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This is the scenario: There's a valve controlled by what it looks to be an AC motor, specified as 220V 50Hz and 7W. That's all, there's no datasheet available and there's nothing else on the labels that talks about stall current or anything like that. The way this seems to work is that the motor shaft, through a whole lot of reduction gears, ends up moving a lever, which opens a valve (Normally closed with no current). The end position of the lever is determined by a metal plate that prevents it from going further. Hence I understand that at this point, the motor stalls. Once current is not applied to the motor anymore, a spring brings back the lever to its original position, which in turn closes the valve.

I said "it seems to" a few times because the motor is currently broken. Actually, there's two, and they are both broken, so a lot of this is my theory. Furthermore, this is all from a videocall with my dad who has the problem and is waiting for the replacement motors, but I'm trying to make sense of it, and even what the manufacturer told him doesn't click in my head very well.

So, two questions:

  • If a system is designed to stall the motor while the valve is opened, shouldn't there be something to limit the current to make sure it doesn't go past the stall current? This motor connects straight to the mains (220V 50Hz), no other electronics.
  • How is this design justified? Isn't the motor going to be consuming the maximum current while keeping the valve open? In this application it's supposed to be open for hours at a time*. I can only think that maybe the torque of the motor is needed to fight back the 1.6bar max pressure that it might encounter to keep the valve open, but even then, surely a design with some electromagnets or relays that fixes the lever mechanically, or a stepper motor or something like that would only require current flowing during the actual opening of the valve? There's even a way to move the lever manually and leave it latched so that the valve stays open.

It might be that my understanding of what I think it's an AC motor is not complete and actually when stalling there's really not that much current consumption?

If someone has experience on a scenario like this or could point to some good reading about this, I'd love to learn more about it.

*I mentioned earlier that its supposed to be opened for hours at a time. My dad had a chat with the manufacturer and he was told that they are not really designed to be used for those longer periods of time, therefore it is normal that people need to replace them. I can't think that this is just a case of something designed to be replaced, that doesn't make sense to me. The application is a time controlled heating system. It could run for an hour, or for 48 hours, why not?

Thanks!

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  • \$\begingroup\$ Are you sure it closes with a spring? The combination of a spring, with reduction gears, sounds odd. \$\endgroup\$
    – user253751
    Dec 11, 2020 at 17:40
  • \$\begingroup\$ yes, 100%, I've seen it. When removing the motor, the spring brings the lever back instantly (snaps), and with the motor, it takes about 6 seconds to complete the movement because of all the reductions. This part makes sense to me because this way the motor doesn't need to spin in the opposite direction to close. Remember there's no electronics. \$\endgroup\$
    – palako
    Dec 11, 2020 at 17:43
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    \$\begingroup\$ I've seen these in combi boilers. In that case they are ON when supplying hot water, and return to OFF when heating the house. The ON time will be long enough to fill a bath, max. So, yes they can just survive stalled operation for a while. Hours maybe, but thousands of hours cumulative may be pushing it. The thing is, unlike high performance high efficiency motors, the stall current won't be 10x the operating current; it may be barely more, relying mostly on the winding resistance to limit power. \$\endgroup\$
    – user16324
    Dec 11, 2020 at 17:50
  • \$\begingroup\$ AKA diverter valve. See viessmann.co.uk/heating-advice/fix-a-diverter-valve \$\endgroup\$
    – user16324
    Dec 11, 2020 at 17:56
  • \$\begingroup\$ Thanks Brian. Yes, this is very similar, it's a zone control valve, dad uses them to send heating to either the lower or upper floors. From my back of the envelope calculations, if the motor says 7W and it's running a 220V, it would consume 30mA while keeping the valve open. \$\endgroup\$
    – palako
    Dec 11, 2020 at 17:59

2 Answers 2

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If a system is designed to stall the motor while the valve is opened, shouldn't there be something to limit the current to make sure it doesn't go past the stall current?

No. Motors can be designed to operate stalled for a long period of time, essentially continuously. They are sometimes called "torque motors."

How is this design justified? Isn't the motor going to be consuming the maximum current while keeping the valve open?

The design is justified by saying that 7W is not very much power. The motor probably does not consume more than that when stalled.

There is not necessarily a better way to do this job. There may be products that last longer, but do the job in a similar way using a similar amount of power. It would probably be best to ask local service people if there is a product that lasts longer.

A zone should not need to be heating all of the time except during the coldest night of the year. If it does, it will be unable to maintain the desired temperature some of the time. If there is one zone like that, it may need more radiator or supplemental heating. If all of the zones are like that, th whole system is inadequate.

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This will be a shaded pole AC motor. These have the advantage that do not require a capacitor for starting and the locked rotor current is not much greater than the run current.

They suffer from poor starting torque hence the gearbox.

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