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14 hours ago, Thruster88 said:

A question for the electrical engineers,  are these alternators switched off or disconnected when the master switch is turned off in flight (smoke in the cockpit). 

It depends on what you mean 'swtched off'.

 

And it  depends on how the aircraft is wired.

The general scheme of things is that the 912 alternator produces AC, which is then rectified to DC and regulated to a given voltage by the voltage regulator module. This voltage is then passed to the DC circuitry of the aircraft: lights/instruments/avionics/battery etc.

The battery connection provides 2 quite different important functions:

1. Charging the battery (obviously!)

2. Stabilising the voltage in the circuit by providing a steady load: in effect the battery acts as an electrical damper and helps iron out sudden changes in circuit voltage due to changes in engine/alternator speed etc.

 

Some aircraft are wired so that turning off the master switch isolates the battery from the charging circuitry.

If the master switch is turned off in those aircraft with the engine still running, the engine will continue running (912 ignition has nothing to do with the alternator etc), but the damping effect of the battery is lost, which may result in a sudden sharp rise in the DC voltage, and damage to equipment.

For this reason, many builders fit a large capacitor at the voltage regulator: the idea being that the capacitor will absorb the power spike in such an event.

 

I fitted the capacitor during my own build (Savannah) but now think it was not necessary: so far as I can make out, the Savannah master switch does not isolate the battery from the charging circuit.

 

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FYI One agin people are being sloppy with technical English -  Electrical "Generator" is the common / generic name for a deice generating al electrical current. ALTENATORS and DYNAMOS are both generators

 

Taken from Wikipedia:

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If you turn the Master switch off, you are disconnecting the battery from the alternator, generator or dynamo and also disconnecting the battery from all loads. There should not be any connection between alternator or whatever and the usual loads.

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14 minutes ago, Yenn said:

If you turn the Master switch off, you are disconnecting the battery from the alternator, generator or dynamo and also disconnecting the battery from all loads. There should not be any connection between alternator or whatever and the usual loads.

That would require two master solenoids, I am not sure if that is common practice.  In a GA type the alternator is deactivated by the master switch but remains connected to the load, only one solenoid used to disconnect the battery.  

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17 minutes ago, Yenn said:

If you turn the Master switch off, you are disconnecting the battery from the alternator, generator or dynamo and also disconnecting the battery from all loads. There should not be any connection between alternator or whatever and the usual loads.

That may be so in your aircraft, Yenn, but I can assure you it's not so in the Savannah, where the alternator remains connected to the rectifier/regulator, which remains connected to the battery.

What the Master switch does in the Sav is make/break the DC connection to all loads except the battery.

 

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On 24/09/2020 at 8:16 AM, Thruster88 said:

It is an alternator but no slip rings. The coils are fixed with permanent magnets on the rotating flywheel.  

 

A question for the electrical engineers,  are these alternators switched off or disconnected when the master switch is turned off in flight (smoke in the cockpit). 

In GA aircraft, generally the alternator is swiched off especially in Cessnas there is an interlocked dual switch with the master.

Normally the alternator comes on with the master but in case of runaway regulators, overvoltage etc. it can be isolated.

My older Cessna had a generator controlled by the master pull switch so if you shut down the electrics in flight the generator field would also be disconnected.

 

Depending on airframe, microlights that have a PMA and associated rectifier-regulator often do not isolate any of these items from the battery.

In many respects this is quite satisfactory, as the amount of current and voltage available even with a failed regulator is not as much and provided the battery is sized correctly (18 AH minimum for Jabiru).

 

There is also serious complications in adding isolate swiches and overvoltage cutouts to these simple systems, for instance if you are going to separate the regulator sense wire from the regulator output and battery connection then you have to allow for a situation where the sense wire has low voltage but the regulator output is still connected to something. Newer Jabiru Stator coils are wired in series you might be looking at 55 Volts or so.

 

Aeroelectric connection for advice on crimping etc. - use proper compression crimpers not the $2 store ones that come with assorted lugs

 

Ralph

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38 minutes ago, jetboy said:

In GA aircraft, generally the alternator is swiched off especially in Cessnas there is an interlocked dual switch with the master.

Normally the alternator comes on with the master but in case of runaway regulators, overvoltage etc. it can be isolated.

My older Cessna had a generator controlled by the master pull switch so if you shut down the electrics in flight the generator field would also be disconnected.

 

Depending on airframe, microlights that have a PMA and associated rectifier-regulator often do not isolate any of these items from the battery.

In many respects this is quite satisfactory, as the amount of current and voltage available even with a failed regulator is not as much and provided the battery is sized correctly (18 AH minimum for Jabiru).

 

There is also serious complications in adding isolate swiches and overvoltage cutouts to these simple systems, for instance if you are going to separate the regulator sense wire from the regulator output and battery connection then you have to allow for a situation where the sense wire has low voltage but the regulator output is still connected to something. Newer Jabiru Stator coils are wired in series you might be looking at 55 Volts or so.

 

Aeroelectric connection for advice on crimping etc. - use proper compression crimpers not the $2 store ones that come with assorted lugs

 

Ralph

Rotax wiring calls for a 2 pole single throw switch, both on and off (opened / closed) at same time.

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Most of the GA planes I have flown have a separate switch for the alternator. The reason being that it is not unknown for alternators to fail in the highvolts / amps load and quickly ruin a battery and also your day.

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I have some things to say about this.

 

There is plenty of energy to generate a fire with a fault in the alternator diode pack and or wiring to the battery, which in most situations , remains connected to the battery unless specifically isolated by a contactor or relay.

 

In general, the regulator does NOT get involved with the high current path, it controls the field current on NON permanent magnet alternators

There are SOME newer alternators that use permanent magnetic rotors, they  MAY have a regulator in the current path. 

 

Again, any path to earth by whatever means can be a hazard if there is a short  / fault condition. 

 

The 18AH jabiru battery can generate quite easily, for 10 minutes at least, a heating power of about 2000 watts ! That's plenty to start a fire.

 

I would be more comfortable with TWO isolation switches- because  an alternator that is CAPABLE of outputing 50V (they can unloaded)  if the regulator goes bad AND the battery is disconnected too big a risk for the electronics,  ie: battery disconnected, alternator connected to loads.

 

IE battery and alternator isolation switches, Both going to the battery terminal, so that the alternator contactor and the LOAD contactor off means the alternator CAN NOT get connected to the load without the battery holding it down to 12V in case of a regulator failure or other.

 

Else where I have covered the issues of arcs sustaining (and arcs are thousands of deg C in temperature) due to the  inductive nature of the alternator stator windings which is VERY similar to a stick welder !!!!

 

-glen

 

 

 

 

 

Edited by RFguy
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Yes, the master needs to disconnect the battery because that is the most likely source of enough current to start a fire. The contactor should be as close as possible to the battery.

 

My understanding is that the Rotax uses a permanent magnet alternator. There is an option for an external alternator, but I don't think I have ever seen one.

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