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leaving alternator breakers on all the time
Hi - My PA23-250 POH uses strong wording to insist that whenever the engines are running, the alternator breakers should be on. This contradicts some checklists for this plane (and those of a few other single-engine Cessnas) that suggest that those breakers be open during engine start and after shutdown. On the twin, one checklist includes an alternator load check during runup that includes opening one then the other breaker, to ensure that the electrical load can be carried by either unit - that sounds valuable but appears to go against the POH. Can someone offer an explanation of the relative wisdom of two approaches? - FChE |
#2
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If you open the alternator output breaker while the alternator is
generating power then you get what is known as a "Load Dump". this will generate a short inductive voltage spike of several hundred volts followed by a steady +90 volts. This really stresses the diodes in the alternator. Most 12 and 24 volt circuit breakers that are used in GA aircraft are not designed to open 90 volts DC. (There is a big difference between the AC and DC voltage ratings of circuit breakers, switches and fuses.) The breaker either burns up due to the constant arc, the contacts melt and short or the breaker finally opens but the contact surfaces are damaged. The more current that is flowing when the breaker opens the worse the result. Some part of this voltage will feed through the breaker while it is opening and put a several hundred volt spike into your expensive radios. On my C172 when the old tired 60 amp alternator breaker opened one night it burned up the alternator field, the regulator, the over voltage warning light, and the over voltage relay. The alternator stator and diodes were ok. This cost me about $500 because the old tired 60 amp breaker would open at 35 amps after a few minutes. Cessna screwed up when they wired the alternator and the regulator on the "M" model but that is another story. John On 20 Oct 2003 16:53:18 -0400, (Frank Ch. Eigler) wrote: Hi - My PA23-250 POH uses strong wording to insist that whenever the engines are running, the alternator breakers should be on. This contradicts some checklists for this plane (and those of a few other single-engine Cessnas) that suggest that those breakers be open during engine start and after shutdown. On the twin, one checklist includes an alternator load check during runup that includes opening one then the other breaker, to ensure that the electrical load can be carried by either unit - that sounds valuable but appears to go against the POH. Can someone offer an explanation of the relative wisdom of two approaches? - FChE |
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#4
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The correct way to take an alternator off line is to open the field circuit, not
the output circuit. Electro-mechanical regulators do exactly that many times a second. The alternator can only produce an output if the field is energized. Basically the field (which is actually the rotating part of the alternator) when energized is a rotating magnet which then induces currents in the stator windings. Open field means no induced currents, which mean no output. This also avoids the load dump problem. "G.R. Patterson III" wrote: wrote: If you open the alternator output breaker while the alternator is generating power then you get what is known as a "Load Dump". So, Based on this (and the rest of the info in your post), it sounds like it would be perfectly ok to start the aircraft with the alternator out of the circuit. I'm not sure it would be a good idea to cut one out in a twin. Does this load dump occur if there are two alternators feeding the system and you cut one out? George Patterson To a pilot, altitude is like money - it is possible that having too much could prove embarassing, but having too little is always fatal. -- --Ray Andraka, P.E. President, the Andraka Consulting Group, Inc. 401/884-7930 Fax 401/884-7950 http://www.andraka.com "They that give up essential liberty to obtain a little temporary safety deserve neither liberty nor safety." -Benjamin Franklin, 1759 |
#5
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The correct way to take an alternator off line is to open the field circuit, not
the output circuit. Electro-mechanical regulators do exactly that many times a second. The alternator can only produce an output if the field is energized. Basically the field (which is actually the rotating part of the alternator) when energized is a rotating magnet which then induces currents in the stator windings. Open field means no induced currents, which mean no output. This also avoids the load dump problem. You can help extend the life of your alternator by not having the alternator field energized while starting the airplane. The starter presents a load of over a hundred amperes when it is cranking the engine. If the alternator field is energized, the alternator is trying to supply current into a load that far exceeds its capacity, which stresses the diodes in the alternator. If the alternator field is open during starting, the alternator produces no current, therefore the diodes remain unstressed. Your car charging circuit is similar to an airplane's circuit except that in a car, turning the key to the start position deenergizes the alternator field so that it does not produce any output while the starter is being cranked. If you have a split master, you can approximate the action of the automotive starter lockout by leaving the alternator side of the master off until the engine is started, then turning it on. This also has the benefit of not putting the field load (up to a couple of amperes) on the battery before and during the start. If you don't have a split master, I wouldn't advise using the field circuit breaker to turn it off, as those are not designed for the amount of operations you'd subject them to pulling it on every engine start. The other thing, is make sure you turn the alternator side of the master on once you do have the engine running, otherwise you'll find yourself with a dead battery before long. "G.R. Patterson III" wrote: wrote: If you open the alternator output breaker while the alternator is generating power then you get what is known as a "Load Dump". So, Based on this (and the rest of the info in your post), it sounds like it would be perfectly ok to start the aircraft with the alternator out of the circuit. I'm not sure it would be a good idea to cut one out in a twin. Does this load dump occur if there are two alternators feeding the system and you cut one out? George Patterson To a pilot, altitude is like money - it is possible that having too much could prove embarassing, but having too little is always fatal. -- --Ray Andraka, P.E. President, the Andraka Consulting Group, Inc. 401/884-7930 Fax 401/884-7950 http://www.andraka.com "They that give up essential liberty to obtain a little temporary safety deserve neither liberty nor safety." -Benjamin Franklin, 1759 |
#6
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Ray Andraka wrote:
: If you have a split master, you can approximate the action of the automotive : starter : lockout by leaving the alternator side of the master off until the engine is : started, then : turning it on. This also has the benefit of not putting the field load (up to a : couple of : amperes) on the battery before and during the start. Another little tidbit on this idea is to wait for a little bit before energizing the alternator field (with split master). The alternator actually presents a fairly significant amount of torque load when putting out full output, which it does briefly when flipped on after starting. I generally wait until the oil pressure has built up and stabilized for a bit before flipping on the alternator and watching the ammeter spike. I'm not familiar with the PA-23 electrical topology, but someone who does feel free to correct this thinking. It's probably just like the PA-24 and older PA-28's, which have a single toggle 'Master' switch. In this case, that master switch turns on the main contactor relay at the battery. The problem with this is that if the field stays energized while the master is turned off, the main power sink and buffer in the system (battery) is now disconnected. If there wasn't much current going into it, it's not a problem. If the battery was low, had just been started, etc, etc, there could be 50A going into the battery that disappears from the bus. This will cause the "load-dump" transient voltage spike mentioned before. If the field(s) can be turned off individually, I would think that's the appropriate technique. Kinda like power increase/decrease is mixture/prop/throttle and throttle/prop/mixture, respectively. Electrical startup would be master/starter/alternator field and shutdown the reverse. -Cory -- ************************************************** *********************** * The prime directive of Linux: * * - learn what you don't know, * * - teach what you do. * * (Just my 20 USm$) * ************************************************** *********************** |
#7
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#8
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Hi - papenfuss wrote: [...] I'm not familiar with the PA-23 electrical topology, but someone who does feel free to correct this thinking. It's probably just like the PA-24 and older PA-28's, which have a single toggle 'Master' switch. [...] The PA23-250 is the Aztec, a twin. My model has a split master, but it's a Left-Right split. There doesn't appear to be a normal setting that corresponds to a C172's battery-power-only configuration, just the dual-alternator-failure emergency case (masters on, alternators off). - FChE |
#9
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Ray Andraka wrote: You can help extend the life of your alternator by not having the alternator field energized while starting the airplane. The field breaker in my aircraft cannot be pulled. The only way to disable the alternator is with the split master. George Patterson To a pilot, altitude is like money - it is possible that having too much could prove embarassing, but having too little is always fatal. |
#10
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If you want to take an alternator off line remove the field current to
that alternator. Do NOT open the output of the alternator. The alternator is a rotary current amplifier. Most 12 volt 50 to 60 amp alternators have about a 2 amp field current which implies the current GAIN is around 30 amps per amp. On a piston engine the cranking speed is much too low to generate enough output voltage to get much current generated until the engine starts. No output current means no torque required to spin it. However you will be drawing full field current of 2 amps or so which is about 1% to 2% of the cranking current. This additional current must be supplied by the battery which does not help but don't hurt much either. At idle speed and full field current ,which the regulator will give since the battery voltage is low, if the output is not connected to the battery the alternator voltage will rise to 50 volts or more. The open circuit alternator output voltage depends on RPM and field current. The voltage is proportional to the magnetic flux and the rate of change of that flux. which is the alternator field current and RPM. This extra voltage will not help extend the life of the diodes in the alternator or the circuit breaker contacts as you will get a good arc when the breaker contacts bounce open upon closing of the breaker A turbine engine is an entirely different story. At the end of a turbine start cycle the turbine has to be running at about 50% - 66% RPM before the engine loop gain exceeds one. At this speed the alternator would be spinning fast enough to be generating a lot of current if it had field current. If the alternator were 100% efficient it world not matter if it was generating or not however I have yet to see one that even approaches this efficiency in an aircraft. This means that if the alternator field is enabled the starter may not be able to spin the engine past the point where the engine gain is at least one. This will yield a hung or hot start. Some turbines have separate starters and alternators. However in many cases the alternator/generator on a turbine is also the starter motor so this is a moot point as it is impossible to have both functions at the same time. On Mon, 20 Oct 2003 23:33:16 -0400, "G.R. Patterson III" wrote: wrote: If you open the alternator output breaker while the alternator is generating power then you get what is known as a "Load Dump". So, Based on this (and the rest of the info in your post), it sounds like it would be perfectly ok to start the aircraft with the alternator out of the circuit. I'm not sure it would be a good idea to cut one out in a twin. Does this load dump occur if there are two alternators feeding the system and you cut one out? George Patterson To a pilot, altitude is like money - it is possible that having too much could prove embarassing, but having too little is always fatal. |
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