Variable speed vacuum pump

[diyfarmer]

I am trying to figure out a way to make a constant speed vacuum pump variable speed. The pump in question is used on a dairy farm and is sized for maximum vacuum requirements. Because maximum capacity is rarely required, the pump is often sucking air through the regulator, wasting a lot of electricity. Currently, the pump is belt driven by a 10hp single phase motor. The obvious solution, which is currently used on some farms, is to install a 3 phase motor with a VFD that will run off single phase. A pressure transducer is used to control the VFD.

The main drawback of this is if the VFD ever quits for some reason, the farm is SOL because there is no way to run the three phase motor off single phase. At 5 in the morning on a long weekend, the cows will not be happy waiting for a new VFD.

After some searching, I've concluded there is not really any single phase output VFD's available. Just an aside here though, could a single phase VFD work, if it was only brought in after the start winding was disconnected. ie, start the motor normally, but once the start switch opened, switch the supply to the VFD?

I also thought of using variable pulleys, controlled by a vacuum cylinder. As the vacuum level increased, the cylinder would "reduce" the size of the driving pulley, slowing the pump down. After more searching, it appears that pulleys like this are not very common. I'm also not sure how quick this system would respond or if it would oscillate at all.

My last possiblity would be to add a large vacuum resevoir, and put a simple pressure switch on the current vacuum pump, much like any air compressor. Not sure how big the tank would have to be to avoid constant cycling, and starting the motor often would likely waste the potential savings.

I'm drawing a blank now, does anyone know of other possibilities? I would prefer not to change the pump and it needs to be very reliable, or at least have a "limp mode" that would still allow the cows to be milked if something failed.

Thanks in advance for any responses!!!

 

[eromlignod]

Since it is a pulley drive there might just be a simple solution. How about a Lovejoy variable-speed pulley drive? They are used on all sorts of machinery. The driven pulley is a deep V-groove that has an adjustable width. As you crank the pulley wider, the groove opens up and lets the belt seat deeper, reducing its pitch diameter and speeding up the driven shaft. You have to adjust it while it's moving though.

There may be several options to adjust the speed of your motor, depending on what type it is. But most of those methods are inefficient and if it's electricity costs that you're worried about, they probably wouldn't help much. Is it a wound-rotor motor (with brushes)?

Don
Kansas City

 

[btrueblood]

The farm already owns a single phase motor and contactor. Add in your fancy new VFD+3ph motor without eliminating that existing fallback. Create a drive belt system with a pony shaft that lets the existing motor sit there, with the belt looped over the shaft but not attached to the motor pulley, or let the existing motor spin along. If the VFD ever faults, just fire up the old motor (making sure the vacuum regulator is re-commisioned too, if it was planned for deletion (I wouldn't)).

 

[Compositepro]

1. A vacuum pump with no air flowing through it is under no load so the motor is just idling and drawing little power.

2. There are different ways to regulate vacuum. One is to bleed air into the vacuum when there is too much vacuum. This loads the motor and is not energy efficient. This is used when pumping condensible vapors that you don't want condensing in the pump. Or a vacuum regulator can be a valve that closes the intake of the vacuum pump. When there is little to pump the valve will be closed and the motor will not use much energy. It sounds like you need to change from option one to option two.

3. Varying the speed of a vacuum pump is almost never done because one of the options above is usually a better choice.

4. VFD's will not work with most single phase motors because the motors have starters like centrifugal switches, etc. However there are vfd's that can operate on single phase and run 3-phase motors.

5. You can use a D.C. motor for variable speed.

 

[MintJulep]

A couple of automotive AC clutches and maybe an idler shaft and you could rig a two-speed setup pretty easily.

 

[diyfarmer]

Thanks for the responses so far!!

Eromlignod, I don't think the pulley you mentioned will work, because I need something that can be adjusted asap in response to changes in vacuum load, which are quite frequent.

I like the idea proposed by btrueblood, as long as the pump is ok, there are two potential power sources. I agree with leaving the regulator in, I would just set it slightly higher (lower?) to act as a safety.

Compositepro's idea of closing off the intake is very simple and would be much cheaper to implement than a VFD setup. Can choking the intake negatively affect the pump? The exhaust is already quite hot (just asking for a heat reclaimer, but that's another topic), would choking the intake build more or less heat? I'm fairly sure it's a lobe pump. Could you elaborate on why vapors would condense in the pump? Would this be a problem with atmospheric air?

I thought of something similar to MintJulep's idea by using a two speed motor, and switching back and forth between the two speeds. I think the constant switching would be quite hard on drivetrain components though, and accelerating the rotating mass frequently would waste energy.

Thanks again, and keep the comments coming!!

 

[badservo]

I don't know if it is true, but have a look at Valvecrazy's post here

http://www.eng-tips.com/viewthread.cfm?qid=203504&page=1

 

[btrueblood]

Uh..."A vacuum pump with no air flowing through it is under no load so the motor is just idling and drawing little power."

Not quite. The motor is loaded by BOTH the differential pressure and the flow (in fact PxV) across the pump, which can be highest when the vacuum is maximum - because tanks and lines leak and leak the most at high vacuum, and all pumps have in-built leakage or "slip", so there will always be some leakage-derived "flow" across the impeller/piston.

I think it would be safer to say that "variable-speed vacuum pumps are not used more often because of the cost of implementing the variable speed", not because valving is somehow more efficient.

 

[Compositepro]

It would be good to know what kind of pump we are dealing with. A Roots blower would not normally be called a vacuum pump nor is it a good choice except for contiuous low vacuum and high flow. Such a blower would indeed use less power if the inlet and outlet are unrestricted, resulting in high flow but low delta P.

In response to the original post about "you can't run a 3-phase motor if the single phase vfd breaks", there are phase converters that can make 3-phase from single phase. But it sounds like you may need a different vacuum pump.

A Roots blower used in this application would always have to run at maximum power even if there is no need for vacuum.
Variable speed would save energy in that case.

 

[diyfarmer]

More good responses, thanks to all that posted.

I did not consider the possible effects of a VFD on animal health, that is definately something to consider, as the slightest negative effect would nullify any energy savings pretty quick.

I'm 95% positive it's a lobe pump, and changing it would be more work than it's worth.

 

[berkshire]

diyfarmer (Mechanical)
I have not been around this stuff since I was a teenager ( My dad was an ALFA LAVAL service rep.) But I recall that on some farms they would have 2 pumps, a large one and a small one. If they just had 3 or 4 cows to milk the small pump would run, then when the whole herd came in the big pump would run. The systems were set up for the small pump to run after the big one kicked off.
B.E.