When troubleshooting a faulty electric motor, time (or in this case downtime) is money. This notion was drilled into my head in my early years and has proven itself over and over again. Isolating the problem in the shortest amount of time saves costly downtime (and maybe even your job). The other point that has been permanently burned into my consciousness involves a different type of self-preservation: always control the electrical circuit you are troubleshooting by following proper lock-out/tag-out procedures. Observing safe maintenance procedures must always be given proper attention. For these reasons, it is imperative that the maintenance electrician, technician or engineer uses a troubleshooting methodology that enables them to isolate the problem in the shortest amount of time. Still, the logical process of elimination used to identify the root cause of an ac motor’s malfunctioning can differ. At times it is more effective to start at the motor and work back to the starter; in other instances it’s better to begin at the starter and move towards the motor. My general preference is to begin at the excitation source and work towards the motor. For the purpose of this article, I’m addressing typical starting problems only. Running problems can be numerous and varied, and in many cases can be traced all the way back to improper motor specification for a particular application. Starting problems with three-phase ac inductive motors can be grouped into three primary categories: voltage source problems; motor excitation problems, which include the motor starter and ac drive; and application or mechanical problems. Troubleshooting voltage source problemsUse a process of elimination to isolate and diagnose voltage source problems. Refer to “Voltage source problems flowchart” on page 66, which illustrates the steps of the following troubleshooting procedure:
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