question about torque vs RPM/HP

[Max_Andrews]

I'm doing a project involving an electric motor, and have the option of either using a 2.5HP motor from a saw or a 2.5HP motor from a router. The saw spins at 4000RPM and draws 16 amps at 120V. The router spins at 23,000 RPM and draws 15 AMPs at 120V.
If I gear both motors down to 2000RPM (2:1 for the saw, 11.5:1 for the router), will there be any difference in torque between them? Reason I aks is that the saw is $50, the router is $100
Thanks!

[nick29498141]

> I'm doing a project involving an electric
> motor, and have the option of either using a
> 2.5HP motor from a saw or a 2.5HP motor from
> a router. The saw spins at 4000RPM and draws
> 16 amps at 120V. The router spins at 23,000
> RPM and draws 15 AMPs at 120V.
> If I gear both motors down to 2000RPM (2:1
> for the saw, 11.5:1 for the router), will
> there be any difference in torque between
> them? Reason I aks is that the saw is $50,
> the router is $100
> Thanks!

In this case, dealing with the given info, Torque (T) is equal to...

T=(HP*5,252)/RPM

So, in theory, two motors with he same HP and the same RPM will produce the same rotational force (T).

This is all theory, in the real world, one may be able to hold onto a speed better, one may be more efficient, ect, ect.

But using that equation, I'd say you're good with either.

NK

[Jeff B.]

> In this case, dealing with the given info,
> Torque (T) is equal to...

> T=(HP*5,252)/RPM

> So, in theory, two motors with he same HP
> and the same RPM will produce the same
> rotational force (T).

> This is all theory, in the real world, one
> may be able to hold onto a speed better, one
> may be more efficient, ect, ect.

> But using that equation, I'd say you're good
> with either.

> NK

No, there is a big difference in torque. Your equation is correct for lb-ft of torque, so lets back the torque out of the ratings:

The saw is rated at 2.5HP at 4000 RPM

and the router is also rated at 2.5 HP but at 23,000 RPM

The formula for Torque is:

T (lb-ft)=(HP*5,252)/RPM

For the Saw it is:
T= (2.5 * 5252)/4000
T= 3.28 lb-ft of Torque

For the Router it is:
T= (2.5 * 5252)/23000
T= 0.57 lb-ft of Torque

The torque of the Saw's motor has 5.75 times more torque. This is why the router is rated at the same HP, but must spin much faster for that rating.

Jeff B.

[nick29498141]

Point taken. But in his post, he said that he would be gearing them both down to the same RPMs.

Then one is theoretically equal to the other.

NK

[Jeff B.]

> Point taken. But in his post, he said that
> he would be gearing them both down to the
> same RPMs.

> Then one is theoretically equal to the
> other.

> NK

But at the same RPM they are not the same HP or Torque, pick your RPM the same 5.75:1 ratio will still stand.

[mikec]

Go with the saw motor unless size is a problem.

The bigger motor will be less likely to stall, lose less power in the gear reduction and be easier to listen to.

Also it will have a standard output shaft.

But if space/weight is a constraint you might be better off with the router.

If you really want torque look at a 1/2" drill, especially the old "spade grip" ones. They are SLOW but will spin the world if you don't hang on!

Give us a few more details and we might be able to help more

________
California Medical Marijuana Dispensaries

[mike1234]

There are other issues to consider though. How quickly do you need the motor to wind down and stop? The ultra-low gearing of the router motor will likely cause a slower stop time... it'll also be much noisier. Does the saw motor have an electric brake? You might want that. Plus you need to consider the life of the bearings and brushes, which will be markedly shorter in the router motor.

Just some random thoughts...

> Point taken. But in his post, he said that
> he would be gearing them both down to the
> same RPMs.

> Then one is theoretically equal to the
> other.

> NK

[nick29498141]

That's why I said "theoretically"

NK

[mike1234]

...no offense, Jeff.

[mike1234]

[nick29498141]

10:35... amazing... lol

NK

[Max_Andrews]

Thanks everyone!
Well for more info about the project, as many of you know I live in New York City, making it impossible for me to afford a car. I don't need a car for most of my traveling, but some sort of motorized transportation would be fantastic for lugging project stuff around (the nearest hardware store isn't anywhere close by, so getting 100 pounds of MDF from place to place is not an easy task), as well as for quick jaunts around the city.
Being the industrious fellow that I am, I thought I'd have a try at building a small, light, single-person electric go-kart thingy. AC motors are attractive because of their low cost and weight to performance ratio, but after looking up the cost of high power inverters it's not really worth it at all (if using four 1.8kw motors, I'd need a $700 inverter...no way jose). So I spent the last few hours looking up DC options instead.
I've found two options:
C40-300 neodymium 24V magmotor, 4.5HP, 20ft-lbs, and only 12 pounds weight, but unfortunately $350 each. So two of these would be $700, and my budget is more like $1000 for the whole kit and kobbodle, I'll have to think about this (like whether I prefer a full stomach or a fast go-kart haha). Despite the high cost I'd still get twice the horsepower and spend less than using AC motors and an inverter. These are also easy to gear because at 4000rpm a 2.5:1 ratio chain and sprocket would do the trick. <A HREF="http://www.robotcombat.com/marketplace_magmotors.html">http://www.robotcombat.com/marketplace_magmotors.html</A>

Other option is four of the 24V dewalt hammerdrill motors, which put out 2HP and 8ft-lbs (16ft-lbs stall). Two would be geared to a single shaft on each left and right rear wheels, making them behave like a 4HP motor with 32 ft-lbs of torque (before gearing). They are a lot cheaper at only $55 each, so for four of them I'd be spending $220 rather than $700 for the magmotors, and I'd get a bit more torque. The downside is that they spin at 21000rpm, needing a much more severe gearing down to 2000rpm (unless I want my go-kart to go 200 miles per hour haha). But all said and done they are a LOT cheaper. <A HREF="http://www.robotcombat.com/marketplace_dewalt.html">http://www.robotcombat.com/marketplace_dewalt.html</A>

Which would you choose?

As for the other details, the frame I'll make from heavy duty cardboard tubes leftover from our print reels at work, wrapped in a few layers of fiberglass and impregnated with epoxy. The suspension will be based on the lotus elise: <A HREF="http://www.sandsmuseum.com/cars/elis...ion/index.html">http://www.sandsmuseum.com/cars/elis...ion/index.html</A> with the parts coming from a junked ATV I should be able to find in a junkyard. Power will be provided by four or so car batteries that I'll charge each night. If I use the cheaper motors I should be able to do it for under $1000
Total weight without the driver should be under 400 pounds, giving this thing more torque than a high performance sports car on a pound for pound basis.

Cool idea or silly?

[Max_Andrews]

for $1000, I could use one of these 34HP montrosities: <A HREF="http://www.robotcombat.com/marketplace_motors_perm.html">http://www.robotcombat.com/marketplace_motors_perm.html</A>

It produces 200 foot pounds of stall torque! Ahhhhhh!

[mike1234]

You'll need some sort of controller even with DC. Otherwise you'll burn rubber everytime you hit the go button. This is a serious issue with a high torque setup. You won't be able to control your acceleration with just a huge switch or relay. You could have an accident or, even worse, injure someone.

If you build it well and add a hefty speed control you just might have a very cool transport. Oh... but stay out of the rain. Water can and will find its way to the cardboard tube frame even if you seal it REALLY well.

Umm... and make sure NY will let you drive it where you need to go. I assume you'll be driving it on the streets?

> Thanks everyone!
> Well for more info about the project, as
> many of you know I live in New York City,
> making it impossible for me to afford a car.
> I don't need a car for most of my traveling,
> but some sort of motorized transportation
> would be fantastic for lugging project stuff
> around (the nearest hardware store isn't
> anywhere close by, so getting 100 pounds of
> MDF from place to place is not an easy
> task), as well as for quick jaunts around
> the city.
> Being the industrious fellow that I am, I
> thought I'd have a try at building a small,
> light, single-person electric go-kart
> thingy. AC motors are attractive because of
> their low cost and weight to performance
> ratio, but after looking up the cost of high
> power inverters it's not really worth it at
> all (if using four 1.8kw motors, I'd need a
> $700 inverter...no way jose). So I spent the
> last few hours looking up DC options
> instead.
> I've found two options:
> C40-300 neodymium 24V magmotor, 4.5HP,
> 20ft-lbs, and only 12 pounds weight, but
> unfortunately $350 each. So two of these
> would be $700, and my budget is more like
> $1000 for the whole kit and kobbodle, I'll
> have to think about this (like whether I
> prefer a full stomach or a fast go-kart
> haha). Despite the high cost I'd still get
> twice the horsepower and spend less than
> using AC motors and an inverter. These are
> also easy to gear because at 4000rpm a 2.5:1
> ratio chain and sprocket would do the trick.
>
> <A HREF="http://www.robotcombat.com/marketplace_magmotors.html">http://www.robotcombat.com/marketplace_magmotors.html</A>
> Other option is four of the 24V dewalt
> hammerdrill motors, which put out 2HP and
> 8ft-lbs (16ft-lbs stall). Two would be
> geared to a single shaft on each left and
> right rear wheels, making them behave like a
> 4HP motor with 32 ft-lbs of torque (before
> gearing). They are a lot cheaper at only $55
> each, so for four of them I'd be spending
> $220 rather than $700 for the magmotors, and
> I'd get a bit more torque. The downside is
> that they spin at 21000rpm, needing a much
> more severe gearing down to 2000rpm (unless
> I want my go-kart to go 200 miles per hour
> haha). But all said and done they are a LOT
> cheaper.
> <A HREF="http://www.robotcombat.com/marketplace_dewalt.html">http://www.robotcombat.com/marketplace_dewalt.html</A>
> Which would you choose?

> As for the other details, the frame I'll
> make from heavy duty cardboard tubes
> leftover from our print reels at work,
> wrapped in a few layers of fiberglass and
> impregnated with epoxy. The suspension will
> be based on the lotus elise:
> <A HREF="http://www.sandsmuseum.com/cars/elis...ion/index.html">http://www.sandsmuseum.com/cars/elis...ion/index.html</A>
> with the parts coming from a junked ATV I
> should be able to find in a junkyard. Power
> will be provided by four or so car batteries
> that I'll charge each night. If I use the
> cheaper motors I should be able to do it for
> under $1000
> Total weight without the driver should be
> under 400 pounds, giving this thing more
> torque than a high performance sports car on
> a pound for pound basis.

> Cool idea or silly?

[Max_Andrews]

Yeah I'm thinking I'll have to up the budget a bit which means delaying it for a while, A good speed controller is about 500 bucks.
Here's a guy who built something very similar: <A HREF="http://www.youtube.com/watch?v=CykCx1xLEzQ">http://www.youtube.com/watch?v=CykCx1xLEzQ</A>
Mine would be a bit larger to have some cargo space in the back, and would have a roll bar, sort of a mix between an ATV and a go-kart.
And yeah I'll have to see what I'd need to do to make it street-legal, it may not even be possible but it's the weekend and I'm bored

[mike1234]

If you can find some huge high-current wire wound ceramic resisters of appropriate values, you could build a multi-step speed ontrol with a multi-position switch and massive relays. If you're not familiar with the resisters of which I speak, these are several inches long and up to an inch in diameter with the large guage wire visibly wound around the exterior and then sealed.

It's just a thought. A true variable speed control is a much better solution.

> Yeah I'm thinking I'll have to up the budget
> a bit which means delaying it for a while, A
> good speed controller is about 500 bucks.
> Here's a guy who built something very
> similar:
> <A HREF="http://www.youtube.com/watch?v=CykCx1xLEzQ">http://www.youtube.com/watch?v=CykCx1xLEzQ</A>
> Mine would be a bit larger to have some
> cargo space in the back, and would have a
> roll bar, sort of a mix between an ATV and a
> go-kart.
> And yeah I'll have to see what I'd need to
> do to make it street-legal, it may not even
> be possible but it's the weekend and I'm
> bored

[Max_Andrews]

would supercapacitors such as this one have any significant performance advantage?
<A HREF="http://www.prestomart.com/results.php?search=BCAP1200">http://www.prestomart.com/results.php?search=BCAP1200</A>

[mike1234]

...he'd need nine of those to attain the 24 volts he needs... $675 plus shipping.

> would supercapacitors such as this one have
> any significant performance advantage?
>
> <A HREF="http://www.prestomart.com/results.php?search=BCAP1200">http://www.prestomart.com/results.php?search=BCAP1200</A>

[Jeff B.]

> ...no offense, Jeff.

Nick is correct. I completely missed the part about gearing down to the same RPM and in my mind had the motors simply turned down to lower RPMs. If they are at peak power and geared down then the torque multiplier is in the gearing and the torque will be the same. Sorry Nick, I missed that crucial point.

Jeff

[mike1234]

I've been called a lot of things but "a curse" is not one of them. Thank you for expanding my repertoire, Nick :^)

> 10:35... amazing... lol

> NK