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

[fixitflav]

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

I don't know whether this might explain it, bit I'll offer it.
In an ordinary induction motor, if you measure the DC resistance of the windings it would indicate a much higher current than observed. The reason the current is lower is not because of inductance, it's because the motor produces a back-emf, opposing the applied voltage. If the synchronous motor produces back-emf 180 volt, that would give the current figure seen.
Problem is I think the motor must be turning to produce the back-emf (but maybe not for a synchronous motor). If so the theory doesn't work when it's stalled. What do you think?

 

[bernardgreen]

The reason the current is lower is not because of inductance,

Wrong. The inductance creates an impedance to AC voltages, the impedance is higher than the resistance for DC voltages

I think the motor must be turning to produce the back-emf

You think correctly, to be precise it is a counter EMF and not a back EMF.

A Back EMF is typically a single cycle event such as a relay coil being de-powered
A Counter EMF is produced for as long as the motor is rotating

 

[fixitflav]

Wrong. The inductance creates an impedance to AC voltages, the impedance is higher than the resistance for DC voltages

I know impedance comes into it. I should have said "not only because of inductance"

You think correctly, to be precise it is a counter EMF and not a back EMF.

A Back EMF is typically a single cycle event such as a relay coil being de-powered
A Counter EMF is produced for as long as the motor is rotating

OK I'm not an electrical engineer. You know what I meant!

Do you have an explanation for JohnW2?

 

[fixitflav]

OK I'm not an electrical engineer. You know what I meant!

Do you have an explanation for JohnW2?

I thought I'd sussed out multi-quote, but that's gone slightly wrong.

 

[bernardgreen]

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 )

 

[fixitflav]

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 )

OK thanks for the explanation. Does it explain JohnW2's observations?

 

[bernardgreen]

Does it explain JohnW2's observations?

It might explain them.

 

[JohnW2]

In an ordinary induction motor, if you measure the DC resistance of the windings it would indicate a much higher current than observed. The reason the current is lower is not because of inductance, it's because the motor produces a back-emf, opposing the applied voltage.

I don't really understand that "not" - the "back-emf" is surely a consequence of inductance, isn't it?

What other than something with inductance can result in a back-emf, and how?

Kind Regards, John

 

[JohnW2]

Problem is I think the motor must be turning to produce the back-emf (but maybe not for a synchronous motor).

That was one of my problems. Even if 'it is different for a synchronous motor', rotation of the motor must result in at least some 'counter-emf', which is why I didn't understand why I didn't see at least some difference in current between 'running' (turning) and 'stalled' (not turning).

Kind Regards, John

 

[fixitflav]

I don't really understand that "not" - the "back-emf" is surely a consequence of inductance, isn't it?

What other than something with inductance can result in a back-emf, and how?

Kind Regards, John

Yes, you're right, I hadn't thought of it that way. Of course it's the back- (or counter-!) emf is why the impedance is higher with inductance in there.
Maybe bernardgreen's explanation covers it?

 

[JohnW2]

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 therefore a complex impedance of a motor and transformer in parallel.

It certainly seems as if the explanation must be somehow related to that sort of mechanism, but I still don't understand, particularly when I think about my 'black box' approach.

Decades ago, I did experience situations in which I was literally given a 'black box' with two or more terminals, a voltage source and a pile of test equipment, and was required to work out 'what was in the box' (or, at least, what the contents of the box was 'equivalent to'.

If, in that situation, I had found that applying 240V 50Hz resulted in a current of 30mA, with that current in phase with the voltage, I would have concluded that the box contained the equivalent of an 8kΩ resistor - and that if any reactive components were present, the magnitude and phase of their reactances at 50Hz (i.e. magnitude and phase of currents through them) were such that the net reactance (and reactive current) was zero. If I had measured the DC resistance of the box and found it to be only 2kΩ, I think I would then have been very confused.

Put another way, I can't see how 'something' can add a component of impedance to the resistance such as to increase overall impedance from 2kΩ to 8kΩ whilst maintaining a situation in which the overall current is in phase with the voltage.

Can you help me any further?

Kind Regards, John

 

[JohnW2]

Yes, you're right, I hadn't thought of it that way. Of course it's the back- (or counter-!) emf is why the impedance is higher with inductance in there.

Indeed.

Kind Regards, John

 

[ericmark]

As I understand it there are two ways to control how much a boiler is modulated, one as John says is the return water temperature, the second is a "Thermostat" connected to the boiler bus. As to if one would really call the device a thermostat depends on manufacturer, as I understand it EvoHome uses OpenTherm and it collects the temperature readings of all the TRV heads and from that decides the output required from the boiler.

Using the bus connection as the house warms up the circulating water temperature is reduced, however using return water temperature the reverse is true, as with any heating system the losses are calculated by time system running to time system required, as if at switch off the water is cool there are less loses than if at switch off the water is hot, so whole idea is as the system starts to reach satisfied there is the minimum possible of heat stored in the system.

Same goes for turning a boiler off, once switched off any heat in the boiler is lost through the flue, so want to switch off as few times as you can, and on switch off have boiler as cool as one can. So although a boiler can use return water temperature that is not the best option, although likely the cheapest option.

The problem is over the last 40 years the whole idea of central heating has been turned on it's head, first central heating I had was hot air, 1970's it heated the house fast, and all rooms at the same temperature with a single thermostat, however circulating the air caused huge looses as it moved air across cold single glazed windows and the walls without cavity wall insulation, and no loft insulation. So expensive to run, however today same system would work far better, there are fan assisted radiators which today turn a water system into a hot air system with 5 fan speeds auto selected according to heat output required, main advantage is speed the rooms are heated, switch heating on as you get home and up to temperature within an hour, however these also turn the whole idea of modulating a boilers output into a nightmare. They also have the advantage with heat pumps they can cool the room as easy as heat it.

But all the new ideas, well my Myson fan assisted radiator is 30 years old so not that new, but the ideas seem to work against each other, so 20 years ago zones were bees knees, but the wifi connected TRV head has turned the whole system on it's head which is not helped by TV adverts pushing antiquated systems like Hive which have now been superseded with systems like Nest and EvoHome which use OpenTherm, that is of course if the boiler supports OpenTherm, seems some large manufactures like Bosch have "Wave"ed the idea away with their own system, with limited interconnection with the TRV heads or non.

As a result I will wait until the industry has got their act into gear before upgrading my boiler, maybe we will do away with modulating boilers and fit sterling engines to the flue and pump what would be waste energy back into the system using micro generation?

 

[JohnW2]

As a result I will wait until the industry has got their act into gear before upgrading my boiler, maybe we will do away with modulating boilers and fit sterling engines to the flue and pump what would be waste energy back into the system using micro generation?

... isn't that, in concept, what condensing boilers seek to do (albeit I doubt that they do much condensing when connected to an existing system of radiators)?

However, rather than developing ways of recovering otherwise lost heat from flue gases, I would have thought that the medium-/long-term aim ought to be to move completely away from the idea of burning hydrocarbons 'locally'.

Like you, I have an ~30 year-old boiler which I am attempting to 'nurse' into a few more years' life whilst I observe what is happening to the technology.

Kind Regards, John

 

[bernardgreen]

Can you help me any further?

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 ).

Even a simple incandescant lamp changes its resistance as the filament heats up.