Sessions Repairing Sessions electric Banjo clock motor

Discussion in 'Electric Horology' started by focusrsh_b07732, Jan 31, 2020.

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  1. focusrsh_b07732

    focusrsh_b07732 Registered User
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    Dec 17, 2009
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    I'm repairing a friends Session electric Banjo clock. The clock has essentially no worth except for sentimental. I've not found any replacement motors although I can buy a complete working clock on Ebay for about $65. But just for the heck of it I thought I'd take a crack at repairing it. I disassembled the motor and found the coil open circuited. I used my wife's yarn knitting winder to carefully unwind the coil, which I estimate as having about 4800 feet of wire. I found that the wire had broken down at the very end, so I created a new pigtail of a few inches at the end, then carefully rewound the coil. After assembling it, the motor ran.

    I tested the motor a few times. One time, however, I unplugged it from the AC and was rewarded with a flash, a bang and a puff of smoke. Taking it all apart again, I found one of the leads burned through. I know the internal leads to the coil were not touching and it was working fine until I unplugged it.

    Being an electrical engineer, I suspect inductive kick-back. The winding coil is fully charged at one point in the AC cycle, and when I unplugged it, I think what happened is the magnetic field collapsed and, like the coil in a car's ignition, it created a huge voltage that arced-over.

    Has anyone else experienced this?

    I know how to prevent this problem today using a pair of high-voltage diodes, but how was this prevented originally? When I disassembled the motor, there were just two large lead wires soldered to the pigtail ends of the coil wires. Any thoughts?
     
  2. Uhralt

    Uhralt Registered User
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    Sep 4, 2008
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    I don't think that there was originally anything to prevent this. I'm not sure if what you describe happens in an AC situation. I'm only familiar with the effect in DC applications.

    Uhralt
     
  3. davefr

    davefr Registered User
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    Nov 29, 2008
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    Semi Retired Engr. Mgr - Currently Rebuilding Tele
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    I disagree. Given that it's AC, the magnetic field is always collapsing at a rate of 60 Hz. Besides that, electric clocks have always been plugged in and unplugged and I've never seen an issue.

    My theory is you have a coil with additional damaged insulation in it's winding. Was the coil ever powered on without being inserted into it's iron frame? If it was, that will fry a coil because it needs the iron frame's inductance. Was it ever used at a higher voltage then it's rating?

    As long as you have the means to rewind the coil, just use brand new magnet wired of the same gauge and number of turns.

    In my experience the vast majority of bad coils are the result of damage in the connecting point to the coil's internal wiring.
     
    THTanner likes this.

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