Turbine charging systems...

[kodiak1]

Hi all, new to PPRuNe and hoping to get some clarification. I'm a piston pilot and working to transition to turbine. I'm curious about the typical turbine charging system and how it is configured do to conflicting information I'm getting around the airport.

I understand the ships batteries for PT-6 turbines are 24 v and around 40 to 50 Ahr and the charging system is 28 volt which makes sense given the generator needs to power the avionics at 24 Vdc as well as have reserve capacity to charge the battery during flight (similar to an automotive system that is 12 v with charging system that is 14 volts of output capacity).

My question relates to ground power supplies. I hearing that GPU's need to be 28 volt for starting but the ships battery is only 24 v? If that were the case, why wouldn't the ships battery be 28v too? Any insight would be greatly appreciated.

Thanks

[grounded27]

The configuration changes per craft, on large turbine aircraft the battery is 28 vdc usually only used for emergency power or an APU start if a GPU is not available the a/c generators power transformer rectifiers that supply dc power for normal instrumentation use a static inverter is available with the loss of a/c power. I got messed up the first time I worked a corporate size jet as each engine had a dc generator and a ac alternator?

[cockney steve]

The 12v /24v are NOMINAL....As pointed out , the standard automotive lead-acid battery is charged to 14.2 volts...it has a very heavy discharge-capability , so , with it's designed full-load (starting) drain-current, it will dip momentarily to ~8v ,stabilising at10-11v just double-up for a 24 volt system.


Were the generator-voltage any higher, it would fully charge the batteries and then proceed to electrolyse the water-content of the battery....thus you would have a highly explosive gas emission of Hydrogen and Oxygen (with a bit of vapourised sulphuric acid entrained for good measure!)
Most modern L-A batteries are semi-sealed and have bits of filtering and baffle gubbins that seeks to recombine the electrolyte before it's vented.

Too low a voltage output and you won't "push" any charge into the battery.

Alternator is usually 3-phase internal output, rectified by a diode-pack and outputted as DC....the voltage normally controlled by a built-in electronic regulator which varies the field-voltage to the rotor...this, in turn, determins the voltage generated by the stator.
External regulators are often seen in Aviation use,also a few Japanese cars have used them (don't know why, I only see that as a disadvantage0
A dynamo is really an "inside-out"alternator...the field current magnetises the pole-shoes in the carcass and the rotating armature generates the output which is picked off a segmented commutator by carbon brushes.....the field current is normally controlled by a Current-Voltage Controller an external device consisting of coils, contacts and compensating components......Dynamos are heavierand lower-output at low speeds compared to alternators.....because the commutator has some sparking, RF can be a problem. Also, if run at max. output, the dynamo armature can overheat, melting the solder which secures the coil-end to the com. segment....open-circuit failure and often the tails then fling-out and wreck the brush-gear.
Sorry, I've drifted a bit off topic but hope this is of interest,

Addendum.- It's likely the GPU only delivers the same open-circuit voltage as a fully charged battery, but at it's rated output, the voltage would "sink" to the nominal 24v

[kodiak1]

Got it, thanks for the clarification.