Starter Motors, Alternators and Generators

[puntosaurus]

Why do turbine helicopters tend to have combined starter generators, whereas piston helicopters tend to have separate starter motors and alternators ? Is there something about the starting characteristics of turbines vs. pistons that tends to drive the configuration ?

[Bertie Thruster]

Less sparks from the brushes of an alternator? (the induced current doesn't have to pass through the brushes and slip ring, as on a generator) So alternator safer in an avgas environment?

[rick1128]

I believe it has to do with weight and the operating properties of generators and alternators. Many years ago pistons used generators also. There were not any explosions or fires. Pistons use alternators as they can deliver constant power at different RPM setting as the piston engine operates in a wide range of RPM settings. The turbine engine can use a generator since the turbine engine operates in a narrow band of RPM settings. The starter-generator is used primarily to reduce weight. Since turbines tend to have higher electrical demands they need a larger and more powerful generator. Plus the engine and its start procedures require a more powerful starter to rotate the engine to the degree necessary to enable it to start. A starter and a generator are essentially the same with just minor differences in how the electrical system is attached to it, which is not the case with an alternator.

[skidbiter2]

I believe it has to do with weight and the operating properties of generators and alternators. Many years ago pistons used generators also. There were not any explosions or fires. Pistons use alternators as they can deliver constant power at different RPM setting as the piston engine operates in a wide range of RPM settings. The turbine engine can use a generator since the turbine engine operates in a narrow band of RPM settings. The starter-generator is used primarily to reduce weight. Since turbines tend to have higher electrical demands they need a larger and more powerful generator. Plus the engine and its start procedures require a more powerful starter to rotate the engine to the degree necessary to enable it to start. A starter and a generator are essentially the same with just minor differences in how the electrical system is attached to it, which is not the case with an alternator.



Hmmm, not sure I totally agree with that.

For one, the piston powered craft has to have a gen/alt PLUS the added weight of a starter, where the Turbine just has the one unit, although it isn't that light.

This is a guess as I am not an expert by any means, but do have a little history with starters/gens and alternators.

Pistons normally need a fast starter to throw out the drive and turn the engine over at a good pace right away, the brushes in a starter normally have less carbon content than an alternator/Generator.
Carbon has a higher resistance than copper etc, so if you had high carbon brushes in your piston starter it would probably turn over to slow initially?

With a turbine, the starter doesn't have to fight against a whole bunch of compression in the cylinders, so can build up speed gradually (relatively speaking), so higher carbon brushes can be used.

Now I'm not exactly sure why Gens/Alts use higher carbon brushes, I think partly because they don't wear down as fast as starter brushes, and probably with the higher carbon content, they wont cause as much arcing and radio static etc.


Sorry for the long explanation, I'm not good at putting thoughts to paper, and I am home bored waiting to go to work :(

[Rigga]

...you can't fit a bendix to an alternator.

...you don't need a bendix to start a jet.


simples!

(apologies to any Meerkats out there)

[puntosaurus]

Hmmm. Some pointers there but not sure it's nailed. My guess is that a starter/generator with enough torque to turn over a large piston engine via the direct drive that would be required, would be heavier and bulkier to mount than the combination of lightweight bendix starter and alternator.

Maybe that's what Rigga was saying !

[blackhand]

Think series wound, shunt wound and compound motor theory.

Series wound for high initial torque - required to get that Otto cycle rotating past compression.

Shunt and compound to get the Brayton cycle spinning around at high speed.

Cheers
BH

[HeliComparator]

... or, keeping it more simple, perhaps its because on a turbine the starter doesn't need much torque but has to turn the engine at around half its max speed, therefore the speed range it has to work over in starting mode and generating mode are not that far apart (1:2 or so) whereas a piston starter has to have lots of torque but only needs to turn at about 10% of max speed - ie a 1:10 speed ratio.

So its all about the required gearing!

HC