Permanent Magnet Charger from the Starter Motor

[MacMcMacmac]

Seeing how a dc motor and a dc generator are physically one and the same, would it not be theoretically possible to lock the starter gears into permanent engagement and use the rotating motor armature as a permanent magnet generator without modification? Of course you would have a kick only machine at that point, and I'm not sure how to regulate the output since I think most PMA regulators toggle the ground.

I am thinking you would have a single wire 13.5v positive output. The motor is rated to 500w I believe. Would that directly translate into a 500w generator?

Big questions:

Could the motor take the rpm?

Would it run cool enough to survive?

How long would the brushes last? Although they seem to last just fine on older attraction/repulsion type electric motors.

Would the output be dc or ac?

I seem to remember a DT125 that used to use its charging system as the starter as well, or I may be misremembering.

Anyone with a lathe and some free time willing to give it a whirl (literally)?

 

[Jim]

Big questions:

Could the motor take the rpm?

I've considered this before and I don't think it's doable. As the starter is currently geared... and just a guess since there's no hard numbers available.... the starter motor is spinning at probably 1500-2000 rpm to get the engine up to 200-300 rpm for starting. Lets call it a 10 to 1 ratio. That ratio at our 7500 redline would have the starter motor spinning at 75,000 rpm. Well beyond the point of grenadeing in my opinion.

I seem to remember a DT125 that used to use its charging system as the starter as well, or I may be misremembering.

Yes, they used what's called a "starter generator." It was a direct drive off the crank and, as you said, a DC motor is identical to a DC generator. It used a relay to direct power too it for starting. When you let go of the start button, the relay switched it to power out to a regulator. Can't remember which... but the RD200 or 250 also used the same starter/generator.

 

[Jim]

Oooof.... that was a piss poor guess on my part. The manual actually gives us the ratio.... it's 20 to 1.
So at the 7500 redline the starter motor would see 150,000 rpm.

 

[MacMcMacmac]

So you're saying brush wear would be an issue....

 

[gggGary]

'Bout the only dual starter generators in recent memory were lawn mowers and they mostly gave it up about 3 decades ago. A 20 to one reduction wastes a lot of energy, OK for a limited use item, like a starter not so much for an always turning alternator/generator. "Specially when one to one on the end of the crank is so easy to do.
The compromises to make one machine do two different functions usually results in it doing neither particularly well. A recent example would be the Osprey.... Before that the Harrier.

 

[Jim]

Among other problems, the biggie would be a steel armature running inside dry copper bushings. It would most likely never make it to 150,000 rpm. I'm guessin' they would fuse solid around 50K on the first go.

 

[jetmechmarty]

Some turboprop aircraft still use starter/generators. IIRC, they use separate windings for each function.
They’re pretty big units.

So, fuggetaboutit!

 

[GLJ]

Could the motor take the rpm?

So you're saying brush wear would be an issue....

Well before it got to 150k rpms the armature would grenade.

Some turboprop aircraft still use starter/generators. IIRC, they use separate windings for each function.

Marty is correct. The mo-gens that Gary referred were made by Delco-Reme. 2 field coils 1 for starting one for generating. It took 2500 rpms in order to quite discharging and start charging. I rebuilt 100s of them.