Direct Drive Genset

[adhall]

Has anybody looked into building a direct drive generator with a slow speed diesel?

One possibility is an 8-pole generator head coupled to a 1000 RPM rated engine running at 900 RPM. I have to admit I've had no luck finding an 8-pole generator yet.

Best regards,
Andy Hall

[biobill]

  8 pole 2.5 kw posted middle of November
http://listerengine.com/smf/index.php?topic=1274.0
  Good luck, Bill

[adhall]

Bill,

Thanks for the link. Sorry I have been slow to respond.

Unfortunately, that is somewhat smaller than what I am looking for.

Best regards,
Andy Hall

[rcavictim]

Since direct drive is the most efficient way to couple a gen head to a prime mover this is the `way to go` in a big generating plant where max efficiency is most important.  A three per cent improvement in fuel economy can represent a lot of saved $$$$ in the life of the plant.  Because it costs much more to make a 8 pole head than a four pole this is justified as the really big engines just don`t spin fast. Since most prime movers in the small sizes run fast enough to spin 1800 RPM, finding small 8 pole heads is tough.  There is little need for them to justify the higher manufacturing complexity and cost.  As a result they are quite rare.

I am looking for a 6 pole (1200 RPM) alternator head in a 20-30 kW size for my big wind turbine and haven`t tripped over a used one I can afford yet.

[rcavictim]

Toothed Gimler belts as used to drive camshafts and dragster superchargers are a good way to speed up a slow engine shaft RPM to drive a higher speed alternator head.  I believe the losses in this kind of belt are quite low compared to the alternatives.  Pulleys for the toothed belts are really pricey though but depending on ratios and belt widths required there are alternatives for the determined but frugal DIY`er.

I made a 1 to 1  drive for my 350 Chevy SB engine out of the camshaft timing belt pulleys salvaged from four Chevy Vega gas engines.  Put a V-671 GMC blower on a 3/8 inch thick steel plate brazed to the quadrajet hole in a cast iron stock manifold.  My stacked Vega pulleys reliably ran a 2 inch wide, 1/2 inch pitch Gimler belt.  This package ran great!  I got 8 PSI boost at 5000 RPM.

[adhall]

Regarding 8-pole generators:

Because it costs much more to make a 8 pole head than a four pole this is justified as the really big engines just don`t spin fast. Since most prime movers in the small sizes run fast enough to spin 1800 RPM, finding small 8 pole heads is tough.  There is little need for them to justify the higher manufacturing complexity and cost.  As a result they are quite rare.

I don't know very much about how generators are constructed, could you explain why an 8-pole unit would be more complicated than a 4-pole? I am aware that slow speed motors tend to have larger frame sizes for the same Hp, and I assume this would be the case with a generator too, but what other factors would go towards the higher cost?

Regarding toothed belts:
I once coupled a 150 Hp electric motor to a 250 KVA generator using a 4" wide cog belt. (This made a rough and ready frequency convertor with a variety of output voltage options--and it's still in service some 8 years later.) Even though the shaft speeds were relatively low (6-pole motor running on 60 Hz with a 4-pole generator producing 50 Hz power), the "singing" of the belt was amazingly loud and was unpleasant to be around. I do understand that there are a variety of different tooth profiles available and it may be possible to minimize that problem.

Also regarding toothed belts:
It would be easy to make a 2:1 ratio using standard automotive camshaft drive parts. This would work OK with the higher speed Listeroids (900 RPM engine speed). One would have to investigate the power handling capacity, of course. On the other hand, my 4" belt setup was rated to carry 300 Hp, so a 1" belt could probably handle 6 Hp easily even with the handicap of small pulleys and the consequent low belt speed.

A final question about toothed belts:
How do the loses compare between serpentine and toothed belts anyway?

Best regards,
Andy Hall

[Doug]

More poles means more coils to spin, more coils means less turns per coil and possibly Parallel circuts. The we get into issues of pole slot ratio, possible odd grouping of coils because a standard stator may not have the correct number of slot. A small machine with that many poles needs to be very well designed because the amount of iron and copper per kW is much higher than normal and the possibility of a messing up the numbers can lead to dramaticaly lower efficiency.

As it stands that many poles is in efficient and expensive to build. If you realy want to build a direct coupled machine with a roid the solution isn't more poles it and inverter generator setup. I doubt any one here realy has the stomach for trying to build such a machine I don't

Doug

[listeroidsusa]

I built a direct drive system several years ago. I used a 40 KW 400 HZ frequency converter as a core. By the time I derated it to 60 cycle it only put out around 5 KW. I just about went broke winding that monster. It was 2' in diameter and my front end loader wouldn't pick it up! I once worked in a motor rewinding shop and still had my coil winder. It would be totally unreasonable in today's market to rewind one of these given the price of copper magnet wire today. The 400 HZ units are 14 pole which give a 60 cycle output at 500 rpm. It might be possible to use one as it is but with the V/F curve it'd only put out around 30 volts at 60 cycle and would require a special transformer to get it back up to 115/208, catch 22, a special transformer costs as much as the converter rewind.....oops........

Mike Montieth
Listeroids/USA