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    Question Designing a snubber circuit for protecting triacs

    Hi all,

    I have built a 400Vac 3ph solid-state relay using BTA41-600 triacs as shown in the attached schematic, to drive a 6.6KW motor which has a LRA of 80Amp, but draws around 5Amps while running...

    During the first test run the motor started, the current was normal 5A, but when I wanted to switch it off the motor kept running, but the current went up to 7.5Amp, I guessed that only one phase had been switched off, so, I switched off the main braker to stop the motor then switched it back on the motor wouldn't start with a weird grinding sound so I switched it back off and unplugged everything. After inspection I found out that two out of three triacs were shorted, and the "golden_rules" app note said that it was a result of a too high current rise rate dI/dt.

    Well I am lost between the app notes, some said that a 10n 300ohm snubber resulted in 400A/µs, knowing that these triacs are rated for 50A/µs I am not even sure that the snubber killed the triacs instead of protecting them.

    Should I add inductors in series with the load as suggested in the "ten_golden_rules" by winding each phase supply wire or is it the snubber that needs redesign ?

    Please help, I am too green at all this & definitely not an EE, all data around there is for 240Vac and too many formulas that seem to be conflicting and I can't locate any Transil diodes rated above 400Vac. don't wanna fry other triacs.

    Thanks !
    cnc2.

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    Default Re: Designing a snubber circuit for protecting triacs

    Up !



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    Default Re: Designing a snubber circuit for protecting triacs

    How can you tell if it is the snubber?
    What is your PWM signal for?



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    Default Re: Designing a snubber circuit for protecting triacs

    Hi KOC62, sorry for the very late reply.

    I'm not sure it's the snubber, but it might be the cause, because a bad snubber can create very high dI/dT current rise rates that will fry the triacs which will become shorted...(from the ten golden rules). and two out of three triacs have become shorted after the first try, the third one which came online after the motor started moving survived the test.

    The PWM signal is supposed(no oscilloscope) to be increasing from 0 to 100% duty cycle, so it will act as a soft start for the motor and reduce the LRA which is rated at 80Amp max.

    Thanks !
    cnc2.



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    Default Re: Designing a snubber circuit for protecting triacs

    I haven't dabbled with PWM circuits for 3 phase motors, but thought it odd that your circuit used the same PWM signal to all 3 phases. I thought that each phase had its own PWM to create a 3 phase control, hence my question.
    The snubber is a passive device and doesn't increase the di/dt, it shunts the voltage spike created by a dv/dt due to current being cut suddenly, as I understand it. Designing a controller without proper test equipment is asking for trouble as you can't see the possible damaging signals. Even a circuit board layout can create extra capacitance that could assist spike generations,. You are then left to guess - and I never did have a crystal ball that worked for me. Using a known working circuit is a start, as you are relying on a proven design. But scratch building your own without an oscilloscope is a risk for failure, as I see it.



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    Default Re: Designing a snubber circuit for protecting triacs

    Hi,

    if you think about it, the PWM signal doesn't need to be 3 phase related, it's just like a stupid dimmer/chopper turning on&off to linearly increase the current & try to suppress the 80A current spike of the locked rotor (LRA).

    Thanks !
    cnc2.



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    Default Re: Designing a snubber circuit for protecting triacs

    I did think about it (briefly ) and thought that each triac would turn on at the same time since the PWM is exactly the same for each triac. That part didn't make sense to me.
    But, ... I then looked up the MOC3061 and it has a "zero crossing" circuit inside, meaning it waits for that phase to reach zero volts before triggering the "big" triac..Since each phase has it's own zero crossing time the firing is also "3 -phase".
    So, as I see it, your controller can only do a "burst fire" mode and not a "phase control" mode. See this link as a concept view for each control mode; https://openenergymonitor.org/emon/mk2/switchdev



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    Default Re: Designing a snubber circuit for protecting triacs

    It's not even a controller, so, no phase control. it's supposed to be a very basic soft starter to cut down on LRA current spikes...



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    Default Re: Designing a snubber circuit for protecting triacs

    See the application note from fairchild AN-3006
    Also ..
    Al.

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    “Logic will get you from A to B. Imagination will take you everywhere.”
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    Default Re: Designing a snubber circuit for protecting triacs

    Have you tried using an 800V device instead of the 600V version? If your using a 400VRMS power supply you get a peak voltage of about 565V, which is not far from the 600V rating of your triac, especially if there is also a voltage spike from the motor being turned off.



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    Default Re: Designing a snubber circuit for protecting triacs

    Thanks for the reply Al, I'm happy you showed up

    I've gone through too many application notes but missed that one, and I have to say that I'm lost at least the one you pointed shows that it is possible to have the snubber off board so I might try higher capacitance values according to the table, with no space constraints. This app note says to use non inductive resistors while another one says that using wire wound resistors (6µH) limits dI/dT current rise rate.

    All of them app notes & white papers bring in useful information which add to my confusion, I was about to add in Transil diodes but didn't find any with the needed rating, locally.

    A little list of papers I've been through (not that I fully grasped everything):

    TRIAC Specifications and Recommendations (Turn‐On Behavior)
    AN-3003
    AN-3004
    AN-3008
    AN-437
    AN-392
    AN-439
    AN-8008-D
    AN-Golden-Rules
    AN-328/0393

    Thanks !
    cnc2.



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    Default Re: Designing a snubber circuit for protecting triacs

    Quote Originally Posted by KOC62 View Post
    Have you tried using an 800V device instead of the 600V version? If your using a 400VRMS power supply you get a peak voltage of about 565V, which is not far from the 600V rating of your triac, especially if there is also a voltage spike from the motor being turned off.
    I used 600V triacs because it was the only option available, but after the fail the local supplier had a new stock of 800V devices so, I now have 800V triacs but a little investigation about the failure mode showed that according to the golden-rules app note, the shorted triacs mean too high dI/dT current rise rate...and two triacs were shorted which correspond to the two leading phases, the third triac seems fine (not shorted) and I suspect that it is ok because during my only test, when I turned off the circuit the motor continued turning but the current went up from ~5A to about 7.5A which means it was running on two phases(the shorted ones) while the third phase got turned off by the still working triac.

    So, I didn't try the 800V ones yet.

    Thanks !
    cnc2.



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    Default Re: Designing a snubber circuit for protecting triacs

    Well, if you don't have the right parts then yes, you may be in trouble. Without test equipment to give us the actual current flow, we are only guessing what it might be. I suppose you could put a resistor in series with each triac to limit the di/dt somewhat for experimental purposes. The snubber can only work on dv/dt (voltage), so I don't see how it would help to control the di/dt. Engineering paper designs have their limitations.



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    Default Re: Designing a snubber circuit for protecting triacs

    Quote Originally Posted by KOC62 View Post
    Without test equipment to give us the actual current flow, we are only guessing what it might be. I suppose you could put a resistor in series with each triac to limit the di/dt somewhat for experimental purposes. .
    Current flow was measured with a digital clamp meter (AC amp meter) at around 5A of course it doesn't show the LRA nor a graph of dI/dT....good luck finding resistors rated for 5A@400VAC.

    The golden rules say that an inductor of few µH in series with the load reduces dI/dT but I'm not quite sure about any side effects which could put more strain on the motor or fry it....Seems to be an easy(ugly) fix when space isn't a problem...I thought about coiling each phase wire according to an online inductor calculator but I really have no idea about the side effects of such a setup (not an EE with very limited electrical knowledge)

    From an other paper, the discharge of the snubber capacitor might exceed dI/dT of the triac and be in the range of 1000A/µs if I recall....look at Fairchild's AN-3008 page 7 ....and of course I have no way of measuring the load's(motor) inductance.

    Thanks !
    cnc2.

    Last edited by cnc2; 06-22-2016 at 11:47 PM.


  15. #15

    Default Re: Designing a snubber circuit for protecting triacs

    Hi,

    I must be harsh with You.
    Mistakes:
    1. Zero Voltage Crossing triac driver is NO-NO for soft starting inductive loads (NO-NO is the technical term for challenged people )
    2. No Circuit Breakers or fuses , REALLY ?
    3. PWM control signal ? REALLY ?
    4. Missing info about funeral services costs in your town...

    Why on earth do you want to design a soft-starter when you don't have basic knowledge ?
    Obviously , you don't know what you do !

    For 140 $ buy a soft-starter like this (Way cheaper than number 4 above !) :

    https://www.automationdirect.com/adc...-_40A)/SR22-09



    Or try this softstarters selector :

    https://www.automationdirect.com/selectors/softstarters


    Find a qualified electrician and pay him to connect your motor !
    Here are some definitions to help You :

    Application
    When selecting the application, the harder the application the larger the Soft Starter that is required.
    Supply Voltage
    This can be either: 110, 200, 208, 230, 380-400, 440-480 Volts.
    Motor Size
    AC Induction Motor Size is measured in Current, kW (@400V) or HP (@460V). The bigger the motor size, the larger the Soft Starter that is required.
    Connection
    Stellar Soft Starters can be wired In Line or In The Delta. When the Soft Starter is wired In The Delta, a smaller unit can often be selected.
    Ambient Temperature
    The default ambient temperature around the starter is 104°F which requires no de-ration. For every degree above the 104°F the Soft Starter needs to be de rated by 1.1% of the Full Load Current.
    Altitude
    The Default height above sea level is 3280feet. For every 328feet above 3280feet the Soft Starter needs to be de rated by 1% of the Full Load Current.

    Application Definitions

    Agitator
    An agitator is a mechanism to put something into motion by shaking or stirring. Used in the water sector among other industries. Overload Class 10.

    Air Compressor – Equalised
    A compressor takes in a gas at a pressure and delivers it at a higher pressure. Equalised Compressors have the same pressure on both sides of the system so it is a fairly easy start. Used in many applications including air conditioners and heat pumps. Overload Class 5.
    Air Compressor – Loaded
    A compressor takes in a gas at a pressure and delivers it at a even higher pressure. Loaded Compressors have different pressures on both sides of the system so it is likely to be a fairly hard start. Used in many industries including air conditioners and heat pumps. Overload Class 10.
    Ball Mill
    A ball mill is a type of grinder used to grind materials into extremely fine powder for use in paints, pyrotechnics, ceramics and cement. Overload Class 20.
    Centrifuge
    A machine that rapidly spins liquids and separates out the particles by their density. Used in the food industry among others. Overload Class 30.
    Chiller
    A chiller is a machine that removes heat from a liquid via a vapour-compression or absorption refrigeration cycle. Overload Class 10A.
    Conveyor – Lightly Loaded
    A Mechanical device used to move materials between operations. Lightly Loaded items include bottles and food stuffs. Overload Class 5.
    Conveyor – Heavily Loaded
    A Mechanical device used to move materials between operations. Heavily Loaded items include Rocks and Concrete. Overload Class 20.
    Crusher – Lightly Loaded
    A machine that breaks rock or other items into smaller pieces by crushing it like a jaw or spinning it in a circular motion at high speeds. For example, lightly loaded crushers could be crushing clay. Overload Class 20.
    Crusher – Heavily Loaded
    A machine that breaks rock or other items into smaller pieces by crushing it with a jaw action or spinning it in a circular motion at high speeds. For example heavily loaded crushers could be crushing heavy rocks. Overload Class 30.
    Escalator
    An escalator is a conveyor transport device for transporting people. Overload Class 5.
    Fan – Low Inertia
    A device for creating a current of air. Low inertia would be a fan with a small mass. Overload Class 10A.
    Fan – Medium Inertia
    A device for creating a current of air. Medium inertia would be a fan with a medium mass. Overload Class 20.
    Fan – High Inertia
    A device for creating a current of air. High inertia would be a fan with a large mass. Overload Class 30.
    Feeder – Screw
    Screw feeders are used in various industries for the metering of material into a production process, usually consisting of a cylindrical tube with a screw shaped object in the centre which moves material from one end to the other. Overload Class 10.
    Grinder
    A spinning disk used to grind materials. Used in mining and forestry among other industries. Overload Class 20. Hammer Mill A high-speed machine that uses hammers and cutters to crush, grind, chip, or shred solid waste. Overload Class 20.
    Lathe Machines
    A machine tool for shaping metal or wood; the work-piece turns about a horizontal axis against a fixed tool. Overload Class 10A.
    Mills Flour Etc
    A mill uses two large disks to grind the materials to a finer material. Overload Class 20.
    Mixer – Lightly Loaded
    Equipment used for mixing or blending the materials used in the manufacture of a number of items. Lightly loaded mixers generally mix materials such as liquids. Overload Class 10B.
    Mixer – Heavily Loaded
    Equipment used for mixing or blending the materials used in the manufacture of a number of items. Heavily loaded mixers generally mix materials such as concrete or high viscous materials. Overload Class 20.
    Pelletisers
    Pelletisers are machines that feed in materials extrude and cut the output into pellets. Overload Class 20.
    Plastic and Textile Machines
    These may be Injection moulding machines and other types of plastic and textile machines. Overload Class 5. Press,
    Flywheel
    A flywheel is a rotating disk used as a storage device for kinetic energy. Overload Class 20.
    Pump – Centrifugal <100A
    A pump consisting of rotating impeller enclosed in a housing and used to impart energy to a fluid through centrifugal force. Overload Class 5.
    Pump – Centrifugal >100A
    A pump consisting of rotating impeller enclosed in a housing and used to impart energy to a fluid through centrifugal force. Overload Class 10.
    Pump – Positive Displacement Unloaded
    A positive displacement pump causes a liquid or gas to move by trapping a fixed amount of fluid or gas and then forcing (displacing) that trapped volume into the discharge pipe. Overload Class 10.
    Rolling Mill
    A rolling mill is a machine or factory for shaping materials by passing them between rollers. Overload Class 20.
    Saw – Band
    An endless saw consisting of a toothed metal band that is driven around two wheels, usually used for cutting metal or wood. Overload Class 10.
    Saw – Circular
    A power saw that has a steel disk with cutting teeth on the periphery; rotates on a spindle. Overload Class 20.
    Screen – Vibrating
    A vibrating screen is a large sieve used to separate different sized materials. Vibrating screens are used in many industries including mining and building. Overload Class 20.
    Tumblers
    Tumble polishing, or tumbling, is a technique for smoothing and polishing a hard substance. It consists of a large drum that rotates causing the materials in the centre to grind against each other. Overload Class 10
    Wood Chipper
    A tree chipper or wood chipper is a machine used for reducing wood (generally tree limbs or trunks) into smaller parts, such as wood chips or sawdust. Overload Class 30.



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    Default Re: Designing a snubber circuit for protecting triacs



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Designing a snubber circuit for protecting triacs

Designing a snubber circuit for protecting triacs