Motor failure, Need help!

[rwacan]

We are on a 600VAC delta system. There is this one motor that runs a saw arbour. The motor is 200hp, 1800 rpm, 189 FLC. We run the feeders through 400:5 CTs and the secondary on the CT's to a eutectic overload device which is set to 2.4 amps which should make the overload current value for the motor at 192 amps. This motor definately sees its share of overloading but for some reason does not trip any of the safety devices. I watched in a video recording that the motor had been stalled for approx 30 seconds after which it became a big ball of flame. During this time neither the fuses or the overload protection triggered a shut down. I did confirm the CT's are actually working and the overload control contacts are good also. I also inspected the motor after the fact and it showed signs of extreme overload but no signs of winding faults or shorts.

Does anyone have any ideas I can check as to why we might have not tripped anything on this type of fault.

 

[Dickylad]

Hi rwacan,
you could inject a current into the overload to see if it opens the contact within the correct time. These are thermal overloads and by there very nature are not very accurate, as a rule of thumb I apply a 20% accuracy to them. For this reason I always lean toward an electronic overload for protection of a motor of this size. If over time the overload has tripped out (possibly legitimately) there is always a temptation to increase the setting (if these are adjustable). As you can see if the setting was adjusted from 2.4A to 3.0A for instance the tripping current would have to be 240A to operate the overload. Because they are thermal devices a correction factor (temperature) is normally used Dependant on the specific application, the manufacturers website should have details of this and the relevant characteristic curves for the heater elements which are fitted in the eutectic overload.

Regards,

Dickylad.