240v magnetic switch on 120v?


I am now running my table saw and dust collector on the same 20a circuit, but will be buying a larger dust collector. Eventually I will put in a new 240v circuit, but for the moment I can change the TS back to 120v. Problem is the magnetic switch as a 240v relay in it.
Is the relay likely to work properly on 120v? When my 120v relay burnt out and I switched the whole thing over to 240v it was only like $10 for the relay, but it was a lot of work that I would just as soon avoid.
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Toller wrote:

The coil won't work on 120V. (You can likely buy a 120V coil)
The starter ratings may too low at 120V - the motor current will double.
If the starter has "overloads" that cause the starter to trip on overcurrent they will be the wrong size (they can probably be replaced with the correct size ones). Instead the motor may have built-in thermal protectors that disconnect it if it overheats (it will then be marked "thermally protected").
-- bud--
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Thanks.
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Hmmm, the fact that it take half the current at twice the voltage to generate the same number of watts doesn't seem to me to mean that the current will double if the voltage is cut in half.
It seems to me that if the voltage is cut in half, the current will also be cut in half also (E=IR), and the wattage will be only a quarter of what it was. (I'm not sayihng that wouldn't work. I don't know.)
Is there any particular reason I"m wrong (AC current?)?
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quarter of the power, it might not work.
But the big problem is with the current running though the relay contacts. It will be twice the current at half the voltage; and will likely burn out or fuse the contracts. Relays are rated by the size of the motor they will start without failure; typically they will start twice the hp at 240 as at 120v. Unfortunately. Since I don't want to replace the relay, I will have to put in the new circuit sooner than later.
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Considering you will have to change the motor wiring, the power cord or plug on the end of it. and still have to deal with this relay, wouldn't it be easier to just run some 12-3 or 10-3 romex from your panel and put a 220 outlet on the end of it? Unless you are lacking space in your breaker box or have to fish the wire thru a whole bunch of stuff, it just seems easier to run the 220 now. You only need 3 parts, the wire, the breaker and the 220 outlet. (plus screws and wire staples) If it's just a clear run across the basement ceiling of about 50 feet max., you're talking 2 hours labor. You'll spend more time than that on all that other stuff.
For the relay, you probably could buy a small step up transformer 120 to 220, but it would probably cost as much as the stuff to rewire. And while this may not be legal, you could make up a rubber coated long extension cord an just put the 220 outlet right under the breaker box. If you own the house, I'd wire it permanently, but a renter might prefer the ext. cord. (of course that rubber cord costs a fortune).
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quad breaker, and 50 joists to drill through. But yes, since the relay won't work, I have to do it now.
BUT, why do you say 12-3 or 10-3? Why three rather than two?
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Yeah, the old full box issue. Of course there are add on boxes that will hold 4 or more single breakers. Sometimes the box is cheaper than those special breakers. All you need is a couple inches of conduit and some heavy cable to put these boxes right next to your present one. If this is an unfinished basement, you can always nail a 1x4 to the joists and nail the cable to that. No drilling that way.

For something thats only 240, you only need 2 wires +gnd. Things like a kitchen range use some 120v things inside such as the light bulbs and clock, need 3 wire cable +gnd.
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has to be changed or rewired so that its impedance (basically AC resistance) is reduced to one-fourth of its 240V value. This is commonly done by switching from two coils in series to two coils in parallel. So you get twice the current with one-half the voltage. If you do not change the load the current IS reduced to one-half and the power IS reduced to one-fourth.
Don Young
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