John Herrera wrote:
> I used to teach Aircraft Electrical Systems. Every textbook, manual,
> pamphlet, video tape filmstrip, etc. I used as a resource warned against
> running an alternator without a battery or mentally-challenged
> load attached
> to the output. But, again, they didn't say why.
I think it has something to do with the open circuit voltage you will
develop without a battery connected that could result in overvoltage damage
to the rectifier diodes. Remember the bronze tinted Ford Instaclear
windshields? These had a relay that disconnected the alternator from the
rest of the vehicle and connected the alternator output directly to the
windshield. The car would run off the battery while the windshield was
energized to clear the ice from the glass. The service manual I saw warned
of voltages up to 70V during operation. There was a voltage sensing circuit
that would switch the alternator from the windshield back to the rest of the
vehicle if the battery voltage dropped too low.
If you look in the Bentley TR7 manual (p 457 to 459) it shows that the 20ACR
alternator has a battery sensed regulator "with machine sensed safety
control European terminations". This regulator has an extra resistor and
diode in the circuit to shut off the field current supply if the battery
sensed lead is disconnected. These regulators may have been used on earlier
alternators since I have an spare 17ACR alternator in my garage that will
not produce output if the battery sense lead is disconnected from the
output.
The wiring diagram for the 14TR regulator with this failsafe circuitry shows
that the yellow regulator lead connects to a free-wheeling diode across the
field winding and is connected to one of the brushes. The metal strap to
the other brush connects to the circuitry that switches current to the field
winding. The white lead is a battery sense lead that goes to one of the
large Lucar connectors on the alternator, the red lead is the machine sense
lead that goes to the other large Lucar connector on the alternator, and the
black lead is the ground connection. It appears that if your regulator
doesn't have the red lead, it won't have the "fail-safe" circuitry to
prevent open circuit operation.
Tim Buja - Rockford, IL - 80 TR8, 73 Stag, 72 TR6
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