Question: Thermal Magnetic Motor Circuit Breaker

The reason is probably that the contacts of the overload protection switch are not capavble of handling the current of the motor. The break away current can pull a large arc which produces excessive heat and contact welding, after which it will no longer work to break the current next time. That is why a relay or contactor is used to switch the larger motor current. Mel

These motor circuit breakers can be used without contactors where you only require local control. Most applications require a contactor so that the motor can be stopped/started by an electrical signal which you cannot do with the circuit breaker, this is especially true for emergency stop and ESD circuits.

The manufacturer in question may not feel the product would be reliable if used as a switch due to the previously mentioned arcing during breaking. Contactors are relatively inexpensive and often come with replaceable contacts. If high cycle rates would cause problems in reliability then consider using solid state relays which are available with ac and/or dc control inputs. I have used them in harsh environments with excellent results.

I switch my industrial compressor on/off by controlling the contactor.

Dedending on the size of the motor,Square D makes a 20amp single pole starter switch that you can insert an appropriately sized Thermal unit (such as a B 45) in. Themal breakers are expensive and the contacts will wear out if used routinely to start and stop a motor. Also a contacter will allow for additional emergency cut off applications.

This device is fitted to disconnect and protect the motor from burning out due to overheating as a result of electrical overload. Frankly I don't know how or why this different or better from the little built in reset able thermal overload breakers that are common in the motors themselves. I'd be interested in hearing about the difference in them and the necessity of a contactor over and above a built in thermal breaker as well.