Fellow Listmembers,

We need to control the speed (inclusive of a good acceleration
and deccelaration) of our hydraulic powered bottle filler and three (3)
proposals were being evaluated and are as follows:

A. Installation of a proportional flow valve and controller on
the existing and very reliable
hydraulic system.

B. Modification of entire drive system using a direct-drive
motor with a VFD;

C. Install a VFD for the existing 30 Hp drive motor for the
hydraulic pump to control the speed.

If you were in my position which one would you choose? Please
state pros and cons.

 

Hello Jose,

Here are my opinions about the following:

> A. Installation of a proportional flow valve and controller on
>the existing and very reliable
> hydraulic system.


Pro: Probable the cheapest solution.
Quick response time.
Con: Motion will not be as repeatable not accurate.

> B. Modification of entire drive system using a direct-drive
>motor with a VFD;


Pro: If Motor is matched to the inertia of the load for excelleration
required, then it could be the most accurate method.
Con: Expensive and requires more of engineering (i.e. time) than method A.


> C. Install a VFD for the existing 30 Hp drive motor for the
>hydraulic pump to control the speed.


I wouldn't even consider this. Too much inertia. Plus I don't think that
the
pump would have a nice linear flow-to-speed ratio.


Just for curiosities, can you explain how the current system operates?

Dan B.

 

José,

If you take this solely on economic considerations, then it depends on
where you are. If you are in the US, it is probably best to replace the
hydraulic system with the VFD. Hydraulic systems, except on mobile
equipment, are becoming rarer primarily because of EPA regulations. They
also cost more initially and they risk being declared a National
Environmental Emergency if you have an oil leak or spill. VFDs have become
inexpensive and reliable.

The proportional valve and controller sound like the least expensive
solution. The only difficulties with proportional valves are non-linearity,
hysteresis, and the lap of the valve causing a "bump" during opening and
closing of the valve. If you're currently just using a bang-bang valve, and
the speed regulation only needs to be relative, these should not be galling
problems. A good controller/ramp generator with dither normally reduces
these to an acceptable level anyway. A servo valve, although initially more
expensive, will be a better choice if any of these considerations could
create a problem in your installation.

My only comment on C is, if you are going to buy a VFD to run the pump, why
not just use it instead of the hydraulic motor?

I hope this helps somewhat.

Willy Smith
Numatico SA
Costa Rica

 

We use for our Stripping Machine (Copper Operations) 5 proportional flow
valve and controller (VICKERS) with good acceleration and deceleration)
normally a Hydraulic system are very reliable.
The alternatives B & C are very expensive and complicates

Regards
Eduardo Ku

 

The fastest and probably cheapest solution is to just change the
existing valve from directional to proportional. You do not state if you
feel the need to close the loop (accuracy), so there will be some
variation in speed and acceleration with the proportional valve (+ 1%)
in an open loop application.

 

A:
Likely to work the best since you are only changing the controls on an
existing, well functioning mechanical system.

B:
VFD is probably a BAD choice. Can't get the performance out of such a
system. A servo motor system would probably work fine.

C:
Bad news. don't even try such a thing. there are so many bad things with
this idea that i can't even start.

(A) seems the least likely to give you serious headaches. (B) using a servo
system is probably Ok but sizing it properly could be a pain. I am presuming
this is a high performance type system. before i made any serious
suggestions I'd want to know a whole lot more though.

 

I vote for "A". There's an old saying: "If it's working, don't fix it". If the hydraulic system is working and you can put up with the leakage, filtering, and noise, why change?

Alternative B may seem cleaner, but you may not get the accelerations you are presently enjoying. This is because as soon as a flow valve is cracked, the maximum pressure is immediately available for acceleration. With electric drives, there is a performance lag depending on the drive design. Also, when you say direct drive, that implies that you are going to try to save the cost of a gear reduction. Be
sure the lost inertia isolation (ratio squared) isn't significant and remember that torque available at the load will also be reduced
by the ratio. It will also have detrimental effects on resolution and stiffness, but these may not be as important.

Alternative C is a no-no, especially if you are closing a velocity loop because there is too much "trapped oil" which will make the hydraulic resonance low and make it very difficult to stabilize any type of loop.

Tom Bullock
President
Bull's Eye Marketing, Inc.
Industrial Controls Consulting Division
[email protected]

 

Hi, All:

I'd check out the proportioning valve, since it lets you keep pretty much everything that's working now.

If you need to look at plan B, variable frequency is hard to beat. The only application where hydraulics outperforms it is in positioning where
motor inertia may prevent really quick stops.

With a VFD, remember the application rules:

1. make sure motor torque is adequate for load at all speeds
2. use input filtering to protect your power system
3. make sure the output protection addresses motor lead length
4. beware of overheating the motor at low speeds
5. keep a spare VFD
6. remember that a new VFD duty motor will have much greater resistance to VFD voltage waveform nasties than your old motor.

All the best!

Rick...................