Low speed (rpm) gen head?

[AdeV73]

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

...

So I suspect that my math is incorrect, I am not understanding how your sprocket arrangement works, the sprocket diameters you provided are incorrect, or the wheel/generator RPM estimates you provided are not accurate.

I'd have thought the 800mm sprocket is running on the same shaft as the 2.5" sprocket; i.e. your sprocket ratio as above is 1 (not 0.0794).

Divide 34.6rpm by 0.08 gives 432rpm; so that's within the (probably quite large, given all the rounding 'n stuff) range I'd expect to see.

[sailawayrb]

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

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

Regards, RAB

Yes, doing ratios using number of teeth would be preferable for the sprockets.  However, using diameter is well within the ballpark for this calculation (plus teeth numbers were not provided) and there is something else fundamentally wrong with my calculation or the data provided.

[sailawayrb]

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

...

So I suspect that my math is incorrect, I am not understanding how your sprocket arrangement works, the sprocket diameters you provided are incorrect, or the wheel/generator RPM estimates you provided are not accurate.

I'd have thought the 800mm sprocket is running on the same shaft as the 2.5" sprocket; i.e. your sprocket ratio as above is 1 (not 0.0794).

Divide 34.6rpm by 0.08 gives 432rpm; so that's within the (probably quite large, given all the rounding 'n stuff) range I'd expect to see.

Yes, that makes perfect sense!  Thanks Ade!!!

OK, so let's re-work and re-state this:

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 and 800mm (31.5 inch) pulley are on the same shaft and the ratio is therefore 1.0 (i.e., the 800mm pulley rotates at same RPM 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.

You stated that your wheel is rotating at 3 RPM and your existing generator shaft is rotating at 300 RPM?

Wheel RPM times 24 times 1.0 times 6.06 should equal your existing generator shaft RPM.  If I use 3 RPM for the wheel speed, I would calculate your existing generator speed to be 436 RPM.  Alternatively, if I use 300 RPM for the your existing generator shaft, I would calculate 2.1 RPM for the wheel.  Perhaps that is within the tolerance of the data provided and the assumptions made in this calculation?  However, you seem to believe that you need a 60mm or less pulley?

[guyd]

Sorry sailawayrb - I wasnt clear.

Lets say the wheel turns at 6 rpm
The big sprocket (60") drives a small sprocket (7") on a 2.5 inch shaft. So we have about 10:1 - so the 2.5 shaft turns at 50 rpm. I have a tachometer and have checked this.
The 800 pulley turns @ 25rpm as its on the same shaft. This turns a 132mm pulley currently - which in this case would be turning at 300rpm.


The wheel is turning far far too fast - about double - so not only do I need to spin the end result up by double - I also need to slow the wheel down by half - so I need a 1:4 ish gearbox.



edit - theres some pics on here http://www.navitron.org.uk/forum/index.php/topic,3113.0.html if you want to see it.
Guy

[mike90045]

>  and if anyone can get me a chain with this size, let me know. Its inch and a half pitch

If/when the chain goes, look into the Harley Motercycle flat cog drive belt.  You would have to make new "gears" for the belt, and stay within it's length limit.

[mobile_bob]

whats special about the chain pitch 1.5 inch pitch = #120 chain

other than being rather spendy, should be available

bob g

[MacGyver]

I see 1-1/2" pitch chain at McMaster-Carr...

http://www.mcmaster.com

Take a look and see if they have what you want. If so, then you know it's available and you just need to find a local source or have it shipped across the pond

[guyd]

The chain is nominally inch and a half - but the roller diameters no longer match the current standards. Its also thinner than the current standards. I had the bloke from Renold Chain come and try to work out if they could make a chain fit - but no. Renold own / took over the Coventry Chain Company (the makers of the original chain)  back in 1950 odd - so would be best placed to find me a replacement.

Harley flat belt. Um. You are not serious are you? Harley - 3 ft lbs on a  good day. 6 with a nitrous oxide kit. This wheel - 38,000 ft lbs. An inch and a half pitch chain is HUGE. Anyway - its fine at the moment.


I am also looking at a huge, 50hp DC motor I can get cheaply - 300 quid. Its plated speed is 1100 rpm at 88 amps (iirc) / 300 volts. Im thinking of just rectifying the mains (230 / 240v here) and feeding that to the field winding directly, and using my clever inverter to control the amount of power generated. OK - so the field winding voltage is slightly lower than the original spec - but it saves me 500 quid on a DC motor controller. As the motor is being used at well less than 1/3rd of its power, but at 1/2 its speed (maybe?) it should be happy.

[sailawayrb]

Sorry sailawayrb - I wasnt clear.

Lets say the wheel turns at 6 rpm
The big sprocket (60") drives a small sprocket (7") on a 2.5 inch shaft. So we have about 10:1 - so the 2.5 shaft turns at 50 rpm. I have a tachometer and have checked this.
The 800 pulley turns @ 25rpm as its on the same shaft. This turns a 132mm pulley currently - which in this case would be turning at 300rpm.


The wheel is turning far far too fast - about double - so not only do I need to spin the end result up by double - I also need to slow the wheel down by half - so I need a 1:4 ish gearbox.



edit - theres some pics on here http://www.navitron.org.uk/forum/index.php/topic,3113.0.html if you want to see it.
Guy

The wheel is turning far far too fast - about double - so not only do I need to spin the end result up by double - I also need to slow the wheel down by half - so I need a 1:4 ish gearbox.

No worries, I'll work the numbers again…this is an interesting project.  The pictures help and look great.

The wheel (60 inch sprocket) to 7 inch sprocket ratio is 8.6 (i.e., the 7 inch sprocket rotates 8.6 times as fast as wheel).  The 7 inch sprocket is on a 2.5 inch shaft.  You have measured the 7 inch sprocket (and hence the 2.5 inch shaft) to be rotating at 50 RPM.  This would imply that the wheel is rotating at 5.8 RPM.  This is consistent with your 6 RPM estimate for the wheel.

There is also a 800mm (31.5 inch) pulley on the 2.5 inch shaft.  The 800mm pulley is therefore rotating the same speed as the 2.5 inch shaft, 50 RPM.  The ratio between the 7 inch sprocket and 800mm pulley is therefore 1.0 as previously stated.  Is the 25 RPM you stated a typo or am I missing something?

The existing 800mm pulley to existing 132mm pulley ratio is 6.06 as previously stated.  This would imply the 132mm pulley is currently rotating 6.06 times as fast as the 800mm pulley or 303 RPM.  This is consistent with your 300 RPM estimate.

So you have several options.  You could continue to use pulleys, but replace them with a new set of pullies that provides your required ratio.  To go from 300 RPM to 500 RPM at generator shaft, seems to me you still need to increase the existing 800mm/132mm pulley ratio from 6.06 to 500/300 x 6.06 or to 10.1 as previously stated.  Please see my previous recommendations for doing this.  You could also scrap the pulleys completely (i.e., the 800 and 132mm pullies) and replace them with a gear box that is connected directly to the 2.5 inch shaft which you appear to prefer.  However, this gear box will still need to provide this 10.1 ratio, right?  You are still trying to transform your 2.5 inch shaft 50 RPM (i.e., input to gear box) to 500 RPM at generator (i.e. out put of gear box), right?  Is the 4.0 ratio (i.e., "1:4 ish gearbox") you stated a typo or am I missing something?

Maybe the "something" I am missing is your desire to slow the wheel from 6 RPM to 3 RPM?  I assume you will accomplish that by controlling the flowrate of water into the wheel.  If the wheel will be rotating at 3 RPM (i.e., half of 6 RPM) and you want the generator rotating at 500 RPM, seems to me you would need to essentially double the previous 800mm/132mm pulley or gear box ratio calculation from 10.1 to 19.4  (i.e., 3 RPM at wheel x 8.6 x 1 x 19.4 = 500 RPM at generator).  So I am thinking you need a 1:20 gear box?

[sailawayrb]

One more thing.  The reason I am going to great length trying to sort out these RPM ratios is that presumably you will also be considering the torque being transmitted at each point to ensure that you have adequate strength components at all points.

[guyd]

I cannot increase the 800 pulley - theres a wall in the way.

I am reluctant to make the small pulley massivly smaller, as the forces become huge, and the losses bad (think of a 0.1" pully - the belt wont turn it, more pull it to one side).  90mm would make me wince, let alone anything smaller. Designflex web on gates belt drives' website refuses to work out a 9:1 with 8kw on a 30rpm shaft with a max 800 mm pulley on the driveR.

so I need to speed it up somehow that is quiet. The easiest way is a gearbox on the floor next to the huge, half ton, motor.




Any DC motor experts please comment on my huge shunt wound motor idea please. I am to pick it up, with cash in my grubby mitt, on Tuesday.

[sailawayrb]

I cannot increase the 800 pulley - theres a wall in the way.

I am reluctant to make the small pulley massivly smaller, as the forces become huge, and the losses bad (think of a 0.1" pully - the belt wont turn it, more pull it to one side).  90mm would make me wince, let alone anything smaller. Designflex web on gates belt drives' website refuses to work out a 9:1 with 8kw on a 30rpm shaft with a max 800 mm pulley on the driveR.

so I need to speed it up somehow that is quiet. The easiest way is a gearbox on the floor next to the huge, half ton, motor.

Agreed, you should try to minimize belt slippage.  If you truly can't replace the 800mm pulley, then you will likely need to replace the 132mm pulley with something (perhaps additional pullies and another shaft) and add a gearbox so as to get a combined 19.4 ratio (or whatever ratio you eventually decide works best).  Probably best way to do this would be sort out what ratio gear boxes are readily and cheaply available in your area, and then work out what you need in lieu of the 132mm pulley to best accomplish what you want.

[rcavictim]

Any DC motor experts please comment on my huge shunt wound motor idea please. I am to pick it up, with cash in my grubby mitt, on Tuesday.

I am certainly far from being an expert on DC or AC motors but I think your idea will work well.  I have a very similar sized DC generator here now that I am playing with thinking of using it as a welding head and driven by belts that can be tightened when welding or battery charging function is desired while the big Changfa 1115 engine is directly coupled to a 3-phase AC head spun at 1800 RPM to do what the plant will do 99.95% of the time, make AC power for the shop and for outages.  My DC gen is 1800 RPM, 400 ADC continuous at 50 VDC.  That is 20 kW output or more than 40 HP input, probably more like 50 HP.  It has a 8 row B-belt shieve attached as acquired. The unit is slightly larger than the 100 HP, 3600 RPM, 575volt, 3-phase induction motor I recently acquired and must weigh 800+ pounds. It has four rows of 3 massive parallelled brushes at 90 degree spaced stations around the armature's commutator.  It has interpoles amongst the field through which output current is routed to reduce brush sparking. The brushes are about 1" x 2" contact area.  I am exciting the field with a variable DC power supply, a variac into a bridge rectifier and smoothing capacitors 0-40 VDC at the moment driven from the mains. The field current is not great but I have not measured it yet.  I'd guess under 10 ADC. I have total control of output voltage at any reasonable speed of say 1/2 of nameplate or over 900 RPM. I haven't bothered trials under 900 RPM as belt slap and genhead creepage is exacerbated the loosening of my C-clamp's grip on the genhead to the I-beam channel base rails and resulting in belt slippage.  I need to tie this down better before I can proceed.  For any given speed I can dial in the voltage I want by changing field excitation up to the maximum that is in art determined by the speed of the armature at the moment.

I plan to experiment with overexciting the field to increase output voltage as part of my experiements.

[guyd]

Well - I picked it up - my god its heavy!

A bit of a struggle, and its in the laundry room / mill room.

Pulled the brushes (theres 12:-  3 at 12 o'clock, 3 at 3 o'clock etc) and 3 and 9 are new, 12 are knackered 6 havent been investigated.


Brush company called and sample sent to copy / evaluate. They suggest clean the commutator, and replace brushes - BUT only use 4 not 12 for the lower current use. You can (contra-instincivly) over-brush a genny....


Plate to mount this little lot ordered, adaptor ordered, rubber mounting blocks ordered etc etc.