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maytag (Industrial) (OP)
20 May 07 10:14
How much energy savings, if any can be derived from replacing a 40 hp 1200 rpm motor with a VFD motor-this is to run a hydraulic pump-system pressure is 2000 PSI and most of the time only about 15-18 gpm makeup fluid is needed.  The original system keeps the 1200 rpm constant and loads/unloads the pumps on demand(45 gallons of accumulators, fixed displacement pumps, piggyback arrangement, amp draw is about 23 amps when the 5 gpm pump is loaded and about 50 amps when both the 5 and 35 gpm pumps load). The proposed new pump would require approx. 500-600 rpm to produce the needed 15-18 gpm of makeup fluid.  
  I'm having a little trouble seeing any savings as the load remains the same-only the pump control method and the rpm change.  Thanks for any direction, advice,  Maytag
itsmoked (Electrical)
20 May 07 22:41
Of course anytime you're bypassing at the limit you're wasting money with hydraulics.  If a new system could be configured with a VFD to always run just below 'relief valve' you will have savings.  With a hefty accumulator in the system this would ease the control problem greatly.

As for the RPM aspect I agree.  I don't see any difference as your loads are defining the required power and bypassing is sizing the waste.  A slower motor just means a bigger motor and a different pump. ??

Keith Cress
Flamin Systems, Inc.- http://www.flaminsystems.com

maytag (Industrial) (OP)
22 May 07 19:59
Keith,
  Thanks for your response-I was in hope that this might generate more interest.  As far as the wasted energy from bypassing over a relief valve that is not the way this is setup-when the pumps reach a setpoint below relief valve setting they are unloaded back to tank at very low pressure(about 100 psi).  This is an old design and somewhat undersized electrically as the need for additional flow and higher pressure tax a 40 hp motor
(HP= PSI * GPM / 1714 * pump eff.)  This new approach-the VFD is one of the options being looked at.  Thanks, Maytag
itsmoked (Electrical)
22 May 07 20:23
I am surprised at the lack of response also.  There is a hydraulics forum but this question seems more motor/VFD centered in my opinion as is your opinion too, I imagine.

Keith Cress
Flamin Systems, Inc.- http://www.flaminsystems.com

dcasto (Chemical)
25 May 07 21:53
Most small motors can be used directly with a VFD.  The hydraulic system uses a PD pump an so a VFD will work.  The motor may have a lower speed limit of say 60% of max speed.  The net is you are only out a VFD, not the whole loop.
LionelHutz (Electrical)
28 May 07 15:07
From your description I would think you could potentially save the energy required to move the fluid at 100psi. So, use 100psi at whatever flow you have to calculate the power being wasted.

Hmm, maybe I don't really understand the system. Are there times the system runs on the relief valve too?
PNachtwey (Electrical)
28 May 07 17:51
"  I'm having a little trouble seeing any savings as the load remains the same-only the pump control method and the rpm change.  Thanks for any direction, advice,  Maytag"

You are right. However, a VFD can be turned of when the system is idle and then brought back to the required speed fairly quickly.

I see two main advantages.  Better response and possibly cheaper because the VFDs can be paired with a fixed displacement pump and are getting to be so cheap now relative to all the swash plate and its control.

I have customers that use VFDs and fixed displacement pumps.  They work well.  The VFD is controlled by a motion controller that also monitors the pressure.  Ultimately it would be good for the same motion controller to control all the actuators too.  Then the motion controller can use the oil demands of the actuators as a feed forward to the VFD so this part can anticipate the load instead of just respond.
maytag (Industrial) (OP)
29 May 07 22:37
Lionel, as long as the systems functions correctly relief valve setting is never reached.  As far as the energy savings relating to moving the fluid back to tank at low pressure
(100*gpm/1714) I don't see any big saving as the amp draw just to run the motor uncoupled is about the same as returning fluid at 100 psi.
  Peter on this application precise control is not a big factor-this is on a caster straightner stand-16 different stands and all with different pressure requirements-P 2 will vary from 650-800 while casting with about 15 gpm of make-up fluid needed.  I have to sell this project on energy savings alone.  Tom
LionelHutz (Electrical)
30 May 07 9:06

Quote:

As far as the energy savings relating to moving the fluid back to tank at low pressure (100*gpm/1714) I don't see any big saving as the amp draw just to run the motor uncoupled is about the same as returning fluid at 100 psi.

Well, I didn't say there would be much energy to save or that it was worth trying to save it. I was just saying that I would figure on that being the amount of energy you could save.

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