There is an overhead door at work that will start to open but then stops. I don't know all the details because I haven't been out to the door yet, but the information has been given to me by one of my guys. The door is driven by a 3-phase 480 volt motor. I don't know anymore information than that. I don't know the hp or if its delta or wye connected or anything else, I have not seen it. The guys tell me they don't see anything mechanically wrong with the door, and that it operates smoothly by the manual chain. I am told that the C-phase thermal overload element was found completely fried. I found this to be unusual being that the overload relay's purpose is to open up the control circuit to protect the motor, in this case the heater couldn't even protect itself! This is a bi-metal type OL set at 115%. So aside from assuming the OL relay took too long to trip, they did the following checks: They took Meggar readings phase to ground from the load terminals and the insulation for all phases was acceptable. They did a continuity check phase to phase and said there was continuity. The workers proceeded to replace JUST the C-phase thermal element with a new one.(without replacing the entire O/L relay) They started the motor and the same thing happened, the motor started, the door started to open and then stopped, it tripped on thermal OL, but yet again the C-phase O/L heater was fried. They took resistance readings again this time from the motor with the line connections determed. They paid particular attention to the exact ohm readings this time and got 20,20,40. The electric brake was determed as well, and was not a factor into these readings. One of the guys explains the readings by saying it is a delta motor with one winding burnt open. I asked him if he could read the nameplate, but he could not. I understand that a single-phased motor will continue to run once started due to inertia and the one phase being energized, BUT I though it could NOT re-start. His theory was that an "opened" delta could re-start if it had all three-phases supplied to it, it would still energize 2 out of 3 phases, energizing enough poles to give it the necessary magnetic fields. Can anyone explain some theory behind single-phasing, and if it is possible to restart a delta-wound motor that has had one winding burnt open?