Exactly! Replacing components just for the sake of replacing something which solved an issue, sometimes is the answer and sometimes just hides the real issue. Unless you can put your finger on something and say "Ahah, That's it", you will never be certain until at a most inopportune time and place the original issue shows up again.Did you keep the old alternator? Did it have bearing issues? Did you split the case and see if anything was loose inside?
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I believe it is a Denso 021080-0760 with an built in regulator.
Did you find out if this is the alternator? I have one of these was going to use on a project, maybe I wont.
Ok. Here's why it's a bad idea to use automotive alternators with built in regs, in aircraft.
The regulator picks off power from the output of the alternator. The controlling element of the regulator is a transistor. Semi conductors, such as transistors, almost always fail in the shorted state, not open. In normal ops, the reg pulls power from the output circuit of the alternator and sends the current through the field coils. If the voltage is low, the reg allows more current flow in the field. When voltage rises the reg will reduce the current flow in the field. This happens dozens of times a second.
If something causes the reg to fail, that transistor will go to a shorted state. This means it turns into a piece of wire. So, now current flows from the output circuit, straight through the transistor (shorted), into the field coils. There is NO control of current flow in this condition, which means that output voltage will rise and keep rising as long as the alternator is turning. Since there is no field breaker for this system, there is only the output breaker.
But pulling this breaker does nothing to stop the path of current flow into the field coils. Since the alternator is already putting out power, the reg will continue to pull power from the output. That output breaker only isolates the alternator from the bus. The result is an uncontrolled runaway. In my past life, I worked on several cars that had this same scenario happen. The result was melted solder and copper slung around the engine compartment. Nothing that I would ever want in an aircraft engine cowling.
This is the reason that most aircraft alternators are still externally regulated. In those systems the regulator pulls power from the bus, through the field breaker, and sends it to the field coils via a wire. In the case of a runaway, if the field breaker is pulled (or the master switch turned 'off', as they should be in series), the path of current flow to the field is PHYSICALLY opened. With no current flow in the field, the output of the alternator will go to zero.
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It depends which wire has the bad connection on which way the amp(indication) will go.Agreed, that is valuable information.
In respect to the two small shunt wires, in a conversation with Dynon, they recommended to check/re-do the small wires that sense the V and amps if the alternator did not fix the problem. The Dynon tech person did mention that a faulty small wire could cause neg amps. I agree that if the circuit is 'open' then is should show 0 amps/volts.
I do have my visual and audio alarms set in case of 0 amps or volts. I can disconnect the alternator from the system via a 5amp breaker for the relay on the main 50amp breaker. Then I'm on just the battery.
That is assuming it is a clean break which it appears not to be. What if it is a poor connection that is vibrating and giving a reduced reading (similar to a PWM). If it is the lead on the alternator side of the shunt it would show negative.
A voltmeter reads an open as 0, sure. But my first hand experience and Dynon tech comments say that a sensitive Dynon input is not a general purpose voltmeter. If you want it to read 0 you have to make it 0.000000 with a short.
Because an ammeter can indicate current in either direction which would reverse the polarity of the voltage differential.If an intermittent connection, it would vary from the correct reading to zero, not into the negative range.
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Because an ammeter can indicate current in either direction which would reverse the polarity of the voltage differential.
An ammeter is a passive instrument. It cannot reverse a signal. Yes it can indicate a negative (reverse) current flow but only if the polarity on the sense wires was reversed.
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You don't need reverse current, just a negative voltage differential. It could be as simple as a cold solder joint somewhere in the Dynon or in the fusible links.Doesn't matter. For a shunt to cause a negative reading of current flow, the current flow would have to reverse. This is the only way the sense wires will read a negative flow.
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