One of the biggest problems faced in analyzing electrical equipment is determining where the true problem is located. Fault Zone Analysis should be utilized to ensure that the actual problem is being detected and not just a secondary problem caused as a result of the real problem. This paper will discuss the diagnostic process for electric motors and outline six specific fault zones that should be looked at, which include the power circuit, insulation condition, stator condition, rotor condition, air gap and power quality.
Electrical maintenance personnel have for years been limited to troubleshooting with no more than a multimeter and a megohmeter. Unfortunately, this does not provide enough information to allow most technicians to feel totally confident in determining if an electrical problem exists or not. A mechanical operator once said, “If a problem exists with a piece of equipment and there is an electric cable within fifteen feet of it then it must be an electrical problem!” If you are involved with electrical maintenance you have probably heard at some time in your career, “It must be the motor.” If you are into mechanical maintenance you have likely heard, “It’s probably the pump. Let’s uncouple it.”
This has been an on-going battle and, up to recently, technology has been primarily developed for the mechanical side. Vibration shows a two times line frequency (2FL) spike and that must mean it’s electrical. Right? …. Wrong!!!! There are so many variables producing a 2FL today that removing a motor from service for an electrical repair due only to a high 2FL is a mistake, possibly an expensive one. The best thing you could hope for is that the repair facility will call back asking, “What do you want done to this perfectly good motor?” More recently the battleground has moved from motor vs. pump to motor vs. drive. It seems as soon as technology is developed to solve a debate that another debate appears, advancing technology even further.
“Resistance to ground testing is all we need.” I find this statement hard to believe. How many times have we been nervous restarting a tripped motor after verifying with our trusty megohmeter that, “The motor is fine.” The fact is, numerous reasons can exist which cause a motor to trip that will not be seen by a megger, such as a turn-to-turn short. Breakdown in the insulation between individual turns of a winding can occur inside a stator slot or at the end turn and be completely isolated from ground. Phase-to-phase shorts can occur the same way. If these faults are left unattended, they can result in rapid deterioration of the windings, potentially ending in a complete motor replacement. Restarting a motor that has tripped should be considered only after these faults have been factored out.
Troubleshooting an electric motor that is suspected to have an electrical problem should not result in the statement, “The motor is fine.” Although someone with years of experience and tons of credibility may be able to get away with such a simple statement, most technicians won’t find the same positive response from their supervisor, engineer, or plant manager. To confidently report the electrical condition of a motor and ensure that your recommendation is taken seriously, there are six areas of interest known as Fault Zones that must be looked at during the troubleshooting effort. Missing any of these zones could result in missing the problem and losing credibility in your troubleshooting skills.
