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    Regulating temperature of mains connected heating element: Skipping mains half cycles

    Hello,
    We have an electric heating element connected to the mains. We wish to regulate the temperature by skipping half mains cycles, as in the attached waveform (LTspice sim also attached).
    The power factor is great because mains current and voltage are always in phase. But is there any problem caused in the mains electricity system by this ON/OFF type waveform?

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    Re: Regulating temperature of mains connected heating element: Skipping mains half cy

    Full wave switching is commonly used for heaters, half wave should be avoided due to possible DC load. Flicker limits according to IEC 61000-3-3 must be observed.



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    Re: Regulating temperature of mains connected heating element: Skipping mains half cy

    Thanks, you mean by full wave that a full 20ms must be switched out, not just 10ms?



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    Re: Regulating temperature of mains connected heating element: Skipping mains half cy

    Always try to draw equal current from each polarity of cycles so the average over a short length of time is always zero. If you draw half cycles, particularly if you do it repeatedly, makes the load appear asymetric, more like a DC load than an AC one. If you switch out a full cycle, especially at zero crossing and into a non inductive load like a heater, you eliminated the DC shift and also EMC from switching at high voltage.

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    Re: Regulating temperature of mains connected heating element: Skipping mains half cy

    Hi,

    Yes, exactly.
    The idea behind this is that the positive halfwave should be equally loaded as the negative halfwave in either case.
    This avoids DC residuals in mains grid system.

    Klaus
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    Re: Regulating temperature of mains connected heating element: Skipping mains half cy

    Thanks, if you look at the waveform of the top post, it skips only 10ms intervals, but the overall DC is zero......it skips a positive half cycle, then the next time a negative half cycle is skipped, ....then a positive half cycle is skipped...Etc etc...so overall its OK?

    If you look at the attached, then yes, it is lop-sided. So the top post one is good?...the one attached is bad?



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    Re: Regulating temperature of mains connected heating element: Skipping mains half cy

    Both are bad, especially the one in post #6. By alternating the half cycles you are creating a 25Hz (I think the fundamental is 50Hz in your diagram) offset, in truth it probably wouldn't cause too much trouble but it is bending the rules. If you need to drop the power even more by dropping further half cycles, which may result in more of one half cycle than the other, then you would run into problems. Along a lossy power cable you might find the residual half frequency offset increases current drawn by other appliances, for example inductive loads.

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    Re: Regulating temperature of mains connected heating element: Skipping mains half cy

    Thanks,
    Presumably the attached two mains input current waveforms to the heater are perfect, and would cause no problem in the mains supply system?

    - - - Updated - - -

    Flicker limits according to IEC 61000-3-3 must be observed.
    …Thanks
    https://www.emcstandards.co.uk/files...s__flicker.pdf
    ….Thanks FvM, because from page 9 of the above, I see that this way of controlling heaters (by burst firing the mains) is likely to always be illegal. Do you agree?
    Though the whole thing seems to be based on flicker of highly inefficient filament lamps….which are widely illegal anyway….so why are flicker laws still so tight? Why aren’t flicker regulations relaxed?

    Supposing that a heater has a burst fire duty of skipping one mains cycle every six mains cycles………..and then suppose that by coincidence, every single customer on that phase, possibly 100’s of users, also had their heater on the same burst fire duty…and suppose by coincidence that they were all synchronized…then its possible to see how the mains voltage level could be a bit up and down. But how would this be damaging to any other equipment on that phase?…..we already appreciate that it could cause flicker in highly inefficient filament lamps, but they are illegal anyway.
    Are these flicker laws simply irrelevant to today’s situation?

    - - - Updated - - -

    The thing is , we don’t see big mains fluctuations anyway even when people all use 16Kw Electric showers at the same time every morning, so I really cant see how we would see widely undulating mains voltages as a result of electric heaters burst firing.
    This looks like a case of an irrelevant regulation on flicker, wouldn’t you agree?
    The flicker laws clearly arise form the 19th century when lamps used to flicker when carbon lamps malfunctioned due to inadequate maintenance……..so really, this regulation on flicker needs updating surely? …..and burst fire heaters are realistically no problem whatsoever?



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    Re: Regulating temperature of mains connected heating element: Skipping mains half cy

    I see that this way of controlling heaters (by burst firing the mains) is likely to always be illegal. Do you agree?
    Not at all illegal. But the switched power has to be limited and the switching pattern should be optimized according to the 61000-3-3 spectrum mask.



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    Re: Regulating temperature of mains connected heating element: Skipping mains half cy

    "Burst firing" in this context is no different to mechanical thermostats but operating a bit faster. The alternative is phase control as in conventional lamp dimmers but for heavy loads that introduces its own set of problems.
    The issue is whether equal load is placed on positive and negative half cycles, not the absolute AC load though.

    16KW showers? That implies a load current of about 65A, my whole house is on a 50A circuit breaker!


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    Re: Regulating temperature of mains connected heating element: Skipping mains half cy

    Most instant water heaters have 18 to 27 kW power rating, of course 3 phase supplied. Newer models use electronic temperature controllers with full wave switching.



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    Re: Regulating temperature of mains connected heating element: Skipping mains half cy

    Thanks, it looks like electric showers are 7.5 to 10kW......this comes off single phase mains, but it has its own thicker wiring...and comes off its own breaker.
    https://www.mirashowers.co.uk/blog/m...ngs-explained/

    - - - Updated - - -

    Newer models use electronic temperature controllers with full wave switching.
    Thanks, and i take it that "full wave switching" means burst firing but the bursts are always even numbers of half mains cycles?....ie full mains periods.

    I wonder what badness happens in the mains supply system if we do the "uneven" 10ms switching type thing?



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    Re: Regulating temperature of mains connected heating element: Skipping mains half cy

    Unbalanced switching just creates distortion in the waveform and a DC offset toward the voltage you are not pulling down. In most cases it would be insignificant but for appliances at the end of a long cable where there was a significant inductance and/or resistance it could increase the chances of core saturation and increased current draw.

    Many years ago, in the early days of color television, there was a problem caused by unbalanced current draw. The TVs often consumed 400W or more and used half wave rectifiers directly from the AC line input. As they became more popular they created a problem with street lighting! At the time, in the UK it was common practice to turn street lighting on and off by applying a DC bias to the AC power lines. It was a cheap and simple way to activate/deactivate many lamps at once from a single point. The load of several TVs was enough to turn street lighting on! Regulations were quickly introduced that forced the use of filters and full-wave rectifiers. Thankfully modern TVs use PFC driven switch mode supplies and they all use bridges at their inputs.

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    Re: Regulating temperature of mains connected heating element: Skipping mains half cy

    OK Thanks very much, i now appreciate that we cant do 10ms "gaps".

    According to pg 12 of the following on EN61000-3-11, for the attached LTspice sim and schem, the difference in mains peak-to-peak in the burst fire mains waveform (of the attached simulation) is 7% (ie its 668vpkpk in the burst fire “gap”, and 624vpkpk when bursting). This is using the mains impedances from the following document…
    https://www.emcstandards.co.uk/files...s__flicker.pdf
    This means that it is illegal. Here in the attached schem and LTspice simulation it is supplying a load that is 6.3Kw. (32.3Arms)
    I just can’t understand how people are selling electric heaters with full-wave control. They are going to be illegal by EN61000-3-3 or EN61000-3-11 ?

    - - - Updated - - -

    Also what is the problem of doing Burst fire mains?......(with full wave switching)
    I appreciate that the mains will go up and down in level when it burst fires due to its impedance, but why is this a problem?
    What badness is it that EN61000-3-11 is protecting against?



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    Re: Regulating temperature of mains connected heating element: Skipping mains half cy

    The commercial products I have seen use a fixed cycle time of about 40 seconds. The regulation is done by changing the "on" time while keeping the cycle time constant.


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    Re: Regulating temperature of mains connected heating element: Skipping mains half cy

    Your flicker assessment is a kind of shallow. IEC 61000-3-3 isn't applicable for > 16 A load current. You have to refer to IEC 61000-3-11. It's however correct that the burst period has to be well considered.

    In case of instant water heater, 40 s is too slow, but 2 to 10 s may work.


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    Re: Regulating temperature of mains connected heating element: Skipping mains half cy

    Thanks, do you think the attached mains input current waveform would pass EN61000-3-11?
    Its 32ARMS.
    If this were the mains input current waveform to a device, would it fail regulations?



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    Re: Regulating temperature of mains connected heating element: Skipping mains half cy

    I don't have the requirements at hand right now. Does your application require this short burst period?



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    Re: Regulating temperature of mains connected heating element: Skipping mains half cy

    thanks, yes because its an electric shower.



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    Re: Regulating temperature of mains connected heating element: Skipping mains half cy

    I would expect a larger time constant of the heater with at least < 1 Hz cut-off frequency.


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