Am 12.01.2014 01:48, schrieb DaveC:
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?"

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 are the arguments for using a contactor as opposed to a simple on-off
switch of equivalent rating?

Thanks.

1.) Safety issues. It is a lot safer to have the machine shut off, when
the power is cut. Otherwise the saw might start up again, while you have
your fingers on the blade or so. Or the blade is stuck and starting the
saw can ruin the blade/motor/mechanics.
2.) When using a simple on-off-switch, there is really no good way to
have it stop, other than turning the switch.
When you use a contactor, you can start and stop the saw with multiple
switches, for example a two-button-start, to ensure both hands are away
from the blade, or one stop-button and a sensor/switch, that stops the
saw, when it hits a certain point or so.
In our company we have special motor-switches, that have an under
voltage-switch attached. The switch has two buttons (on and off; who
would have thought :-) and the under voltage-switch is attached on the
side and turns the switch off, when the voltage drops below a certain
value, via a mechanical rod assembly. And that on machines, that,
theoretically will just give you a good whack if it starts up and you
have a body part in there. Might also be a good reason to install
something like that in a band saw.

Greets

Tom