when spinning. Countervailing anecdotes were also provided.

I think the wise approach is to always turn on both sides of the master switch for a "normal" start. This allows the alternator to spool up its output as it comes up to rotational speed following engine start. If, on the other hand, you are in an "abnormal" condition where every last available coulomb of juice is required to just barely turn over the engine, turn on the alternator only after that very dicey start has been accomplished, understanding that by doing so there is more than a tiny risk that the sudden turn-on of the alternator may produce a puff of expensive smoke.

In the case of our aircraft with a B&C alternator and B&C linear regulator this process works quite well.

As always, your mileage may vary, don't try this at home, filmed using trained stunt dummies, etc etc etc.

[rocketman1988]

also consider that the alternator in your car doesn't get switched off prior to start...

I am not advocating either way, just presenting some data points...

[mfleming]

Originally Posted by Canadian_JOY

This has been discussed / debated at length here in previous threads.

In short, several good anecdotes were told that supported the theory that alternators don't like to be turned on when spinning. Countervailing anecdotes were also provided.

I think the wise approach is to always turn on both sides of the master switch for a "normal" start. This allows the alternator to spool up its output as it comes up to rotational speed following engine start. If, on the other hand, you are in an "abnormal" condition where every last available coulomb of juice is required to just barely turn over the engine, turn on the alternator only after that very dicey start has been accomplished, understanding that by doing so there is more than a tiny risk that the sudden turn-on of the alternator may produce a puff of expensive smoke.

In the case of our aircraft with a B&C alternator and B&C linear regulator this process works quite well.

As always, your mileage may vary, don't try this at home, filmed using trained stunt dummies, etc etc etc.

While reading your post, I had to look up two words, countervailing and coulomb. Thank you for making my day I always enjoy expanding my vocabulary....back to the topic at hand!

[Canadian_JOY]

Originally Posted by mfleming

While reading your post, I had to look up two words, countervailing and coulomb. Thank you for making my day I always enjoy expanding my vocabulary....back to the topic at hand!

LoL - thanks for the giggle - glad to know I sent somebody diving for their dictionary!

[uk_figs]

Thanks for the feedback, think I will stick with my current approach which has worked OK for 500 hours.
Figs

[Mich48041]

With the engine off, the regulator sends maximum current to the alternator field, about 5 amps. As the engine is cranked, the field current does not increase because it is already at maximum at about 5 amps. During engine cranking the alternator is not hard to turn because it is producing very little electrical power at engine cranking speed. Five amps of alternator field current is only about 2 percent of starter current. That is not enough to significantly affect engine cranking. Trying to start an engine with a weak battery is not recommended. Sometimes starter contactor contacts get welded together due to arcing caused by trying to start the engine with a weak battery. If the contactor chatters, arcs are jumping across the contacts.

[Mel]

Just as a point, when I built my -6 (First flight May of 1993) I started with the alternator off, turning it on after start. After several years and alternator changes, I removed the alt switch and left the alternator on continuously. No more alternator changes.