Overload prot?

TimeFluxCap

Member
Join Date
Nov 2002
Location
Australia
Posts
321
Hi everyone

have a curious question regarding a star delta panel that I rewired the other day. The motor had two types of protection, thermistors for over temp and and overload setup that consisted of three CTs for the supply mains with a overload factory wired to them. The overload had a range of 1 - 6 A. I assume that this is for phase inbalance but I have never seen this used before. Is this common overload protection control?
 
Hi,

I think the overload protection of 1-6 A is made for te CT´s.
I asume they are *** to 5A. So the protection has to be set as a percentage of the CT current.
 
Hi TimeFluxCap

We use this kind of protection very often.
In our case the machine will run in star without the protection of the overload protection, due to the long start-up time (3min.). The overload protection would not hold that long, therefor we use also a thermistor relay.
 
The use of C/Ts for over current sensing is almost universal for solid state starters, and is very common on conventional starters with solid state overload protection. I first saw it on some Siemens starters around ten years ago. The advantage, obviously, is lower cost because you don't have to run full motor current through the overloads, plus you can use the same overload device for a variety of starters just by changing the C/Ts.

A thermistor isn't actually an overload device, it is an over-temperature detector. The high temp may be due to high current, or it may be due to plugged ventilation louvers, or it may be due to high ambient temperature, etc. Actual temperature sensing and shutdown is getting very common on VFD applications, but it is a good idea on any motor where the ambient is harsh or the motor is located remote from the over current protection.
 

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