VFD or variable displacement? 1

[BrianE22]

I'm looking to purchase a hydrualic power supply, approx 25 HP. Just wondering, won't a fixed displacement pump and VFD accomplish the same thing as a variable displacement pump?

 

[micalbrch]

Yes and No. That depends upon what you want to do with the drive. Can you provide some more info?

 

[BigInch]

Mostly NOT.

"We have a leadership style that is too directive and doesn't listen sufficiently well. The top of the organisation doesn't listen sufficiently to what the bottom is saying." Tony Hayward CEO BP
"Being GREEN isn't easy." Kermit

http://www.youtube.com/watch?v=hpiIWMWWVco

http://virtualpipeline.spaces.liv

 

[BrianE22]

The HPS would power a hydraulic actuator on a test stand. So constant ouput pressure (3000 psi) for flow rates from zero to the max output. Seems like pressure feedback for the VFD would accomplish that. Of course accumulators would be used to smooth the flow.

 

[vpl]

Brian

No one will stop you from buying a VFD and a fixed displacement pump. It's your money.

Patricia Lougheed

******

Please see FAQ731-376: Eng-Tips.com Forum Policies for tips on how to make the best use of the Eng-Tips Forums.

 

[micalbrch]

You need a variable displacement pump. That's for sure. Pressure feedback to the VFD? That will theoretically work but it depends upon the speed of your actuator and about the cycles per minute or hours. Don't really think about it.

 

[hydtools]

Use a pressure-compensated variable diaplacement pump. It will vary flow to maintain a set pressure. That's what they do.

Ted

 

[hydtools]

Just a for instance:

http://www.parker.com/literature/Hy...iterature/PAVC/PAVC Complete HY28-2662-CD.pdf

Ted

 

[LionelHutz]

I wouldn't want to sell you a VFD in an attempt to do that, at least without assurances I won't be dragged into the project to help make it work. The motor speed would likely have to change very quickly to hold a constant pressure and the control loop in the VFD just would not respond quick enough.

 

[BrianE22]

So it's a time response issue. That's the info I was looking for. Thanks people!

 

[LionelHutz]

I was thinking that since the hydraulic fluid can't compress that the pump speed has to change instantaneously to keep up with and flow changes.

I didn't post that you could add a pressure storage device (or an accumulator) to help this and looking back I now see you mentioned that in your second post.

 

[BigInch]

Lionel, We'll let you off this time, but for future reference, you're not allowed to say that hydraulic (or any) fluid can't compress in the Pipeline and Fluid Mechanics forum.

If you can compress steel and other solids, compressing any fluids is relatively a piece of cake.

"We have a leadership style that is too directive and doesn't listen sufficiently well. The top of the organisation doesn't listen sufficiently to what the bottom is saying." Tony Hayward CEO BP
"Being GREEN isn't easy." Kermit

http://www.youtube.com/watch?v=hpiIWMWWVco

http://virtualpipeline.spaces.liv

 

[BigInch]

sorry this is pumps forum, but same difference.

"We have a leadership style that is too directive and doesn't listen sufficiently well. The top of the organisation doesn't listen sufficiently to what the bottom is saying." Tony Hayward CEO BP
"Being GREEN isn't easy." Kermit

http://www.youtube.com/watch?v=hpiIWMWWVco

http://virtualpipeline.spaces.liv

 

[ccfowler]

BrianE22,

I suspect that you have been uncritically reading too much VFD sales literature. VFD's have their places and can serve some needs very well, but contrary to the sales pitches, they are not the best answer to every imaginable problem. This is REALLY not a VFD friendly application. Do yourself (and everyone else that will have to deal with the device that you have apparently described) a favor and carefully consider the excellent advice offered above.

It is a near certainty that if you insist on using a VFD for this application, you will find yourself reading sales literature much more critically in the future.

Valuable advice from a professor many years ago: First, design for graceful failure. Everything we build will eventually fail, so we must strive to avoid injuries or secondary damage when that failure occurs. Only then can practicality and economics be properly considered.

 

[LionelHutz]

Biginch - in the context, ie using the PID loop of a VFD to control a hydraulic pump so it produces a constant hydraulic pressure in a system without an accumulator, the compressability of the hydraulic fluid really has no bearing.

Now, if we were discussing an air compressor...

There actually is a second problem with an accumulator and VFD controlling pressure - the system will fail to maintain a constant pressure when the accumulator reaches it's limits. Maybe using an accumulator signal feedback to the VFD could work - keep the accumulator at the middle of it's travel.

 

[BigInch]

Yes, I agree. Just a technical warning.

"We have a leadership style that is too directive and doesn't listen sufficiently well. The top of the organisation doesn't listen sufficiently to what the bottom is saying." Tony Hayward CEO BP
"Being GREEN isn't easy." Kermit

http://www.youtube.com/watch?v=hpiIWMWWVco

http://virtualpipeline.spaces.liv

 

[BrianE22]

A quick check on google shows that this discussion topic (VFD vs. variable diplacement) has been going on for quite some time. There's even a thread in the Electric Motors forum on eng-tips.

From what I have gathered so far, VFDs offer quick response to flow changes (when configured in vector control) and increased efficiency. The drawback is the initial cost.

There's even several companies that offer VFD/fixed displacement pumps as standard products.

 

[vpl]

I thought the drawback was that VFDs seldom work as expected. But, as I said earlier, it's your money. How you waste spend it is your choice.

Patricia Lougheed

******

Please see FAQ731-376: Eng-Tips.com Forum Policies for tips on how to make the best use of the Eng-Tips Forums.

 

[LionelHutz]

Brian;

The VFD will always have a certain response time to a change in flow. The VFD is monitoring pressure. When a valve opens or closes the flow changes which causes the pressure to change. The VFD then has to adjust the speed of the pump until the pressure returns to the pressure setpoint. There will be large over and under pressures while the VFD plays catch up. The higher quality the PID control loop (the better written the internal VFD code) and the ability of you to properly program it can determine how well it responds.

An accumulator can help but is not a simple plug and play solution. An accumulator could be though of as a hydraulic cylinder with a spring trying to force the ram into the retracted position (the spring tries to empty the cylinder). In your case, lets say the spring applies exactly 3000psi of pressure to the ram. The problem is that if the pump outputs just over 3000psi then it will fill the accumulator. Once the accumulator is full it can not buffer sudden reductions in flow demand (there is no place for excess fluid to go). The opposite applies if the pump outputs slightly less than 3000psi. When the accumulator is empty it can not buffer sudden increases in flow demand (no excess fluid to supply).

Now, as I posted before. If you tied the VFD feedback to the accumulator and kept the accumulator 1/2 full then it might work. I have never heard or read about this being done but in theory it might work. Basically, use the VFD to keep the accumulator 1/2 full and then the accumulator will be what actually maintains the pressure.

I think you could also use a full accumulator and an over pressure relief valve, both combined to keep the pressure within some working range.

Maybe you could use 2 accumulators, say at 3100psi and 2900psi, with one taking care of pressure surges and the other taking care of pressure dips.

It also might work if the changes in flow are gradual allowing the VFD time to keep up to the pressure changes.

There is yet another issue in that a positive displacement pump likely will not allow excess pressure to dissipate. If the flow goes to 0 and the pressure spikes, the VFD and pump could drop to 0 speed and still not drop the pressure back to the setpoint.

All of these potential and application problems are solved by simply buying a variable displacement pump.

If a variable displacement pump matches the fluid pumped to the flow demand then the VFD might add a little to the efficiency but certainly not enough to make it a valid option based on possible energy savings. Despite the general image in the marketplace that any VFD installation will save energy, a VFD is often not an energy savings device.

 

[zeusfaber]

I'm interested in what you're doing to this poor actuator that means it needs to see a constant pressure across the whole flow range.

Does the application involve multiple actuators moving independently off the same HPU?

A.