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  1. #1
    Join Date
    Jul 2011
    Location
    Adelaide
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    837

    Default Unimac DC motor controller issue.

    I finally decided to have a close look at my Precision Grinding Unimac. IMG_20200909_121028.jpg

    It has a parallel wound DC motor. The speed controller circuit speed control.pdf shows a half controlled bridge rectifier providing power for the armature and a bridge rectifier for the field coils.

    Most of the time the speed controller works fine but sometimes at slow speed it will give a burst of full speed for ½ a second or so then for a second run at the speed it is supposed to run at then repeat until I get scared enough to turn it off.

    I think I basically understand the circuit but can't think of a component failure that would lead to the mode of failure I am seeing.

    Anybody have any ideas before I open her up and start taking measurements?

  2. #2
    Join Date
    Mar 2011
    Location
    Southern Flinders Ranges
    Posts
    1,536

    Default

    Sounds like one of the SCR’s is being gated when it goes full noise, figure out which one and have a look around the Op Amp driver for it. Nice circuit to fault find, nearly everything is duplicated.

  3. #3
    Join Date
    Jul 2011
    Location
    Adelaide
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    837

    Default

    Quote Originally Posted by racingtadpole View Post
    Sounds like one of the SCR’s is being gated when it goes full noise, figure out which one and have a look around the Op Amp driver for it. Nice circuit to fault find, nearly everything is duplicated.
    I have opened it up. Both SCRs are gated when it goes full noise the problem seems to be a spurious input to the op amp comparators from the one bit of circuit that isn't duplicated. I am tending to think it is probably aged capacitors.

  4. #4
    Join Date
    Oct 2007
    Location
    Alexandra Vic
    Age
    69
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    654

    Default

    Another possibility would be an intermittent short between field and armature terminals/cabling. Because the field is always fed half wave rectified derived from the mains supply, and the armature is fed from the same source via SCR rather than simple diode, the armature voltage would normally be somewhat lower than the field voltage. Some form of intermittent short would raise the armature voltage to the same level as the the field voltage (full bore) until the the short dissipates. I'm thinking cables that have frayed insulation in a conduit etc, some form of moisture/pests on some contacts, or if not that then I would be checking SCR's and driver circuits, could be a scratchy pot if you have user controlled speed through some of the pots in the circuit, scratchy pots are a plague in older AV equipment and an intermittent wiper connection could cause the issue.

    I suspect you will need to see waveforms etc to diagnose if the issue is electronic rather than something related to cabling, terminals etc. Beware connecting a CRO or similar, output is effectively a hot chassis situation with nowhere to connect the grounds on the CRO probes, 50% chance of connecting mains active to CRO Ground and 50% chance of connecting mains neutral to CRO Ground. Either way will trip RCD's etc and isolate the circuit under test. True isolation transformer to power the speed control would be useful and eliminate grounding issues.
    I used to be an engineer, I'm not an engineer any more, but on the really good days I can remember when I was.

  5. #5
    Join Date
    Sep 2012
    Location
    York, North Yorkshire UK
    Posts
    6,439

    Default

    Hi Guys,

    I suspect you will need to see waveforms etc to diagnose if the issue is electronic rather than something related to cabling, terminals etc. Beware connecting a CRO or similar, output is effectively a hot chassis situation with nowhere to connect the grounds on the CRO probes, 50% chance of connecting mains active to CRO Ground and 50% chance of connecting mains neutral to CRO Ground. Either way will trip RCD's etc and isolate the circuit under test. True isolation transformer to power the speed control would be useful and eliminate grounding issues.
    I Strongly agree with using an Isolation transformer, I've seen scopes destroyed from not doing this ! You could also easily electrocute yourself. Remember "one hand in pocket" when working on live gear !
    Best Regards:
    Baron J.

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