CENTER
Lifetime Supporting Member
Good evening gentlemen. I just want to start off by saying that I have learned a lot by reading these posts just about every day. But I have two questions that are not PLC related. I hope one of you engineering gru's may be able to answer for me. Got a typical service call today on an old digital 150 hp DC drive. Circuit breaker trips immediately every time they jog. I get there and there are two other drives doing the same thing two more drives that are shutting down for no reason and two other drives that are running fine. To make a long story short after doing all the normal checks and of course when you run out of ideas start changing parts and then get on the phone. So after going over everything with the tech he told me to check phase to ground. Which I would have never thought of doing. Phase to phase was 480, phase to ground was 180, 720 and 720. Found out that the initial installation of 1000 KVA transformer 208 Delta the 480-volt Y they pulled the neutral into the switchboard but never grounded it. Located a 100 amp circuit breaker for an HVAC unit that was causing the trouble. Turn that off and everything runs fine. Now my two questions are.
1 I can see getting 480 to ground but 720?
2 Why would a drive with no reference to ground at all fire or stay latched 2 SCR of different phases at the same time?
This is been bugging me all day I hope somebody has an answer for me thank you.
1 I can see getting 480 to ground but 720?
2 Why would a drive with no reference to ground at all fire or stay latched 2 SCR of different phases at the same time?
This is been bugging me all day I hope somebody has an answer for me thank you.