What happens when you overspeed an ST head?

[Number21]

I am interested in putting together a large-ish inverter generator that would generate 60v using an ST head modified as described elsewhere on the forum, and then feed the 60v into an inverter. The engine speed would vary depending on load.

What I am curious about figuring out, is what happens when you over speed or under speed an ST generator head modified for 60v output? Obviously the frequency would be all over the place, but that's not a problem if I rectify into DC. What happens to the voltage when you spin an 1800 RPM head faster than 1800 RPM? Or slower than 1800 RPM? Does the head still maintain the same or similar voltage?

And will an 1800 RPM head survive if you ever spin it much faster than that?

[BruceM]

With a faster rpm there's  increased voltage, and regulating the output will require some thought. Mechanically, I don't think the rotor is up to it.  The windings aren't dipped, and the bolt on coils were designed for the rated speed and centrifugal forces, no more.  Quality has always been less than stellar on the ST heads-  I would not want to be run one of these over 65Hz.

For DC generation, a three phase unit would be a better choice, due to the vastly reduced dc ripple. I don't know about getting 60v on the 3 phase units- perhaps someone more familiar with the 3 phase stator winding can enlighten us.

[Tom Reed]

When I was thinking about doing this, it was for fixed load battery charging. As Bruce said I wouldn't go much over 65hz too. Slower is less of a problem. Look to an large truck alternator for a variable rpm unit.

[Number21]

Look to an large truck alternator for a variable rpm unit.

I'm looking to build something in the 15KW range, so I need a BIG alternator.

I assume if you spin them slower than 1800 you would get less than 60v?

The inverters I want to use accept 45 to 90 volts, so I could try to belt drive the gen head within that range, without going much over 1800 max. A 3 phase head is probably the best idea.

[BruceM]

Yes, lower speed will reduce both voltage and efficiency. 

[Tom Reed]

Thinking outside the box here, but how about a power plant from a hybrid car converted to NG? Start with a junk yard engine/alternator combo, shave the deck to  up the compression and fit it with an impco carb? I do believe they are in the correct KW range for your needs.

[Number21]

Thinking outside the box here, but how about a power plant from a hybrid car converted to NG? Start with a junk yard engine/alternator combo, shave the deck to  up the compression and fit it with an impco carb? I do believe they are in the correct KW range for your needs.

I've thought about that - the Prius engine being a modified Atkinson cycle is supposedly 40% thermally efficient on gasoline. My concern is the actual conversion to NG, I think it would be very difficult if I did not completely eliminate the OEM computers. And if I did that, will the engine even run? Something needs to control the VVT, and I really have no idea how that works.

When I do a google search for CNG Prius, all they have are some very expensive factory converted examples simply designed to make an environmental point...I don't think anybody is actually driving one or converting them on any sort of commercial scale. There must be a reason? Surely it's possible I'm just not sure how difficult...

With that said I have seen some youtube videos of Priuses converted into gasoline generators, and I suspect a used Prius with bad batteries doesn't cost very much. If the Atkinson style valve timing could be kept, it might be a great engine to use.