On 2014-01-12, Ignoramus13867 wrote:

[ ... ]

On 2014-01-12, 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 are the arguments for using a contactor as opposed to a simple on-off
switch of equivalent rating?

[ ... ]

The best argument for using a contactor in a small bandsaw, is that
you can get proper thermal overloads to protect the motor.

i

This is more of a factor with three phase motors, where a motor
once started will continue to run if one phase of power fails, but the
current drawn in each remaining winding will be higher, and will be
likely to burn out the motor. (And he was going to add a 240 VAC single
phase motor to the bandsaw in question.)

A proper motor starter (of which the contactor is a part)
includes thermal sensors for the current through each winding, and if
any one of the three goes well above the normal full-load running
current (as will happen if one phase dies), it will interrupt the power
to the motor and stop it before the motor is damaged. And it also
protects the windings from burning out when trying to start with one
phase of three missing.

And actually, some starters only measure the current through two
of the three legs, because if one is out, one of the two monitored will
be over-current enough to trip it.

So -- with a single phase 240 VAC motor, the over-current from
loss of a phase would not apply.

But the other reasons for using a motor starter/contactor still
stand.

Enjoy,
DoN.

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