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Subject: Re: (Avionics) How can this circuit produce an "inductive surge"?
Date: Thu, 26 Sep 2002 18:03:43 -0600
NNTP-Posting-Date: 27 Sep 2002 00:04:19 GMT
X-Mailer: Mozilla 4.7 [en] (Win98; I)
> The normal procedure in light airfraft startup/shutdown is to have all
> possible avionics disconnected when the engine is being started or
> Moreover, one is normally advised to have the alternator field circuit
> broken at these times (there is a switch for this purpose).
The diagram above does not match the OVP/VR/Alternator wiring found in
Cessnas and Pipers.
The biggest errors are:
1. The VR does NOT do "remote sensing". The "sensing" is done via
the same wire that connects the VR to the main bus through
the "field" switch. There is no separate "sense" input to the VR.
2. One of the field slip ring brushes is grounded to the alternator
frame, hence the VR modulates the field current by breaking
the path between the field switch and the non-grounded brush.
Only a few MOPAR alternators used low side switching as depicted
in your diagram. I've never seen one of these on an aircraft.
In automobiles, the "field switch" is a set of contacts in the ignition
switch, which is closed in the RUN position, and is automatically
OPEN while the key is in the START position, effectively shutting
down the entire charging system during cranking. In airplanes, the
pilot gets to decide if he/she cranks with the alternator field
switch open or closed!
The best reason for cranking with the alternator field OPEN is that
by so doing, you save 2 to 3A of load on the battery from the
time you turn on the master, until the engine is running. Further,
cranking with the field open makes a few more ft-lbs of starting motor
torque available for cranking the engine. These effects are minimal
on warm days, and with a near-new battery. They can help a lot during
cold weather starts.
Cranking with the field breaker/switch closed will not generate any
significant transients. What generates the biggest transients is either
the starter motor itself, or the starter solenoid. I have recorded
transients of > 400V across the main bus in Cessnas that came out of
the factory without the proper spike suppressor diodes...
Cranking with the alternator field open protects the OVP/VR from said
transients, just like having all of the avionics off protects the avionics
There is no transient generated while the engine is stopped with the idle-
cuttoff. It doesnt matter if the avionics/alternator are left on while
the engine stops. However, an unsupressed master relay solenoid can also
put out a 400V spike, so it is a good idea to open the alternator field
switch, and turn off all the avionics before turing off the master.
In Cessna with a split master switch, this happens automatically, because
the ALT half of the Master switch is mechanically interlocked with the
The key to eliminating spike-induced damage to avionics and/or the OVP/VR
is having said equipment off line at the instant the spikes are generated,
and also by installing "catch" diodes across the starter and master solenoids,
as well as having a "transorb" (MOV or Zener) across the main bus...
owner Skylane '1MM
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