Coding aThermal Overload Relay?

[Paulusgnome]

I was contemplating the you-beaut software features that are packed into today's PLC hardware, how if one had a mind to in an application with motors, it would presumably be quite feasible to code the thermal overload function if the line current was available as an input.
Feasible, maybe, but does anyone know if this is actually done in practice?

 

[jraef]

As the primary means of protecting the motor, or as a monitoring function? As a primary protection function it's not feasible primarily from a regulatory standpoint. Solid State Overload algorithms have to go through rigorous testing and approval from interested parties such as UL (I can't speak for IEC regulations but I'm sure it is similar). But as a monitoring function it could be done. You ideally would take in all phase currents but from a non-protective function you could get by with 2 (assuming 3 phase here). The basic algorithm is just I[sup]2[/sup]t, but there are tweaks for current imbalance, thermal memory and cool down time constants that I can think of, probably more.

But the next question is; why reinvent the wheel? Many SSOLs are now available with communications ports so they are already doing the protection internally under an approved algorithm and can just send the pertinent information into the PLC.


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[carl12345]

yes it would ...but then you would have to know the current to thermal characteristcs of the motor..which is also related to time......and that relationship is most likely not linear.....so to get a true relationship you would use a look up table to plot the thermal characteristcs or you would have the thermal mathamatical model for the particular motor....just put in a thermal overload...a fuse would do the job