Change motor speed for vacuum control?

[lrob]

I have a project to improve the control of a vacuum tank in my process. Currently I have a proportional valve that opens and closes based on vacuum setpoint. My question is can I change the speed of the vacuum pump and get the same or better results? I have 3 phase 460V motor.

 

[jraef]

Generally, motor speed control often can be used to replace proportional valve control, but not all machines are created equal. It depends upon the type of vacuum pump technology. That question might be better dealt with in the Mechanical Eng. or Pump Eng. forums. If the experts there say that your type of VP will work with variable speed, pose it back here for advice as to how best t0o accomplish that.

"Venditori de oleum-vipera non vigere excordis populi"

 

[jbartos]

Suggestion: There are several ways to change the speed of the motor, e.g. via:
1. VFD; however, it needs a inverter type duty aligned with the pump
2. Motor input voltage regulator, e.g. autotransformer/variac
3. Electromagnetic coupling, e.g. MagnaDrive (if the motor-pump alignment permits it)
However, it is better to consult the equipment tech support to have it approved

 

[PwrCntEE]

I agree that you should get a mechanical opinion. I will offer my thoughts, just thinking aloud. Depending upon the amount of range you need and the min/max targets will most likely determine the easy/cost effective way (not that one is much more dificult than the other).

My experiance with the valve approach is poor control. Basically ends up with an off/on effect. Meaning that I could get a little change on the high end, but could not control very well on the low side. My guess is that a pump control would provide better control at both ends, but I have never had the need for that much control.

Might be worth a call to a rep for you application. Filter though the sales pitch and review thier literature.

 

[bentov]

We routinely retrofit dairy operations with VFD's and vacuum transducers to control their vacuum pumps, fairly simple and relatively inexpensive, generally provides substantial engergy savings for them . . . just depends on your performance requirements (i.e. how closely you must hold to a setpoint and how quickly the system must respond).

 

[rmw]

Here is a mechanical opinion. You can absolutely control vacuum (no pun intended) by varying the speed of a vacuum pump. The selection of the method is up to you electrical guys. I am a mechanical guy, I prefer steam turbines.

rmw

 

[jbartos]

Suggestion: If electrical speed drives do not fit well, consider mechanical speed drives, e.g.

http://www.andantex.com/speedcon.html

 

[jraef]

Based upon rmw's statement then, replacing your proportional valve control should be fairly simple. Look for a VFD that has PID (or at least PI) loop control capability built in. Take your vacuum measurement (assuming you have a transducer of some sort now) into the PI control loop as the PV (Process Variable). Program your other functions as appropriate to the VFD manufacturers instructions and away you go. There are other considerations such as distance from the drive to the motor, effects of harmonics on your plant environment, ambient conditions etc. that need to be investigated of course.

One major pitfall to be aware of though is lubrication of the vacuum pump. Check with the manufacturer to assure that lubrication is not speed dependent. If it is you may need to consider upgrading to a separately powered lube system for it.

"Venditori de oleum-vipera non vigere excordis populi"

 

[jbartos]

Suggestion: For process control and PID, see reference (somewhat aging but the theory and applications hold):
Yasundo Takahashi (died in 1996), Michael J. Rabins, David M. Auslander "Control and Dynamic Systems," Addison-Wesley Publishing Company, 1972