VFD drive for a pond pump?

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VFD drive for a pond pump?

Home Forums General Questions VFD drive for a pond pump?

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  • #250462
    norm norton
    Participant
      @normnorton75434

      I want to control the flow from my pond waterfall pump. It is 175w and moves 7,000 l/h. It has an encased 250v winding of some sort that induces rotation in a 4" did iron rotor that has a number of inset copper segments.

      Now, I don't understand electric motors in detail but I presume this one rotates at a set speed that is derived from the 50hz supply. I thus assume that the only method of speed control will be to adjust the frequency. If I did this and ran it at, say, 25hz, would it draw less current, which I am hoping it will, or the same as it maintains torque?

      I have found pond pump speed controllers for sale but they seem to be triac type that effectively reduce the total current – I can't see that these are suitable for my induction pump.

      I would appreciate any thoughts on the subject…..

      Norm

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      #24679
      norm norton
      Participant
        @normnorton75434
        #250464
        MW
        Participant
          @mw27036

          Hi Norm,

          I'm not totally sure this is the case but from my experience of induction motors only 3 phase ones have the potential to be controlled via delta star wiring. Single phase can only be operated as ON/OFF type function. So the pump would need 3 phase or 4 wires, 1 for each phase(U,V,W) and earthing lead. A "CY" type? shielded cable will do the trick, with the plastic foil and steel mesh insulation.Also helps to see each lead marked 1, 2 and 3 so you don't mix them up. 

          The VFD would take the single phase current, expressed in it's wave form, interpret it and digitally emulate a new 3 phase output waveform, so it's kind of like a real 3 phase mimicry.

          A phase converter traditionally did a worse job because you'll only get 75% of its output at best from a rotary, a poor quality static converter could actually damage the motor by frequently overheating. Having said that, their popularity remains because if you have 3 or 4 machines to hook up then it's still economically efficient way of getting 3 phase without buying it from the grid. 

          I would prefer to do it this way rather than go down a DC controller route.

          Michael W

           

           

           

          Edited By Michael Walters on 12/08/2016 10:58:18

          #250466
          Speedy Builder5
          Participant
            @speedybuilder5

            Rather than control the speed (expensive for a VFD etc), why not control the flow by either restricting it, or making a bypass for excess water, or were you thinking of using it for a different application?
            BobH

            #250471
            Martin Connelly
            Participant
              @martinconnelly55370

              What you have sounds like a shaded pole motor. The speed of rotation is linked to the frequency but since it is a single phase motor you cannot drive it with a 3 phase VFD. The VFD would see an imbalance in the output and shut off due to a perceived fault. BobH's solution may be a better option.

              Martin

              #250473
              JasonB
              Moderator
                @jasonb

                I'm with Bob, a simple Tee and ball valve will let you adjust how much goes over the waterfall and what does not goes straight back to the pond.

                With the money saved get yourself a less power hungry pump, My 7000lts/hr one only uses 75W and still pumps to a good head.

                #250478
                Bazyle
                Participant
                  @bazyle

                  When you reduce the flow on a centrifugal pump it actually uses less power…..

                  #250484
                  David Jupp
                  Participant
                    @davidjupp51506

                    As suggested by others, just put a valve in outlet line, partially close to reduce flow – this will also reduce how much power is drawn. This is a standard way of controlling output of a centrifugal pump.

                    #250487
                    Richard Marks
                    Participant
                      @richardmarks80868

                      If your are using restriction to control flow then put your valve in the inlet not the outlet otherwise you could build up pressure inside the pump and outlet pipes.

                      #250488
                      Bazyle
                      Participant
                        @bazyle

                        mmmm not so sure about that as it might promote cavitation.

                        #250489
                        JasonB
                        Moderator
                          @jasonb

                          Agreed I have always been lead to believe you restrict the output

                          #250490
                          David Jupp
                          Participant
                            @davidjupp51506

                            Richard – the centrifugal pump won't generate any more pressure than it is designed to generate, it isn't a positive displacement device. The restriction should go in the outlet. Large centrifugal pumps starting DOL are often started against a closed (or nearly closed) outlet valve to limit switch on surge current.

                            An inlet restriction may well lead to surging, vapour locking, or erosion due to cavitation.

                            #250498
                            Martin Kyte
                            Participant
                              @martinkyte99762

                              Stick a bypass on it.

                              If you really want to get fancy put a turbine generator in the bypass and use the output to drive the pump.

                              ;0) hee hee hee

                              #250502
                              norm norton
                              Participant
                                @normnorton75434

                                Thank you for all the replies, very helpful. I wanted to cut down the flow at night, thus a timeswitch and power control would be helpful, plus I might reduce the energy consumption.

                                Yes the 15year old Tetra pump is power wasteful and new Oase pumps at circa £200 are half the power use at 80w for 8,000 l/h. Perhaps the cost effective solution is to replace the pump, and perhaps put a gate valve that I have spare in the output line. But there would be no time automation.

                                I was very interested in the comment that putting a restriction on the pump will reduce energy consumption. Is this because as it works harder the back emf rises and thus its resistance?

                                I spent some time looking for web answers before I asked this question, and I have looked again this morning. You can buy fancy pumps with their own speed controllers (£600 plus) but there is no information on DIY speed control. There are stories from the USA about a triac fan speed controller sold for pond pumps that didn't always work.

                                You can get fan motor controllers, they are used in a lot of horticultural set ups. About £30, but no clue on how they work. The fans seem to have inductive motors so perhaps I should speak to a UK manufacturer that I have found regarding if they work for pond pumps.

                                There also appear to be no 250v single phase VFDs as boards or box devices down at the 1amp rate. You can get nice, assembled motor controllers from China but these are around £100 and 10amp. Again, I don't know enough to understand whether they would drive a 'shaded pole' motor (is that a specific type of inductive motor?).

                                Edited By norm norton on 12/08/2016 14:42:09

                                #250505
                                Vic
                                Participant
                                  @vic

                                  Central heating pumps are cheap and can have up to 4 speed settings. I know some folks have used these for fish keeping and you used to be able to get plastic components to replace the Iron/steel parts to prevent rust being a potential problem. No idea what the flow is though, it may not be enough for your pond.

                                  Edit: some info here.

                                  http://www.ukaps.org/forum/threads/central-heating-pump-conversion.26709/

                                  And here.

                                  http://www.ebay.co.uk/itm/Aqua-Turn-conversion-kit-for-Grundfos-Pumps-Koi-Ponds-and-Filter-LARGE-and-small-/271191559769?pt=UK_HomeGarden_Garden_PondsWaterFeatures_UK&hash=item3f2446ce59

                                  Edited By Vic on 12/08/2016 14:45:28

                                  #250507
                                  Martin Connelly
                                  Participant
                                    @martinconnelly55370

                                    If you choke off the flow from a centrifugal pump the impeller starts to stir the some of the fluid instead of moving it through the pump. As a result it does less work so the speed increases and back emf rises. The power requirement is a function of mass flow through the pump, the rate of change of momentum of the fluid is equal to the force that causes the change. Just keeping the same bit of fluid spinning does not change the momentum of that fluid so zero rate of change no force required (apart from friction losses). Think of the note change on a vacuum cleaner when the suction is blocked off, it will rise as the motor spins faster.

                                    Martin

                                    #250515
                                    David Jupp
                                    Participant
                                      @davidjupp51506

                                      Add time automation to the restriction by putting a solenoid valve in parallel with it – solenoid open = full flow, solenoid closed = flow allowed by restriction.

                                      OR small pump plus existing pump in parallel (check valves to prevent reverse flow) – run large pump in day, small pump at night.

                                      #250516
                                      JasonB
                                      Moderator
                                        @jasonb

                                        Typical Grundfos CH pump will put out about 800gph over a 3ft waterfall, as they are reasonably cheap you could have two pumps and simple time switches to run one pump during the day at full setting and second pump at low speed at night.

                                        Bear in mind fish need more added oxygen at night so if you are using waterfall to oxygenate the pond then lowering the flow at night is not good. Hence bypass to just splash into pond at low level if you are wanting to reduce noise at night

                                        #250529
                                        duncan webster 1
                                        Participant
                                          @duncanwebster1

                                          Old washing machines used to have solenoid valves inside, so can be got cheaply (or free), not sure about new ones, which appear to have more computing power than an Apollo moon rocket.

                                          #250530
                                          norm norton
                                          Participant
                                            @normnorton75434

                                            Thank you Martin, you have kindly confirmed my thoughts on conservation of energy – no fluid moving thus energy used only to heat the motor plus heating the churned, trapped fluid. I have just had my electric meter compulsorily replaced by one of those 'indoor readout sim card things' and it records right down to a single watt (W) being consumed. My lowest domestic consumption in the morning is 650W of which 250W is the two pond pumps (one waterfall and one filter circuit). The remaining 400W seems to be going into all the standby and computer devices. But I can now choke the flow on one pump and see what the change in power consumed is.

                                            I like the dual pump and solenoid idea. But, perhaps I need motorised gate or ball valves to achieve that?

                                            You are quite right about the danger of oxygen depletion at night, but I have never understood why some say MORE oxygen is needed at night. I have always suspected that the cases of fish death at night in hot weather are due to the pumps being turned OFF completely. I can't use basic CH pumps as my upper and lower ponds sit below the filter house and the lower one is below ground level, thus I need submersed pumps.

                                            #250539
                                            not done it yet
                                            Participant
                                              @notdoneityet

                                              The power requirement is a function of mass flow through the pump,

                                              Not quite true. The amount of fluid pumped will depend on the head. At maximum head, the pump will provide no flow but will still consume power. The stated flow rate is likely at zero head, hence a high figure for prospective purchaserscto ogle over! Impellor pumps are notoiously inefficient once off the pumping curve. Water providers now save huge amounts of power by speed regulation, rather than throttling the output.

                                              Actual power used will be a function of running the water pump plus mgh, the potential energy imparted to the water, where m=mass, g= gravitational field strength, a h = the lift height. Values in SI units, of course. At maximum lift, the mass of water pumped will be zero.

                                              #250541
                                              JasonB
                                              Moderator
                                                @jasonb

                                                Any plants or algae in the pond will give off oxygen and take in CO2 during the day but at night they do the opposite so can deplete the oxygen level in the pond.

                                                Easy enough to run CH pumps on that setup, you need to take a feed through the pond base or wall and into a pump chamber. Ideally pump wants to be 12" or more below pond level to avoid cavitation. Also put a screen on teh inlet as they tend to get blocked by frogs in the spring. I used to run two C/H pumps on my koi pond until I changed filters a few years ago now have a 10000lph for the filter rated at 60w and 7500lph rated at 75W for the waterfall, both Messeners

                                                Old plumbing

                                                New plumbing

                                                #250565
                                                Roger Head
                                                Participant
                                                  @rogerhead16992
                                                  Posted by duncan webster on 12/08/2016 18:11:08:

                                                  Old washing machines used to have solenoid valves inside, so can be got cheaply (or free), not sure about new ones, which appear to have more computing power than an Apollo moon rocket.

                                                  Waaay more. For those who are interested, have a read of this book. The hardware description reads like a modern-day kindergarten introduction to computers. ISBN 978-1-4419-0876-6.

                                                  cover.jpg

                                                  Edited By Roger Head on 13/08/2016 01:40:05

                                                  #250595
                                                  Les Jones 1
                                                  Participant
                                                    @lesjones1

                                                    Washing machine solenoid are much too small in flow rating for a pond system. Motorised central heating valves may be large enough.

                                                    Les.

                                                    #250602
                                                    Tim Stevens
                                                    Participant
                                                      @timstevens64731

                                                      If you do add a bypass between pump and pond, you could always add a further water feature rather than just 'wasting' the extra flow.

                                                      Regards, Tim

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