[alpinelakespilot2000]

1. In layman's terms, can someone please explain the purpose of an alternator field breaker? What is it protecting?

2. What is the relationship between this breaker and the alt.side of the Cessna split master switch that turns the alt. on and off?

3. How does one determine the appropriate size for the breaker?

4. I've seen a number of panels or wiring schematics that don't seem to have an alt field breaker. Is there some appropriate substitute?

Thanks.

 

[lucky333]

Quick answers

1. In layman's terms, can someone please explain the purpose of an alternator field breaker? What is it protecting?

2. Aside from wire, what is connected immediately upstream and immediately downstream of the breaker?

3. What is the relationship between this breaker and the alt.side of the Cessna split master switch that turns the alt. on and off?

3. How does one determine the appropriate size for the breaker?

4. I've seen a number of panels or wiring schematics that don't seem to have an alt field breaker. Is there some appropriate substitute?

Thanks.

1) The field current determines the output of the alternator and runs in the several amps range. Like any fuze/CB, it protects the wiring against overloads and short-circuits. It is also the preferred way to implement overvoltage protection schemes (see below).

2) Wiring (generalized):
Internal regulator: bus-> breaker -> switch -> wire -> field supply terminal on alternator.
External regulator: bus-> breaker -> switch -> field supply terminal on regulator -> regulator innards -> field output terminal -> wire -> field terminal on alternator.
Note that in either case, the breaker/fuze is way upstream to protect the downstream wiring.

3) Both remove field current from the alternator which kills it. The breaker/fuse is there to protect wiring, the switch is there so that you can shut the alternator down if necessary due to overvoltage or other problems.

3a) The wire size in the circuit (which must handle the field current, of course) determines the size of the breaker. Check the specs to see what the field rating is. Pick a wire size and spec the breaker accordingly.

4) Like ANY circuit, the alternator field circuit should be protected against short circuits to protect wiring. Like ANY device capable of developing considerable power, the alternator needs a way to turn it off hence the need for the field switch.

I once had to do just that when the condenser (filter) on the alternator quit. The noise in the radios caused the poor guys at ATC earaches from the squeals when I transmitted. (And NO, I wasn't smart enough to just turn the ALT off, I used a handheld. We found the problem on the ground.. another lesson learned).

Another consideration is what happens when the ALT dies due to diode failure, belt breakage etc. Its no longer producing power but the regulator will drive the field to the max (wasting battery power) trying to get something out of it. Turning off the field means you have battery power longer.

SO.. the rule is drop the field whenever you lose regulation, high or low. If you have a fancy gadget to do that, fine. Have a switch and a voltmeter for backup.

Overvoltage protection is simply a matter of turning off the field with a switch. What this switch is is a matter of design. If you monitor the volts either with a voltmeter or idiot light and detect that the voltage is too high, turn off the alternator field and the alt will cease producing current.

'Crowbars' and other OVPs should work the same way i.e. detect an overvoltage and turn off the field. The main problems in OVP design are:

1) when the OVP drops the field drive, the voltage will drop down to the battery voltage and the overvoltage condition will cease. If the OVP is not smart enough, it will then reapply field current and the OVP condition will cycle on and off. A 'crowbar' is a crude but time-honored way of handling the OVP. When an overvoltage condition is detected, it deliberately short-circuits the field circuit downstream of the breaker. It pops and the field (and alternator) is dead until the breaker is reset. A crude, 1-bit memory circuit! Newer OVP circuits remember the problem electronically and hold the field OFF until manually reset but the idea is the same.

2) the other OVP issue is transient tolerance. A momentary spike in voltage should not trip the protection. All workable OVP schemes have some degree of delay built in (and of course, if all you have is the idiot light, the delay depends on your scan )

An internally regulated unit will have OVP built in. External regulators - depends on the unit.

The idea of using any OVP to crowbar the 'B'attery lead is not good one. First, removing the field does the same thing. Second, you get real serious transients when you do that. Count on blown diodes and serious hits on the busses. 'B' leads are fuzed,breakered etc to protect the wiring against shorts or diode failure in the alternator. Overcurrent at the output of the alternator may happen in rare cases as well but not often.

John

 

[alpinelakespilot2000]

Thanks a lot John for taking the time to explain it. At least for the moment , it's much clearer!

Initially I had the alt. field breaker between the alt switch and the o.v./regulator but, since all my other fuses go ahead of their respective switches, it makes much more sense that the alt. breaker would go between the main power feed and the alt. switch as well.

Still have more overvoltage protection questions, but I'll do some searching to see if I can find some good threads describing more about when they are required and when not as well as what makes for good and bad o.v. protection.

Many thanks again.

 

[sprucemoose]

and of course, if all you have is the idiot light, the delay depends on your scan )

I know you said this tongue in cheek, but unless your scan speed and reaction time are measured in milliseconds, you'd do well to have an OVP. I've come across more than one builder who thought along the lines of, "If I see the voltage go to OV, I'll just switch off the alternator switch." A voltage runaway is likely to happen a bit too fast for that.

Again, I know it was in jest, but I think it bears expanding on a bit.

A copy of the Aeroelectric Connection should be included with every finish kit

http://www.aeroelectric.com/

 

[AlexPeterson]

SNIP

The idea of using any OVP to crowbar the 'B'attery lead is not good one. First, removing the field does the same thing. Second, you get real serious transients when you do that. Count on blown diodes and serious hits on the busses. 'B' leads are fuzed,breakered etc to protect the wiring against shorts or diode failure in the alternator. Overcurrent at the output of the alternator may happen in rare cases as well but not often.

John

John, excellent writeup.

If there was a contest, I'm not sure which would win - prim$rs or alternators! There are thousands of posts on other forums on this topic and probably also on this one!

There exist many alternators in which the actual field (juice going to the commutator to energize the rotating magnetic field) is powered internally to the alternator. The "field" in these cases is just an enable signal. There are known failure modes where disconnecting the enable (called, somewhat erroneously, field in these alternators) will not stop a runaway voltage condition. There are also people who have demonstrated that, once running, shutting off the "field" does not stop certain non-failed alternators from putting out juice.

I have an internally regulated ND 40 amp, and one test I keep wanting to get at is to measure the current going through the "field" circuit. If it is actually powering the field, I would expect, as you pointed out, a couple amps. If it is an enable circuit, probably only milliamps. It does, however, shut down the alternator's output when I disconnect it.

 

[lucky333]

Tounge removed!

I agree, Spruce. A manual OVP is not enough with todays electronics. In some failure modes, the battery can hold the voltage rise down to a manageable level, giving you time to react (as it bubbles unhappily) but in others (loose battery connection comes to mind) the bus voltage can go crazy fast and an automatic OVP is necessary. In the old vacuum tube radio days, it didn't matter as much. It does now and OVP should be standard equipment.

As Alex correctly points out, there are alternators which derive the field current internally. I confess, I don't know how they do OVP in those. Looks like its pretty common, though. This AM I got out and snapped on the ol' DC ammeter to the 'enable' wires on the Honda (Denso alternator) and Tahoe (Delphi) and BOTH are like Alex describes. No appreciable current into the control terminals. In these, one would wonder how they make the OVP reliable and what kind of failures would render it inop. I imagine that the designers have taken all of that into account, though. Modern autos also have lots of expensive electronics to be covered under warranty..

I (*really*) HAVE seen units (including some NDs) which get the field current from outside and whose internal OVPs short that terminal, blowing a several amp field fuse. Plane Power's description implies that that's how their unit works. Looks like I'll have to give them a jingle to be sure.

I appreciate the feedback and new information. Always learning.. The big reminder here is that while all alternators work on the same principles, the implementation varies. Awhile ago, I helped a builder troubleshoot his NipponDenso alternator using the schematics from Van's. Turns out that although the connector looked the same, the alternator was different internally and if you hooked it up according to the diagram, it blew the regulator. Expensive lessons.

Whatever unit you use, its important to be sure you have the correct documentation for that particular one and understand how it works and how the various features are implemented.

John

 

[elippse]

Something to consider if you have an external regulator: If your alt. field supply comes from the battery-side of the alt. breaker, then in the event that for some reason the alt. breaker opens, the regulator, sensing the lower battery voltage with the alt. not charging, will apply full field voltage to the alt. Its output will rise to 100V or more, and then, if you close through the alt. breaker, you will have a really big transient applied to your system. It's better to attach the field supply to the alt.-side of the alt. breaker, and that way, if the breaker opens, the regulator will still be attached to its output and will try to keep it in regulation. Then when you close through the breaker, the transient will be far, far less.

 

[MCA]

I have an internally regulated ND 40 amp, and one test I keep wanting to get at is to measure the current going through the "field" circuit. If it is actually powering the field, I would expect, as you pointed out, a couple amps.

Alex, I have the Vertical Power system installed and it measures the current of each circuit, including the alternator field. With a B&C 40A alternator installed on Bus A on my -7, the field draws 3-4 amps without the engine running (it's trying its hardest to charge but can't) and in normal cruise it draws between 1 and 2 amps.

Marc

 

[lucky333]

Good point

Something to consider if you have an external regulator: If your alt. field supply comes from the battery-side of the alt. breaker, then in the event that for some reason the alt. breaker opens, the regulator, sensing the lower battery voltage with the alt. not charging, will apply full field voltage to the alt. Its output will rise to 100V or more, and then, if you close through the alt. breaker, you will have a really big transient applied to your system. It's better to attach the field supply to the alt.-side of the alt. breaker, and that way, if the breaker opens, the regulator will still be attached to its output and will try to keep it in regulation. Then when you close through the breaker, the transient will be far, far less.

It took a minute to figure out what you were saying but that's correct, the field source has to be on the alternator-side of the 'B' breaker. I grabbed a couple of old Cessna manuals (C150 and T210) and they are wired as you point out which is of course the correct way.

Thanks for the amplification. I would add that before re-closing the 'B' breaker, you should turn off the field as well. Close the 'B' breaker then reapply the field.

BTW: I find that looking at production aircraft POHs can be most useful. Cirrus and Columbia have nice diagrams of their dual-redundant systems. I got both from their sites. And it goes without saying that aeroelectric connection is a great resource as well.