Shopsmith Forums

everettdavis wrote:I am trying to mitigate the damage caused by contact arcs present when inductive load of the motor starts and stops. Some sort of Arc Suppression similar to that used in relay circuits which often switch rapidly, is very similar to a power switch in that the relay manually engages and disengages contacts by action of a magnetic field pulling on a spring loaded actuator, while switch contacts are manually moved by a lever mechanism which manually engages and disengages contacts.

Below I have placed an illustration that shows the result of switching loads with and without suppression. My point is only that if we can determine an affordable option for similarly arc suppressing Mark V Shopsmith power switches, we can significantly prolong their life.

Arc Suppression.jpg

I suggest a capacitor with a bleeder resistor across the contacts.

However My power switch on my Goldie has only been replaced once in a half century.

i.e. Not worth the trouble.

BTW to me an arc suppressor is not the same as a varistor.

These days, industrial relay circuits typically use DC coils. Those are very easy to tame by simply adding a freewheeling diode across the coil. Modern relays often have plug-in diodes as an option.

AC is a little more difficult to work with, as you can't use freewheeling diodes. In designing CNC upgrades for older machine tools, I've had good success with putting RC snubbers like these across the relay and contactor coils. I've attached a data sheet -- note the preferred snubber placement in the circuit. Calculating the snubbed waveforms is complex -- it's easier to just try one (or two, or three ..) and measure the results with a scope.
Another way to reduce contact erosion is to simply wire both sets of DPST contacts in series. That way, each contact only has to dissipate half as much energy from the inductive arc, which should more than double the contact life. For this exact reason, I suspect that it was a mistake on Shopsmith's part when they quit wiring the switch to break both the line and neutral connections to the motor.

Edit: I see that JPG beat me to the punch. These snubbers are indeed just RC circuits, but in a single package.

Is the snubber the same as a MOV or a metal oxide varistor? An MOV is a very high resistance until the applied voltage rises above the trip point and then the resistance goes low to bleed off the excess voltage. These are more used for spike suppression that occur as a result of lightning strikes or other overvoltage glitches.
Bill V

wa2crk wrote:Is the snubber the same as a MOV or a metal oxide varistor? An MOV is a very high resistance until the applied voltage rises above the trip point and then the resistance goes low to bleed off the excess voltage. These are more used for spike suppression that occur as a result of lightning strikes or other overvoltage glitches.
Bill V

No, an MOV is a different technology. RC snubbers can last indefinitely if used within their ratings. MOVs have wear-out modes which limit the number of surges that they can withstand, and are best used to protect against abnormal events. But because the absorbed energy is distributed throughout the device (instead of only at a small junction as in a zener diode), they can absorb a much larger single energy pulse than can a semiconductor device of a similar physical size.