Pulley Question from a Farmer

[ars3]

Hello,
Is it possible to move a 50HP motor requiring 3600RPM with a 1HP 1800RPM motor?

I'm not an engineer but I have a big irrigating water pump that requires 50HP 3600RPM to move and I've heard of people doing it with only a 12 VDC 1HP motor by using pulleys.

Does this sound possible? If so, can someone tell me how or where I could find how to do it?

I already have the pump which I move with a gas powered 45HP engine but would love the idea of doing it with a 12 VDC 1hp motor.

Thanks,
Andrew

 

[amorrison]

What do you mean by "to move" . . .turn over? start?

 

[ars3]

Sorry, yes, to turn the shaft. The current gas motor I have came with the pump as a separate power option when I bought it. The first power option was a 240V motor.
I don't have any electricity but have plenty of 12DC batteries.

Thanks,
Andrew

 

[ExRanger]

Well if the pump uses 50HP at 3600RPM, then it takes
73 ft-lbs of torque to turn it at that speed. A 1 HP motor could be connected to a pulley system to deliver that much torque at a lower speed, but it will never be able to deliver 73 ft-lbs at 3600 RPM. So you can probably use the 1 HP motor with pulleys to run the pump at a lower speed. Just figure out what speed the 1HP motor runs at, calculate the torque at that speed, then create two pulleys with a ratio that will increase the torque to what you need to turn the pump - which will be something less than 73 ft-lbs at a speed lower than 3600 RPM.


Power = torque * speed
1 HP = 550 ft-lbs / second
3600 RPM = 377 1/sec

50 HP = (50 * 550)ft-lbs/sec = 27500 ft-lbs/sec
torque = (27500/377)ft-lbs = 73 ft-lbs


For the 1 H

 

[ars3]

I apologize for not being too knowleadgable but here's my question now:
If I increase the RPM on the 1HP could I turn the pump? I kinda get the formula but not sure how to figure it out.

Thanks,
Andrew

 

[haggis]

ExRanger is pointing you in the right direction for running the pump, as he states, a pulley arrangement to attain the output torque required would slow the pump RPM to around 140 RPM and the pulley arrangement while in theory works, would be impractical. Something like a 4" pulley on the motor and a 100" pulley on the pump.

But...the fact that the output torque of the 50 HP motor produced 73 ft-lbs of torque at 3600 RPM means this drive was designed to deliver so mant gallons of water per hour at a certain pressure. By turning the pump with the small motor at the greatly reduced speed would reduce your flow rate to trickle if it move the water at all depending on how far the delivery is.

 

[ARenko]

50 hp and 1hp are pretty far apart. Unless the pump is way oversized for your needs, it doesn't seem like it will work. Using the affinity laws (assuming a centrifugal pump) you could run the pump at 1 hp, 977 rpm, which delivers 5.4 ft-lbs. This is probably not enough torque to even turn your pump, but even if it was your capacity (gpm) would be 7% of the original and the head ("pressure") would be 27% of the original.

To get your torque up you'd have to slow the pump speed even more and you would probably end up with almost no flow by the time you got something to work.

Find out more about the other applications - I think there is a big difference between yours and anyone's who is using a 1 hp motor (maybe they have a much smaller pump).

 

[stephenweinstein]

If you are not in a hurry, you could use the small motor to slowly lift a weight (using a pulley of one size), and then let the weight drop quickly and turn the pump (using a pulley of another size) as it falls, then (using the original pulley) lift the weight slowly again, then (using the other pulley) let it fall quickly, etc.

 

[scarecrow55]

Are we talking about actual pumping with thwe 1hp, if it's only to turn the shaft surely you are only turning against the bit of water in the pump.

Can you turn it with a bar or anything?

 

[Warpspeed]

A lot depends on how the pump was originally rated for flow and pressure head. There will be a whole range of different operating conditions depending on how the pump is being used.

The original pump ratings may be for maximum combined pressure and flow. If the pump has a much easier job to do than that, then much less power may be required to get the actual pump performance you require.

See if you can get some pump curves from the original manufacturer. That will show how much Rpm and drive power is required for different combinations of flow and pressure head.

Hook it all up and see how it goes. There will be a rating plate on the motor that tells you how much current the motor will draw at full rated horsepower.

Measure the current and see how that compares to the motor rating. That will tell you how heavily loaded the motor is. If it is overloaded (too high a running current) slow down the pump. If running current is very low, speed up the pump a bit to get the motor working harder.

A small speed increase will give a fairly large change in motor current. It may be roughly cube law. So 20% more pump Rpm may be roughly (1.2 x 1.2 x 1.2 = 1.73) 73% more motor current.

As you can see motor power varies a lot with only small changes in Rpm. It may be quite possible that a 50Hp pump can run at only 1Hp and still work quite usefully.

If you cannot find any pump curves to initially get you started, jut try it and see what happens at a fairly slow pump speed. As long as you check actual motor current, there is no harm in trying.

All motors draw very high current when first starting up, it is the current draw once it has reached final steady speed that is important.

 

[Warpspeed]

If you run the pump at 1,000 Rpm instead of 3,600 Rpm to start off with, that will be fairly safe as a first guess.

1000/3600 = 0.277

0.277 x 0.277 x 0.277 x 50Hp = 1.06 Hp

Then experiment from there.

 

[sbi]

How high are you lifting the water ? from a pond over the top to a field or from a underground well 250 feet down?
are you useing a single impeller pump or a multie stage centrifical?

answer some of these and them some of the calculation will tell you that 1hp probibly will not work, it may take 5 hp just to turn the pump

SBI
Central Ne.,USA