CardinalNSB(at)aol.com
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 Posted: Mon Apr 05, 2010 9:33 am Post subject: External vr runaway "B" alternator protection |
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(Please don't make this into a discussion of what an idiot I am for believing there might be such a thing as a runaway alternator, or that I should just use a particular product and don't worry about it.)
I had ass-u-me-d that the runaway alternator problem was limited to the internally regulated alternators, because the B (Ford) style external voltage regulators/alternator controls would shut off the alternator field voltage (and shut down the alternator output) if the power to the vr/acu was cut off (either manually such as with a Cessna split master switch/pulling the breaker on the vr/acu power wire, or through an automatic system that would sense high voltage and shut off power to the vr/acu).
I have just read that that some of the vr/acu's are set up that the voltage feed wire (from the split master for example) is only necessary for the initial energization of the alternator field, and from there the field energy is derived from the voltage (sensing) wire. Then, a diode regulates how much voltage goes from the sensing wire to the field wire.
The issue presented is if the diode inside the vr fails, and shorts out, there will be full voltage from the sensing wire applied to the Field, even if all power to the vr/acu is disconnected.
Question: is the above correct, at least in some cases? is there an issue in an external type B regulator that a failure "inside the box" can result in a runaway alternator, and that simply pulling off power to the vr will not de-energize the alternator? what happens if I simply pull the 60 amp B lead breaker while the alternator is running away? should I have a pullable breaker between the vr and the Field terminal on the alternator, that can be manually switched off? is this the best way to assume control in the unlikely event of a failure to short in the vr switching diode?
Thank you, Skip Simpson
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