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SCR triggering for high current inductive loads

Discussion in 'Electronic Design' started by Paul E. Schoen, Jun 16, 2006.

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  1. The circuit breaker test sets I have been involved with use an AC switch
    consisting of two SCRs wired in reverse parallel connection. For some test
    sets, a fixed voltage of 240 or 480 VAC is switched, while in others it is
    a variable voltage of 0 to 560 VAC. The currents involved may vary from
    only several amperes to about 200 amperes continuous, and pulses of 100
    mSec or so up to at least 1000 amperes. The load is a step down transformer
    with an output of 6 to 15 VAC or so, and it is connected to circuit
    breakers up to 6000 amps. Breakers are typically tested at 3x for time
    delay (18,000 amps for 30-90 seconds), and instantaneous at 10x (60,000
    amps for 0.02 to 0.05 seconds).

    The load is primarily inductive, so the SCR firing circuit is adjusted for
    an initial firing angle of about 70 degrees. It is adjusted so that all
    half-cycles are approximately the same peak value. Also, the controller is
    set to produce an integral number of cycles (typically 5), when pulsing the
    output current below instantaneous trip value. If an extra half-cycle
    occurs, there is a net DC component, and the transformer is essentially
    magnetized (remanent magnetism). When this happens, the inrush current of
    the next pulse is extremely high, sometimes enough to trip a 200 ampere
    mains breaker, and causing a very audible noise in the conduit as the
    cables slap against the pipe.

    I have used several designs for firing circuits. Originally we used
    commercially available controllers which used high frequency pulses to fire
    the gates, but quite often we would see waveform distortion because the
    current was not enough to keep the SCR in conduction. I designed several
    circuits that used DC for the gates, and made sure that the current was
    applied only when the anode had a positive voltage. However, because of the
    inductive load, there was current still flowing when the gate current was
    turned off, and distortion was seen.

    Finally, about ten years ago, I designed a simpler board which kept gate
    current on continuously, and these boards have been used in hundreds of
    test sets with no apparent problems related to the gate drive. I use a
    simple constant current source with a PNP transistor and a 2.0 ohm
    resistor, and diodes, which limits the current to one junction drop (0.6V)
    on 2 ohms, or about 300 mA. It is sourced from about 12 VDC.

    I am now designing a new board which will use a PIC to control the SCR
    firing. I plan to use DC/DC converters (12V to 5V at 200 mA), rather than
    transformers for the gate voltage supplies, to save size and also allow the
    circuit to run on 12 VDC. It will also have sensors to determine if the SCR
    is actually turned on when gate current has been supplied, and it will have
    other bells and whistles such as programmable phase angle, overcurrent
    detection, etc.

    In researching gate drive requirements, I found a specification for the
    SCRs we now normally use, giving a guaranteed turn on current of 200 mA, at
    a voltage of 1.0 to 4.0 volts, but there was also a specification
    indicating that the gate should not have current applied when the anode is
    negative with respect to cathode. The previous design applies the 300 mA
    current during the full conduction cycle, which does not meet this
    specification. However, the other SCR is conducting at that time, so the
    anode to cathode voltage is only a couple of volts. I am assuming this
    condition does not do any harm except waste power. With my new board, I may
    be able to detect actual current flow and turn off the gate drive when
    current is flowing in the opposite direction, but it is easier to leave
    things as they are. Any comments on this?

    I also found that the recommended gate drive consists of an initial high
    current pulse, followed by a "back porch" of lower current. I can do this
    fairly easily by adding a capacitor across the current limit sense
    resistor, but it will produce this waveform only for the first phase
    delayed firing. After that, I prefer to leave it on continuously. If I turn
    gate current off during the time of reverse conduction, I would need to
    retrigger at a very critical point just after the zero crossing, and any
    delay will cause distortion, and early firing will waste the peak current
    pulse.

    Another problem I have seen is that, at very high current levels, there is
    often an additional half-cycle of current, which magnetizes the transformer
    and causes subsequent inrush problems on the next pulse. I found that this
    effect could be minimized by reversing the phase of the incoming power, or
    by reversing the gate connections to the SCRs. The 480 VAC supply is
    produced by an autotransformer on a 208 VAC source, so the voltage to
    ground on the two inputs is 360 VAC and 60 VAC. The case that works best is
    where the line side of the SCR is at the 360 VAC potential, and the load
    side switches from -60(off) to +360(on). The extra half cycle occurs when
    the line side stays at -60 and the load switches from +360(off) to -60(on).
    I think there is some turn-off transient that is being conducted into the
    firing circuit, and probably it is the one that is changing voltage to
    ground. If this circuit controls the gate of the SCR that sees a positive
    line voltage excursion, it fires and causes the extra half cycle. The SCR
    board is normally mounted on the SCR heat sink, which may be at the line or
    load side of the switch, but I think I have also seen this problem when the
    board has been mounted remotely. This may take more research, but, again,
    any comments or suggestions will be appreciated.

    Thanks for taking the time to read this long post. Perhaps you may find it
    interesting, and responses may be helpful to anyone dealing with similar
    applications. More information on the general technology of high current
    primary injection testing is on my website.

    Thanks!

    Paul E. Schoen
    www.pstech-inc.com
     
  2. Just one random thought about high inrush currents.

    Would it not be appropriate to limit them using a resistor in series,
    and another antiparallel switch to bypass resistor after a couple of
    cycles:


    your original SCR switch S1 --+------/\/\/\resistor/\/\/\/\-------+- load
    | |
    +______________/ ___________________+
    another switch S2

    S2 would be switched on 1/20 second after S1 is turned on. Resistor
    could be watercooled or whatever, to cool it after every cycle. (you
    can probably take some length or 10 gauge hookup wire and submerge it
    in water). 20 ft of it would have resistance of about 0.6 ohm and
    limit current in a 480V circuit to no more than 800 or so amps.

    800 amps conducted for 1/2 cycle is

    800*800*0.6/120 = 3200 joules

    3200 joules is not much if it is water cooled.

    Assuming your normal current is 100A, as you mention on your page, it
    would be nothing for this submerged hookup wire.

    i
     
  3. It would probably not even need to be water cooled, which would be a real
    problem implementing in a portable (sort of) test set. There are also some
    big thermistors which start at a fairly high resistance and quickly drop to
    a few tenths of an ohm after they heat up. However, I tried them on a
    smaller test set, but they were not too effective, and they burned up. A
    contactor for your suggested S2 above would be quite expensive. The SCR
    switches are actually much cheaper, at about $400.

    Also, adding this much resistance will seriously affect the initial
    waveform of the current pulse, and that will adversely affect test results.
    If 48,000 amps output is required, the primary current must be 1000
    amperes, but the 0.6 ohm resistor would drop 600 volts. What it actually
    does is limit the output current to about 24,000 amps until the S2 pulls
    in, and then you get the required current. I saw that sort of effect with
    the thermistors. Also, they reduced the surges until they got hot, and then
    were ineffective.

    With my new SCR board, I am thinking about implementing a demagnetizing
    routine, which will detect an odd number of half-cycles, and then apply a
    series of phase-delayed pulses to eliminate the magnetic charge. I have
    proven that this concept will work, by reducing the primary voltage and
    initiating a short pulse of lower current. Then I return to the setting for
    higher current, and the input surge is less than it would have been
    otherwise.

    Thanks for your idea.

    Paul
     
  4. Roger

    Roger Guest

    Years ago when I worked in power electronics I had a similar problem
    with 300A SCR's. I was just a young fledging assistant at the time. The
    original design used a little trigger cct for each SCR which had its
    own floating supply and an opto which was used to switch a power
    darlington onto the gate. I remeber that there was a resistor in the
    emitter to limit the current (the base voltage was diode clamped. The
    trouble was that the current was not high enougth to ensure that the
    SCR's remained in conduction, but the designer was reluctant to simply
    beef up the circuit because he was worried about having too much
    current in the gate.

    The solution came from the technique that is used in many simple
    opto/triac firing schemes, take the gate current from the anode. If you
    take a resistor from the anode and through a diode to a floating
    switch, then the current will automatically depend on the voltage
    accross the SCR. That way you can make the resistor very low as any
    situation where large currents start to flow into the gate will ensure
    that the SCR is fired and hence the voltage across it is low, hence no
    drive.......a very simple automatic gate drive system.

    It worked well on that circuit anyway ;-)
     
  5. Guest

    The simplest way to drive scrs like this is with an opto triac (2 in
    series and resistor) between the gate leads but this may not work very
    well if you have a very low mains voltage. Fireing at 90Deg should
    eliminate inrush. As far as remance is concerened I doubt very much if
    this has any sizeable effect. Recently I built a PIC contrlolled system
    just like yours for controlling a welding machine, (2scrs, transformer,
    isolated dc gate drives) and had similar problem (random fuse blowing)
    it turned out to be software problem (not turning off scr gate drive
    correctly) thats where I would look first.
     
  6. I would personally be very interested to hear about that welding
    machine project. I am foing something similar now (but in 3 phase).

    i
     
  7. Fred Bloggs

    Fred Bloggs Guest

    Your description is too nebulous to understand the setup with any
    certainty, like for example your mention of distortion while firing at
    70o phase angles...makes no sense. You might spring for a technical writer.
     
  8. I could have written more details, but the post was too long already. With
    a partially inductive load, the phase angle of current lags the voltage by
    up to 90 degrees. We have determined that 70 degrees is characteristic for
    the case of a shunt placed across the output connections (essentially a
    bolted short), and we examine the waveform with an analyzer using the
    millivolt drop. This is also how we calibrate the unit. We have also
    analyzed the waveform with the shunt in series with the test loads, which
    may be any one of thousands of different circuit breakers, each with
    varying amounts of inductance. Some show evidence of saturation, so the
    waveform is distorted with characteristic peaks and high crest factor.

    The initial waveform into the circuit breaker must be such that the first
    half-cycle is about the same as those proceeding it. If it is much lower,
    the instantaneous trip element will ignore it, and will act upon the next
    half-cycle, which will actually be higher than those proceeding it. As the
    measurement circuit reads the entire waveform and performs a true-RMS
    computation on it, this creates a timing error of about 1/2 cycle and a
    corresponding measurement error. The new SCR board will be capable of
    adjusting the initial firing angle to produce the best waveform possible.
    Some of this may be incomprehensible to someone who is not familiar with
    circuit breaker testing.

    The distortion I was referring to occurred mostly on test sets where the
    voltage applied to the SCRs was low (perhaps 10-20 VAC), and the current
    also fairly low (5 amperes or so). These are large SCRs, and especially for
    older ones, the holding current is probably several amperes. There is some
    unavoidable distortion at these levels due to the 1 or 2 volt drop on the
    SCRs, but we were seeing distortion that indicated the SCR was dropping out
    of conduction well before the normal zero crossing. The loss of conduction
    coincided approximately with the removal of gate drive at the zero crossing
    of voltage, which occurred about 70 degrees before the zero crossing of
    current. When the current dropped below holding current, the SCR went out
    of conduction, causing a drop in current. This was sometimes followed by
    one or more current spikes, due to inductive kick and dV/dT triggering.
    Using DC for the gate drive solved this problem.

    However, that could explain some problems that we have seen when we try to
    obtain the exact number of half-cycles. If the gate drive is removed too
    long before the end of the current, an effect as described above could
    occur. If it is removed after the zero crossing, then the SCR might be
    triggered and conduct for the following half-cycle. It may be critical to
    detect the current zero crossings and remove gate drive at the optimum
    time.

    I hope this explains it.

    Thanks,

    Paul
     
  9. Guest

    It was for a resistance welder, tin can seams, single phase AC output.
    I have done 3 phase controllers in the past for motor soft starters, I
    doubt that would be much use for your application.
     
  10. John - KD5YI

    John - KD5YI Guest

    Hi, Paul -

    This gate drive characteristic was found to be critical for the
    McMurray-Bedford thyristor inverters we manufactured years ago. It is all a
    bit hazy now, but as I recall, the initial "hard-fire" portion helped to get
    as much of the silicon triggered as possible in the early stages of firing.
    As a result, di/dt handling characteristics are improved, I think.

    Did you see in the recommended gate drive paper about compliance voltage? As
    I recall, the Westinghouse (later, PowerX) recommendation was about 30 or 40
    volts. It turns out that the gate can rise into the tens of volts at the
    beginning due to high anode current. This part was very surprising to me
    when I first came across it.

    I don't remember the gate current we supplied during the hard-fire time, but
    it was in the amps (at 10 or more volts). The back porch was in the hundreds
    of milliamps.

    Good luck.

    John
     
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