VFD to drive a fixed displacement hydraulic pump

[szpak]

I want to use a VFD to drive a fixed displacement pump to control the speed of a hydraulic cylinder in a very wide range of pump speeds(flow) (10%-100%).
What type of a pump should I use ( gear,vane,piston)?
Any suggestions reg. manufacturer and model ?
What are the problems when running the pump at low speeds?
Any publications, articles on this topic?

Thanks

Joe

 

[ve7brz]

Hi Joe,

I'm sure there are lots of people out there who've had a great deal of experience with this, but I have to ask an obvious question. Why use a VFD in this application, unless you have a set laying around for which you can find no better use? A simple motor driving a variable piston pump might be more economical, in spite of the higher cost of the pump.

Which type of pump would be best is entirely dependant upon the particular application; it's impossible to make any recommendation without all the facts.

As to manufacturers, your decision should perhaps be influenced more by availability of service and parts than finding the absolute best manufacturer of the particular type of pump you need.

Bruce

 

[ve7brz]

Sorry Joe, I just re-read your question and realized my reply sounds contradictory, so I'll try to clarify it.

Looking at you range of volume requirement my instinct says fixed speed drive - variable displacement piston pump. If you must use a VFD or it looks like the the variable piston would be too expensive then the type still depends on the application (flows, pressure etc).

Bruce

 

[rbcoulter]

The use of a VFD is usually for an AC induction motor and is usually the best way to control pump speeds in hazardous (flammable) environments. If there is no flammable atmosphere hazards then other means of controlling pump speed may be better options (DC motors, etc.)

 

[szpak]

Thank you for your comments

In my opinion a configuration with a VFD and a fixed displacement pump is superior to any other configuration including a variable displacement pump (VDP) driven by an induction motor. I assume that both systems are configured without PID loops. The VFD option is more expensive than others due to the high price of the VFD.

There are many advantages of this system:
1. Speed (flow) control is about 25 times better than with
a VDP (VFD speed control is 0.1%,no slip)
2. Pressure control is easy by putting the VFD in torque
mode with an external torque reference. (I can not
do it at all with VDP without a pressure PID loop)
3. The drive controls both positive and runaway loads
(four quadrant operation) if I use a reversing
pump which can function as a motor. (it can not be done
with a VDP)
4. A fixed displacement pump is simpler and more reliable
than a variable displacement pump.
5. No electronics and precise components (proportional
valve with LVDT) in the hydraulic circuit.

I need to operate the pump at low speeds and this is my dilemma.???
Joe

 

[gbent]

10 to 1 is a wide range for a VFD, isn't it? A VDP will handle this range with no problem. You don't say what flow units or amounts you will use. The solution may be different for 1-10 gal/min than 10-100 gal/min.
There are a number of pressure and flow compensating systems that can be purchased complete.

 

[bhwtam]

Hi all,

Without knowing the pressure/flow characteristcs required, it is rather difficult to suggest a particular type of pump. A progressive cavity pump may be a good choice if you would like a reliable flow/pump speed characteristics to control without an actual PID loop.

The problem associated with running the pump at low speed with VFD is the possibility of overheating the motor if it is an air cooled type (one with a fan attached at the end of the motor).

The easiest way to help yourself out is to contact the pump manufacturer for the speed limits of their pumps. They do have lots of technical data (including power consumption with different motor speed, etc.)