Why use a contactor?

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On 1/12/2014 1:56 AM, DaveC wrote:

Yes.
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    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.
    Enjoy,         DoN.n
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http://www.sensata.com/klixon/thermostat-precision-7bt2.htm
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 to suit.
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    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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No, on bandsaw, the main problem is that it runs unattended and can easily get stuck.
i
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    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.
    Enjoy,         DoN.
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Am 12.01.2014 01:48, schrieb DaveC:

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
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How does a low-voltage switch work via a mechanical rod assembly?
Can't picture this...
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"DaveC" <

Picture this: a "linear rheostat". ;>)} The shoe could be moved by the rod ass'y. pdk
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