Re: Why use a contactor?

On 1/11/2014 10:44 PM, Lew Hodgett wrote:
> This may be a duplicate post
>
> Lew
> ---------------------------------------------------------------
> "DaveC" wrote:
>
>> A friend (in USA) with a 220v, 1-phase horizontal bandsaw in his home
>> machine
>> shop wants to replace the 1/4 hp motor with 1/2 hp and rewire with
>> an
>> auto-off switch (when the blade cuts through the metal).
>>
>> I always default to using a 3-wire control with contactor, but he
>> asks the
>> logical question: "Why?"
> --------------------------------------------------
> What you are describing is known as under voltage protection.
>
> Safety pure and simple is the best reason for using it.
> ---------------------------------------------------
>> Other than the "If the power fails while cutting" answer (which he
>> says seems
>> of limited additional value on a saw with auto-stop feature) I
>> wasn't very
>> convincing.
> -----------------------------------------------
> What happens in the event you lose power and as a result you lose
> lighting
> as well as power to the saw?
>
> Without under voltage protection, when power is restored, the saw
> will restart automatically which can be disastrous, especially if the
> lights
> haven't restarted.
>
> Your insurance company will also like you.
> ------------------------------------------
>> What are the arguments for using a contactor as opposed to a simple
>> on-off
>> switch of equivalent rating?
> ---------------------------------------------
> See above.
>
>
> Lew
>
>
>
I suspect that because most 220 volt machines are used more in an
industrial setting, in the US, than in the typical home work shop. In
an industrial setting there are numerous machines that are operating all
the time. With a power outage all machines stop but do not come back on
after workers leave. Imagine all of them powering back up with no one
standing around to turn them off when the power is restored.

With the lower voltage machines typically found mostly in home shops it
is unlikely that more than one would come back on by itself nor a group
of people that might be around when the power is restored, hence you
don't often see a magnetic switch being used.

I suspect the magnetic switch is more if an industrial safety measure
that many of us enjoy the benefits from in our home shops.

Having said that and IIRC my relatively new Delta 15 inch stationary
planer does not have a magnetic switch although it is wired for and runs
on 220. I suspect that particular model with its switch configuration
is not available as an industrial unit.