Direct drive vs. chain drive for a 4800 rpm treadmill motor?

[AdamM]

This is a PM DC motor from an 1980s vintage Vitamaster treadmill. The nameplate says its 15 amps, 90 volts, and 4800 RPM. By my calculations, if the relationship between voltage and rpm is linear, the motor would have to be going about 693 rpm before it would produce 13 volts. (I would chuck it into a drill press and test its output, but my drill press is too small.)



My questions are:



1) Is there a linear relation between voltage and rpm? If a motor goes 4000 rpm at 100 volts, will it produce about 10 volts at 400 rpm? Or does it produce more/less voltage per rpm at lower speeds?



2) Is it realistic to build a direct drive windmill at 700 rpm, or is that way too fast?



3) How nasty are chain or belt drive windmills? What are their main drawbacks? They would be less efficient and require higher start-up wind speed, but by how much? They also would probably be higher maintenance, but also by how much?



The first application for this windmill will be as a backup generator for power outages on a small island on the coast of BC, Canada. Eventually it or a later model will probably be installed as part of a full-duty alternative power system.



We've considered building a axial flux alternator for the project, but our shop capacities are limited at the moment, so we're going for the quick and simple approach for now.



Any suggestions from all you otherpower wind whizzes would be wonderful!



Thanks,

Adam



P.S. Here's a picture of the motor:

[Flux]

There seems to be two threads running on this issue. I have already started with Tballer2 in a couple of posts down. Perhaps he can also follow this to save me repeating it.

Your concept of cut in speed is right and it is very fast even for a small prop, just about possible at 4ft in a windy area.

Gearing is possible.  "V" belts are nasty, timing belts are only a little better, they can be very efficient at full load but are poor in the region where you will be working most often.

Gears, hardened and ground and in a proper gearbox are efficient but it's not a simple home project. Roller chain is the best choice and again in an enclosed case with proper lubrication it is about the best you can do. Exposed gears have limitations in hostile climates, they don't like our climate in the uk and dust may be a problem in dry desert areas, but otherwise in reasonable climates they may be ok for quite long periods. Look at the common bicycle or motorcycle chain, it works under hostile conditions and still gives good service.

For that size motor you can probably cope with about a 6ft prop and that will give you far more potential than a 4 ft prop even allowing for gear loss.

Ideally you would be looking at about 3:1 speed increase but the greater the ratio the worse the starting. You need to balance this against a faster prop and less ratio and there may be little difference.

Cut in at about 250 rpm with a decent wooden 3 blade prop and gearing about 2.5:1 may be a good choice. I would steer clear of pvc props.

Many of these motors are limited by shaft size and gearing is one way round it. I strongly advise using some form of furling to limit peak power in storms. At 4 ft you could possibly manage without but 6ft going wild is not fun.

Flux

[hvirtane]

Hello,

my idea about the gearing is exactly the same as that of 'Flux'. To use a roller chain is cheap and not difficult. You can make the box for the chain even of wood. A box is a must. To make the chain to last and to make it silent.

I would use even a bigger blade and a higher gearing. You would get the power from lower winds. To protect the generator from overheating, put it furling early.

- Hannu

[bob g]

i would not use that motor at all,

it has a value of about 200bucks used as it is,

perhaps you might consider selling it and going a different route?

i am with the other two on gearing with a chain drive

and if you are going to use this motor why not gear it up and charge

into a 24 volt bank?

i can't tell, but does the motor have a C face on it as well? if so that will make it easier to mount to a homebuilt gear case.

bob g

[DanG]

Roller chains do not mix well with continuous duty and wind power designs ARE for continuous duty. If a chain drive fails it likely will leave the turbine uncontrollable. Look at a motorcycle application for an example.

21" diameter tire rotates 960 times per mile, (12k miles = 11520000 revolutions) so at 60mph 12000 miles is 200 hours operation. At your minimum working ~700rpm that equals 275 hours service; or at 7.5 mph thats 90 days service. If you get real wind with normal high RPMs you could see a years service in well under a weeks 168 worth of hours!

Of course the turbine won't have the link stretching torque or road grit wear the motorcycle chain has - but for lubrication and adjustment schedules and metal-to-metal wear it's directly comparable. Please re-think using a chain drive. Even an automotive timing chain style is meant for intermittent operation in very specific environment.

[feral air]

You can get decent treadmill motors for as little as $20, what makes that one worth $200? Just curious.

[Flux]

I have some respect for your views. I don't like any form of wind turbine that doesn't have some other method of stopping it.

Even brake switches are not completely foolproof but normally it is sufficient. Wires don't often break or become disconnected. I don't consider any dc motor foolproof with direct drive or gearing, brush and commutator failure can always happen.

Belt and chain drives can fail and 6ft is about the limit of size without some alternative means of stopping it. Gears can also fail but you normally can't put up with the noise months before failure.

It is a little unfortunate that those least able to build satisfactory alternators are the most likely to revert to gearing or other doubtful things, it is likely to be the lack of decent engineering that will lead to failure. Roller chains can have a very long life under continuous duty under correct conditions but the average constructor is unlikely to work it under good conditions.

Although I don't really encourage any speed increasing device it can still work effectively. Nothing is foolproof for wind power, regular checks and maintenance are desirable but few of us get round to it.

At least you have given a warning, the builder has to decide what risks are reasonable and decide to incorporate alternative shut down mechanisms if deemed necessary.

Looking at pictures here I often see things that I am certain are a failure waiting to happen and some are potentially far worse than a failed chain drive.

Flux

[AdamM]

A big thanks to Flux, Hannu and bob g and Dan for your excellent responses to my questions.  I'll probably try the nearest metal recycling yard to see if I can find a direct-drivable PM DC motor before we start experimenting with chain drive mechanisms. If we're successful, the current motor may end up powering a nice variable-speed lathe or some such.

If we do go with a chain drive, what have people found makes the best support for the prop and gear? An obvious solution would be to cut the pedal bearings out of a bicycle, then use the ready-made sprocket on one side and mount the prop onto the other side. But would this be strong and durable enough? Bicycle sprockets are designed to handle serious shear loads as someone thrashes around on the pedals, but using for a wind generator would certainly exceed the designed rpms and duty cycle.  It's more tempting because if we used a 21 or 24 speed bike there would be three or four different gear ratios to choose from. You could even leave the derailer in place and change the gear ratio depending on wind speed!

Any thoughts?

Thanks again,

Adam

[feral air]

Bike parts could work but probably not for too long before they need service/replacement. I wouldn't even consider leaving the derailer on because of the added friction - an internally geared hub would be better if you need to be able to change gears.

take it easy

[joestue]

Have you actually measured how much friction is in a bicycle derailer?

I'm trying not to laugh but if that will stop your wind turbine.

I would take it off, due to the high rpm, but first I would increase the tension in the spring by a factor of at least 3 and see what happens.

One bicycle chain is probably good to about 200-400 watts. A 12 foot diameter HAWT would need at least 5 chains on a fixed 3-5:1 ratio.

I thought about doing this, but it does get a little more complex because the chains will not share the torque evenly unless all the sprockets are new and aligned properly. I don't think it would take too much effort to weld a limited slip spring coupling to each sprocket... but thats me.

[feral air]

I used to ride bikes all day, every day so I've got a strong opinion; derailers suck. That's all.

Oh, if you have to bring out the WD-40, use it as a flame thrower...'cause that's just way cooler than a rusted out chain. Er..

Instead of trying to resurrect an old rusty chain, get a new one and give it a light coat of petroleum jelly once in a while. take it easy

[QUAZ HOLT]

Feral Air. I have the same motor and what makes it good is continues duty. The 20 dollar type have a heat problem. This type of motor is best suited for R.E. applications. driving a drill press or lathe. Not the best choice for a wind generator. Bill

[Gog]

A couple of thoughts.

I used derailer chain on a 40 watt water wheel for a while

and had the links snap clean in half way more than once.

A far harsher environment than you are talking about for sure,

but I would be inclined to go for small motobike or at the very least single speed style bike chain.

As for braking , my ( to be restored ) old pumper windmill has a spring that clamps a disk

 mounted on the hub when a rope is released.

cheers

Keith

[feral air]

The $20 treadmill motor I got a couple months ago is continuous duty at 1.5hp and intermittent at 2.5hp. The only catch is that it didn't come with a fan (or flywheel). It puts out 15v at 390rpm. take it easy

[elvin1949]

Adam

  Bicycle crank bearings WON'T hold up.

If you have to gear it us motorcycle gears.

 Get them from a high hp bike.And mount the jack [prop] shaft in pillow block bearings.Then you can put a brake on the other end,should the chain break.

later

Elvin

[Tballer2]

If you want to check my diary for what I found, it may work for you also, I now have mine set upwith a starter motor gear and a ring gear off an old briggs and straton lawn mower, 8 hp motor, so far I am pretty happy with the results, also I went with the pillow block method for the blade shaft, tomorrow I will try to get a pic of what I have so far, they say a pic is worth a thousand words!