Master Switch

Install a light so display when the alt system is not charging. Inexpensive
(as aviation goes) and bonus is the light comes on in flight when the alt
goes off line. Been there. Nice to know before the battery is exhausted.

--

Thx, {|;-)

Victor J. (Jim) Osborne, Jr.
wrote in message
oups.com...
I was told the same thing from a UPS pilot friend of mine. While the
current draw to energize the alternator field is minimal it could make
a difference on a cold day(?) Just don't forget to hit the ALT switch
after engine start and verify the draw on the amp gauge.

Victor J. Osborne, Jr. wrote:
Install a light so display when the alt system is not charging.

Not only does the Maule have such a light, but the fuel gauges are on the field
coil line. Alternator goes off line, the fuel gauges read empty. Really gets
your attention!

George Patterson
Why do men's hearts beat faster, knees get weak, throats become dry,
and they think irrationally when a woman wears leather clothing?
Because she smells like a new truck.

"Larry Dighera" wrote in message
...
On Mon, 18 Jul 2005 12:57:42 GMT, "Lakeview Bill"
wrote in
::

On an aircraft with separate "battery" and "alternator" switches, should
you
turn on BAT and leave ALT off during engine start, or should you turn them
both on prior to start?

A little bit of the "why" on this would be nice also...



Of course, you should follow the starting procedure stated in the POH.
That usually has you turning on the alternator field winding after the
engine starts. Energizing the field winding causes about a 5 ampere
current draw. That five amps is better used for turning the starter
motor until the alternator is able to spin fast enough to produce
useable electrical output current.


1. Does not the starter circuitry include a "contactor" solenoid that pulls
off the power to most everything else during the time that the starter is
engaged?

2. The generic 1976 C172 POH, Section 7 says "Normally, both sides of the
master switch should be used simultaneously". So that's how I start a 1976
172. I am sure another POH may recommend something else.

3. How does pulling the BAT half of the master during engine-running,
reconcile with the caution about pulling battery power on a boat/automobile?
( There ...boats/cars..., isolating the battery from the alternator while
running, is said to cause the alternator to blow diodes and fail. )

Icebound wrote:

3. How does pulling the BAT half of the master during engine-running,
reconcile with the caution about pulling battery power on a boat/automobile?
( There ...boats/cars..., isolating the battery from the alternator while
running, is said to cause the alternator to blow diodes and fail. )

You should not do it, unless in an emergency situation.

The battery buffers the voltage spikes originating
in different electro-magnetic equipment on board. The
avionics are in danger if the spikes are not discharged
into the battery.

Pretty often the master switch halves are interlocked
so that it's not possible to keep the alternator on
without the battery.

The alternator diodes are not the only endangered
semiconductor components. There are many other
diodes, transistors and integrated circuits feeding
from the power bus.

HTH

--

Tauno Voipio (CFII, avionics engineer)
tauno voipio (at) iki fi

Lakeview Bill wrote:
On an aircraft with separate "battery" and "alternator" switches, should you
turn on BAT and leave ALT off during engine start, or should you turn them
both on prior to start?

A little bit of the "why" on this would be nice also...

What does the POH say? The instructions (and posssibly some insight)
will be there.

As a rule, the ALT is left off until the engine is running to avoid
power spikes.

What does the POH say? The instructions (and posssibly some insight)
will be there.

The instructions will be there, They may or may not be optimal. They
are unlikely to offer any insight.

As a rule, the ALT is left off until the engine is running to avoid
power spikes.

Correct procedure (field off until engine start), wrong reason. You
are not going to hurt your alternator field with a voltage spike
produced during starting - it's not at all delicate. Neither will the
alternator produce much in the way of a spike at starting RPM's. I've
heard that reason passed on from CFI to student for years, and it's
just an aviation OWT.

The real reason is far more prosaic - the alternator field coil draws
3-6 Amps (depending on the alternator) that are better used driving the
starter motor. Aviation batteries are sized to be just barely big
enough to start the engine to keep the weight down, and in terms of
quality and design they are the worst batteries available, far inferior
to any reasonable quality lawn tractor battery. As a result, available
starting power is often marginal and anything done to reduce it is a
bad idea.

It is the avionics master is off during engine start to keep spikes
from getting to the avionics. I suspect that's the cause of the OWT.
In airplanes lacking an avionics master, optimal procedure is to shut
down the avionics prior to engine shutdown and turn them on only after
engine start. In cars this is accomplished by means of an automatic
contactor, but in the aviation world we're not up to that level of
technology.

Michael

Michael wrote:

What does the POH say? The instructions (and posssibly some insight)
will be there.


The instructions will be there, They may or may not be optimal. They
are unlikely to offer any insight.


As a rule, the ALT is left off until the engine is running to avoid
power spikes.


Correct procedure (field off until engine start), wrong reason. You
are not going to hurt your alternator field with a voltage spike
produced during starting - it's not at all delicate. Neither will the
alternator produce much in the way of a spike at starting RPM's. I've
heard that reason passed on from CFI to student for years, and it's
just an aviation OWT.

The real reason is far more prosaic - the alternator field coil draws
3-6 Amps (depending on the alternator) that are better used driving the
starter motor. Aviation batteries are sized to be just barely big
enough to start the engine to keep the weight down, and in terms of
quality and design they are the worst batteries available, far inferior
to any reasonable quality lawn tractor battery. As a result, available
starting power is often marginal and anything done to reduce it is a
bad idea.

It is the avionics master is off during engine start to keep spikes
from getting to the avionics. I suspect that's the cause of the OWT.
In airplanes lacking an avionics master, optimal procedure is to shut
down the avionics prior to engine shutdown and turn them on only after
engine start. In cars this is accomplished by means of an automatic
contactor, but in the aviation world we're not up to that level of
technology.

But the _real_ reason is that when the field winding is energized
(current flowing), the alternator is bogging down the airplane engine.
Generating electricity requires lots of torque. And, the total amount
used to start the plane must be pumped back into the battery. That is
the "short term spike" that people confuse with some sort of static
discharge. Once the battery has been recharged, the torque required
tapers down to just enough to run the plane, but not the starter.

Turning off avionics master, the Pitot heater, and excess loads of that
nature also decrease the mechanical load during starting.

But the _real_ reason is that when the field winding is energized
(current flowing), the alternator is bogging down the airplane engine.
Generating electricity requires lots of torque.

Actually, I think you will find that at the RPM's involved in engine
start, negligible amounts of electricity are being generated so the
bogging down of the engine is negligible compared to the loss of power
to the starter.

Michael

This'll add some more fuel to this fire -

There are three differences between car and airplane electrical systems
that apply here -

1) Only aircraft have a master switch relay - which may stutter if the
battery is low, and
2) Only aircraft have an alternator stator circuit breaker that if
tripped causes the alternator to disconnect abruptly.
3) Aircraft also have a circuit breaker and switch on the alternator
rotor.

Given number 1, if the avionics are on, the starter could briefly still
be connected to the avionics with no battery in the circuit allowing
the starter collapsing magnetic field to put a helluva positive voltage
spike into the avionics stack. That's why everyone is supposed to turn
off all electronics before engaging the starter.

Cessnas have a starter circuit activated drop out avionics relay, but
the avionics relay mechanical timing just might still allow the spike
to get to the avionics. Only if the battery stays in the circuit, does
the collapsing starter field pulse stay confined to arcing the starter
contactor.

Remember that there are no faster fuses than silicon cuircuits. I
would guess most 14 V electronic stuff could withstand say 20 V, but
certainly not 100 V

Number 2) causes the current in the alternator stator to collapse
putting a major (negative I think....) voltage spike into the
alternator regulator. If the regulator is solid state, it could be
wrecked. If it is mechanical, it could probably withstand this.

I don't think Number 3) presents a theoetical problem to the system.

You don't get voltage transients when connecting up inductors. You get
them in spades when you quickly interrupt the current flow to them.

I'll bet Jim Weir has some thoughts on this.

"Michael" wrote in message
ups.com...
....snip...

It is the avionics master is off during engine start to keep spikes
from getting to the avionics. I suspect that's the cause of the OWT.
In airplanes lacking an avionics master, optimal procedure is to shut
down the avionics prior to engine shutdown and turn them on only after
engine start. In cars this is accomplished by means of an automatic
contactor, but in the aviation world we're not up to that level of
technology.


I am reading from the generic POH for 1976 Cessna 172, section 7, Airplane
and Systems Descriptions, Electrical System:

quote:
Both sides of the bus are on at all times except when either an external
power source is connected, or the *starter switch is turned on*; then a
power contactor is automatically activated to open the circuit to the
electronic bus. Isolating the electronic circuits in this manner prevents
harmful transient voltages from damaging the transistors in the electronic
equipment.
:unquote.