Motor Trip on Instantenous OC

[burntcoil]

200hp, 480V motor tripping on instantaneous RS510 DigiTrip unit. Motor winding Insulation resistance above 50G-ohm and healthy winding resistance. Trip Unit is healthy and no issue with the cable. What else can cause this?
Note: Motor runs for 20-25 min before tripping.

 

[jraef]

You really didn't provide anywhere near enough information for anything more than guesses.

Running for a length of time and then tripping on IOC generally points to something in the circuit. For example it could be moisture getting into a conduit or sheath, but only after whatever process involves this motor gets under way for a period of time, i.e. it takes 20-25 minutes for the moisture to reach a spot in the cable insulation that leaks. Anything inside of the motor is generally going to be immediately evident. So I would start with a good insulation test on your motor leads, without the motor connected since you already did that.


"You measure the size of the accomplishment by the obstacles you had to overcome to reach your goals" -- Booker T. Washington

 

[oldfieldguy]

If it's doing this on start-up, chances are good that the instantaneous setting of the trip unit is set too low. According to NEC, instantaneous can be set to 1300% of full load current. We generally set ours at the next increment the trip unit allows below that value.

If it's gone through start-up successfully and then trips on instantaneous, serious stuff is afoot, as jraef says. It is all too common to have a winding shorted and NOT have a ground path. Less common ar intermittent shorts in motor leads or in the feeders.

old field guy