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  1. #1
    BadBeta is offline Stainless
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    Default VFD Electromotor driving hydraulic fixed displacement pump? Any experiences?

    A hydraulic servovalve setup is expensive, so I tried to think of alternatives.

    It occurs to me that a VFD driven electromotor driving a hydraulic fixed displacement pump shaft could possible give the same or even better volume regulation for less money? Some must be using this already, or at least have tried it? What are the experiences and suitable applications? (If any?)

  2. #2
    robmc is offline Cast Iron
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    How are you going to control the VFD. I think if you don't want servos or a load sensing variable displacement pump and have pressure or volume sensors
    that it could talk to the VFD then it might be a viable solution. The constant displacement pump is a definite cost saving over load sensing variable displacement pumps but the VFD and sensors vs. servos is something you'd have to research as far as available components and cost is concerned. I don't have any first hand knowledge of that type of system.

  3. #3
    BadBeta is offline Stainless
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    I'm thinking about running hydraulic winches this way. I might not even need any feedback. If the pump is fixed displacement, and the hydraulic motor is fixed displacement, I should be able to come pretty close to any wanted winch rotation speed just by sensorless vector control on the inverter drive?

    It should be interesting for use with several winches that are used one at a time. The VFD could be used for ramping speed up and down and a bank of simple close center directional valves could be used to activate each winch and direction in particular?

  4. #4
    mainewillys is offline Plastic
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    At a previous job we used a VFD to drive a motor coupled to a fixed displacement pump. The pump was running a hydraulic cylinder with a load cell attached to it for a tensile testing rig.

    We used the VFD to set pump speed (fast for jogging, slow for fine load adjustments), and actuated valves manually while keeping an eye on the load cell readout. The VFD was a sensorless vector drive, and it gave acceptable low speed control for what we were trying to do.

    We eventually went to servo valves and wired both the VFD and the load cell to a logic controller, which allowed us to type in a setpoint load. The valves would open, VFD would ramp up the pump speed, then slow down again when it approached the setpoint load.

    So using the VFD for manual speed control can work nicely, but the functionality can really be expanded with some valves and logic.

  5. #5
    robmc is offline Cast Iron
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    You are going to physically control the winches speed by using the VFD yourself, but I don't know about the closed center spool valve assembly. You might need an open center to ramp up the motor and pump and then operate each winch, however you want, with the spool valve. You don't want to lock up the pump depending on how you control the VFD, but if you do this you would control the speed by feathering the spool valves and don't need to change speed with VFD. The closed center system uses a variable displacement pump that shuts down by hydraulic pressure that tells it what to do and is made to so called idle (oil bypasses internally) like this without damage. You would need that closed center system to instantly shut the motor off and on. If this was some type of computer controlled system that found a VFD to be more cost effective than a variable displacement pump by ramping motor speed up and down I could see it. Maybe I don't understand exactly what you mean and am by no means an expert on any of this.

  6. #6
    robmc is offline Cast Iron
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    mainewillys..... was your first system open centered or closed centered? Open centered I can understand.

  7. #7
    mainewillys is offline Plastic
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    Open centered in both cases - the pump ran continuously.

  8. #8
    BadBeta is offline Stainless
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    Quote Originally Posted by mainewillys View Post
    At a previous job we used a VFD to drive a motor coupled to a fixed displacement pump. The pump was running a hydraulic cylinder with a load cell attached to it for a tensile testing rig.
    I believe that is more precision than I'll need, so I guess there is hope.

    Quote Originally Posted by mainewillys View Post
    So using the VFD for manual speed control can work nicely, but the functionality can really be expanded with some valves and logic.
    It will certainly be automated (no manual valves), and sensor feedback can be added as needed. I just basically wanted to know if this is a feasible way to go, and it seems it is.

    Rob, I don't intend to run the pump continously. I mainly want to be able to ramp volume up and down in a controlled fashion. For small volumes this might be cost effective - I can buy a VFD and a directional valve for a lot less than a proportional or servo valve. (The last ones I bought cost about USD 3000 a piece, while a VFD and a directional valve would be about 300-400 or so).

    Maybe we speak around each other here, but if I have a bank of open centre valves all the oil will just go back to the tank unless all the valves are activated simultaneously? (And then move the lightest loaded actuator).

  9. #9
    robmc is offline Cast Iron
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    The open center spool valve assembly can have many separate spool valves in
    one assembly. Yes, the oil is constantly circulating back to the reservoir, but you don't have to open all the valves simultaneously to use it. You can open one or two or three, etc. They go to your individual winches. You can plumb it any way you want. Run two at same time, etc. Yes... an OPEN CENTER
    spool assembly is about the cheapest way to go. I'm not sure why you even need the VFD, just an on/ off switch. The motor and pump run at constant speed and the OPEN CENTER spool assembly controls everything, vol., speed etc.
    A CLOSED CENTER system is more exotic and is usually more expensive.

  10. #10
    BadBeta is offline Stainless
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    Ok, now I'm certain we talk around each other. When I said a bank of valves I was thinking of a manifold with Cetop modular valves attached, and usually with a top stack valve with a closed or floating centre. (Maybe "bank of valves" was the wrong english term for that).

    Either way, as mixing open centre valves and several actuators doesn't make sense to me with the above setup, I assume you talk about another type of spool assembly. I'm thinking likely a parallell or series valve setup of some kind. (Mobile equipment, likely manual handles and operation?)

    As for the VFD I need that to get a controlled and easily rampable pumped volume. An on/ off switch or directional valve can't do that on its own.

  11. #11
    robmc is offline Cast Iron
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    Yes.... that was what I was talking about. A manually controlled type of spool valve. You're are talking about something more sophisticated. I have an idea
    of what you mean but I'm not going to speculate.

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