Not really. The thermal overloads sense the current in each
winding, and generate heat to soften either a wax or a low temperature
solder to allow a wheel to rotate and a low-current contact to open.
All of the phases have their contacts in series, and in series with the
coil of the contactor. Any one of them opening drops the contactor out
to protect the load.
With single phase, a slow-blow fuse in series with each hot lead
should sufficient, but with three phases, you need to sense any
condition of over-current in any single phase, and to drop everything
once it is sensed. (Of course, if you are running the three-phase motor
from a VFD, that incorporates the functions of the starter (current
sensing and automatic shutdown) in the electronics of the VFD.
they describe it as a "thermostat" and rate the contacts at 10A 240V.
probably enough for a 1/2 HP motor.
but if you need the cut-out to switch both terminals that's not going
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
O.K. I've not experienced that on mine -- but given some of the
motors on the import H/V bandsaws, that is a possibility. And *that* is
a good reason for replacing the motor with one which delivers the
nameplate horsepower, instead of being mostly empty air in a too-large
housing trying to look like an adequate motor. :-) I've read the reports
of motors getting so hot that they burn the hands of the user, but mine
never gets that hot. MSC got a proper motor put into it, apparently.
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.
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