How to downrate a circulator pump.

[Agile]

Viessmann boilers use standard pumps but clip the waveform to reduce the power output as the boiler modulates back.

Bit like the triac drill speed control before B & D learnt how to fit them inside the case!

Tony

 

[Paul Barker]

That would just clip the amplitude which is not much better than reducing the voltage.


the best way to slow down an ac motor designed for a given voltage is to
constantly stop then start it, by chopping the power to it, and the letting it back in. That way it always has sufficient torque to start. With amplitude method you would reach a point where the motor doesn't turn but the winding heats up.

Though voltage reduction is clearly the only available option to the diy plumber.

 

[chrishutt]

.. has all the air been eliminated from the circuit. I fitted one on a 15mm circuit a while back and set the pump on speed 1 and it operated quietly.

It's not really noisy, just the slightest, barely audible whirr as one would expect. But the copper pipework (including a heating coil in the airing cupboard which it serves) acts as bit of a sounding board near the bedrooms. It's not really a problem since the timer will cut out operations at night anyway

I was just curious about the possibility of downrating the pump since it's far more powerful than the situation requires. Had I known that the gravity idea wouldn't work I'd have done the return loop in 10mm plastic pipe!

 

[ChrisR]

Er, I thought the capacitor was just to assist the motor in starting up from rest.

No that would be a capacitor-start motor, these are capacitor-run motors...!
The cap is there to provide permanently provide a different set of windings with power at a different phase angle to the directly connected ones, so the field rotates.

SO if you reduce the capacitor value, the main coils take the same current and produce the same field strength, but the cap'd ones make a weaker field, and at a differerent angle. Not really efficient, but the upshot is a motor running slower.

Crude triac type voltage reducers switch the power on part way through the sine wave, thus:

With a "Universal" motor like in an electric drill you can make a circuit alter the point where the switching happens to increase starting torque and maintain a particular, reduced speed, but not with a CH pump.

Other ways to reduce the speed would be to reduce the voltage, using a transformer, possibly an auto type (as in puma fans) or simply bung in a resistor, as with many fans, though it would have to dissipate a lot of power and get jolly hot unless huge. A slightly eccentric but effective option, can be to use a light bulb. You need your pump to use of the order of half power, so of the order of 50 watts, so it might be worth a try. Complication with light bulbs is that they aren't linear, the resistance changes with power, so you'll have to experiment.

"Switching" power controllers turn the mains on and off very fast through the sine wave (say 2,000 times a cycle) , so once smoothed, you can get whatever voltage you want, but they're expensive.

 

[staffs]

Great bit of kit and a good idea.If it is in your own house and you are doing it then why not give it a go ?

The only problem from an installers point of view is that if any problems occured due to fitting this controller ie : pump failure...fire !!! then i have no doubt that any manufacturers warranty would be null and void as the unit has not been fitted in accordance wit MI and also in a worst case scenario The installers Public Liability may take a battering if that was not voided also.

us Installers are sometimes prone to doing someone a favour and leave ourselves wide open ..." Go on hit me in the stomach?" arent we ??

IMO the public are getting worse and are only too keen to find a reason to salte us off or want some form of financial reduction and when quite unwarranted.

Cover your own back and get things signed and in writing. maybe i am a little paranoid but ny fingers have been burned one too many times doing someone a favour. It always come back and kicks you in the NUTS.


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[ChrisR]

Quite right staffs. Maybe with reduced water flow through it, the pump wpould overheat and catch fire!

There's always the time-honoured method: At least the flow in the main circuit is reduced, which was the intention...

 

[Softus]

Thanks for your explanation ChrisR. It was clear and it was marvellous.

 

[chrishutt]

The cap is there to provide permanently provide a different set of windings with power at a different phase angle to the directly connected ones, so the field rotates.

SO if you reduce the capacitor value, the main coils take the same current and produce the same field strength, but the cap'd ones make a weaker field, and at a different angle. Not really efficient, but the upshot is a motor running slower.

That's what I wanted to know. I'm very grateful to ChrisR (as ever) for taking the trouble to supply such a detailed explanation.

As I've said, the original problem is very minor and I'll probably leave it as it is (unless the customer complains), but I was curious to know what might be done if the problem were significant. The secondary return is the first one I've ever installed so I've been experimenting (at my not inconsiderable expense) to see what set-up works best.

The way I have it the pump should run for only a matter of seconds before the hot water reaches the pipestat and switches it off, so overheating isn't going to be an issue, although frequent start-ups (say 15 minute intervals?) might be so I wouldn't want to do anything that increased the start-up load (like increasing circuit resistance too much?).

 

[Agile]

If you reduce the flow through a pump by closing a valve the motor current actually decreases.

At the original Droitwich LW transmitter installed in 1933 the cooling pumps were started with the valves closed and took about 2.2 Amps per phase and the valves were opened until the current was 6.0 A which was the time honoured flow setting point.

About 600 kW was dissipated in cooling ponds where the 32°C allowed the goldfish to grow to about 300 mm !

The replacement switch mode transmitters were well over 90% efficient and were cooled by air.

Tony

 

[chrishutt]

If you reduce the flow through a pump by closing a valve the motor current actually decreases.

Are you sure about that? I always thought it was bad to pump against a dead-end due to the load put on the pump motor. All the motor does is rotate the impeller. The resistance to that rotation will increase with the differential pressure across the pump which will be greater with increased circuit resistance. Or have I got that wrong?

 

[Agile]

It might not be the same for all types of pump and certainly not for piston pumps but I think it applies to most impeller types. Do a test!

However, think about it from another point of view!

If the valve is closed no work is being done. If its doing a lot of pumping more energy is required to lift the water.

Tony

 

[Agile]

That would just clip the amplitude which is not much better than reducing the voltage.

No they clip ( switch ) the current as in Chris' second graph. Same current for half the time is less power. Saves electrical power.

Tony

 

[Paul Barker]

yes I saw that I would call that chopping I call shortening of the peaks clipping.

but we now know what each other means. You meant the thing I was also thinking of as the solution.

 

[Paul Barker]

It might not be the same for all types of pump and certainly not for piston pumps but I think it applies to most impeller types. Do a test!

However, think about it from another point of view!

If the valve is closed no work is being done. If its doing a lot of pumping more energy is required to lift the water.

Tony

I have a clamp meter so if my gate valves at home were any good I could let you know, but as they are 30 years old and never used no such luck. Next new install I'll find out for you.

 

[ChrisR]

I don't know much about motors but I am sure you can't generalise from one type to the next about what happens under load.
Fascinating things, invented by jolly clever people several generations ago, mostly, and developed and tinkered with ever since. The maths gets boggling, all too quickly.

One thing you do get is "slip", which is an angle between the position of the rotor and the rotating field.
Rather well demonstrated in this demo of one of the iconic, wonderful, motor designs, the squirrel cage (look for a diagram of a 3 phase one):
http://www.scienceshareware.com/squirrelcage_DL.htm

I had to persevere, to get the unzipped files all into the same directory. It's at its best when you change the "slip" to less than one.

This page is pretty good. If you never made things like this at school with elastic bands and paper clips, you don't know what you've missed. Better than stinks lab.
http://www.physclips.unsw.edu.au/jw/electricmotors.html#schematics