Thermal Overloads

[kody916]

Grreetings everyone.

How do thermal overloads(in a magnetic motor starter) and in general operate?
can they be reused once they operate or they are like fuses, once they operate they have to be replaced.

how do electronic overloads operate and how are they different from thermal overloads?
Thanks

 

[petersonra]

Grreetings everyone.

How do thermal overloads(in a magnetic motor starter) and in general operate?
can they be reused once they operate or they are like fuses, once they operate they have to be replaced.

how do electronic overloads operate and how are they different from thermal overloads?
Thanks

thermal overloads work like old style thermostats. they heat up as current flows through them and eventually the heat causes mechanical motion that opens up a contact.

electronic overloads work by monitoring the current electronically and simulating the way thermal overloads work mathematically.

 

[Jraef]

They are not like fuses, once they cool down again they return to normal. If they have tripped hundreds of times, some types might develope what's called "thermal memory" where it takes less current to trip them, but that still protects the motor.

Electronic OLs never do that, plus most come with real phase loss protection.

 

[templdl]

Commonly the OLR consists of a 'heater' and a bimetallic thermal element. The 'heaters' are selected base upon the motor being protected based upon a chart of heater sizes as would be included with the OLR. The current for the motor passes through the heater element which will increase in heat as the current increases. The heat warms the bimetalic element which is located close to the heater causing the bimetallic element to start to bend. If the bimetallic element is deflected enough if opens a N/C that is wired in series with
the coil of the contactor which intern disconnects the power to the contactor coil dropping out the contactor which controls the motor.