Bidirectional DC motor protection

Discussion in 'General Electronics Chat' started by dannomassey, Jul 28, 2010.

  1. dannomassey

    Thread Starter New Member

    Jul 28, 2010
    7
    0
    Hi Guy's
    I am building a bidirectional motor controller to run multiple motors using solid state relays (60V max) and a PLC. My original circuit diagram is attached, (half-bridge). The issue I have is controlling the inductive voltage as the relays are opened. I wanted to use Diode / Zener back to back configuration, but running at 5V to 30V and up to 4A I cannot find a zener rated to this. Any suggestions or circuit mods to work around this?
    Thanks all
    Dan
     
  2. retched

    AAC Fanatic!

    Dec 5, 2009
    5,201
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    Here is the image, so folks dont have to load WORD or OO to see the diagram.

    [​IMG]
     
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  3. dannomassey

    Thread Starter New Member

    Jul 28, 2010
    7
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    Thanks Retched, will use a better format in future
     
  4. ifixit

    Distinguished Member

    Nov 20, 2008
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    Hi dannomassey

    You could use a reversed schottky diode across each of the switch outputs to limit the back emf to just a volt or so above the rail voltage. Alternately, you could use a Metal Oxide Varistor (MOV) across the motor.

    Google is good,
    Ifixit
     
  5. timrobbins

    Active Member

    Aug 29, 2009
    318
    16
    Do you have any access to (say) 1W zeners? You could use eg 3x 12V in series to spread the peak dissipation. You should be able to operate the zeners at a high peak level (eg. they often have a surge current spec) as they only work with one transition, and you probably won't be switching too frequently.

    You should also use an RC snubber placed as close as possible to the motor terminals.

    Are you only switching when the rotors are at rest? Ie. is 1 sec deadzone long enough?

    Ciao, Tim
     
  6. dannomassey

    Thread Starter New Member

    Jul 28, 2010
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    0
    Thanks for your time guy's
    Timrobbins, I had thought of using an MOV accross the motors but due to their life expectancy I have ruled them out as this rig will be running hundreds to thousands of forward / reverse ops every day. Thats why I have switched to SSR from mechanical, as the relays were wearing out or becoming sticky.
    I will take a look in to the Schottky method, I haven't used these before so will have to read up.
    Thanks Tim, a snubber will be a good idea, not too sure about a string of zeners though, initial surge maybe a bit high. I'm looking at running ~ 40 motors simultaneously and while the lower voltage range is no issue, up around 28 - 30V there could be a problem.
    Dan
     
  7. timrobbins

    Active Member

    Aug 29, 2009
    318
    16
    Dan,
    Is your aim with the overvoltage protection to just protect the SSR's (ie. limit the switch peak voltage to <60V), or are there other concerns (eg. commutator arc-over)?

    If you just want to protect each SSR then you can add a bi-directional voltage clipper across each of them as well.

    I'd be assuming the peak current into a protection zener was 4A at a transition. A 5W 27V zener has a surge current capability of 4A (eg. 1N5361B) - which is a non-recurrent 8ms half sine - so you could derate from that limit. I suggest you could get a thermally performance from say a 36V 'zener' (eg. a string of 3-4 5W zeners in series) in series with a diode and a resistor - the resistor could be sized to pass say 4A at 20V peak, and could be common to both zener arms.

    The snubber would assist shunting the highest rise time portion of the freewheeling current - do the SSR's have a dV/dt limit?

    Imho MOVs have too large a voltage tolerance and 'working voltage span' for this application.
     
  8. dannomassey

    Thread Starter New Member

    Jul 28, 2010
    7
    0
    Tim
    Yes my aim is just to protect the SSR's switch peak voltage to <60V as tehy are easily degraded this way.

    Can you explain bi-directional voltage clipper to me? Is this similar to a DAIC? I am thinking a couple of 2A DIACs accross the load may be the simplist solution now.

    As you said, yes peak current into a the zener would be 4A, less most of the time. A string of Zeners could dtill be the way to go, though I think the DIAC may be more straight forward, unless you can see an issue with using these? ie:NTE6408
    I will apply a snubber also, and I can't see any dv/dt ratings on the data sheet.
    Thanks again, really usefull
     
  9. beenthere

    Retired Moderator

    Apr 20, 2004
    15,815
    282
    Those appear to be DC motors. Won't you have problems using SSR's? They latch on when passing DC.
     
  10. timrobbins

    Active Member

    Aug 29, 2009
    318
    16
    Sorry for the confusion Dan. 'Bidirectional voltage clipper' was long-hand gobbledigook for your zener clipper circuits.

    I hadn't thought of a diac - and haven't looked at one of those for decades! Will have to do some revision of that thought.

    Beenthere - you can get dc load versions of ssr's.

    Ciao, Tim
     
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