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electric motor rpm question


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we have a table saw 10 inch that is run by a 3500 rpm motor, its starting to have problems, only a 1 horse motor, small craftsman saw, we have a lot of 1750 rpm motors laying around, question is if the 3500 I horse motor has a 2 inch pulley on how big of a pulley would you need on a 1750 rpm motor, we have a few 1 .5 horse motors . thanks for help

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15 minutes ago, Mike H said:

double the size of the shiv on the motor.

Mike

Ya BUT ,  now when you start to push a board through check  the amperage under a full load .  You might find it’s not big  enough. 
 

Horse power ratings don’t mean what they used to .

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For the cost of a new motor it is likely you could buy a nice portable contractor saw, however, that said, be aware that getting the RPM out of it is only half the battle, just like climbing a hill in high gear in your truck, that saw will stall quickly and easily, 3600rpm motors are fairly inexpensive, TSC used to have a decent selection, I would suggest looking into a new saw or at least the appropriate motor before cobbling something up. Also my FIL has  a craftsman saw, his is direct drive, does yours in fact have pulleys? As @lorenzo said pay close attention to nameplate ratings. I’ll cite the 6.5hp shop vac we have all seen. Anyone who has moved around a 5 hp electric motor knows that ain’t no 6.5 hp on top of that rolling trash can. Also a quick look at an ohms law chart would tell us this is not feasible, so how do they get away with calling it so? They are using locked rotor amperage to rate horsepower, not full load amperage. It’s the equivalent of calculating a tractors top speed based on how fast it would travel if dropped out of a plane. It’s utterly fraudulent, yet somehow legal. 

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Mathematically a 4" pulley will accomplish the same thing (2 x 3.1415 = 6.283 x 2 =12.566 radius). My concern would be horsepower. The increasing load will "cube" or times three the brake horsepower. Not sure a 1.5 HP motor with a 4" pulley will handle the load. So in theory you need a 3-5 HP motor to run the load with twice the pulley diameter.

Dang! Now you are taxing my old brain. Haven't had to think about BHP and RPM for quite a few years!

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It's early and I've just started coffee but my gut reaction is that you'll be fine with the lower RPM motor and larger sheave. The motor doesn't know what it's doing - just rated HP @ RPM. Just because you essentially gear it up to make the blade turn at the RPM it needs makes no difference. The saw probably uses the 3500 motor just because they're cheap. The 1740 motor makes more torque in order to have the HP at the lower RPM. The 1.5 @1740 motor is going to be bigger, heavier and should do the job just fine.

I've got a 10" table saw with 2HP 1740 motor. My problem is that it's on a 15 amp circuit with some lights on it as well. When I load it down the 15 amp breaker will pop. It's usually only a problem when ripping 2" stock, so I haven't been aggravated enough to run a 20 amp circuit.

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current saw is not direct drive has a belt on, on this farm you seem to  acquire all these small motors saw is old , more or less for rough sawing, do have acscess to a nicer saw when needed , plus this 3600 is wired 110, would a 220 make a diffence?

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1 HP is small for a 10" saw. I don't think wiring it for 220 will make any difference as long as you don't have significant voltage drop at the outlet. I find 2 HP on mine is just adequate so I'd go for the bigger motor if it's the same frame size and simply needs a bigger pulley. If it doesn't work out you're out less than 10 bucks and your time.

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12 minutes ago, pt756 said:

current saw is not direct drive has a belt on, on this farm you seem to  acquire all these small motors saw is old , more or less for rough sawing, do have acscess to a nicer saw when needed , plus this 3600 is wired 110, would a 220 make a diffence?

Nope. Now if it is truly a 1.5hp motor I dont have a chart in front of me but my gut says your gonna need 240v.  Wiring at 120v your amperage might be too close to 20a under load. In other words if the motor draws 18a at full load your probably going to trip the breaker every time you send a board through. Wiring it 240v doesnt change efficiency or gain any power or speed 

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On 9/17/2020 at 9:53 AM, Missouri Mule said:

Nope. Now if it is truly a 1.5hp motor I dont have a chart in front of me but my gut says your gonna need 240v.  Wiring at 120v your amperage might be too close to 20a under load. In other words if the motor draws 18a at full load your probably going to trip the breaker every time you send a board through. Wiring it 240v doesn't change efficiency or gain any power or speed 

Yep, the reason for 220 is the system wiring can be smaller for the same HP rating. Same reason our bus runs on 24 VDC  starter cables and terminals and motors can be smaller and therefor less cost and weight (but they actually charge more).

I think I probably have the same 10 inch sears saw, Keep the blade sharp,  It doesn't like 2 inch hard pine but is ok with 3/4 plywood except old cdx.  The only issue I have with it is the zinc pulleys keep working loose ( I promised to replace them with cast iron 20 years ago) and the tilt mechanism has to be oiled by 5 years in the garage. Has been a good saw for the little work I have done with it, and it only cost $70 new at a Sears dent and store use sale.  I have never had it trip a 15A breaker or did I probably swap to a 20A breaker, forget!

I keep looking on purple wave for a commercial 1 phase 240 V saw (most are 3 phase) but they are too big for the little space I operate from.

The attached icon is my plate for knowledge.

image.jpeg

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17 hours ago, messer9696 said:

Mathematically a 4" pulley will accomplish the same thing (2 x 3.1415 = 6.283 x 2 =12.566 radius). My concern would be horsepower. The increasing load will "cube" or times three the brake horsepower. Not sure a 1.5 HP motor with a 4" pulley will handle the load. So in theory you need a 3-5 HP motor to run the load with twice the pulley diameter.

Dang! Now you are taxing my old brain. Haven't had to think about BHP and RPM for quite a few years!

The formula he used is what you need. Circumference is 2 X pie X radius. Which messer9696 figured to be 6.283 inches. Double that to get 12.566 circumference. As stated working backward from here gives a 4 inch pulley.

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You guys are making it more difficult than it is. When calculating the circumference, pi is a constant. We would multiply both sizes of pulley by pi so they cancel each other out. This makes it very simple. Half the speed requires twice the diameter to produce the same belt speed. 1/4 speed would be 4 times the diameter. One fifth would be 5 times the diameter and so on. Easy 

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16 minutes ago, pt756 said:

I have been using a thinner blade, carpenter got me onto them years ago.

Same here.Thin blade is on it instead of thick combination blade. I still need a dedicated 20 amp circuit.

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At 240v the motor will run cooler, it will also run at half the amperage and experience less voltage loss, not only does this allow it to start easier, it allows it a significant “buffer” for minor overload (like a dull blade or difficult piece of wood) before tripping. 
i maintain my position, get the motor it is intended to have, preferably one with Integral thermal overload protection. 

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