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Posted by Toller on December 10, 2006, 9:12 pm
>
>>
>>> Toller wrote:
>>>> 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.
>>> Three possible problems
>>>
>>> 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.
>>>
>>I didn't think of the starter rating; probably end up burning it out.
>>Thanks.
>
> 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?)?
>
You are right about the current to the coil; since it will be pulling a
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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