External Alternator Overvoltage Problems

[neville]

reading your post it appears that when you start the engine the external alternator has no load until you turn a switch on. Alternators do not like it when they generate voltage with no load (such as a battery). The alternator will go over voltage and usually destroys the diodes.

[Esser]

reading your post it appears that when you start the engine the external alternator has no load until you turn a switch on. Alternators do not like it when they generate voltage with no load (such as a battery). The alternator will go over voltage and usually destroys the diodes.

Interesting. How do you get around this problem with dual alternators as one of the, ussually won't be contributing to the loads.

[neville]

most dual alternator systems operate with the alternators directly in parallel. Because of slight differences in impedance between units one alternator will carry more load than the other, however, as load increases the output of both units approaches equal load. In the old brush type generators dual systems had provisions to balance the output loads.

As an example I refer you the the Rotax install manual 24-00-00 page 16, discussing added external alternator, load distribution; " Due to slightly different output voltages of the regulators the power is drawn by the generator (alternator) with the highest output voltage at low load". Also in the same section is a generic Rotax wiring diagram which shows both internal and external alternators connected to the 12v buss. The diagram is for the 912iS. The A alternator output is used exclusively by the engine computers while the B alternator is routed the the 12V buss along with an external alternator if installed.

[amerkarim]

Many thanks Neville,

Thats just what I wanted to know.

Kindest regards

Amer

[Esser]

As I dive more into the electrical stuff, I learn more/confuse myself more.

Correct me if I am wrong, but with the 40a ext alt, if you turn off the power supplied to the field generator, the alternator will not make power(5A line on 5e schematic). If you break the connection between the alternator supply line and the bus/battery(50A line o. The schematic), the alternator will still be making power with no where for that power to go which damages the alternator.

[jrevens]

Having “no where for that power to go” does not damage the alternator, Josh.

[amerkarim]

I am not sure why the regulator blew on the alternator. Perhaps it was the charge remaining inside, or it was disconnecting the field wire. Or something to do with the way it was wired into the bus and battery?

Rewiring it has solved the problem so far. Leaving it permanently connected and not having a switch for it at all. I also upgraded the rectifier to the all aluminium Silent Hectic German rectifier that has a 37A rating, and have mounted that on the firewall where there is good air circulation. It looks a really solid piece of engineering and a definite upgrade to the standard Ducati one.

Sorting the alternator created more problems however. The grounding of the alternator charge through the engine resulted in over reading of all the resistive senders by over 20 degrees C. So oil, CHT and EGT all went up while the engine was running and then would fall instantly by 20 C when the engine was off.

This has been partially fixed by adding a large 30mm grounding cable directly from the alternator casing to the negative battery terminal.

[Esser]

Having “no where for that power to go” does not damage the alternator, Josh.

This isn’t my original thought so maybe I didn’t clarify myself. Here is an excerpt from a post at Rotax owners.com

“When disabling an alternator from the system, you remove power from the field windings.
The Output remains attached to the Battery.
If you are removing the Output from the battery with the field still energized, The regulator loses its reference and the voltage can rise high enough to destroy the regulator.
By removing the Field supply the Alternator output goes to zero.
The regulator Diodes prevent any reverse currents to the alternator from the battery.
It just quietly goes to sleep.”

[amerkarim]

Hi Guys,

Thanks for the replies. It seems that I have not understood how alternators work or that the external alternator is actually "permanently on and generating current " when the engine is running despite having disconnecting it from the battery with a relay. I was just concerned that by having 2 generation systems connected at the same time, that one will end up charging and burning out the other, much like having 2 different batteries connected together.

I have a 3 axis autopilot, dual panels and the usual other electrics in my build, and the alternator was recommended to me as a necessary addition to prevent overloading the internal generator. For reference, my battery is a 36Ah 12V lithium battery and I have a smaller back up battery for the panels as well.

I think the regulator on the alternator has blown from being disconnected from the load while the engine has been running, hence the over voltage now. This has been sent for repair and the mechanics have assured me it is very easy and cheap to just replace the diode plate and regulator and it will then work as normal.

I presume that when the alternator comes back fixed, that I should have it and the generator both switched on all the time so that they each have a load to the battery and system to prevent damage to either of their systems. Probably the alternator will supply most of the load compared to the generator. is there any harm in doing it this way?

My rectifier has been mounted with heat paste and I have fitted a computer fan to the unit to help it cool.

Any other tips or suggestions would be very welcome.

Thanks in advance