The purpose of a magnetic power switch, or magnetic starter, is to prevent machinery which has had the power removed, usually by a circuit breaker tripping, from starting up again when power gets restored.
I bought a cheapie magnetic switch from Woodcraft for $50 and put it onto my 220 V jointer. It lasted about a year. The official rating on the switch said 10 Amps, and my jointer said 8.5 A, so I figured I was safe. When it failed, I found two problems. First, the Start switch was stuck in the On postion. Second, one of the relays' contacts was stuck closed. (This is the usual failure mode of relays which switch high current to inductive loads: the coils want to keep current flowing, so when an attempt is made to break the flow, large voltages appear across the relay contact, leading to arcing, high temperatures, and welding of the contacts.) Contacts on all relay wiring were charred, which makes me think that the parts and/or wiring can't handle the advertised Amperage. I replaced the relay, unstuck the switch, and beefed up the wiring. While I was as it, I added a remote set of start/stop switches on top of the jointer, where they're easier to reach.
This got me to thinking that I could make my own magnetic switches, better and cheaper. Higher-current-rated relays and reliable switches would be a good place to start. Some type of protection for the relay contacts, such as a MOV (Varistor) should also be added. Here are schematics of what I came up with:
The parts list contains a rundown of all parts for either voltage. I ordered things from Digi-Key, but the list has a column of manufacturers' part numbers (or trade numbers), so that you can get them from other suppliers.
By the way, there's a good article Robert Reed wrote for Woodwork magazine's October 2004 issue, in which he describes a magnetic switch that stays latched in response to current flowing through the load, rather that just the presence of voltage, as in basic designs such as this one above. The current detector (really just a transformer with a high turns ratio and fat wires in the primary windings) triggers a triac, which holds the relay latched. Reed's magnetic switch has the additional safety feature that it will shut off in the event of any interruption of current to the load, not just an overload tripping the breaker.
