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Microwave transformer inrush current solution?

Discussion in 'Electronic Design' started by Richard Rasker, Jun 26, 2013.

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  1. Hello,

    I've been asked to look into a problem with an industrial installation with
    3 kVA HV magnetron (microwave oven) transformers, where mains fuses keep
    tripping due to the inrush current.

    Normally, I'd look into NTC's or power resistors with bypass relays;
    however, those solutions are useless in this case, because the magnetron
    units are switched off and on very frequently (up to a dozen times a
    minute). NTC's need at least a minute to cool down, and power resistors
    (e.g. 5 ohms) would be dissipating a few dozen watts this way, even when
    bypassed after a mere 100 ms.

    So I wonder if it's possible to switch off just the magnetron's cathode
    filament instead of the whole supply transformer. Obviously, the relay would
    need to be a special type, with at least 6 kV insulation voltage between the
    coil and the switching contacts, but for the contact rating itself a few
    volts and amperes would suffice.

    Does anyone have any experience switching magnetron elements this way?
    If so, are there any problems to be expected? And where can I find relays
    that can handle the required insulation voltage?

    And oh, I also looked into a triac-based solution, but that has its own
    snags: the need for zero-crossing current(!) switch-on timing, and bypass
    relays to prevent several dozen watts of power dissipation due to the
    triac's on-state voltage of up to 1.5 volts @ 16A -- yet this relay in turn
    complicates switching off during zero crossing ...

    Thanks in advance for any insights,

    Best regards,

    Richard Rasker
  2. I presume that they're using a mechanical relay to switch the primary
    of the transformer, and that simply switching to a slow(er)-blow fuse
    or circuit breaker is not an option for some reason.

    High inrush current is a function of the magnetization of the core
    from the previous cycle and the point in the AC cycle when transformer
    is energized.

    You could use a commercial product such as these ones:-
  3. Hello Vladimir,

    That is very interesting to know. Can I assume that this also happens when
    switching the transformer including magnetron element off and on? Because
    that's what these people are doing at the moment, and they may be interested
    to hear that they can prolong the life of their magnetron elements.

    If this only happens in my scenario (so turning the filament off and on with
    HV present), then why is that?

    You mean switching just the HV off and on, instead of the whole magnetron
    element, and keeping the cathode filament powered-up?

    One problem is of course that they already have a complete set-up,
    transformers and all, and don't want to replace most of the hardware. But
    I'll certainly look for a HV switching solution.

    Anyway, thank you for your advice already,

    Best regards,

    Richard Rasker
  4. yes, they make/made HV reed relays for this. Old microwaves had these to
    switch the HV side from the transformer on and off for the defrost cycle.
    I don't recall if the AC or side or the doubled up DC was switched though.
    The transformer and heater coils were powered up the entire time, so you
    never got crazy inrush current problems like you're facing.
  5. Bill Martin

    Bill Martin Guest

    I've wondered about how that can work...maybe not so big a problem at
    kilowatt power levels. I can assure you that it is very bad at megawatt
    power levels! Magnetron life can be shortened to approximately zero in a
    hurry if HV is applied when ther cathode is not at full temperature.
  6. They're already on the slowest circuit breakers feasible, and 99 out of a
    100 times things switch on without tripping. But this still means that thigs
    go wrong every 15 minutes or so.
    I know. Remanence combined with same direction field from new inrush current
    = saturated core = no more induction, just DC resistance (0.5 ohms). Hence
    the triac solution, with switch-on at mains voltage zero-crossing, and
    switch-off at current zero-crossing (plus perhaps a small extra timing
    factor to allow for magnetic hysteresis). Now if only those triacs didn't
    have this >1 volt voltage drop ...
    Thanks, I'll contact these people for some pricing information (although
    experience tells me that these things won't be cheap).

    Best regards,

    Richard Rasker
  7. These things will be running 8 hours daily, with an estimated 500 up to 2500
    on/off cycles. Indeed a bit of a waste if they needs replacing every two
    weeks or so.

    I'll look for switchmode (and switchable) HV generators with separate,
    continuous filament power.

    Thanks for all the help.

    Best regards,

    Richard Rasker
  8. Guest

    try a suitable sized incandesent light bulb across the on/off switch/relay.When the switch opens the filament will be cold so the current through the xformer will continue and will gradually reduce as the filament heats. In other words you demagnatize the core each time it shuts off so that it won't be able to saturate at the next turn on.

  9. Is the dissipation in a triac really that much of a problem?

    lots of stuff run of SSRs like:

  10. Nico Coesel

    Nico Coesel Guest

    How about timing the relay so it switches at the zero crossing? Switch
    on when the voltage is zero and switch off when the current is zero.
    With a constant voltage the time a relay needs to close the contacts
    should be pretty constant. With a microcontroller and volt / current
    sensing you could make it self-adjusting.
  11. Mechanical relays are too inaccurate for this kind of timing -- typically up
    to 10 ms (1/2 mains period) activation and switch-off time, with quite a bit
    of tolerance (20% easily), not to mention a bounce time of at least half the
    activation time. And indeed this would require a controller with software,
    current and voltage sensing, a low-voltage supply ...
    For now. it's probably easiest and cheapest to follow Spehro Pefhany's
    advice and simply buy a couple of commercial transformer relays, especially
    designed for the job.
    For the next generation of magnetron drivers, I'll try and find switchable
    HV supplies with a constant cathode filament voltage.

    But thanks for thinking along anyway :)

    Best regards,

    Richard Rasker
  12. Does the magnitron have a torroid transformer? There are gapped
    torroids just for this problem.

    You could also try bleeding off the core to reset it during the OFF
    time. This may be the easist to try. Once the core is reset, you dont
    have to worry which AC cycle you start on.
    Likey the core is not reset and starting again on the same AC cycle
    saturates the transformer and pops the fuse.

  13. Jamie

    Jamie Guest

    We run into that problem many times with inrush currents due to poor
    contactor closures for motors and transformers.

    You may have noticed that you don't always get the big initial surge
    but when you do, it is very noticeable. Poorly or cheap designed contact
    closures can cause bounce and depending where you are on the curve, if
    can cause strong mag current.

    What works well is an electronic closure with mechanical closure to
    follow. This not only helps with this problem but also extends the life
    of the contacts.


  14. Phil Allison

    Phil Allison Guest

    "Richard Rasker"
    ** Switching on at a zero crossing creates the LARGEST possible *magnetic*
    inrush surges with transformers - this is so because the AC wave takes a
    whole cycle to average to zero volts.

    For minimum magnetic surge current, you switch on at a voltage peak - this
    way the AC wave averages to zero in a mere half cycle.

    To make a transformer that does not surge with random switch on points,
    simply wind the primary for double the AC supply voltage - this has the
    same effect as using the transformer at double the usual supply frequency.

    NB: In cases where the transformer is loaded with rectifiers and large
    capacitor bank, surge current due to charging the caps dominates if you
    switch on at AC supply peaks. This is not the case with the OP's question.

    .... Phil
  15. Phil Allison

    Phil Allison Guest

    "Richard Rasker"

    ** Are the 16 amp breakers a part of the microwave units OR just installed
    in the AC supply to each oven ? If the latter, then simply use a larger
    breaker, ie 20amp.

    Remember - supply breakers are only there to protect cabling, not

    IME the instantaneous tripping current of magnetic/thermal breakers is about
    10 times the nominal amp rating and it seems your oven need a bit more than
    160 amps.


    Old fashion wire fuses have much higher tolerance to very short surges like
    transformer magnetic inrush.

    .... Phil
  16. Guest

    Since when does a fuse "trip"??? They either open or not, but in English the word, trip, implies it is resettable, so maybe you're talking about circuit breakers??? Who would know? And who would know of it's a plurality of "fuses" or a single fuse, wherein a singular fuse is indicative of a miswiredinstallation.
    Not practical for the usual duty cycle modulated installation.
    Absolute last thing anyone would want to do.
    Yep, TRIACS are hardly ever used for power control, it's amazing the component even exits it's so cumbersome to use.
  17. Guest

    Saturated transformer peak inrush current is winding resistance limited so it makes no difference whether it got there by being cycled through zero or peak reinforcing residual magnetization. The only difference is a cycle of exra integration time.
    The installation power distribution and circuit protection are miswired and selected badly.
  18. josephkk

    josephkk Guest

    The correct answer is to quit screwing around and upgrade the wiring to
    properly support the load. Failure to do so can have legal consequences,
    particularly in case of a fire.

    Just ask your local fire marshall.

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