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I inherited a very old drill press from my father (120V single-phase motor). I don't use it a lot but it's always worked OK.

Recently, however, it sometimes won't turn on. If I keep flipping the toggle switch, it will (usually) eventually start. Once started it seems to run normally. Also: when I'm flipping the switch trying to start it, sometimes when I flip the switch OFF the motor kicks a fraction of a revolution.

The problem with disassembling it to check things is that many of the screws and bolts are rusted and frozen; so far penetrating oil hasn't helped. I'd like to minimize radical surgery like breaking things in order to get it apart.

I'm thinking the top 2 suspects are the power switch and the starting capacitor. The switch seems unlikely, but could a failing capacitor cause the symptoms I described? There is a cylindrical housing attached to the motor case, which I assume is the capacitor.

Any other possibilities I haven't thought of?

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    \$\begingroup\$ Needs a new capacitor. 99% for sure. The specs should be printed on the old capacitor. They are easy to source, look for "motor start capacitors"... Any old cap will not do. Yes, the cylindrical housing is most likely the cap - Note it might be INSIDE that housing. You'll figure it out. \$\endgroup\$
    – Kyle B
    Commented Feb 7, 2023 at 23:58
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    \$\begingroup\$ I think it's bad contacts on the switch. Capacitor start motors have high inrush current. \$\endgroup\$
    – PStechPaul
    Commented Feb 8, 2023 at 7:07

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The cap should be replaced . I use metal film caps for reliability and very long life .Bipolar electrolytics are less reliable and have limited life .Use approx the same value origional start cap .Tolerances are broad so there is little to gain by being exact .If your new cap is much smaller your starting torque will be lower which will not be a problem with a fan load but will be bad on say a piston compressor .If your cap is too much bigger the start windings will have more copper loss and get hotter.It is fine to use multiple caps in parallel to get your value .Your metal film caps should have a proper AC voltage rating.Check the mechanical switch because cap failure can sometimes ruin the contacts.

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  • \$\begingroup\$ Have ordered a new cap. The original 5KΩ bridging resistor seems unusually low; is there a problem making it bigger? \$\endgroup\$
    – Rob Lewis
    Commented Feb 8, 2023 at 1:38
  • \$\begingroup\$ 5k dissipates about 3W. I'd try a 10k or 15k. It's probably only to discharge the cap. 5k and 340 uF is about 1.5 seconds TC. \$\endgroup\$
    – PStechPaul
    Commented Feb 8, 2023 at 7:05
  • \$\begingroup\$ Excessive cycling by the oP is likely to have reduced the lifetime of the contacts... \$\endgroup\$
    – Solar Mike
    Commented Feb 8, 2023 at 8:49
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@PSTechPaul nailed it: flaky power switch. I found that if I flip the toggle switch up and it doesn't start, just pulling the switch handle down a fraction of an inch will usually make it go. This also explains why sometimes switching it OFF makes the motor kick for an instant. Unfortunately I'm going to have to chisel or grind off the frozen screws that hold the switch mounting plate to the chassis.

In any case, the motor now sports a brand-new starting capacitor to replace the original one that is probably close to 80 years old!

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  • \$\begingroup\$ Angle grinder, drill and tap for new switch and plate screws. I'm going to go out on a limb here and wager your new switch will not last the same 80 years as the last.... Glad you got it sorted out. \$\endgroup\$
    – David C. Rankin
    Commented Feb 12, 2023 at 6:57

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