VFD Centrifugal Pumps How? Marketable!

Discussion in 'General Electronics Chat' started by kingull, Jan 1, 2012.

  1. kingull

    Thread Starter New Member

    Jan 1, 2012
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    Hi Members,

    I have a pool pump application where I would like to vary the speed of the motor. This is a highly marketable application with accent on power saving. "Save our planet" Eco system.

    Pool (centrifugal pumps) are usually driven by 2 pole PSC motors.
    (Permanent Split Capacitor).
    A dual wound type motor with one main and one auxiliary winding.

    The application is "Variable torque" RPM X HP is not a linear relationship.
    Double the RPM requires eight times the original horsepower.
    Reducing the RPM is in the opposite direction.
    Half the speed requires 0.125 times the original horsepower.

    Speed change effects: Flow is directly proportional.
    Pressure is to the square of the difference in speed.

    The pool pump application therefore only needs about 100 RPM reduction in speed to drop load from 1500 watts out to about 600 watts out.
    A VFD controller therefore requires only a small reduction in frequency for a pool pump application. It needs to be varied via a potentiometer.

    My specific application is two horsepower (1500 watt) 2 pole (2900 RPM).

    Power input: 240\1\50 to output 240\1\45-50 approxomately.
    A VFD of this size would suit all pool pump motors below two horsepower.
    (As long as they are PSC motors).

    Where can I buy one? Dont suggest building one. I am too old for projects. TIA. Frank.
     
  2. thatoneguy

    AAC Fanatic!

    Feb 19, 2009
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    kingull likes this.
  3. jimkeith

    Active Member

    Oct 26, 2011
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    My gut feel is that it is cheaper and more efficient to cycle the motor on and off, and that the power savings will never pay for the controller.
     
  4. kingull

    Thread Starter New Member

    Jan 1, 2012
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    There are all sorts of reasons for slowing a pool pump:
    Noise level. Match performance. Improve efficiency. Energy conservation.
    There are other forums looking for a suitable VFD.
    A speed controlled pump is available from Hurlcon:

    The Viron P280 Vari Speed Pump is designed to save you time and money while helping the environment. This advanced three speed pump adapts to the task at hand while other pumps run at full speed all the time this reflects in decreased operating costs by up to $580 per year, less Co2 emissions in the environment and a reduced noise level.

    The reason I want a suitable VFD is I dont want to alter the pipe work and I want infinitely variable control and not a three speed unit.

    Incidentally, Centrifugal fans follow the same physical laws.
    Horsepower is changed X the cube of the difference of speed.
    Diameter of impellors are to the power of five.
     
  5. kingull

    Thread Starter New Member

    Jan 1, 2012
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    So far, all these VFD`s are single phase in and three phase out.
    I refer to every VFD on Ebay that I have checked.
    Has to be one phase in and one phase out. But appreciate links. frank.
     
  6. praondevou

    AAC Fanatic!

    Jul 9, 2011
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    What's the problem with using only one phase of a three phase VFD? I've never seen a VFD with a monophase output.

    I wonder if this works though since the phase shift the capacitor in the motor causes will change with frequency.
     
  7. thatoneguy

    AAC Fanatic!

    Feb 19, 2009
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    There is a way to do it using a microcontroller and a solid state relay.

    It isn't a VFD drive, but a PWM drive, it turns the motor on and off thousands of times per second so the total power can be adjusted from off to 100% capacity.

    Details from Motorola/Freescale

    I'm assuming this is an induction motor? If so, there's a way to wire the 3 phase output of the VFD to power your motor, but you'd need an electrician to do the setup, it's not a very straightforward task. Either between phases (resulting in a higher voltage) or one of the phases to ground. For the latter, you'd need to contact the VFD manufacturer to see if the VFD could handle such an unbalanced load, as 3 phase normally needs all 3 phases to draw equal power.
     
  8. kingull

    Thread Starter New Member

    Jan 1, 2012
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    A PSC motor is a standard induction motor, virtually but it can be called a two phase motor. The switching method, (Triac) does not reduce power consumption and is unreliable at moment of connection (start up) plus they can generate a lot of radio hash.
     
  9. thatoneguy

    AAC Fanatic!

    Feb 19, 2009
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    The drive linked actually pulses the voltage as the sinewave and doesn't create the interference that a Triac "chopper" drive does, such as is used in a standard light dimmer switch.
     
  10. praondevou

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    I just read a catalog of commercially available VFDs. They would accept three-phase induction motors as well as PSCs. So they don't care too much about the necessary "correct" phase shift for the second winding.

    For the correct phase shift at varying frequency there seem to be methods like a "adjustable capacitor".
     
  11. kingull

    Thread Starter New Member

    Jan 1, 2012
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    Thanks, Your first link wont open. Your second is a different type of motor.
    The PSC Motor has two windings. Main and auxiliary.
    Virtually a standard single phase motor but minus the O\C switch.

    The auxiliary winding and the capacitor must be matched for the unit to run at maximum efficiency. Reducing the cap size can influence RPM but is likely to cause overheating as torque is reduced and the frequency still attempts to gain synchronous speed minus standard slip.

    Variable frequency is the key to proper speed control without endangering the unit by over heating. The centrifugal pump is favourable for VFD due to load falling off to the cube of the difference in speed. (Variable torque)
    Power input is reduced as load is reduced accordingly
     
  12. strantor

    AAC Fanatic!

    Oct 3, 2010
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    you can't reliably or efficiently vary the speed of a single phase motor by changing the frequency with a VFD. Reason being, the capacitor, whether it be capacitor run or capacitor start, is sized for the intended frequency to offset the the phase of the second winding to 90 degrees. If you wanted to vary the speed by varying the frequency, you would need to vary the capacitance at the same time (That's why you won't find a single phase VFD)- that's for a capacitor run motor; for a capacitor start motor, it would probably work, but then you have the issue of "at what point does the centrifugal switch cut out?" - which complicates things. My suspicion is that the centrifugal switch cuts out well below rated speed, so if you used a capacitor start motor and set the VFD (mythical single phase VFD) to run 45hz all the time, it would probably work. But then, where do you get the mythical single phase VFD? you make it. I doubt you will be able to find a commercially available VFD that will let you run a single phase motor, because they are "smart" - in that they will detect the missing 3rd phase and shut down.

    I may be wrong about the single phase VFD being nonexistent, but the question comes up all the time and I always say the same thing, and I've never had anyone show me to be wrong.
     
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  13. praondevou

    AAC Fanatic!

    Jul 9, 2011
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    Strange. Just opened the first link. It's a pdf file. Second link is IMO exactly the motor you were talking about. On page 2 figure 1, main winding and auxiliary winding with series capacitor... :confused:
     
  14. jimkeith

    Active Member

    Oct 26, 2011
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    This is sales hype! A more realistic energy cost reduction would be $58 per year--how many years would it take to pay for a $600 controller? Note that such a motor /control would likely consume 10% of the power when considering control losses and additional motor harmonic losses. A two phase motor has a limited VFD speed range because the capacitor is optimized for standard line frequency.

    Remember the wonderful NOLA power factor correction controller that was invented by NASA? While offering power savings by under-exciting a less than optimally loaded motor, it added harmonic losses and really never became economically feasible, even though it was very cheap to implement.

    CO2 emissions as related to global warming is a fraud because they have to insist that solar radiation remains CONSTANT to make their model work.
     
    Last edited: Jan 2, 2012
  15. jimkeith

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    Oct 26, 2011
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    I agree with this statement!
     
  16. GetDeviceInfo

    Senior Member

    Jun 7, 2009
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    you'll want to pay particulat attention to Stantor's previous post. Changing frequencies changes the caps impedance and subsequently the aux/main winding relationships. Impose this over the the pumps efficiency curves and you'll find that operating away from peak efficiency, you'll be wasting a lot of energy. Yes, if you require less you'll use less, but you'll waste more. Any good salesman will peg you down real quick.
     
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