pump design help

Karl, Since you are drawing 2 amps that sounds like it's truely operating at

1/2 hp.

I have heard of "constant torque" VFD's. Never seen "constant horsepower" in the literature.

So, since hp =3D rpm x torque, if you cut the rpm in half, you are going to get only 1/4 hp.

I think the best bet is to get the pulleys for the 3.5 reduction, and use a 1 hp motor and VFD. Or, if $ is most important, rig up a jack shaft, for a secondary reduction.

If you go to the 1 hp & VFD, it's best to get an "inverter duty" motor. My motor man tells me the insulation on older/standard motors will break down over time. I have one "inverter duty" motor on a VFD with no probs, one unknown on a VFD that has been fine for years (intermittent duty), and had one old motor that wound not work on a VFD.

Dave J.

Sounds reasonable to me. Lower pitch is like more reduction on the pulley. If the noise comes from the cavitation then the lower pitch will solve it. If the noise comes from vibration or gear noise just from turning 800RPM then lower speed won't help at all, of course.

How noisy is it with no prop?

I need to run lower RPM to get rid of the cavitation. I'll need a higher pitch to still move enough water. My question is do I need a larger motor and VFD.

Karl

Cavitation comes about because the pressure on the lifting surface of the prop blades falls below the vapor pressure of water*, and the water "boils".

For a given pressure differential across the prop, a higher pitch prop is going to have to generate more lift, because it'll be working at a less advantageous angle.

All else being equal, if playing with pitch and speed without adjusting diameter will get you there, a lower pitch, higher speed prop should be the way to go. Theory says you should be able to do this with a larger diameter prop, to get your volume at a lower pressure differential followed by a nozzle to get the pressure up -- but this is where the fluid dynamics part of my brain starts flashing a red light next to a sign that says "ask a real hydrodynamicist". So I may try it myself at home, but if I were going to take your money to try it I'd be playing with scale models at a wet bench -- and I'd start by advising you that you'd spend your money more effectively with a real hydrodynamicist.

I'd suggest that you consult a boat racing site or expert, because they deal with cavitation all the time. But expert racers can have notions that are quite divergent from reality; myths and legends that were the best that science could come up with in 1908 somehow get carried forward to 2008 as if hundreds of physicists and mechanical engineers hadn't spent their entire careers improving the state of the art.

So, consult a boat racing site or expert, but bring your salt shaker.

• Ya, ya -- I'm simplifying. So sue me.

My thoughts are reduce pitch to reduce cavitation. Increase diameter to increase flow. Since you are running in a tube, increasing diameter might be difficult. So another off the wall idea would be to run two props on the same shaft. Maybe with the first prop slightly lower pitch than the second prop. This is likely to be a poor idea as I am more of a EE than ME. I am sure someone will pipe up with why this is a good or bad idea.

Dan

I might start with the VFD. If it's cavitating I'd expect it's being driven too fast, and slowing it down to stop the cavitation may still give you the same flow since the cavitation isn't helping the flow.

It may work if you run vanes to straighten the flow between stages.

But it'd take a ton of work (or an expert) to tell you for sure, and how to do it in a way that wouldn't just compound your problems, instead of simplifying them.

So Karl, how much time do you have on your hands for experimentation?

Call amusement ride builders like Arrow Dynamics, S&S or Vekoma, or your local amusement parks that have these ride systems - does the Mall Of America have a log flume or 'Tunnel Of Love' style dark rides?

The standard propulsion method for the boats in a flume ride is a small propeller in a cast aluminum or bronze vortex cone, direct coupled to a submersible well pump motor. Four stand-offs between the vortex base and the mounting bolts on the shaft end of the pump, debris screen optional. Creates a nice current, and motor cooling is never an issue.

And they do make inverter duty submersible well motors.

-->--

Na, I'm not totally re doing something that has run 20 years. But I'd like to try a different prop - it might be better. If not, I'll go back. My only question is if this motor will have enough power at 1/2 speed. Do you know how to figure that?

It likely will be fine. You could also change the pulleys to get the ratio you need. Indeed, you could find the correct speed using a VFD and then replace the VFD with the correct pulleys to get the prop speed you need at the full motor speed and put the VFD on the shelf for the next project.

Indeed.

Ive had the same issues with 3ph fans..run them too fast and they simply stop pushing air, so had to throttle them back to a lower speed and voila! they started pushing air again.

"Obama, raises taxes and kills babies. Sarah Palin - raises babies and kills taxes." Pyotr Flipivich

First, the mass of water moved and the head determine the hp, ignoring frictional losses, so gearing down and changing the prop pitch to move the same amount of water should require the same hp so I'd start with the motor you have now if you gear down with pulleys. If a vfd gives constant torque but now the motor is running slower, then it will develop less hp so you need a bigger motor. hp=torque*rpm/5252 in sae units, so right now at .5 hp at the motor you are getting 1.52 ft-lbs at

1725 rpm. If you wanted .5 hp at 863 rpm that means you need 3.04 ft-lbs of torque, and if you want to get that by running a 1725 rpm motor at half speed with a vfd that would be a motor that makes 3.04 ft-lbs at 1725 rpm or a 1 hp motor. Clear as murky mud? :-)

-- Regards, Carl Ijames carl.ijames at verizon.net

You mean mass flow rate and head determine the HP, ignoring all efficiency contributors.

By my calculations you should need 1 lbf-ft/sec to move 100gpm at 1" head, or about 1/550th of a horsepower.

At 100% efficiency he should be able to gear down a gerbil to do this;

0.5HP should be overkill by a factor of about 270:1.

(1" head)(.43 psi/" of head)(144 sqin/sqft) = 5.2 lb/sqft of pressure.

(100gpm)(0.125 cuft/gal)(1min/60 sec) = 0.208 cuft/sec

(5.2 lb/sqft)(0.208 cuft/sec) = 1.09 lb-ft/sec.

So how much noise _does_ 0.499HP worth of cavitation make?

If you need 1/2 Hp delivered to the prop, then you need a 1 Hp rated motor at 30 Hz to deliver 1/2 Hp. Below rated speed, all induction motors are constant torque. Above rated speed you get constant HP output. So, running a 1/2 HP motor at 30 Hz, it will pull rated current at only 1/4 Hp, and long operation at 1/2 Hp load will smoke it.

Jon

I really doubt you can make an outboard motor prop cavitate with a 1/2 Hp drive. It takes real HP input to the prop to do that. My guess is it is just sucking air due to turbulence, I don't know what sort of deflectors you are using, maybe some screens one inside the other would stop the rotation. Making the intake the largest diameter you can would help the air to not be trapped in the water.

Jon

I can guarantee you will not get 1/2 Hp out of a 1/2 Hp-rated motor at

30 Hz. You will get 1/4 Hp at rated load. It scales linearly with frequency (speed) below the rating point. If you try to load the motor to the rated mechanical output, it will draw twice the rated input current (roughly) and smoke.

Jon

In this case available torque is more or less proportional to current. Hence the need to halve the HP when you halve the speed.

Before you change the prop, try wiring in the VFD and seeing if you can get a decent flow at a lower speed with the existing setup. If it's either cavitating or (worse) pulling in air, it may well pump better at lower speed anyway.

Is the lead in to the prop properly streamlined and gently divergent? Pressure drop before the prop is your enemy. Higher net-positive-suction-head is your friend to avoid cavitation.

Mark Rand RTFM