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

[JohnW2]

Temperature sensitive resistors. Increase voltage and the resistor takes more current and heats up. The increase in temperature can increase the resistance of the device ( positive co-efficient ) or decrease the resistance ( negative co-efficient ).

I find it very hard to believe that such is the explanation for what I have observed. The current I observe does not change at all with time, and is at its constant value when first displayed, within a fraction of a second from application of the voltage. It is also unchanged many minutes later if I leave the power applied (after motor has stalled).

In any event, I would rather doubt that there is any material whose temperature coefficient is such that its resistance would increase 4-fold in response to a change from ambient temp to a credible 'running temperature'.

I imagine that there must be an 'electromagnetic' explanation fro what I have observed.

Kind Regards, John

 

[JohnW2]

Temperature sensitive resistors. Increase voltage and the resistor takes more current and heats up. The increase in temperature can increase the resistance of the device ( positive co-efficient ) or decrease the resistance ( negative co-efficient ).

Further to my recent post which seriously doubted that as a relevant mechanism ....

... after running for 2 hours 'stalled', the DC resistance had increased from 2.02kΩ to 2.44kΩ (hardly 8kΩ!) but, interestingly, the current remained stubbornly unchanged at 31mA and, even more interesting, the PF had crept down from 0.98+ to 0.87 (so true power consumed had fallen a bit, despite the unchanged current). After 15 mins cooling, the DC resistance was back to just over 2kΩ and on powering up again, we were back to 31mA and a PF of about 0.98.

Kind Regards, John

 

[SimonH2]

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.

No, zone valves tend to be either open or closed, as previously mentioned they were perhaps the first iteration of adding controls to an otherwise uncontrolled system. One house we used to live in was built in '69, oil boiler, and no controls other than a time clock. The design principle was that the boiler runs when the time clocks says so, once the system is hot it starts cycling, and by balancing the system you got the rooms to "reasonable" temperatures. Dad added a room stat as one of the first things he did, and the boiler engineer used to comment on how clean the boiler was inside - thanks to the lack of short cycling.

I looked at fitting all electronic TRV heads but it works out rather expensive.

Indeed, waiting to see what the next batch of OpenTRV (aka Radbot) heads turn out like, supposedly due out around October - should cut the cost down significantly. But if you ignore the modulating for a moment, you can think of these as "a zone valve per radiator" - ie extending the idea of zones down to room level. As an aside, one of the other houses (actually a bungalow) in our new street was customer fitted for the purchaser - he had a wall stat and zone valve for each room which was "really cutting edge" for the early 70s.
TRVs, especially programmable electronic ones, take that one step further by modulating the radiators rather than just being on-off.
You can run the system with the boiler just "doing it's own thing", or it's better if the TRVs signal to the boiler when heat is actually needed - that's fairly easy to do just with a "radio controlled relay" that switches on the boiler when there's a TRV signalling for heat.

The big problem for all forms of TRV is that the flow requirements for the heating are incompatible with the flow requirements for the boiler - and boiler manufacturers seem to still have their heads stuck in the last century Basically the flow requirements are for lower flow as demand reduces, but the boilers are designed for a minimum flow which usually means not far off a constant flow - hence the need for a bypass. But, this means that as load reduces, there needs to be more bypass, and the return temperature to the boiler increases. With a variable flow system, the return from the rads reduces with reducing demand.
When I get to that stage, I'll be fitting a thermal store in our house - decoupling the boiler from the heating (and hot water, I really don't like combi boilers ) which means the heating can use a modulating pump which I can tell you makes for a nice quiet system (non of the hissing so often found as the pressure goes up as the valves close off).
And an interesting arrangement of the plumbing (may need a separate FGHR unit) to maximise condensing in the boiler while satisfying the manufacturer's requirement for constant (excessive) flow.

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,

Indeed, those fan coils are very effective at getting heat from the water to the air in a small space - and that's especially useful if you want to run a low temperature system to make best use of renewables (and a condensing boiler). I would be inclined to use a TRV to modulate the flow rather than an on-off stat - that would avoid the hot-cold-off-hot-cold-off-... cycling.

... after running for 2 hours 'stalled', the DC resistance had increased from 2.02kΩ to 2.44kΩ (hardly 8kΩ!) but, interestingly, the current remained stubbornly unchanged at 31mA and, even more interesting, the PF had crept down from 0.98+ to 0.87 (so true power consumed had fallen a bit, despite the unchanged current). After 15 mins cooling, the DC resistance was back to just over 2kΩ and on powering up again, we were back to 31mA and a PF of about 0.98.

I can't help thinking that the shaded pole is what's doing the bulk of the losses.

 

[ericmark]

Many year ago I worked as an auto electrician, with the exception of electrohaul and straddle carries mainly working with 12 and 24 volt. Often generated as three phase even if used as DC, however some of the very old stuff had some very complex designs.

Buses were really a special case, but dynamos and alternators were designed to give a reasonable output on tick over as well as when running between stops, and they used a system called the bucking coil, any dynamo would have residual magnetism which would start the whole process off, however with a bus this residual magnetism was enough to over charge the battery, so the field winding had so many turns in reverse so it would kill the residual magnetism.

That was well before I got my degree and I don't really know the theory of how it worked, however I do know it did work. The bus as I said was special the alternator CAV 203 was the only alternator I know of with current regulation the 440 regulator was a special instead of Hi, Med, Low it has M1 and M2 terminals and a resistor which was a lump of steel with cuts in it measured the current (volt drop) to both feed the regulator and work the ammeter. Without the special 440 regulator the alternator could burn its self out.

In the same way I would guess the motors in a motorised valve are rather special, and as with the bus charging system we know its special but really as to how it all works we simply don't need to know.

Or maybe we do? I remember a Y plan and trying to fault find, the readings simply did not seem to make sense, circuit diagram showed a single micro switch, stab in the dark I went for a new motorised valve, if I could have bought a new head I would have done, but week end, once changed all worked as expected, opened the old valve, not one but three micro switches inside.

It was clearly the V2 micro-switches which had failed, but could not have bought them on a Sunday anyway so new valve was the right option.

 

[ericmark]

What he said

Yes the systems do seem to work well in theory, however in practice a building management system is required to integrate boiler and fan assisted radiator. Having spent the latter years of my working life writing and modifying PLC programs I am sure I could write a program to run a really good central heating system.

With a multi-million shopping complex I an sure I could save the client a lot of money, but with a private house that's a different story. It all down to installation cost, and I don't think I could write a system which would ever pay for its self within the life time of the client.

 

[JohnW2]

I can't help thinking that the shaded pole is what's doing the bulk of the losses.

You may well be right but, as you will have seen, I'm still struggling with the basic physics of the overall situation (which at first sight makes little sense to me), regardless of the mechanisms.

I think we need a clued-up electrical engineer!

Kind Regards, John

 

[JohnW2]

... Did you measure PF both loaded and unloaded ?

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.

Found it! The spare motor is fractionally, although certainly not significantly, different from the one I tested before (with its valve). The spare motor has a DC resistance (when cold) of 2.04kΩ. With 242.3V applied and running "unloaded" (see **), it draws 35 mA, with a PF of 1.00. Stalling the motor (takes a bit of effort!) makes absolutely no difference - 35 mA & PF of 1.00.

** although externally 'unloaded', there is a substantial gear chain, reducing speed from its 3,000 RPM to about 4 RPM, which presumably introduces some mechanical load/losses.

So, 'loaded' or 'unloaded', running or stalled, the current and PF remain essentially unchanged, that current being around a quarter of what it would be on the basis of the DC resistance (and unity PF) alone. I have not yet made any significant progress in getting my head around all this!

I can't help thinking that the shaded pole is what's doing the bulk of the losses.

I'm not sure that "losses" is necessarily the right word. What I am observing is that the motor is consuming only about a quarter of the power it would be consuming if the load it presented consisted of its DC resistance alone! The assumption therefore has to be that 'electromagnetic' phenomenona (as yet not really understood) are in some way resulting in a reduction in the power consumed.

Kind Regards, John

 

[fixitflav]

You may well be right but, as you will have seen, I'm still struggling with the basic physics of the overall situation (which at first sight makes little sense to me), regardless of the mechanisms.

I think we need a clued-up electrical engineer!

Kind Regards, John

Some synchronous motors have a shaded pole, a magnetic pole with a ring of copper around it, The ring of copper acts as the shorted secondary of a transformer, The primary of the transformer being the main coil of the motor.

This produces a magnetic field in the shaded pole that is out of phase with the magnetic field in the other pole(s) and thus there is a rotation in the magnetic field applied to the rotor.

The over all impedance of the motor is therefor a complex impedance of a motor and transformer in parallel.

The motor is also designed so that DC applied to the coil ( or half wave rectified AC ) will hold the motor stationary against the return spring. ( for holding a three port valve in the mid position )

If anybody's still interested, there's a forum CR4 I get on, with an electrical section, so I posted the question there.

I asked (in case it helps follow the replies)
"I got into a discussion on a heating and plumbing forum about diverter valves and the power consumed by the motor.

The one I have is 3-port, spring return, synchronous motor, stalls at end of travel when energised, which can be for long periods. Some posters queried the power when stalled, and whether that would cause overheating and damage.

Mine is labelled 230/240v AC 50hz motor 5 watt. I guess that is electrical power, as it doesn’t produce any output power when stalled, but in any case it’s clearly not going to do any harm, it will pick up much more heat from the valve body.

I haven’t had much to do with synchronous motors, but I understand the permanent magnet types (as this is) have PF ~ 1.

The query – one of the posters took some measurements on a similar valve, and came up with

“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!
The DC resistance of the motor was exactly 2.0kΩ”

The apparent impedance when running (or stalled) is = 244.3/0.031 ~ 8kΩ, so as the PF is close to 1, shouldn’t the DC resistance and impedance be the same? Or at least not differ by a factor of 4.

We’re all puzzled, can anybody explain?"

Reply 1 (RAMConsult)
"There are actually two types of low power/torque motors used in small actuator duty such as a zone valve. Typically they are impedance-protected shaded pole synchronous motors, not higher torque (and more expensive) permanent magnet synchronous motors. Since most zone valves rely on the impedance-protected feature to prevent the motor from overheating when stalled when the valve reaches its end of travel, I wonder which type of motor you have.

The data you measured indicates it's a shaded pole motor (unless the nameplate suggests otherwise). Although it may appear counterintuitive that a stalled motor would have near unity power factor, that's exactly what should happen. Remember if we stall a motor it does work to overcome the stall and overheats until a circuit protective device operates or the coil burns out.

An impedance protected motor is specifically designed with thin wire that overheats very rapidly, and as the wire heats up, its resistance goes up as well, resulting in lower current being drawn. These motors are designed to reach an equilibrium temperature below the rating of the winding insulation so that nothing burns out.

The key to the near unity power factor is that all the current being drawn is going into the I2R losses in heating the motor, and I2R losses are Watts (Real Power)."

Reply 2 (JRaef)
"I agree, that’s what the description fits, an impedance protected motor, often identified somewhere with the letters “ZP”, Z being Impedence, P for Protected.

One thing worth mentioning here, you (OP) indicated that the valve body may be getting hotter than the motor, which I took to mean the fluid is high temperature. In the design and testing standards for ZP motors the motor is allowed to reach no more than 175deg C for 18 days. This makes this type of motorized valve unsuitable for many steam applications without some means of cooling. If it’s a liquid, don’t worry about it but if it is steam, post back and I’ll tell you a trick!"

JRaef has always seemed pretty good on electrics. I told him fluid temperature isn't an issue here as it will be 90°C tops.

Their explanations seem to have a lot in common with what bernardgreen said. Not sure whether all this explains the 8kΩ apparent impedance vs the 2kΩ measured DC though.

 

[JohnW2]

If anybody's still interested, there's a forum CR4 I get on, with an electrical section, so I posted the question there.

Many thanks.

" ... There are actually two types of low power/torque motors used in small actuator duty such as a zone valve. Typically they are impedance-protected shaded pole synchronous motors, not higher torque (and more expensive) permanent magnet synchronous motors. .... The data you measured indicates it's a shaded pole motor ... An impedance protected motor is specifically designed with thin wire that overheats very rapidly, and as the wire heats up, its resistance goes up as well, resulting in lower current being drawn. These motors are designed to reach an equilibrium temperature below the rating of the winding insulation so that nothing burns out. ... The key to the near unity power factor is that all the current being drawn is going into the I2R losses in heating the motor, and I2R losses are Watts (Real Power)."
Their explanations seem to have a lot in common with what bernardgreen said. Not sure whether all this explains the 8kΩ apparent impedance vs the 2kΩ measured DC though.

I don't believe it does. As I indicated, even after a long period stalled, the DC resistance changes (as a result of increased temp) only by a very modest amount ...

... after running for 2 hours 'stalled', the DC resistance had increased from 2.02kΩ to 2.44kΩ (hardly 8kΩ!) but, interestingly, the current remained stubbornly unchanged at 31mA and, even more interesting, the PF had crept down from 0.98+ to 0.87 (so true power consumed had fallen a bit, despite the unchanged current). After 15 mins cooling, the DC resistance was back to just over 2kΩ and on powering up again, we were back to 31mA and a PF of about 0.98.

Kind Regards, John

 

[fixitflav]

Many thanks.
I don't believe it does. As I indicated, even after a long period stalled, the DC resistance changes (as a result of increased temp) only by a very modest amount ...

Kind Regards, John

Agreed, I don't think we've bottomed it yet. I'm sceptical about the idea that temperature rise is the answer, it doesn't seem plausible and your measurements showed otherwise. On that subject, in your cold and hot measurements, I^2*R*PF doesn't change much - I^2 is constant, R*PF = 1.98 cold, 2.12 hot. Though I can't think of a reason why the PF should fall.

 

[bernardgreen]

Possibly a large change in impedance of a stalled motor could be a solid state self resetting fuse that disconnects one of the windings.

Two windings is parallel, one via a polyfuse, provide enough torque to drive the motor forward against the spring return to the stall point. The stalled motor warms up and that plus the current throught the polyfuse trips the polyfuse depowering one winding. The other winding creates enough torque to prevent the motor being driven back by the spring. Even if the motor cools down the polyfuse will remain open circuit until power has been removed which occurs when the motor is required to allow the spring to drive it backwards.

 

[JohnW2]

Agreed, I don't think we've bottomed it yet. I'm sceptical about the idea that temperature rise is the answer, it doesn't seem plausible and your measurements showed otherwise.

Indeed. As I've said, although I remain far from understanding exactly what it might be, In view of what one reads about these motors it seems as if there must be an 'electromagnetic' explanation.

On that subject, in your cold and hot measurements, I^2*R*PF doesn't change much - I^2 is constant, R*PF = 1.98 cold, 2.12 hot. Though I can't think of a reason why the PF should fall.

I suppose it depends on whether the resistance and reactance behave as being in series or parallel (or somewhere in between!). If one assumes that reactance remains unchanged, if a resistance in parallel with it increases (with temperature), then the impedance would become 'more reactive' (i.e. PF would decrease), whereas if the resistance were in series with the reactance, the impedance would become 'less reactive' (i.e. PF would increase), wouldn't it?

Kind Regards, John

 

[JohnW2]

Possibly a large change in impedance of a stalled motor could be a solid state self resetting fuse that disconnects one of the windings.

Apart from the fact that such sounds a little more sophisticated than one would probably expect to find in what is essentially a 'cheapo' motor, what I have observed is absolutely no change in either impedance (as reflected by current) or PF (which would change if one "disconnected one of the windings") when the motor stalls.

Kind Regards, John

 

[ban-all-sheds]

Doesn't back-emf limit the current when the motor is running, so although you are assuming that no change in current = no change in impedance, that might not be the case? I always though that the problem of motors drawing more current when stalled is not because their impedance drops, it's because back-emf stops?

 

[JohnW2]

That also corresponds to my understanding - so maybe I should have talked of 'effective impedance'. If one regards the motor as a 'black box' to which an external voltage is applied, a change in the resultant current which flows into that black box equates to a change in the 'effective impedance' of that box.

In any event, what I have observed is a failure to draw more current when stalled, so the question of the mechanism of an increased current when stalled (if it arose) is essentially moot.