Speed control of a single phase PSC induction motor

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I'm hoping this might interest some on here...
I'm wondering what the preferred method for controlling the speed of, in this case, a central heating pump.
My initial thoughts were to use a triac, as you would find in a dimmer switch.
However, I found this
on YouTube, where the guy modulates the duty cycle of the a.c. at about 7-8 Hz using a solid state relay, which was interesting.
I've seen a discontinued variable speed Grundfos pump for the American market, which uses some sort of simple triac circuitry, where the speed is controlled by an external signal.

Any thoughts on pros and cons?
 
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My initial thought is that in your shoes the first thing I would do would be to find out how well the pump would tolerate being turned on and off all the time.
 
Years ago if you wanted speed control you used DC motors, be it a conveyor belt or pump does not matter, however as the inverter arrived so the DC motor went and three phase motors were used on inverters, even in domestic we now have fridges and freezers with inverter drive three phase motors, that has now become a way of life.

However with a single phase motor reducing speed without over heating, stalling or other unwanted side effects is not so easy, I needed motor speed control a lot where I worked, and I went for I think from memory it was a SWF motor, gearbox, inverter, brake, and ASii control, as trying to combine different makes of motor, gearbox, and inverter seemed to cause problems.

The brake would not be aligned to the inverter or other faults which would cause the whole lot to trip out. It may work, but why take the chance, if I spent £1000 on a SWF motor rather than £500 on a collection of items I was not really questioned why, however if the machine tripped and I lost one hour of production is was why did this happen, as in the main that would likely cost the firm £5000 so the idea was get the right kit for the job, and watch your back.

So it may work, but I would not use it.

Oh and yes model trains had a controller which would monitor the return EMF from the engine motor to work out if the engine had started to turn, it would allow a very slow start of the toy emulating the real thing, however it destroyed the permanent magnets in the motor, if the motor was that type, so we used a variable voltage regulator not as good controlling but did not kill the magnets, in your case what will the capacitor do? will it stand the pulse width modulation? It may be OK, but unless you have 100's is it worth trying?
 
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My initial thought is that in your shoes the first thing I would do would be to find out how well the pump would tolerate being turned on and off all the time.
Exactly. My thinking was that if the rotor actually came to rest then there would be significant heat losses. But at that rate, the rotor probably would not stop...
 
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My second thought is if you need a variable speed pump you should buy a variable speed pump, not use external gubbins to make a normal one variable.
 
My second thought is if you need a variable speed pump you should buy a variable speed pump, not use external gubbins to make a normal one variable.
That's fair enough. What was interesting about the Grundfos VS pump was that it seems that Grundfos had done exactly that: added external gubbins to a single phase motor.

There was another doc which gave away a bit more about the circuitry, but I can't find it now.(n)
 

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What was interesting about the Grundfos VS pump was that it seems that Grundfos had done exactly that: added external gubbins to a single phase motor.
Quite possibly. But at their risk, after their testing and with their warranty.
 
Found it. There's a diagram near the end of the document which looks very much like a triac-controlled affair...
 

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  • ph_VS_Install_Guide (1).pdf
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A centrifugal pump will run 'happily' against a variable head so you might have better luck installing a throttling valve downstream of the pump and running the pump at a fixed speed (like it's designed for).
 
A centrifugal pump will run 'happily' against a variable head so you might have better luck installing a throttling valve downstream of the pump and running the pump at a fixed speed (like it's designed for).
Yes, the problem is then transferred to controlling the throttling valve. I've not investigated this much.
 

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