Inductive kickback

Discussion in 'General Electronics Chat' started by uriahsky, Aug 21, 2012.

  1. uriahsky

    Thread Starter Active Member

    Apr 27, 2009
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    I have a soldering station that has a strange problem.

    When I press the button on the solder sucker it works just fine and vacuums up the solder but when I release the button there is a huge surge of current that blows the AC power input fuse. a 3 amp fuse.

    The station works just fine except for this one issue.

    The transformer is wired directly from the AC power and then it powers the rest of the system. The motor is wired to the output of the transformer and is controlled by a SCR. I have replaced all of the parts in the motor circuit on this board and I still get the problem.

    Can I just put a Diode or resistor and a cap across the motor leads and have it suppress this kickback?

    I figured I would ask before I did more damage.

    Thanks,
    Russ
     
  2. #12

    Expert

    Nov 30, 2010
    16,284
    6,797
    Sir, you are mistaken...but you already knew that, didn't you?

    You have a failed something-or-other. Redesigning it is the wrong way to go. Putting it back like new is the right thing. For that, we need a schematic, or at least a model number and a brand name. Maybe then we can spot a likely place to get that kind of short circuit.
     
  3. uriahsky

    Thread Starter Active Member

    Apr 27, 2009
    42
    2
    OK, but I double, triple checked all of these parts and changed them out with the exact replacements of the original parts and I still get the same problem, so I kind of gave up on that part and am looking for anything that might make me get some more life out of this. It works real well, except for the fuse blowing every now and then.

    I have asked for help on this before but didn't get any answers. I think it is in the zero crossing of the SCR if I remember right, is that where these kind of problems occur?

    I am wondering if it could be the transformer?

    Attached is the part of the circuit that controls the pump motor and the other one is the main schematic but the pcbs that control the temp are not included.
    Thanks again,
    Russ
     
    CrazyGoblin likes this.
  4. #12

    Expert

    Nov 30, 2010
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    That schematic is so hard to read!

    The way I see it, the motor is powered by AC voltage, right off the power line. The controller must therefore be a triac and the transformer is not supplying the motor. Even if the voltage punched through the triac, the motor should still limit the current. In other words, the motor should be able to run all day and not blow the fuse. You could add a MOV rated at 250 volts AC to protect the triac, but I think the motor is shorting. You could also short the triac with a piece of wire and it shouldn't make any difference.

    Have I read the schematic badly?
     
  5. uriahsky

    Thread Starter Active Member

    Apr 27, 2009
    42
    2
    I found another schematic. I have been at this one for a long time and forgot I had this.

    I changed or tested most of the parts past the MOC3061 going towards the motor. The parts before just send the signal to the MOC to turn on the motor, so I didn't think it was in any of those but I tested them and they checked OK. I thought there was something about zero crossing with the SCR as being a factor in inductive kickback?

    The power hot and neutral just parallel off the secondary from the transformer and they are right next to each other on the pcb.

    Take a look at this schematic if you could, it is much clearer.
    Thanks again,
    Russ
     
    CrazyGoblin likes this.
  6. #12

    Expert

    Nov 30, 2010
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    The hot and neutral to the motor parallel off the primary of the transformer. L and N mean Line and neutral. The pump is so isolated from the rest of the board, and it doesn't even have a fuse. (That thing called, "ZNR" is an optional MOV.)

    I reaffirm my previous statement. That motor should run all day without blowing a fuse, even if you shorted triac 2 with a piece of wire.

    The only fuse I see is a 3.15 amp fuse on a 24 volt heater. Is there another fuse that is not on the drawing?
     
  7. uriahsky

    Thread Starter Active Member

    Apr 27, 2009
    42
    2
    There is a fuse on the socket for the AC power cord input. One of those fuses that you can reach from the outside of the case.

    I was thinking that maybe the transformer was causing the problem? Could the transformer do this?

    When I read the current I cut the wire right after the AC input, right after the fuse on the way to the AC primary into the transformer. When the motor was running it was under an amp but as soon as the motor stopped it spiked up to almost 3 amps, the size of the fuse.

    I did the same thing on one of the motor leads and got the same thing, but they are both in parallel so it which one is causing it?

    The motor can run fine as long as it doesn't stop, when it does then the fuse will blow.
    ???
    Thanks
    Russ
     
  8. #12

    Expert

    Nov 30, 2010
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    I think you just proved the 3 amp current is going to the motor.

    Ideas...a fuse could be pre-heated by dirty connections. That will make it fail easier.
    The motor could be going bad.
    The habit of the power companies to supply higher and higher voltage over the years might be causing a higher current in a perfectly good machine.
    You might have a standard fuse where it should be a slow blow fuse.
    You might have a 3 amp fuse where a 4 amp fuse belongs. (read the label?)
    I found the fuse on the dim drawing, but no label there.

    That's all I can think of right now.
     
  9. uriahsky

    Thread Starter Active Member

    Apr 27, 2009
    42
    2
    Thank you for the help. I will provide a little bit more back ground and one last question. One day I was changing the one of the soldering irons out and I didn't realize that even with the power switch off there was still activity at the plug-in for the soldering iron. I put the wrong plug in and heard a pop. I fixed the SCR that blew out and after that I started having this problem. I don't think it is the fuse because I have changed it out so many times. And you can hear and feel the inductive kickback when the motor stops running. I wouldn't think that I damaged the motor or the transformer because those are beefy parts. I think it might in the group of parts I changed out even thought I replaced them all with the exact same part numbers, or another part in the circuit. But, I have checked them all so many times.

    Isn't there a simple way to stop inductive kickback?

    Perhaps if the circuit was operating correctly it would take care of this but if it is not would a diode on the motor stop that back rush of current or a resistor and a cap?

    Don't they do that for motors in simple circuits?

    Thanks again for the help,
    Russ
     
  10. #12

    Expert

    Nov 30, 2010
    16,284
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    From post #4

     
  11. #12

    Expert

    Nov 30, 2010
    16,284
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    From post #6

     
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