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Reasons for Generator master thermal switch


ctfarmer

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Going through the CTLS checklists has me wondering what is the purpose is for a separate thermal isolator CB/switch alongside the battery master for the generator.

 

This is listed to be turned on after engine start.

 

Any thoughts on the design reasons for this? For example why not run the generator 12VDC supply direct to the battery and which would require only the main battery isolator.

 

cheers peter

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A wild-ass guess would be that having the voltage regulator connected to the battery drains it down if the the engine isn't running.  Another reason is tradition.

 

As to the checklist item, that doesn't seem right.  My voltage regulator has "WITH MOTOR RUNNING, NEVER DETACH BATTERY CABLES" embossed on it.  That's exactly what you're doing if you're starting with the generator breaker open.

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Thanks Jim,

 

 

Researching you wild ass guess reveal you are indeed correct. Thank you.

 

Checking the Rotax manual re disconnecting from the battery while running - that appears to be a Rotax warning and they say to ensure the you do not damage the rectifier in such circumstances, install a capacitor of at least 22000uF/25V to earth which will protect the rectifier and flatten the voltage.

 

Checking my wiring diagram, the CTLS has this capacitor installed. 

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I looked at the wiring diagram. I need the pinout information for the voltage regulator to properly understand what rotax is accomplishing, but I will give an alternator primer. First, batteries act as chemical capacitors, absorbing current surges caused by voltage spikes. An alternator is an electromagnetic generator, relying heavily on regulated current to the field coils to generate power. In a car, you should never disconnect a battery while the engine is running, as there is a high chance of damaging the alternator.

 

If you need to shut down an alternator while keeping the mechanical source operating, the correct way of doing so is to disconnect the field winding before anything else. This shuts down the alternator without surging or arcing.

 

Anyways, alternator/generator switches are a good thing to have. You need a way to shut down the alternator in case of a malfunction. In the best case, it would just be a drain on the electrical system, but in the worst case, it would be a major fire hazard.

 

Finally: rotax does not use an alternator in the 912 uls. They use a generator. They use permanent magnets to induce current instead of a field winding. They also call it a magneto generator, because it also has ignition coils. Therefore, the above primer doesn't exactly apply to the rotax generator.

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The common English meanings for "generator" and alternator" have nothing to do with whether the field is produced by  windings or permanent magnets.  A generator produces direct current and an alternator produces alternating current.  The Rotax/Ducati device produces alternating current.  Rotax calls it a generator but that doesn't mean that they are using the correct English word.

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Jim"s right. We have an alternator.

 

 It produces the 18-22 AC voltage (the two yellow wires that go into the reg/rec) to the reg/rec that gets converted to DC. The ignition system /plugs use AC current. When I check plug wires I use an AC tester.

 

 

p.s.

Does anyone know who makes the ignition modules or what they go to?

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Damn it I had a big old post on the history of alternators and generators, and why the line is blurred when you start looking at things deeply in electrical theory (commutator brushes, vs slip ring AC generators, vs field winding generators, self-exciting generators, etc) and history, and why calling our generator an "alternator" is not very accurate if you try to define it by common terminology. It's a type of alternator, yes, but it's true design is magneto generator. Then I closed my browser while grabbing sources, which had my post tab open, so instead I will now keep it short.

 

Anyways, a magneto is a generating device which uses permanent magnets instead of a field winding for the magnetic field. We can make it output AC or DC, it doesn't matter. But, we cannot regulate the strength of the field, a defining characteristic that we tend to associate with a "true" alternator. That's why I don't really like to call it an alternator, especially since I can show you a slip ring rotor AC generator with a permanent magnet stator and really make things confusing. Yeah it can be categorized as an alternator, but then you start having conflicts with the "old" and "new" definitions, and implied functionality.

 

I agree though, from a waveform standpoint, we have an alternator. But the most correct designation is "magneto generator". I said it that way because my post was about interrupting the field continuity of the field winding to shut down an alternator, something we cannot do with our magneto generator because there is no field winding to disconnect.

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What are those big things connected to turbines at power plants called?

 

Generators and they provide alternating current and have an exciter to create the electromagnetic field instead of permanent magnets. So they should be called alternators but are not.

 

Alternators alternate current and dynamos don't, but they are both generators since they make electricity.

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Soooo...I am thinking, what difference does what I call it make as long as I know what button to push or pull, and when. In addition, I have seen the could and the magnets and have a general understanding of how my generator/dynamo/alternator works. :)

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Guess what? ALL alternators are generators :). "Generator" is defined as a device which concerts mechanical energy to electrical energy, and actually has nothing to do with the waveform output. Common terminology has made a distinction between two technologies that are not actually mutually exclusive. The correct terminology for a generator that outputs DC from a commutator a "DC Generator"; THAT is mutually exclusive from what we call an "alternator".

 

Also, big power guys don't call the generators "alternators" because it's popularized as an automotive and aircraft term. It's used semantically to differentiate between the old style generators (pre 1960) and the new alternators. While calling the big power generators "alternators" would be more precise, even more so would be calling them a "permanent magnet synchronous generator" for most types of powerplant generators.

 

EDIT: I'm not trying to be pedantic, but it looks like I've ended up doing it anyways. Sorry.

 

 

To answer your question ctfarmer: you have the capacitor per the manual, so you shouldn't worry about it. A good reason for having a capacitor is the same reason they exist in aircraft magnetos (called condensers when used in that application). Without the condenser, the switch will arc VERY badly, but with it, it provides a low resistance path long enough for ionization of the air to be minimal while the switch contacts open, keeping arcing to a minimum.

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The quiescent draw of the alternothingamajiggy is practically zero and the cap will eat any load dump spikes from starting or stopping IF the leads are not broken (which they often are). The reason for the switch is to cut out the Reg/rect if the thing goes open loop and tries to overcharge the battery (rare) or internally shorts which could drain the battery into the RR (not so rare).

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I heard an electrical engineer a couple of weeks ago do a short presentation on why you should leave avionics off until you start because the collapsing field when the power stops going to the starter creates a damaging spike.

Going off on a tangent here.

 

It is, in fact, this collapsing field concept that drives aircraft magnetos. There are two windings: the primary and secondary. The primary winding is grounded out through a set of breaker points, and electricity is induced by a rotating magnet. The primary is also attached to a secondary coil, which leads to the spark plugs (after passing through the distributor, which selects which plug to fire). While the breaker points are closed, electrical current can flow unrestricted to ground, through the breaker points, and back to the start of the primary coil. This builds a powerful magnetic field. We interrupt this process by opening the breaker points. The breaker points are timed to a camlobe, driven by the engine crank through gears, so that the camlobe opens the breaker points when the magnetic field stresses has the greatest electrical potential and the piston is in the best place for ignition.

 

When the breaker points open, it begins to collapse the field, and induces a high voltage potential in the secondary coil. The voltage builds up to the point where it ionizes the spark plug gap. When you ionize the air, the resistance drops dramatically, and the spark plug acts as the new ground. While this ionizing is going on, the primary field has not fully collapsed yet. It's still inducing current in the secondary coil, and in itself.

 

With the plug gap ionized, the current in the primary and secondary have a path to ground. Remember, the secondary is attached to the primary, so it will begin to dump the current load through the secondary, and the magnetic field collapse will accelerate. The condenser, which is in parallel to the breaker points, also unloads any charge it has. Since all this current is now flowing through the secondary coil, and the two fields are opposing each other, this causes the collapsing field to "snap", dumping all the current in the system and creating a powerful voltage spike in the secondary coil, which jumps the spark plug gap.

 

Neat huh? :)

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(Yup, same thing with a mechanical distributor, points and coil. )

I felt this was somewhat relevant since it has to do with when you turn on the avionics. It is a difficult with Dynon screens since you need the EMS to see the important info.

(Not sure if you were here when we talked about how many of us are ham radio operators - AA0PT here.)

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I think at least some of the CTSWs came from the factory with a panel sticker saying that avionics should remain off until after engine start, and I know it's in the POH.

 

Another question...are our ignition modules self-powering like a magneto, so that if the power fails the fan keeps turning?  I have heard the answer is yes, but I can't remember who told me so I don't know if the info is reliable. 

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I think at least some of the CTSWs came from the factory with a panel sticker saying that avionics should remain off until after engine start, and I know it's in the POH.

 

Another question...are our ignition modules self-powering like a magneto, so that if the power fails the fan keeps turning?  I have heard the answer is yes, but I can't remember who told me so I don't know if the info is reliable. 

The answer is yes. They have windings on 2 poles of the power generation system to provide power. There is no connection what so ever to the battery. In fact they are designed to operate on the A/C power output.

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...are our ignition modules self-powering like a magneto, so that if the power fails the fan keeps turning?  I have heard the answer is yes, but I can't remember who told me so I don't know if the info is reliable. 

 

 

I have inadvertently switched my gen off and killed my battery in flight, twice, once at 12,500' over Yosemite and in both cases the CTSW kept on flying and even charged the battery back up when I switched gen back on.

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I have inadvertently switched my gen off and killed my battery in flight, twice, once at 12,500' over Yosemite and in both cases the CTSW kept on flying and even charged the battery back up when I switched gen back on.

 

Cool, thanks for being a real-world test pilot!  :D

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Might have been me. I posted a big post when i first joined the site explaining out the rotax 9 series ignition systems work. The iS can run lff the battery if both generators fail, the ULS has two separate coils in the mag generator. Failure of the maggen will not cause an ignition failure as long as the flywheel is intact and turning.

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Might have been me. I posted a big post when i first joined the site explaining out the rotax 9 series ignition systems work. The iS can run lff the battery if both generators fail, the ULS has two separate coils in the mag generator. Failure of the maggen will not cause an ignition failure as long as the flywheel is intact and turning.

 

Thanks for the detail, that helps me visualize it.

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