Author Topic: 28VDC generator  (Read 201 times)


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28VDC generator
« on: November 02, 2017, 08:55:28 AM »

Hi there guys.  I know there are lots of you guys who are very savvy about generators and related (Starfire, you're a candidate for starters)

I recently bought a little generator unit.  It is a four-cylinder flathead Waukesha petrol engine with a four a-section belt drive to a (nominally) 28V generator head which also has a starter winding.  I would guess it was made early in the 1950s, or if it is built later, is of an old design that was kept in production.

I just bought it to use the generator head as it is an open-drive design - I can picture it hanging of one of the  CSs doing a sterling job charging a 24V battery bank

The little Waukesha looks to be maybe 600 or 800cc?  I sold it to someone who likes to play with things like that.

These little units were widely used in aviation, I am told, and this one was on a NZ air base keeping helicopter starter batteries topped up, so the son of the original owner told me.  If I was going to guess I would think the little motor might have worked at around 1500 RPM - maybe a bit more?

The generator drive is geared about 1:2 or maybe 1:1.8, so maybe the generator head is designed to spin around 3000 RPM?

The generator is a solid unit, it'd be 30 Kgs, probably.

I don't know what is the relationship between RPM and output for a generator (as opposed to an alternator) and I'd be interested in any thoughts on what the common wisdom is on this, and on how fast or slow a generator of such an old design should be run

I'd appreciate any thoughts

Cheers, Mike


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Re: 28VDC generator
« Reply #1 on: November 02, 2017, 10:53:03 PM »
Hello Mike. That machine sounds interesting, where do you pick this stuff up from, you have good contacts?
Generators are  DC, alternators are AC.... although this classification has merged over the years.
Generators have no fixed operating speed as there is no AC frequency content to worry about.
They have a minimum speed where they become inefficient, maximum speed is usually limited by the commutator disintegrating, and increased inductance in the windings to increased rotation speed...... they generate less as speed increases over the  sweet spot..
They are also inherently less efficient, the brushes span several commutator segments when switching from one armature winding to the next, significant current is wasted, these are called switching losses. This is why generators are bigger for a similar output to an alternator. An AC alternator uses diodes to do the switching, so there is no conduction overlap.
Thats why I never recommend generators, they are really too lossy with our limited horsepower, and the job can be done much better with AC and diodes.
Unless it has historic interest for a museum, practically it has little value for us off gridders.


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Re: 28VDC generator
« Reply #2 on: November 05, 2017, 04:59:03 AM »
Cool. thanks Starfire

Good explanation

Old shit just comes from keeping an eye on TradeMe

I hear you re the benefits of alternators.

Quite a lot of what little experience I have had of electronics has been the process of finding ways to let the smoke out.  I don't understand them so I fear them, see?  So I am tempted to play with an old generator thinking it might be "idiot proof"

However, as you know I have a couple of brand new Leece Neville truck/bus alternators to go into service one day.  You'll know when that happens as I will be asking some more dumb questions



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Re: 28VDC generator
« Reply #3 on: November 05, 2017, 11:21:53 AM »
Hi Mike

IMHO there's a lot more stupid answers than there are stupid questions!

Cheers Stef
Tighten 'til it strips, weld nut to chassis, peen stud, adjust with angle grinder.