Tuesday, April 15, 2014

Burning up Alternators at Idle....

One thing that has always bothered me was when the engine idles down to a lower RPM and the Alternator is still trying to pump out a massive amount of power - regulator will end up driving the field to Full Power 100% field current trying to keep up. The question is: Are Alternators able to handle this? In some ways it is kind of like lugging an engine uphill in high gear..

I know this is not an Alternator Rotor
But you get the idea..

This last week I had someone contact me.  They have been poring over both this Arduino regulator project, as well as its companion http://smartdcgenerator.blogspot.com and asked the question: "Am I doing anything to reduce Field current at low RPMs". He went on to share an experience of a boat using large frame Leece Neville 4900 series alternators (the same 35lb units I use for our main engine house alternator) driving massive loads  to power engine room blowers via inverters. After an extended period of time at idle (and likely in a very hot engine room - these were MTUs in a $$$ go-fast craft), there was a failure in the field - total melt down of the Field / rotor..

My initial intention of adding the 'Small Alt' mode selectable via the DIP switch was to protect smaller framed alternators from prolonged periods of high current output. Ala, that 200A Hot-Rodded alternator in a standard small frame.. Small Alternator mode will cap an alternators output to 75% of its capability (configurable of course), at all charge points. But it seems perhaps more might be needed - especially given the real life experience of someone melting down my beloved Leece Neville large frame alternators just idling around...

So, I have added a new feature to further restrict how hard the field is driven when the engine is at idle.  By sensing this RPMs vis the Stator wire, I can tell when we are near idle - or at least near the lowest RPMs that the engine has been operated at..  And when at idle I now cap the field to 50% of its max drive.  This % is increased up to the max allowed (100% or 75% depending on the DIP switch, and if the defaults have been adjusted).  And there is a new parameter in the $SCA: command to configure this capability.

Blue line shows default idle capping curve.
Red and green give an idea of some other configurable options via the $SCA: command..
Of course you can also use the $SCA: command to also disable this feature. Right now I have the '50%' hard codded in the firmware; starting to get a bit too many configurable parameters. If someone has any feeling for the right % value to use at idle, let me know.  If you want to change it in the firmware look at the  "set_VAWL(float passedV)"  helper function in "calculate_ALT_targets()" 

I hope this will help out some installs.  Though we never had any issue pulling max amps (and max field) for several hours  at a time while idleing around during Christmas Ships with our LN 4900 270A alternator, clearly someone has..   Perhaps I can feel even better now - protecting the Alternator just a bit more...

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