Vacuum control by changing motor speed?

[lrob]

I posted this in motor and controls forum, then realized this forum is probaly more appropriate. I have a vacuum tank that I currently control vacuum with a proportional valve (with mechnical linkages). I am looking to improve my control. I was wondering if I could control the vacuum by changing the speed of the vacuum pump motor?

 

[koalmnr]

Could you, yes. Efficeintly and quickly, no.

If you are looking to make major step changes this could be used. Just like any pump, the volume head relationship is partly dependent on pump speed. but this wil not give fine level of control, also it would probably impact your volume, which would impact the rate of attaining the vaccum pressure more than the absolute pressure, in fact (depending on the actual vauum pump) lower your total vacuum. This can be determined form the performance curve of the pump.

 

[unclesyd]

What type vacuum pump do you have?

Most laboratory pumps are sliding vane and have to run in a narrow speed range.

What degree of control are you trying to achieve?

 

[lrob]

An example of my vacuum requirements would be that my process is running at 9.1"Hg then I would adjust setpoint to 9.4" Hg. I am looking to control +/- .1" Hg. currently I am controling +/- .3" Hg with a proportional valve that has a motor driving mechnical linkage on 2" ball valve. The system is PLC(4-20 ma) controlled with PID loop.

 

[lrob]

Some additional information. The pump is a C-pump liquid sealed. The liquid being water. I am more on the electrical controls, so I hope this info is helpful.

 

[Torricelli]

Irob,

you could adjust your vacuum by using a pressure control valve instead of the 2"-ball-valve.

ET

http://www.vacuum-guide.com

 

[koalmnr]

As Torricelli commented, your problem is your control element. A ball valve with mechanical linkages is not a very sharp controller. Using a regular control vlave, you should be able to easily control (all else being capable of it) to 1 to 2 % or better.

 

[Torricelli]

...and better install this control valve (of the smallest available size) in a by-pass (not in the inlet line).

ET

http://www.vacuum-guide.com

 

[automatic2]

No, don't consider changing your pump speed. Your feedback loop would be horribly harmonic. Sounds like you want to move to a diaphram type control valve with direct feedback. You could adjust the setpoint, but let the mechanics of the valve do the controlling.