Low speed (rpm) gen head?

[listeroidsusa1]

I went by my local scrapyard and found a nice 20 kw 8 pole generator. Unfortunately I was a day too late. The uninformed at the yard only see scrap copper and they had already destroyed it, in spite of my several requests to keep an eye out for such a beast. I had told them I would pay twice the scrap price but they couldn't be bothered and just cut it up. grrrrrrrrr...........

[rcavictim]

I went by my local scrapyard and found a nice 20 kw 8 pole generator. Unfortunately I was a day too late. The uninformed at the yard only see scrap copper and they had already destroyed it, in spite of my several requests to keep an eye out for such a beast. I had told them I would pay twice the scrap price but they couldn't be bothered and just cut it up. grrrrrrrrr...........

It is a debilitating disease that a unfortunately a lot of people have.  I see a lot of that.  No peripheral vision whatsoever.

[listeroidsusa1]

I would have wagered that it was a good running generator as the only fault I saw was that the splined  2 1/2" input shaft was worn and stripped. I would have just turned it round and keyed it to 2". At 900 rpm it could have been direct coupled to one of my engines.

[sailawayrb]

...My question is - what size head do I need to generate 5 to 10kw at 500rpm - and what voltage and current would I require for the field? I accept that I will need external ventilation / forced air cooling for this. I will assume the original spec for the gen head is 1500rpm (415v / 50Hz)...

You would need a 15-30 kW 1500 rpm machine.  The field would be operated at its normal nameplate-rated current.  Thus, the output voltage from the machine would be about 1/3 of it's rated 1500 rpm value, and so would the power.  And, the frequency would 16.7 Hz.  But that's OK, right? - you'll just run it into a 3-phase full wave rectifier and make DC.  You're right about needing additional cooling air as the internal fan in the machine isn't likely to move enough air across the windings at the lower speed.

Yes, that answers the question and should work very well.  Operating at 1/3 rated power, it might not even require any additional cooling either...especially considering it won't be operating in a high temp engine room environment.  I sure wish I could generate my power this way.

[guyd]

Rab (hello..)

I have scrapped the gearbox as it lost huge amounts of power, and all the straight cut spur gears were very noisy. Currently I have two PM wind turbine heads belt driven from the layshaft - at 300rpm. They are rated at 1500 watts at 500 rpm - but are already overheating generating 900 watts each. I thought I could couple up a bank of these (post rectifier) and feed the resultant DC into the inverter(s), but this doesnt work as the heads are never exactly the same.

Anyway - I have found locally (ish) a very large motor http://cgi.ebay.co.uk/Very-Large-Air-Compressor-Electric-Motor-680rpm-3phase_W0QQitemZ110335934347QQihZ001QQcategoryZ26209QQssPageNameZWDVWQQrdZ1QQcmdZViewItem.

If I spin this at 300 to 400 rpm with 150uF of capacitors in delta across the windings - does the board think that it will generate enough power?

What sort of speeds can a self excited generator run at? the capacitor defines the speed - but +/- how much? 1%? 10%? 50%?

[GuyFawkes]

Dude, I'm local to you and my grandad built most of the waterwheels in devon, when it comes to retrofitting small wheels like yours for generating electrical power there is only one system that consistently works.

1/ Get effective wheel RPM geared up to something more useful, e.g 500 rpm minimum.

2/ generate DC

item #1 ideally you'll find a nice sealed hypoid gearbox, quiet, safe, reliable, if not, you'd be amazed what you can do with a bunch of toothed timing type belts, they handle far more power than people think, and the pulleys are easy / cheap to make.

As an aside I have sat here a 135 VDC @ 900 RPM 3.25 Kw jobbie sat here, which is the sort of thing to be looking for.

https://surfbaud.dyndns.org/sites/photo%20album/index.php/pictures/dynamo/DSCF0011.JPG?action=resize

[Jim Mc]

...Operating at 1/3 rated power, it might not even require any additional cooling either....

I wouldn't assume that.  Remember,  it's still operating at full current.

[sailawayrb]

...Operating at 1/3 rated power, it might not even require any additional cooling either....

I wouldn't assume that.  Remember,  it's still operating at full current.

Yes, I wouldn't assume that either, but I was thinking it might be worthwhile giving it a try.  However, I now realize that I shouldn't respond so quickly late at night...  While power equals voltage times current, power also equals current times resistance...and the latter is more relevant for this heat discussion.  In fact, the heat generated is actually proportional to current squared times (and also proportional to the wire resistance times the wire length divided by the wire cross sectional area…but these parameters are essentially constant for this problem).  So in theory, the heat generated should be the same at full and 1/3 power and solely proportional to the current squared.  However, the internal generator fan will be providing much less air flow at 1/3 power.  So YES, additional cooling will indeed be required to compensate for the lost air flow.

[oliver90owner]

If , as Guy says, you can get to 500rpm, perhaps you can get to 540.  that is the standard pto speed for a tractor, so you might be able to find a suitable 240V/50Hz generator by that route.  Power lost in transforming and feeding batteries/ inverter and selling back through the grid was free anyhow....and possibly quite comparable with any other route around the problem.

Regards, RAB

[guyd]

I already have the inverters and all that kit - which I need for grid tie anyway.

Seeking a gearbox to speed it up - anyone (UK) ? I have extensively searched fleabay to no avail (yet)

[sailawayrb]

I already have the inverters and all that kit - which I need for grid tie anyway.

Seeking a gearbox to speed it up - anyone (UK) ? I have extensively searched fleabay to no avail (yet)

Being a DIY type, I think I would just build by own.  Perhaps something along the lines of what Guy suggested.  Belt/pulleys or chain/gears would work adequately.  One advantage of building it yourself is that you can experimernt, adjust, and set the RPM exactly as required given any variable, seasonal water flow rates (i.e., swap out pulley/gear as required).

[guyd]

Currently I have, starting at the wheel - a 5 foot diameter (ish) sprocket - cast iron, inch and a half pitch - with a special, irreplaceable chain on it. This drives a sprocket on a layshaft (2 1/2 inch diameter) in a moveable housing. This drives a six groove 800mm OD pulley - which currently drives a 132mm pulley on a small genny head. This head currently spins at 300ish rpm.

In order to spin a genny at 500+ rpm I need a 60mm or less pully - which is not possible on a 70mm shaft.. - so getting a step up gearbox will solve this.

[GuyFawkes]

In order to spin a genny at 500+ rpm I need a 60mm or less pully - which is not possible on a 70mm shaft.. - so getting a step up gearbox will solve this.

No you don't, you just need another pulley pair, and since you've got spare torque coming out of your ass, who cares about the 4% lost

[sailawayrb]

Currently I have, starting at the wheel - a 5 foot diameter (ish) sprocket - cast iron, inch and a half pitch - with a special, irreplaceable chain on it. This drives a sprocket on a layshaft (2 1/2 inch diameter) in a moveable housing. This drives a six groove 800mm OD pulley - which currently drives a 132mm pulley on a small genny head. This head currently spins at 300ish rpm.

In order to spin a genny at 500+ rpm I need a 60mm or less pully - which is not possible on a 70mm shaft.. - so getting a step up gearbox will solve this.

OK, so let's work out all the sprocket/pulley ratios:

The wheel (60 inch sprocket) to 2.5 inch sprocket ratio is 24 (i.e., the 2.5 inch sprocket rotates 24 times as fast as wheel). 

The 2.5 inch sprocket to 800mm (31.5 inch) pulley ratio is 0.0794 (i.e., the 800mm pulley rotates 0.0794 as fast as the 2.5 inch sprocket). 

The existing 800mm pulley to existing 132mm pulley ratio is 6.06 (i.e., the 132mm pulley rotates 6.06 times as fast as the 800mm pulley).

To go from 300 RPM to 500 RPM at generator shaft, we need to increase the existing 800mm/132mm pulley ratio from 6.06 to 500/300 x 6.06 or to 10.1

Solving 800/X = 10.1  which results in X = 79.2mm.  So if you replace the existing 132mm pulley with a 79.2mm pulley you will get 500 RPM at the generator.  You could also replace the existing 800mm pulley with a larger size pulley (or replace both existing pullies with some combination of two new pullies) to achieve the 10.1 ratio.

However, you seem to believe that you need a 60mm or less pulley?  You also stated that your wheel is rotating at 3 RPM and your existing generator shaft is rotating at 300 RPM?

Wheel RPM times 24 times 0.0794 times 6.06 should equal your current generator shaft RPM.  If I use 3 RPM for the wheel speed, I would calculate your existing generator speed to be 34.6 RPM.  If I use 300 RPM for existing generator speed, I would calculate your wheel speed to be 26 RPM.  So I either suspect that my math is incorrect, I am not understanding how your sprocket/pulley arrangement works, the sprocket/pulley diameters you provided are incorrect, or the wheel/generator RPM estimates you provided are not accurate.

[oliver90owner]

While considering sprockets, forget the diameter.  Just count the teeth.  KISS

Keep diameters for belt pulleys where you have to use them!

Regards, RAB