lml@lancaironline.net Mailing List Archive

Rick,

I would certainly take the side of Keeping It Simple - and simple especially means keeping the decisions to be made in case of failure as simple as possible. I would leave the diodes feeding the e-buss and avoid any relays or switches in that area (no decision to be made - the important stuff keeps working as long as there is either side functioning). There is no such thing as a "cranking spike" - the starter just pulls the voltage in the system down, potentially causing brown-outs or re-boots in electronic equipment. Shouldn't be a problem for any competently-designed hardware. I would not activate the alternators through oil pressure switches - just more complexity for little or no benefit. One thing that should be taken into account is that an alternator should not be allowed to become disconnected from a battery. Even though it might be able to continue to supply current it will not be able to follow load fluctuations without producing "load dump" spikes. Therefore, the master switch for each battery should also switch the power for the field for that alternator. Unfortunately, that means that each alternator is dedicated to its battery and can't "cross-feed" without the batteries being connected (masters on). I'm ambivalent about the location of the shunts as I would leave them out entirely and just monitor voltage. If I had a current shunt I would put it on the battery as you suggest, monitoring the charge current, not the alternator load or the aircraft load. In my case if the voltage is above 13 everything is okay; below 13 not okay and time to reduce electrical load and land. In your case if the system voltage drops you can turn on the cross-feed and land - you just have more time to land. The decision process is the same. KIS

Gary

Also note, in my simplified sketch, I have an essential/endurance bus which is supplied from both the “A” and “B” side via a diode on each. Thus, the essential bus gets power even if the A-side fails and I haven’t yet activated the cross tie. Thus, the EFIS doesn’t quit and have to re-boot. This also provides some additional EFIS power redundancy in the event of a simultaneous A side and cross-tie failure. Normally, my “A” side voltage regulator will be set to slightly higher than the B side (~0.1 volt). Thus, power will normally flow into the essential bus via the A side. Alternatively, I could have a manual switch on the B-side lead or a normally closed relay on the B side lead which is dis-engaged by the A side (open then A is hot). I opted for the diodes as KISS in this case and am seeking to minimize the number of switches. However, a manual switch would create some additional starting spike isolation. Perhaps a normally closed relay which is powered (dis-engaged, opened) by the starter relay. Perhaps the B side essential feed should be tied to the B-side alternator switch, since it would normally be off during starting (providing isolation).

I’m also considering activating the alternators based on an oil pressure switch (one for each side as redundancy). That way the alternators com on-line automatically with oil pressure (just after engine start). Without oil pressure the alternators won’t work long anyway {g}. Each could also be turned off by pulling the circuit breaker on the respective voltage regulator. I’m undecided on that, but I digress a bit…

2) If I understand your second question correctly… I have a shunt and ammeter between the battery and the bus/alternator (on both the A and B side). These are not shown on my prior sketch. On the B side it is past the starter so the “starting current” does not go though the shunt (a special high current event). Otherwise, both sides are essentially setup the same. Thus, the ammeter(s) show the current flowing into (or out of) each battery. With the alternator on, this current shown is normally very low (just any battery recharging). With the alternator off, the ammeters will show the “drain” being placed by the electrical equipment that’s on at the time. This setup dose not directly show the total load on the alternator. However, it is indirectly available. If you want to know (approximately) how much total load is being placed on the alternator at any point in time, you can look at the ammeter with the alternator on (the current going into the battery) and then momentarily turn the alternator off and see how much current flows from the battery to the electrical stuff. The current flowing out from the battery to the other stuff, was previously being supplied by the alternator. So, the total alternator load is (was) the sum of the two. Note: the needle (flow) will shift from positive to negative, which just indicates current direction relative to the battery. The total current being supplied by the alternator is (was) the sum of the absolute values (disregarding the direction into/out of the battery).

Some folks (manufacturers) favor putting the shunt in the alternator B lead (between the bus/battery and the alternator). That setup shows total alternator load directly, but is useless in an alternator failure scenario, when you might want to really know total current draw to preserve the remaining battery power. It also provides no mechanism for determining whether the battery is charging (health) or draining (unhealthy). Others favor putting the shunt at the head of the bus(es) i.e.before all the electrical stuff. That setup shows total equipment load directly, but provides no mechanism for determining total alternator load, nor whether the battery is charging (health) or draining (unhealthy). Thus, the std battery ammeter setup seems the best approach (as I understand it).

Rick