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Snubber

Discussion in 'Electronic Basics' started by ClueLess, Mar 9, 2008.

  1. ClueLess

    ClueLess Guest

    Hi all

    I want to start a motor remotely.

    The contactor coil is rated 240 volts and I am driving it with the
    contacts of 12 volt DC relay driven by a microcontroller.

    The relay contacts arc and I thought a snubber will reduce the arcing.
    I checked various sites and ended up lots of calculations for which I
    have no proper data.

    Is there a ball park value I can use for the capacitor and the
    resistor?

    Thanks in advance for your time and attention

    ClueLess
     
  2. I would pick a resistor that passed a peak current that is
    about the same as the highest peak current the contactor
    draws. Unfortunately, that number may be hard to come by.
    But you might estimate it as several (say, 3) times larger
    than the rated coil current. So if the contactor coil
    current is rated to be about 0.1 amp, I would pick a
    resistor that is the peak line voltage divided by 0.3 amps
    or 240*1.414/0.3=1.1k, so lets say, 1k ohms. That way, if
    the contact happens to close at peak voltage, the contact
    won't have to deal with a resistor inrush current much
    greater than what the relay will draw within a half cycle.

    Most RC arc suppressors you can buy have lower resistances
    in them, most from 220 to 47 ohms, but most are intended for
    loads heavier than a relay coil.

    Ideally, the capacitor value is based on the inductance of
    the coil and its energy storage (proportional to the peak
    current squared), but too large a capacitance is safe (will
    absorb the inductive energy at a safe voltage peak), as long
    as it doesn't overheat the resistor. So lets say you use a
    1 watt resistor (more the peak voltage rating or 300 volts
    than anything else) and pick a capacitor that will dump a
    half watt into it. by W=I*I*R, 0.5 watt=I*I*1k, so I=22
    milliamps. If that current is limited by the capacitor
    across 240 volts, that implies a capacitive reactance, by
    Xc=1/(2*pi*f*C) and Xc=240/0.022=10700 so C=0.25 uF. I
    would probably go with a 0.22 uF, 600 or 1000 VDC capacitor
    in a pinch, but the right kind is one rated X1, for across a
    240 volt AC line.
    This sort of thing:
    http://www.panasonic.com/industrial/components/pdf/abd0000ce30.pdf

    I hope that helps you (and I didn't hit a wrong calculator key).
     
  3. Jamie

    Jamie Guest

    100F in series with a .1 uf 600v cap maybe?
    You could use just the cap across the terminals
    how ever, if the cap becomes fully charged and then
    you induct even more, it could short the cap.
    Using the R in series helps with a slower charge.


    http://webpages.charter.net/jamie_5"
     
  4. whit3rd

    whit3rd Guest

    Yep, there is switch bounce and the motor has stray inductance.
    Your easiest option is a surge suppressor type device,
    like MOV or ZNR varistors. In the old days, a resistor-capacitor
    pair with some size adjustment to the inductance was recommended,
    but that's too much like engineering...
     

  5. Would still be .1 µF.


    --
    Service to my country? Been there, Done that, and I've got my DD214 to
    prove it.
    Member of DAV #85.

    Michael A. Terrell
    Central Florida
     
  6. I use self-contained snubbers that are 100 ohms in series with 0.1 uF for
    contactor coils, and that seems to be a good combination for most sizes.

    At 240 VAC 60 Hz, the resistor will dissipate about 8.8 mW with 9.4 mA.

    I did a simulation with LTspice for a 240 VAC coil with 5H inductance
    (which draws 130 mA or 31 VA sealed-in). This is probably about right for a
    good-sized contactor rated at 40 amps or so.

    Under best conditions, if you apply the voltage at a waveform peak, the
    inrush current is 180 mA peak, which is the same as continuous waveform. In
    actual proctice, however, the peak inrush will be much higher (perhaps
    5-10x) because the inductance is much lower when the clapper is separated
    from the electromagnet. If the circuit is opened near the zero current
    point, there is just some ringing but no voltage overshoot.

    Now look at worst case conditions. If you apply voltage at the waveform
    zero crossing, you get a peak current of 355 mA, and a "DC offset" of coil
    current that persists for several cycles. This illustrates why
    zero-crossing triacs are not the best for inductive loads. But the worst
    condition is turn-off, when the relay opens at peak current. There is a
    1.25 kV peak, and ringing at about 220 Hz for about 35 mSec until the peaks
    are reduced to 600 volts.

    You can play with this simulation and find a better value for the snubber.
    I was surprised that the model for a voltage-controlled switch does not
    allow conduction in both directions, so I added a second switch in
    anti-parallel. But the simulation appears very realistic.

    In my circuit breaker test sets, I am switching primary currents of several
    hundred amps at 480 VAC into highly inductive loads, so you can imagine the
    transients that can occur. But we use a phase-delay fired SCR switch which
    limits the initial surge, and it continues to conduct until the current is
    nearly zero, so turn-off transients are not bad. But there can be problems
    when the circuit breaker trips and the load instantly changes from nearly a
    short circuit to an open. It is very common to see arcing on breaker
    contacts.

    Paul

    ================== LT Spice ASCII File Follows ============================

    Version 4
    SHEET 1 880 680
    WIRE 64 -176 -208 -176
    WIRE 16 -144 -112 -144
    WIRE 64 -144 64 -176
    WIRE 0 -96 -48 -96
    WIRE 96 -96 80 -96
    WIRE -112 -16 -112 -144
    WIRE 64 -16 -112 -16
    WIRE -208 16 -208 -176
    WIRE 16 16 -208 16
    WIRE 64 16 64 -16
    WIRE -48 64 -48 -96
    WIRE -16 64 -48 64
    WIRE 0 64 -16 64
    WIRE 96 64 96 -96
    WIRE 96 64 80 64
    WIRE 128 64 96 64
    WIRE 256 64 128 64
    WIRE 256 112 256 64
    WIRE -208 128 -208 16
    WIRE -48 144 -48 64
    WIRE 128 144 128 128
    WIRE 128 160 128 144
    WIRE -208 240 -208 208
    WIRE -112 240 -112 -16
    WIRE -112 240 -208 240
    WIRE -48 256 -48 224
    WIRE 128 256 128 240
    WIRE 128 256 -48 256
    WIRE 256 256 256 192
    WIRE 256 256 128 256
    WIRE -48 288 -48 256
    FLAG -48 288 0
    FLAG -16 64 Vin
    FLAG 128 144 Vres
    FLAG -208 240 0
    FLAG 256 64 Vrly
    SYMBOL res 112 144 R0
    SYMATTR InstName R1
    SYMATTR Value 100
    SYMBOL cap 112 64 R0
    SYMATTR InstName C1
    SYMATTR Value 0.1µ
    SYMBOL voltage -48 128 R0
    WINDOW 3 103 175 Left 0
    WINDOW 123 0 0 Left 0
    WINDOW 39 22 174 Left 0
    SYMATTR Value SINE(0 360 60 0 0 0 150)
    SYMATTR SpiceLine Rser=1
    SYMATTR InstName V1
    SYMBOL sw 96 64 R90
    WINDOW 3 -36 -113 VRight 0
    WINDOW 0 24 92 VRight 0
    SYMATTR Value MySwitch
    SYMATTR InstName S1
    SYMBOL ind 240 96 R0
    SYMATTR InstName L1
    SYMATTR Value 5
    SYMATTR SpiceLine Rser=50 Rpar=1Meg
    SYMBOL voltage -208 112 R0
    WINDOW 3 -219 162 Left 0
    WINDOW 123 0 0 Left 0
    WINDOW 39 0 0 Left 0
    SYMATTR Value PWL(0 0 .1041667 0 .104167 5 .5041667 5 .504167 0 1.00 0
    1.001 5 1.50 5 1.501 0 2 0)
    SYMATTR InstName V2
    SYMBOL sw -16 -96 M90
    WINDOW 3 -27 190 VLeft 0
    WINDOW 0 26 16 VLeft 0
    SYMATTR Value MySwitch
    SYMATTR InstName S2
    TEXT -88 496 Left 0 !.tran 2 startup
    TEXT -392 344 Left 0 !.model MySwitch SW(Ron=.1 Roff=100Meg Vt=2 Vh=-.5
    Lser=10n Vser=.6)
     
  7. I tried again with a 0.47 uF capacitor and a 270 ohm resistor, and the
    transients are now limited to 600 volts peak. The snubber power is only
    about 1/2 watt (44 mA), and peak current will be limited to 360/270 = 1.3
    amps. The worst case current for the contacts will be when they first close
    into the initial inductance of the coil, which could result in 3-4 amps of
    inrush current. And the most damage may happen as the contacts bounce on
    closure. But the snubber will greatly reduce the arcing upon opening, which
    is really metallic ions transferring from one contact to the other, or onto
    the surrounding insulation, causing the familiar metallic deposits that are
    seen in relays with pitted contacts.

    There are some trade-offs with puttong the snubber across the coil (as I
    normally do), or across the relay contacts. In this case, when energized,
    the snubber draws an additional 44 mA, but the total load on the supply is
    only 85 mA, while the relay coil itself draws 129 mA. So the snubber
    actually helps the power factor and reduces load on the supply. Placed
    across the switch, it will apply a constant leakage current to the
    contactor coil, which could be enough to cause humming, and an unwanted
    potential when it is supposed to be deenergized. But in other cases, such
    as for SCRs, it is customary to put the snubber across them.

    A better solution might be to use a solid state relay (triac or SCR) which
    can be driven directly from the microcontroller. You can get them for
    inductive loads, but even the zero voltage turn-on types will not cause too
    much problem for a motor load. Of course, a soft-start full featured PWM
    controller is best, but probably overkill.

    Paul
     
  8. (snip)

    I can only guess that all these currents are coil currents.
    What is the worst case snubber current at turn on? It
    should exceed 3 amperes, right when the contacts are
    bouncing closed. For such a light load, I think a 100 ohm
    snubber resistor is well below the optimum compromise for
    contact life.
     
  9. The problem with MOVs and ZNR varistors is that they have a limited
    lifetime, and deteriorate with every surge they absorb. As they begin to
    fail, they get more leaky, and conduct at lower voltages, until they
    overheat and explode. You must put a fuse in series with them, and when the
    fuse blows, you no longer have protection. There are some zener type TVS
    protectors that do not deteriorate, but RC snubbers are still the best for
    controlling transients. The ideal solution, however, is to eliminate the
    transients by using solid state controls such as triacs and SCRs.

    Paul
     
  10. whit3rd

    whit3rd Guest

    This is true, but the surges one usually associates with varistors
    are lightning-strike kinds of events, much higher currents than
    a little spark on a motor-start relay.

    In this duty, I'd expect a long useful component life.

    There is some confusion, though, on the snubber function: this
    calls for a snubber or varistor across the MOTOR leads, not
    across the relay contacts (which would result in running the
    motor if it fails short) nor across the winding (winding current
    wasn't arcing, as I read the situation). The varistor, if it fails
    short circuit, pops the circuit breaker just like a shorted motor
    would.
     
  11. This is true, but the surges one usually associates with varistors
    are lightning-strike kinds of events, much higher currents than
    a little spark on a motor-start relay.

    In this duty, I'd expect a long useful component life.

    There is some confusion, though, on the snubber function: this
    calls for a snubber or varistor across the MOTOR leads, not
    across the relay contacts (which would result in running the
    motor if it fails short) nor across the winding (winding current
    wasn't arcing, as I read the situation). The varistor, if it fails
    short circuit, pops the circuit breaker just like a shorted motor
    would.

    It is not totally clear whether the arcing was on the contacts of the small
    relay being driven by the microcontroller, or the larger contactor that
    controlled the motor. But I think it was the small relay, and for that I
    think the snubber across the contactor coil is best. A motor is a reactive
    load as well as a regenerative load, so it will produce a high voltage
    spike if the AC current is interrupted at a peak, but it may also tend to
    act as a generator and try to inject voltage back into the supply. A
    snubber would function somewhat as a dynamic brake.

    MOVs should be protected by a fuse much smaller than the fuse or circuit
    breaker of the mains supply, as they usually have fairly small leads, and
    if allowed to absorb the full short circuit instantaneous trip current of a
    mains breaker (say 150 amps), at full voltage (240 VAC), that is 36 kW,
    which can do a lot of damage even in a few milliseconds. The MOV protector
    should be something like a 1 amp fast blow fuse.

    If the motor will be turned on and off only occasionally, there may never
    be a problem, but for repetitive use, deterioration of the MOV is
    inevitable, and it is not that difficult or expensive to use a proper
    snubber (or better yet, solid state phase control).

    Paul
     
  12. ClueLess

    ClueLess Guest

    I am really overwhelmed at the many responses my question evoked.
    Thanks to all of you for giving me some leads and I am looking forward
    to the weekend on my workbench toying with what I have learnt.

    Thanks again to all of you

    ClueLess
     
  13. ClueLess

    ClueLess Guest

    Yes, arcing is on the small 12 volt relay. This relay is the totally
    enclosed variety and for testing I cut away the outer casing so the
    contacts are visible. There is arcing when driving the contactor coil
    and I wanted suppress that.

    Thanks for your interest and help

    ClueLess
     
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