Thermostat N/O and N/C is N/C ever used?

[JohnW2]

Just noticed this post. That's right for the mid-position valve, but the additional switch is to stop the motor at mid-position. I meant a switch to start the pump and boiler, for safety reasons. The either/or valve, with body style as your #70, doesn't have either switch.

Indeed - as I thought we had agreed, there would really be no point in a switch with a mid-position valve.

Of course (as I think I must already have said somewhere!), with a 2-port valve the 'auxiliary switch' exists for more than safety reasons - it is usually used as the means of switching on boiler and pump when the valve opens - as well as having the 'safety' function (i.e. preventing the boiler/pump from being on when the valve {possibly all valves} is closed).

Kind Regards, John

 

[SimonH2]

I haven't studied the circuit for the mid-position valve in detail, but I think the motor stops in mid-position, but still stalls when CH only calling.

Yes, the motor stops in the middle - there's diagrams & explanations on the internet, and it's a "very ingenious" design. It motors one way (and stalls at the end) and spring returns the other way. In the middle a resistor and diode put a small DC current through the motor which is enough to make it hold against the spring.

I'm pretty sure some 2-port valves have spring return, so also stall (but not the fully motorised one you described, obviously)

The ones used in domestic systems are generally spring return, with the motor just stalling at full travel.

More surprising, when stalled, both current and PF were completely unchanged, which somewhat offends my understanding of physics!

I rather think that as these are designed to be stalled for some time, they are designed quite differently to normal motors. I suspect most of the losses are down to being shaded pole (often criticised for it's low efficiency) - with the iron losses dominating and hiding the difference between running and stalled conditions.

 

[JohnW2]

I rather think that as these are designed to be stalled for some time, they are designed quite differently to normal motors. I suspect most of the losses are down to being shaded pole (often criticised for it's low efficiency) - with the iron losses dominating and hiding the difference between running and stalled conditions.

As you say, they clearly have been designed with 'being stalled in mind'. I'm just surprised that there wasn't at least a measurable (if not large) change in current when it stalled. I've just tried again with a somewhat higher resolution meter, and it was 31.3mA both running and stalled - so any difference is tiny.

As I also said, I was also surprised at the 0.98 PF - I presume it must be 'corrected'.

Kind Regards, John

 

[SimonH2]

I suspect the "good" power factor is down to the shaded poles. I did a quick search and came across this long thread which has some interesting nuggets in it if you slog right through - just like some threads in here There won't be any correction in there - certainly no caps.
Basic explanation seems to be that the shaded pole motor naturally has very low torque to start with (compared with other motor designs), so when it's stalled it's not going to change input current much. The shaded pole having a high resistive loss - so you have a high PF when unloaded. Did you measure PF both loaded and unloaded ?

 

[JohnW2]

I suspect the "good" power factor is down to the shaded poles. I did a quick search and came across this long thread which has some interesting nuggets in it if you slog right through - just like some threads in here There won't be any correction in there - certainly no caps.

Thanks - I'll have a look at that thread (I'm not frigtened off my long ones ).

Basic explanation seems to be that the shaded pole motor naturally has very low torque to start with (compared with other motor designs), so when it's stalled it's not going to change input current much.

Fair enough. As I said, I'm just surprised that I could not see any change at all - with 31.3mA vs 31.3 mA, any change must be appreciably under 2%.

The shaded pole having a high resistive loss - so you have a high PF when unloaded. Did you measure PF both loaded and unloaded ?

I'm not sure what you mean by 'unloaded' - I've measured it 'running' (loaded by the spring) and stalled (fighting against a brick wall) - and the PF was hovering between 0.98 and 0.99 in both cases. If you mean 'literally unloaded' (i.e. running the motor 'bare' without any external mechanical load), I haven't tried that yet.

Whilst I realise that such a motor probably has substantial resistive losses, a PF of 0.98 - 0.99 implies that it is presenting an almost entirely resistive load - which seems a bit odd for a motor!

Kind Regards, John

 

[SimonH2]

I'm not sure what you mean by 'unloaded' - I've measured it 'running' (loaded by the spring) and stalled (fighting against a brick wall) - and the PF was hovering between 0.98 and 0.99 in both cases. If you mean 'literally unloaded' (i.e. running the motor 'bare' without any external mechanical load), I haven't tried that yet.

I suspect that there is little difference between running against the spring and stalled in terms of load.

Whilst I realise that such a motor probably has substantial resistive losses, a PF of 0.98 - 0.99 implies that it is presenting an almost entirely resistive load - which seems a bit odd for a motor!

Yes it's a motor - but it's a shaded pole motor, and things are somewhat different for them.

 

[JohnW2]

I suspect that there is little difference between running against the spring and stalled in terms of load.

That is why I asked what you meant by 'unloaded'. However, I would have expected that would be an appreciable difference between a motor running at design speed and one which is stalled (hence not rotating at all).

Yes it's a motor - but it's a shaded pole motor, and things are somewhat different for them.

Maybe, but a PF that high implies that the inductance (or, more correctly, the reactance of that inductance) is negligible in comparison with its resistance - which, as I said, seems odd for any sort of motor.

Kind Regards, John

 

[ericmark]

I have been reading with interest, I did at one time consider motorised valves and splitting house upper and lower floor, however it would require that much extra pipe work, I decided it was a non starter.

In the old days, all you wanted from a motorised valve was simple open/closed, however with the introduction of modulating boilers, I would expect you now need to control how much the valve is opened. The TRV with electronic head I hear the motor run from time to time as it adjusts how far open the valve is. I hear it run at 10 pm as the program calls for temperature to be reduced over night. I assume same goes for any zone valves.

I looked at fitting all electronic TRV heads but it works out rather expensive. And in my own house down stairs is open plan, so a single wall thermostat is enough, upstairs standard TRV heads are fitted just to stop rooms getting too hot, but with an open plan stair case any attempt to get down stairs hotter than upstairs is just not going to happen, heat raises.

However my boiler must be getting on for 30 years old, at some point it will need changing, so have been looking at how central heating works. At the moment only one fan assisted radiator is fitted, would like to fit more, specially in the kitchen, they had one huge difference to the non fan assisted, they don't hold much water, this means the circulating water can heat up, and cool down much faster, as there is less of it. So walk into cold house and within 5 minutes of the heating going on the Myson is kicking out hot air. However same in reverse, within minutes of the thermostat switching off the boiler the radiator is cold and the fan auto turns off, and before the thermostat has switch the boiler on again the room is feeling rather cool. But with the non fan assisted the water in the radiator stays warm for some time.

So 0.5°C slewing with thermostat is really too much with fan assisted radiators, the answer I know is fit a modulating boiler with of course a modulating thermostat. However unsure how the old Myson will work with micro bore on a modulating boiler, if the water is cooler will the old Myson work, note old Myson also nearly 30 years old.

This is why I bought the cheapest programmable thermostat in the Electofix store. Some day either I will move to a different house, or will need to fit a modulating boiler with all the control problems that go with modulating boilers. I did at one point consider a water store, father-in-law fitted water solar panels, and I thought great idea heat house from sun, however they have never worked so forgot that idea. And anyway new cistern will likely cost more than a new boiler.

Have looked at the thermostat I use to brew beer with, a MH1210A very like the STC 1000 which has a slewing of just 0.1°C it uses temperature sensor: NTC 25ºC=10K B3435 ±1% what ever all that means, but have wondered if I could actually have more than one sensor? If for example I had 4 sensors, spread around the room, would not matter if sun at front or back, or wind south or north I should get around about the same temperature in the room. However the box I used to house the MH1210A and the STC 1000 would not come up to my wife's exacting standards, so idea shelved.

Since already have one Myson have also looked at heat pumps, would be nice to cool house in summer, however price is some what off putting, think a stand alone a better option, then also there should boiler break down to heat house.

Been out of house for 18 months looking after my late mother, so now looking forward to returning home and getting it A1.

 

[JohnW2]

I have been reading with interest, I did at one time consider motorised valves and splitting house upper and lower floor, however it would require that much extra pipe work, I decided it was a non starter.

I have quite a few motorised valves (I think 6 or so), but they were all fitted together with the pipework.

In the old days, all you wanted from a motorised valve was simple open/closed, however with the introduction of modulating boilers, I would expect you now need to control how much the valve is opened.

I'm certainly no expert in such matters, but I don't really see why that would be necessary. Is not the modulation of the boiler just a 'local' matter, based on the input and output temps of water at the boiler, regardless of 'where the water is going'?

Although there are room stats in my various zones which theoretically close the corresponding motorised valve if the temp gets up to the set level, in practice the MV for a zone is usually continuously open whenever that zone is 'switched on', with (non-electronic) TRVs actually doing the temperature control.

Kind Regards, John

 

[fixitflav]

OK - done, with a Honeywell V4043H spring-return 2-port valve, and I'm rather surprised by the answers ...

The spec and markings say it is 6W. At 244.3V, with the motor running it takes 31mA with a PF of 0.98 (which surprises me, for a motor) - hence about 7.4W. More surprising, when stalled, both current and PF were completely unchanged, which somewhat offends my understanding of physics!

Anyway, as I suppose had to be the case, there certainly does not seem to be any 'horrendous' stalled current!

Kind Regards, John

Good to see some actual current figures, well done!
I haven't had much to do with synchronous motors, but I thought I remembered they run at PF 1. But having a quick look on Wiki, it says ones with DC excited magnets (which clearly doesn't apply to valves we're discussing here) can have PF between 1 and 0.8 leading.
Of course, lower PF means less heat dissipated (for a given current)

 

[JohnW2]

Good to see some actual current figures, well done! I haven't had much to do with synchronous motors, but I thought I remembered they run at PF 1. But having a quick look on Wiki, it says ones with DC excited magnets (which clearly doesn't apply to valves we're discussing here) can have PF between 1 and 0.8 leading.

Hmmm. In the absence of a correcting capacitor, a PF or 1 surely means that there is no inductive component - and I rather struggle to understand how any motor can function without inductance - motors are, after all, 'electromagnetic' devices!

Whatever, my measurements seem to have given us a satisfactory answer to the question in hand.

I can't currently find it but, somewhere, I have a spare of one of these synchronous motors. If/when I find it, I'll do as Simon has suggested and see what happens (particularly as regards PF) when it has no explicit mechanical load.

Kind Regards, John

 

[fixitflav]

Hmmm. In the absence of a correcting capacitor, a PF or 1 surely means that there is no inductive component - and I rather struggle to understand how any motor can function without inductance - motors are, after all, 'electromagnetic' devices!

Whatever, my measurements seem to have given us a satisfactory answer to the question in hand.

I can't currently find it but, somewhere, I have a spare of one of these synchronous motors. If/when I find it, I'll do as Simon has suggested and see what happens (particularly as regards PF) when it has no explicit mechanical load.

Kind Regards, John

This is a link I found http://petrowiki.org/Synchronous_motor, but there's plenty more. It says it can have a leading PF, and can be used with "ordinary" motors to improve overall PF.
A thought I had - normally a wound coil has an inductance, but if a coil is wound on a former, like a solenoid, clockwise from one end to the other, then wound back clockwise from the other end, wouldn't the magnetic fields cancel, resulting in zero inductance? If so, maybe the synchronous motor coils work in a similar way.

 

[JohnW2]

A thought I had - normally a wound coil has an inductance, but if a coil is wound on a former, like a solenoid, clockwise from one end to the other, then wound back clockwise from the other end, wouldn't the magnetic fields cancel, resulting in zero inductance? If so, maybe the synchronous motor coils work in a similar way.

I need top think

On the face of it, quite from anything else, if the arrangement were such that magnetic fields cancelled, it's hard top see how it could function as a motor, since it's a ('net') magnetic filed that makes a motor work.

More after I've had a thunk!!

Kind Regards, John

 

[JohnW2]

This is a link I found http://petrowiki.org/Synchronous_motor, but there's plenty more. It says it can have a leading PF, and can be used with "ordinary" motors to improve overall PF.

I have not yet got my head around this, but ( from here ) .....

Kind Regards, John

 

[JohnW2]

It says it can have a leading PF, and can be used with "ordinary" motors to improve overall PF.

Yes, I'm now reading that all over the place, so I suppose it must be true !!

I've never had reason to think in any depth about, or to understand about, motors, but there is clearly some learning for me to do here!

However, even if I forget that it's a motor and just consider it as a 'black box' with two terminals to which I apply an AC voltage, I still have a problem. The one thing I didn't mention before is that the DC resistance of the motor I was testing was exactly 2.0kΩ. If that were purely resistive, with ~240V AC applied to it, that should result in a current of ~120mA, but what I actually get is a current of ~30mA, which equates to an impedance within my 'black box' of around 8kΩ. However, I am also seeing a PF of around 1.0, which implies a net reactance of zero (i.e. current and voltage in-phase) - so where is 'missing' 6kΩ of impedance within my black box coming from?

Kind Regards, John