Blog post by Scott Sullivan, electrical engineer at Electronic Drives and Controls who specializes in on-site field service of AC drives.
Recently, I was called in to troubleshoot a customer’s air handler drive. This particular unit was blowing fuses on its input from the main line. The customer’s in-house electrician correctly determined that something was drawing too much current. He then decided that the problem had to be with the drive or the motor. For testing purposes he bypassed the drive, which he believed left only the motor. When he attempted to start the motor, the fuses blew again. Since he believed the motor and drive were the only possible culprits and the drive was bypassed, he figured the motor had failed and so he installed a new motor. This time when he tried to start the new motor with the old drive, the fuses still blew. His determination was that both the motor and the drive had failed. At this point I was called in to troubleshoot.
After being given a brief summary of the problem, I started to examine the drive. After a thorough examination I didn’t find anything wrong with the drive that would explain the issue. To test my hypothesis, I disconnected the motor from the drive and tried to start. If the drive had been the problem, the fuses would have blown at this point. The fuses remained fine, so the problem was not between the main line and the drive. This left only the output wiring and the motor. Since the motor was brand new, it was unlikely to be the problem. Therefore, the output wiring was the only issue remaining. With the aid of the customer’s electrician, I ran new wire from the drive to the motor and tried to start. The motor and air handler started up without an issue. After examining the wiring I found that some of the insulation had worn off and part of the live wire was touching the metal conduit. This caused a short to ground and caused excessive current to be produced, thereby blowing the fuses. If the fuses had not blown, the drive would have most likely failed, possibly burnt up the new motor, and maybe even caused a fire.
The lesson to be learned here is that there are many causes for fuses failing. Just because you eliminate one cause doesn’t automatically mean it has to be something else. This customer spent money on a new motor when they probably didn’t need it. When you determine a problem has occurred and you don’t know how to fix it, the best solution is to ask for outside help. It may actually end up saving you money.
