star delta starter , the power plant block out

yes that is for main breaker 800 A,

for that individual feeder , motor protection of 145 A overload relay

we are using incomer for motor is 800A, but the motor has seperate protection such as thermal overload relay eventhough the motor also burnt

may be its bcoz of high short circuit......have u consulted wit anyone regarding breaker sc capacity......?

yes the motor is LT motor (440v),and it is 60 HZ

800 A breaker is the incomer to the compressor feeder. In compressor feeder we are setted 145 A thermal overload relay, single phase preventer. eventhough the motor burnt.

ok,

for the compressor 1 litre oil was filled. this was after happened after 2 times the motor got tripped due to overload.

befor starting the motor we checked the megger>550 m ohm and winding resistance as 0.13 ohms

so we started the motor for the third time . at that time the motor got burnt and our auxiliary transformer got tripped due to earth fault

So that is it.You have started the motor continuously under hot condition.Every motor is suitable for certain number of hot and cold starts .Normally it is 3 cold starts and 2 hot starts .Cold starts means machine under ambient condition.Hot is when machine is swiched or tripped while running with full load and at rated temp rise over ambient.You should have allowed a rest of 45 Min (Check manufacturers cooling curve)Normally in new machines 30 min cooling is ok.Continuous starts means continuously starting current flowing.One question did it trip after switching to delta or in star?some times if switching over is delayed motor cannot accelerate inder star condition and may trip.Since the machine was in hot condition thermal overload must have tripped and repeated attemps to start must have lead to the failure .By the way have u got a fly wheel on the motor shaft?One one by one if we analyse it will not be a simple application .

You have probably exceeded the no of starts permissible under hot condition.Every motor has what is known as no of cold and hot starts.In normal case it is 3 cold and 2 hot starts in an hour.After 2nd start motor should be allowed to cool down .A rest of 30 min to 45 min is essential ( See manufacturers cooling time curve)Further do you have a fly wheel mounted on the motor shaft?Did it trip after changing to delta or in star?I presume you are using a automatic star delta star.With the available details one can conclude that permitted temp rise has been exceeded due to repeated starts leading to failure.Probably the tripping was on thermal overload which was ignored.

we are not not using fly wheel mounted on shaft. we are using automatic star delta starter.after the motor burnt we found our main contactor R,y.B phase was melted and the star contactor also melted. nothing happened to delta contactor. also thermal overload relay also burned.

ok the motor got burnt.because of high current,the incomer breaker for the compressor motor 800 A not tripped. our auxiliary transformer breaker in LT side 4000 A is also not tripped, HT side is tripped due to earth fault

So the motor was in star and could not accelearte the load and failed.As regards other relays and protection one cannot comment unless we go through the whole relay grading and coordination scheme.

how suddenly it cannot acccelerate the load. the motor was run just before 1 day ago

Because starter failed to switch over to delta.The star gives only1/3 torque and in delta motor will give rated torque.You are using the star delta starter to minise duration for which starting current persists.