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10th Sep 2020, 07:13 PM #1Golden Member
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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?
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10th Sep 2020, 10:29 PM #2Most Valued Member
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- Mar 2011
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- Southern Flinders Ranges
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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.
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11th Sep 2020, 12:02 AM #3Golden Member
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11th Sep 2020, 12:21 AM #4
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.
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11th Sep 2020, 06:03 AM #5
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.Best Regards:
Baron J.
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