Interesting call yesterday to a big carpenter shop. I had an Italian made planer that was tripping the main breaker feeding the machine the instant you pressed the start button to bring on the planer motor. I felt that it was most likely a grounded motor winding, and it did show a ground. But I went through the connections and metered it all out, but I didn't see the normal reading you would see on a regular 3 phase motor. The reason? It was wired as a 'wye/delta', soft start circuit. This circuit applies the voltage to the motor windings in a 'wye' connection during initial starting, then after a 3 second time delay, connects the motor as a 'delta' connection, or full line voltage. This can be a bit tricky if you're not used to seeing this. At first, I checked things with my meter and felt that, due to the ground reading' it was the motor for sure, and made the call, billed them, and left.
On the drive home, it was just nagging me about the readings I saw with my meter. I began doubting my diagnoses. This could have been a costly mistake for my customer if I was wrong. A $1000 dollar motor shipped, plus shipping costs! I got home and decided to go all the way back, to Mancos, to re check the circuit!
I got back to the shop and devised a test for the control contactors by jumping out a couple circuits, and was able to press the start button, and not actually apply power to the motor, but just let the control bring on the contactors to simulate a normal start. Everything checked out perfectly! I had made the right call after all! I also learned a great deal about wye/delta starting for large, 3 phase AC motors! A good day after all.
On the drive home, it was just nagging me about the readings I saw with my meter. I began doubting my diagnoses. This could have been a costly mistake for my customer if I was wrong. A $1000 dollar motor shipped, plus shipping costs! I got home and decided to go all the way back, to Mancos, to re check the circuit!
I got back to the shop and devised a test for the control contactors by jumping out a couple circuits, and was able to press the start button, and not actually apply power to the motor, but just let the control bring on the contactors to simulate a normal start. Everything checked out perfectly! I had made the right call after all! I also learned a great deal about wye/delta starting for large, 3 phase AC motors! A good day after all.
