I recently came across a problem of a six lead 3 480 volt 300 HP 340 FLA with the option of an in-line delta connection or a Y-start/delta run configuration motor which powers an Ammonia based compressor for an IQF system at a food processing plant, not wanting to start. Couple minutes of troubleshooting, find that the third leg of a Square-D thermal-magnetic 600V 600A 3 pole circuit breaker with the Mag adjust setting at aprox. 4500A had burned up. A little puzzled, replaced the breaker in order to get the system back up and running. Compressor started, and the system seemed to be running just fine. Started investigating for answers to report to the customer why this $3400.00 (my cost used) breaker had burned up and how to prevent it from happening again. Here's my findings after talking to maintenance personell, visual inspection, studying schematics, and personal knowledge.
Breaker would trip aprox. 4-5 times a month 95% of the time with in 3 sec. of start up. Maintenance personell would just reset it and the system would start the second time until the breaker would give up. Well, answer about the breaker. Also found out that this was the fourth breaker in about 3 yrs. Here's what else I found. It was a clean professional installation. Top of the line SQUARE-D Equipment from the 3000A 480V Distribution through the size 6 which I at first thought was a Y-start/Delta Run set up, 600KMCM copper conductors all controlled by a computer based controller/diagnostic set up on the compressor. Visual inspection of the motor revealed some winding dark in color and apparent insulation break down. Megger confirmed this. OK. Motor degrading this could be the cause of the breaker tripping. Now, why motor degregation. At this point, customer wanted to replace the motor in order to prevent any system down time. (New motor $7000.00 my cost) Time to find out how to prevent reoccurence. Further investigation of the starting cabinet. It's premanufactured by SQURE-D consisting of 3 size 6 starters, a tapped off thermal overload set up, a couple of timing relays, mechanical interlocks between starters, CT on center leg for computer etc.. Wasn't to sure why only three conductors to motor or what the fairly good sized xfmer at the bottom of the cabinet was for. Read schematics and found out that it was a reduced voltage starter set up which is the first time I ever came across something like this. So here's the questions I have for you guys.
1) Why this kind of set up when the motors designed for Y-start/Delta run and all you would really need is three more conductors if you needed a high torque start?
2) If I can turn the shaft of the motor while connected to the load by hand, why not just a regular delta connection?
3) What the heck is the purpose of a reduced voltage start where the xfmer is tapped at 65% of line voltage at start up? Are'nt most motors tolorences with in 20% before damage may occur? Wouldn't the reduced voltage create excessive current and heat to the windings of the motor causing insulation breakdown over a period time to the point of excessive in rush current applied to protective device such as a breaker bypassing the thermal overload protection.
4) Any comments, thoughts, or solutions would be appreciated.
By the way, I called the tech. assist line of the motor manufacturer and asked basically the same questions, but directing it towards what affect and concerns it has on the motor, and I couldn't really get a direct answer. Thanx, Bill
[Moderator's Note: Edited to remove email address.]
[ July 15, 2004, 09:50 AM: Message edited by: charlie b ]
Breaker would trip aprox. 4-5 times a month 95% of the time with in 3 sec. of start up. Maintenance personell would just reset it and the system would start the second time until the breaker would give up. Well, answer about the breaker. Also found out that this was the fourth breaker in about 3 yrs. Here's what else I found. It was a clean professional installation. Top of the line SQUARE-D Equipment from the 3000A 480V Distribution through the size 6 which I at first thought was a Y-start/Delta Run set up, 600KMCM copper conductors all controlled by a computer based controller/diagnostic set up on the compressor. Visual inspection of the motor revealed some winding dark in color and apparent insulation break down. Megger confirmed this. OK. Motor degrading this could be the cause of the breaker tripping. Now, why motor degregation. At this point, customer wanted to replace the motor in order to prevent any system down time. (New motor $7000.00 my cost) Time to find out how to prevent reoccurence. Further investigation of the starting cabinet. It's premanufactured by SQURE-D consisting of 3 size 6 starters, a tapped off thermal overload set up, a couple of timing relays, mechanical interlocks between starters, CT on center leg for computer etc.. Wasn't to sure why only three conductors to motor or what the fairly good sized xfmer at the bottom of the cabinet was for. Read schematics and found out that it was a reduced voltage starter set up which is the first time I ever came across something like this. So here's the questions I have for you guys.
1) Why this kind of set up when the motors designed for Y-start/Delta run and all you would really need is three more conductors if you needed a high torque start?
2) If I can turn the shaft of the motor while connected to the load by hand, why not just a regular delta connection?
3) What the heck is the purpose of a reduced voltage start where the xfmer is tapped at 65% of line voltage at start up? Are'nt most motors tolorences with in 20% before damage may occur? Wouldn't the reduced voltage create excessive current and heat to the windings of the motor causing insulation breakdown over a period time to the point of excessive in rush current applied to protective device such as a breaker bypassing the thermal overload protection.
4) Any comments, thoughts, or solutions would be appreciated.
By the way, I called the tech. assist line of the motor manufacturer and asked basically the same questions, but directing it towards what affect and concerns it has on the motor, and I couldn't really get a direct answer. Thanx, Bill
[Moderator's Note: Edited to remove email address.]
[ July 15, 2004, 09:50 AM: Message edited by: charlie b ]
Or maybe someone got a "deal" on the autotransformer starter being available sooner. Who knows... ?