I have a 7200 volt motor with a 97 amp FLA, that burned up due to single phasing and what I need help on is interpreting the events recordings. This motor is loaded down to about 45% load and I can tell that the customer did not properly troubleshoot the faults leading up to the motor catching on fire, but my question is if my Multilin 369 is seeing incoming voltage on C phase or is the voltage coming from A and B phase? Voltage is present on all three phases but current is not present on C phase.My fuses did not open and now my contactor has A and B phase welded closed and C phase is open, fuses never opened since they are sized for short circuit protection and the current at the time of the motor fire was at 605 amps for 4 minutes. Before the motor was stopped the current was about 125 amps. Normally the motor runs at about 45 amps when everything is normal.
Let me know if more information is needed, thanks and look forward to your comments.
You don't ever have voltage on only a phase, you have current on a phase, but voltage BETWEEN phases, i.e. A-B, B-C, C-A (referred to a Vab, Vbc, Vca).
If you have a 7200V system, the 369 measures voltage through Potential Transformers that step it down to a safer level, typically 120V. Those transformers are usually connected using two PTs in an Open Delta configuration, typically connected to Vab and Vbc, then Vca is derived from those other two. There is nothing wrong with this connection by the way, this is how most protective relaying equipment is connected. But the PTs are fused, so if you blow one of those fuses, the relay will not show one of the voltages. Default settings in the 369 would have it trip off on Phase Loss since it would not know it was just a PT fuse. What I have seen happen is that someone unfamiliar with how this all works will see the relay trip on phase loss, they check the incoming phases and see there is no phase loss, so they ASSume the relay is defective and "fix" the problem by disabling the phase loss trip function! So that's one thing to check first,
what is the 369 SET to do?
The danger of single phasing a motor that is very lightly loaded is that the severe imbalance of current in the stator creates negative sequence current flow in the rotor, which in turn causes counter-rotating torque to be produced. So the motor "fights itself' and the heating effect in the motor is
greater than it should be for the given current flow. So if the motor is very lightly loaded, the increase in current under single phasing may still be below the Thermal Overload Trip setting, but the EFFECT that current has on motor heating is OVER the thermal damage curve of the motor. That is why, in something like a 369 relay, you want to program a current imbalance protection scheme into it. Again, check to see what it is set at. If it is disabled (as I would suspect), that is the root cause of your failure in the motor.