From a previous alternator "challenge" on my 360

Excess vibration being transmitted to to the brushes at specific RPMs caused the brushes to hop away from the commutator just long enough to let the output voltage drop.  Then when the brush again made contact, the alternator output spiked to over 16.1V for >50msec, causing the VR's SCR OVP to trigger, popping the Field circuit breaker.  Since even B&C Specialties could not tell me the time constant for the VR/alternator closed loop system, the differing vibration frequency from certain engine RPMs would cause a long enough hop.

jim...
N1222K  611.1 hours

Paul Lipps wrote:

    I did a test on a Nippondenso alternator the other day. Its no-load voltage with 13.2V on the field and spun at 3000rpm was 40.2V. That's average voltage; its 3-phase rectified peaks are higher. The alternator's pulley OD is 2.75", and the pulley on the starter ring-gear is about 9.5". This means that its output at 2700 rpm is about 125V. Have any of you measured the time constant of your OV crowbar? I would think that it would have a filter in the trigger circuit to prevent false triggering on transients, so there is probably several milliseconds from the appearance of an OV event until the crowbar activates. There is probably a several millisecond L/R field time constant for the decay of the field current. If one of you has the means to test this total loop, it would be interesting to see just how long this magnitude of OV would be present at the alternator output. Keep in mind that there has to be an overvoltage existing on the main buss before it can be detected. I have queried Exide about the L-R-C time constants of a typical 25AH lead-acid battery, and its reaction to a very brief high-voltage transient but have not yet received a reply. Consider: a lead-acid battery generates voltage by an electrochemical reaction, and is charged by the reverse reaction. Wouldn't an electrochemical reaction of ion exchange on the cell walls take a finite amount of time? I wouldn't think it would be on the order of the light-speed of electricity flowing in a conductor. I would hazard a guess that any sinking of a short-duration transient would be more due to battery capacitance than to being through absorption by the chemical reaction. The inductance in the leads from the battery to the buss would also add an additional time delay. There is nothing instaneous in electricity. Even the thought-to-be instantaneity of doppler radar isn't. The processor counts cycles over a fixed gate period, which yields the range change, so its apparent effective time, with constant range-rate, is in the middle of the count period, and the range-rate is range per count-period. So! How long would a 125 V transient from an alternator exist on the buss? Long enough to damage some fine avionics?