VFD drive for a pond pump?

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

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

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

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

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.

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

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.

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.

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

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

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.

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

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

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

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

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.

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

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.

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.

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.