reverse polarity 12dc w/solid state relays

Discussion in 'The Projects Forum' started by gassiebernal, Mar 10, 2012.

  1. gassiebernal

    Thread Starter New Member

    Mar 7, 2012
    2
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    Hi everyone!

    have a 12V DC pump rated max 26Amp
    this pump is controlled by an autopilot(yacht) unit.
    this autopilot unit has a max capacity of 20Amp so after burning several fuses and eventually the unit sent it to repair and technician told me to make external relay box.
    the way it operates is reversing the polarity of the pump to turn port(left) or starboard(right).
    technician suggested using SSR solid state relays 4 of them.
    when i build the circuit the SSR got burnt almost inmediately it seems that when the polarity is reversed the SSR didn't fully discharge and shorted.
    Q:how can i ensure the SSR is empty as soon as the actuator is disconected?

    diode, resistor???? how?

    thanks G
     
  2. gassiebernal

    Thread Starter New Member

    Mar 7, 2012
    2
    0
    option B
    since now in Madagascar and no SSR available.
    temp solution is to use 30Amp car relays in same config.
    i did and they burnt...so changed for 40Amp relays...
    these mechanical devices produce lots of heat so put fan in box...will test soon.
    if doesnt work could i double the amount of relays to 8 and connect pairs in parallel to increase amp capacity? or the inaccuracy of these spst relays will not sync at triggering therefore one single (or pair) Relay will trigger sligtly sooner and all load will be carried by one of them rendering the whole parallel option useless?
     
  3. R!f@@

    AAC Fanatic!

    Apr 2, 2009
    8,750
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    If you were in Maldives, I could have rectified ur problem. :D

    Mind posting pictures if you can. Like to see how u are going about it.
     
  4. wayneh

    Expert

    Sep 9, 2010
    12,103
    3,038
    A 26A nominal max pump shouldn't blow a 30A relay, nor heat up a 40A relay if the wiring is adequate. Is it possible something else isn't right here, ie. could the pump be overloading somehow? Shorted field windings? Just speculating that maybe you're trying to solve the wrong problem.
     
  5. Bernard

    AAC Fanatic!

    Aug 7, 2008
    4,172
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    26A nominal might be around 100A locked rotor which pump is going thru for every reversal. This is out of my field, but would suggest a solid state H bridge with soft start & current limiting.
     
  6. debe

    Well-Known Member

    Sep 21, 2010
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    Is this a Hydraulic pump? If it is its a stange way of doing it. The last Autopilot I installed in a vessel used a 24V DC hydraulic pump that ran in one direction, & used Hydraulic solenoids on the steering ram to change direction. What brand & model is the Autopilot?
     
  7. debe

    Well-Known Member

    Sep 21, 2010
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    This may be some help. The cicuit is for an Autopilot that uses an electric motor to drive the steering. It is the relay unit & they were huge about 80Amp relays. Have hi lited the description of this unit, This particular Autopilot was either Hydraulic or Electric driven.
     
  8. strantor

    AAC Fanatic!

    Oct 3, 2010
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    it sounds to me like you're trying to make an H-bridge out of relays. Like any h-bridge, you have to look out for shoot-through. this could occur if, in when your autopilot switches from port to starboard, there is a brief instant where the starboard relays close while the port relays are still closed or vise versa. The current will flow from one side of the battery, through the two relays, straight to the other side of the battery, completely bypassing the motor, and spiking the amps very high which burns up the relays. you need to make sure that there is some dead time between switching from port to starboard. if using mechanical relays, this can easily be accomplished by running the port relay coil signal through the starboard relay normally closed contact and vise versa (assuming DPDT relay used).
     
  9. BillB3857

    Senior Member

    Feb 28, 2009
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    Looking at the schematic posted, make sure the wiring noted by the * and arrow noting terminals D and E are set to match the motor type you actually have. If wired for shunt motor and you have a series motor bad things can happen.
     
  10. panic mode

    Senior Member

    Oct 10, 2011
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    all comments so far are valid but if circuit is wired according to schematic, there is no wonder that you get short circuit.

    Shunt must be connected to terminals D and E. If shunt is connected to C and D (as shown), you get short circuit every time Rly3 is energized.

    If this is the case, Rly1 and Rly2 will never get chance to do anything, because Rly3 and fuses will be dead.

    Rly1 and Rly2 form relay equivalent of H-bridge. But for this to work, you need negative (in this case though shunt) on terminal E.
     
  11. BillB3857

    Senior Member

    Feb 28, 2009
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    Shunt winding between C and D would supply power to the shunt, not provided a short. Shunt windings in motors are made to carry full voltage unless a variable speed motor, then they put a variable voltage on the shunt to control torque/speed. If he has a series wound motor and the relay box is wired for a shunt wound motor, power would be supplied to the armature only, without the series field being in the circuit. At least that's the way I see it. Correct me if I'm wrong and I'll learn something.
     
  12. debe

    Well-Known Member

    Sep 21, 2010
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    I think you are right Bill. I only posted this as a learning exersise. Its a comercialy made Autopilot I fitted to a 45Ft ferrocement fishing vessel some years ago. The one fitted was hydraulicly operated, switching solenoids. But it could also be used with an electric motor drive using the relay circuit shown. I slill have a copy of the manual.
     
  13. WeeChumlee

    New Member

    Mar 14, 2012
    1
    0
    Hi guys
    It would seem to me that the best solution would indeed be the already mentioned H-Bridge, with integrated delay to prevent shorting.
    There are a few solutions on the market, this being one of them:
    http://www.crydom.com/en/Products/Catalog/d_p4_r.pdf
    More expensive than EMRs but may sort your problem with a minimum of hassle so could be worth a look.
    Cheers
    Stewart
     
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