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Thread: Controlling the speed of a Fan Heater motor

  1. #41
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    a pair of relays switched by the PicAxe
    Geez. That's not even electronic. Something akin to an oxcart pulled by a Japanese robot.
    - Tex
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  2. #42
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    Quote Originally Posted by PhilHornby View Post
    Very droll



    They're going to 'click' aren't they? ... at random intervals
    Well I had assumed you would be selecting either 1/4 heat, 1/2 heat, 3/4 heat or full heat for some continous period of time, not attempting to vary the power to maintain a target temperature. If that is the object of the exercise how about rectifying the AC for the heating elements then use Triacs to PWM the resultant DC?

    Quote Originally Posted by PhilHornby View Post
    Any views on that current flow graph?
    I've seen similar effects when testing the motor drivers at College. I was always told by the *clever people* that this was down to the motor momentarily becoming a generator. As they had the degrees, I had to believe their words.

  3. #43
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    Quote Originally Posted by Texasclodhopper View Post
    Geez. That's not even electronic. Something akin to an oxcart pulled by a Japanese robot.
    I thought PicAxe were British?

    I'm guessing you've never seen how domestic fan heaters vary the power to the elements? Often for the "Frost" settings there would be a series diode to give the lowest possible output from one element that would be bridged by a combination of switches for the other heat settings. Perhaps it's a UK thing?

  4. #44
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    Hi,

    Quote Originally Posted by tmfkam View Post
    how about rectifying the AC for the heating elements then use Triacs to PWM the resultant DC?
    That might be the "correct" way to avoid interference and "flickering" of other lights. But not by using a Triac or SCR/Thyristor because AFAIK they only switch off at the current-reversal of an ac source. From my (relative) youth I remember "gate turn-off" SCRs, but I believe they needed very large reverse gate currents and were little-used. You could use a FET of course, probably with PWM at quite a high frequency to make the filtering easier. But it's still basically a 2 kW (or more) dc power supply, so the mains reservoir capacitor would need to be thousands of uF at 400 volts (with probably a large inductor to smooth the current as well).

    The electricity supply companies "dislike" the use of half-wave rectification and low Power Factors (large phase shift between current and voltage) because you only pay for the power consumed, but their distribution losses increase with "non-resistive" loads. Normally an amateur/hobbyist may get away with doing "wrong" things, but if you have a "weak" mains supply then the practicalities may force you to do things more correctly.

    I'm not sure if you could use Triacs/SCRs to connect the two heating coils in series (to reduce power), but otherwise your Thyristor + Triac method with zero-crossing firing may be the best solution. For continuous (infinitely variable) control you might delay the firing of the Triac, but probably should use a large series inductor to clean up the current waveform.

    Is your current waveform measured with the "clamp" meter or a series resistor? It appears to have an ac-coupled "step" with a time constant around 10ms; the positive step at the start of the power burst, up to approximately the level of the "T" marker (probably just a coincidence), then the negative step where the burst terminates. That might be an artifact of the clamp meter circuit or perhaps a thermal effect.

    13% slip sounds rather high, but it might be caused by the motor cage having a rather high resistance (by design or economy). As was said above the "load" (or amount of air moved) with a fan is proportional to the cube of its rotational speed, so you might not need a large rpm variation. Therefore a simple impedance in series with the motor could be all that's needed. The load (drag) will be so little at lower speeds that there's probably no risk of the motor stalling.

    "Universal" motors (ac/dc with a commutator) are highly dependent on their "back emf" and may well emit some "magic smoke" (or at least smell) if stalled for any significant time. But I don't think this applies as much to induction motors, and even less to synchronous/stepper motors.

    Cheers, Alan.

  5. #45
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    One problem with two triacs in parallel, and may be applicable here, is that as soon as one is on and drawing current, there's not enough current through the other to allow it to turn on.

    It's a common problem discovered by people who try to double-up as they would when paralleling transistors or FET's to increase current capacity. All that happens is one takes all the current, exceeds its rating, over-heats, explodes, or does something nasty. The other does nothing, until the first departs. Then it too takes the full load and likewise swiftly departs.

    Might not be an issue though so long as one and only one is on at the same time.

  6. #46
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    Quote Originally Posted by tmfkam View Post
    Well I had assumed you would be selecting either 1/4 heat, 1/2 heat, 3/4 heat or full heat for some continous period of time, not attempting to vary the power to maintain a target temperature.
    I suppose that will be the best/simplest/easiest case - I'll have to do some experiments (in Winter!) to see if it's possible. The other (convector) heaters in the cottage use 0.3125C hysteresis, achieved by simple ON/OFF control and I'd like to match that. (0.3125C is 5/16 in DS18B20 terms. Dimplex claim 0.3C for their latest heaters, so I'd thought I see if I could do the same)

    Quote Originally Posted by Him as well
    I've seen similar effects when testing the motor drivers at College. I was always told by the *clever people* that this was down to the motor momentarily becoming a generator.
    I thought this was a great answer - for a couple of hours - until I remembered that this effect is not measurable with the fan only ... it seems related to the heater elements, somehow

    Quote Originally Posted by AllyCat View Post
    Is your current waveform measured with the "clamp" meter or a series resistor? It appears to have an ac-coupled "step" with a time constant around 10ms; the positive step at the start of the power burst, up to approximately the level of the "T" marker (probably just a coincidence), then the negative step where the burst terminates. That might be an artifact of the clamp meter circuit or perhaps a thermal effect.
    The graph comes from the output of a 'sensing' coil from a commercial Energy Meter. I only put it in circuit to confirm that the Picaxe/SSR combination were indeed turning on and off for 60mS as designed. Likewise, the 'Sangamo Weston' clamp meter was only there to give a rough indication of current flow - I didn't expect any great accuracy, but it does seem to be of the right order of magnitude. There's no measurable output from either of these devices, when only the fan is in operation.

    Quote Originally Posted by him as well
    13% slip sounds rather high, but it might be caused by the motor cage having a rather high resistance (by design or economy). As was said above the "load" (or amount of air moved) with a fan is proportional to the cube of its rotational speed, so you might not need a large rpm variation. Therefore a simple impedance in series with the motor could be all that's needed. The load (drag) will be so little at lower speeds that there's probably no risk of the motor stalling.
    13% was higher than I'd been expecting ... but that's what I measured . That particular fan heater (the supermarket one) has two large 'lobes' that make up the fan; maybe the fan is oversized for the motor?
    I'm going to try some of the other motor control techniques - and I have brought the Fan heater section of the Dimplex Optymist back to base, so I can do some tests on the actual target hardware.

    Quote Originally Posted by hippy View Post
    Might not be an issue though so long as one and only one is on at the same time.
    Yes that's the plan.
    Last edited by PhilHornby; 07-08-2017 at 18:08. Reason: Add a product link

  7. #47
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    Default Dimplex internals - update

    Quote Originally Posted by "Me" View Post
    My guess is that currently, the air temperature never exceeds 80 C - it seems to use the type of thermostat commonly found in gas boilers ... though the wiring seems a little odd. (The heater has two separate elements and there are two of these thermostats to act as overload cut-outs. However, they're wired in series with just one element, as far as I can tell...)

    See: https://www.dropbox.com/s/o62hrsfyn2...719_211325.jpg

    and some more photos of the innards, for those who are interested...

    https://www.dropbox.com/s/4nxokmszzo...719_211404.jpg
    https://www.dropbox.com/s/miv16tffhb...719_214504.jpg
    It turns out that the thermostats/cut-outs are, in fact, in the common return for both elements. One is rated at 90C and is what I thought it was ... the other is rated at 125C and is a Manual Reset type, that I've not seen before.

    Optymist heater.png

  8. #48
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    Typically, the lower temp one would be a slow action trip with the higher temp one more fast acting. Engineer put the higher temp one there for unintended consequences.
    - Tex
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  9. #49
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    Default Moving on...

    Quote Originally Posted by Me View Post
    Observing the current flow to the heater, in the Picaxe+Heat configurations shows an odd effect :-

    Attachment 21413


    • Why does current continue to flow when the SSR has been turned OFF?
    • Why is the amplitude of the first half-cycle after SSR triggering, lower than the following five?


    Presumably, the substantial amount of current flowing after SSR turn off, is related to the burst firing of the heating elements; on Fan only I can't detect any measurable current at all, using this method.
    Searching around the web, I've found a few references to this phenomenon, when switching large inductive loads. I've been thinking of the heater elements as resistive loads; but given that they're physically formed as coils, they're probably quite inductive as well!

    Of course, being told that the zero-crossing points are different for voltage and current has raised another question ... if the voltage has gone, what's making the current flow?!? I could live with this theory, if the current direction reversed due to the collapsing magnetic field; but it doesn't

    Ah well, more googling required!

    Quote Originally Posted by inglewoodpete View Post
    I'm not convinced that feedback and incremental control of a heater fan is necessary.
    Quote Originally Posted by tmfkam View Post
    In theory, this would only reduce the voltage applied to the motor, this may affect the speed, but the theory would suggest not.
    I decided to try some of the simpler, 'analogue' methods of fan speed control. I used the following circuits (with the cheap 'supermarket' fan heater) :-
    Analogue Fan Speed Control.png (Click to view!)

    • "A" - 240VAC/25W incandescent bulb. Definitely reduced the fan speed (by or thereabouts). However, the bulb was far too hot and bright for this to be useful.
    • "B" - 2F capacitor. This had no discernible affect on the motor speed. It did, however, scare me to death afterwards, when I discharged it! Unfortunately, the only other mains-rated capacitor combination I could come up with, was only about 0.07F; the fan didn't operate with those in series and I have no other suitable capacitors, in my 'bits box' :-(
    • "C" - Conventional wisdom says the fan wouldn't operate with a diode in series - but it did! However, you could easily count the revolutions, it was going so slowly!
    • "D" - 6VAC transformer (of unknown provenance), with fan in primary circuit and diode in secondary. I didn't measure the speed, but I would say it was approx. to full speed.
    • "E" - Transformer as above, with secondary circuit short-circuited through an ammeter. The ammeter displayed about 2.5Arms, and the fan turned at something approaching full speed.
    • "F" - Transformer as above, with 1R resistance across secondary. Voltage across resistor was 1.4VACrms, so current about 1.4A. Fan speed is approx. speed (more detail follows). The resistor got hot ... 80C or so. Despite having an IR thermometer in my right hand, I tested it first with my left thumb


    I decided to shoe-horn circuit "F" inside the 'supermarket' fan heater, to act as a proof of concept. I've disconnected one of the heater elements, so it's a 1KW heater, with a half-speed fan. I want to see how the room heats up with a slow moving, hot air stream - compared with a fast moving, cooler air stream. I note with some alarm, that there are one or two visible hot spots in the heater element - an issue that will have to be resolved before I modify the Dimplex.

    The measured fan-speed is interesting though - because it's definitely not constant. When I first powered it on, it (visibly) accelerated to 1260rpm (full speed is 3000rpm for this motor). When I switched on the heater element[*], the fan speeded up?!? ... eventually reaching about 1900rpm. Removing the heat, made it slow down to about 1600rpm. All quite odd - but even at 1900rpm, the sound output is greatly reduced ... so I can proceed on this basis.

    Edit
    [*] Could this be some sort of inductive coupling effect, between the heater element and my transformer? ... or the hotter, less dense air posing less of a load for the fan

    I'm going to have a go at some more sophisticated TRIAC control of the fan motor (either phase-control or burst firing), having ditched the SSR. This blog has some details that I can use as the basis of a Picaxe control circuit.
    Last edited by PhilHornby; 13-08-2017 at 05:20. Reason: Had another thought!

  10. #50
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    Default Answering a few of my own questions

    Quote Originally Posted by PhilHornby View Post
    I've been thinking of the heater elements as resistive loads; but given that they're physically formed as coils, they're probably quite inductive as well!
    I remembered I have a multimeter that performs inductance measurements. The heater elements have no inductance that I can measure. Furthermore, applying a simple Power=V2/R calculation gives the expected answer; revealing that there's no Reactance involved. So much for that theory!

    Quote Originally Posted by Me as well

    • "B" - 2F capacitor. This had no discernible affect on the motor speed. Unfortunately, the only other mains-rated capacitor combination I could come up with, was only about 0.07F; the fan didn't operate with those in series and I have no other suitable capacitors, in my 'bits box' :-(
    I did some measurements of my two fan heater motors (and did some calculations, with the help of this web page) :-

    The Dimplex motor has a DC resistance of 220Ω and an inductance of 4.8H. 4.8H equates to 1508Ω @ 50Hz, so the combined Impedance works out at 1523Ω. The reactance of a 2F cap. is approx. 1600Ω (as previously pointed out by 'inglewoodpete') - so I would have expected that to be about the right value to half the speed of the motor - I wonder why it didn't? (I checked the cap. ... my meter said 2.2F).

    Yet my (shorted) 6VAC transformer primary (which did have the desired effect) had an impedance of 1398Ω (Resistance = 543Ω, inductance=4.1H (1288Ω @ 50Hz).

    The measured fan-speed is interesting though - because it's definitely not constant. When I first powered it on, it (visibly) accelerated to 1260rpm (full speed is 3000rpm for this motor). When I switched on the heater element[*], the fan speeded up?!? ... eventually reaching about 1900rpm. Removing the heat, made it slow down to about 1600rpm.
    [*] Could this be some sort of inductive coupling effect, between the heater element and my transformer? ... or the hotter, less dense air posing less of a load for the fan
    It can't be inductive coupling, because removing power to the heater would have an immediate effect - and it doesn't. The fan speed does slowly come back down, but not as quickly as it rises when the heater is turned on. Other than as a general effect, there's no real correlation between the temperature of the air coming out and the speed of the motor.

    The motor has definitely lost a lot of its ooomphh running in this mode; it takes almost 18 seconds to reach 1500(ish) rpm!
    Last edited by PhilHornby; 15-08-2017 at 02:54. Reason: Finally found the Ω sign ;-)

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