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Fig 1

The conveyor motor at the heart of this problem is a 4160 volt, 805 HP, wound rotor motor. On the bright side, the wound rotor motor is perfect for a belt application, as it provides good speed control and plenty of torque at low rpm. Looking at the other, not so bright side, the wound rotor motor is expensive and requires significant maintenance to ensure reliability.

At 2:30 p.m. on the Monday following the failure, the crane was finally in place and the damaged motor was lifted off the deck in preparation for replacement with the spare motor. By 7:00 p.m., the coupling had been removed from the failed motor and was being installed on the new motor. At 12:00 midnight, the motor was aligned, wired up for high voltage, and ready to start. This moment would have been an ideal time to apply the first start rule: On the first start of a motor, always let the new guy push the start button.

The motor growled, blew a fuse, and appeared as if it was trying to turn in reverse of the desired direction. The rotation was changed and another start was attempted. Another $1000 fuse blew in a different phase. The resistor bank and relays were op-tested satisfactory. The rotation was reversed again for a third start, resulting in a growl, followed by what appeared to be a near start, followed by another $1000 fuse blowing in phase B.

At 4:30 a.m. on Tuesday, more advice was requested, resulting in the removal of the capacitors to no avail. At 2:00 p.m., low-voltage (480v) three-phase power was applied to the stator windings with the brushes lifted to check for a balanced induction on the three-phase wound rotor. Sure enough, there was an electrical imbalance on the rotor, but it shifted phases with rotor rotation, creating uncertainty as to the root cause. At 7:00 on Tuesday night, it was determined that the spare motor needed to be sent to the shop for further inspection. However, one more idea was presented before going through the long arduous effort of bringing a crane back on site to remove the spare motor. The technicians at Flanders Electric utilize the MCEmax electric motor test equipment for reliability testing and troubleshooting, and they were hoping that the wound rotor motor testing module would offer some assistance in determining the root cause of the fuse-blowing party.

The MCEmax performs tests on electric motors while the motor is running or when de-energized. With the motor de-energized, the test equipment applies high and low frequency AC, as well as high and low voltage DC, for a variety of analysis techniques. With the motor running, the MCEmax enters a passive mode, acquiring AC and DC voltage and current signals to facilitate analysis of the motor, power circuit, and power supply. Fault zones analyzed during this testing include the power quality, power circuit, stator and rotor ground wall insulation, stator and rotor turn insulation, rotor bar and lamination integrity (squirrel cage induction motor), and air gap symmetry.

With hopes high, the MCEmax arrived on scene at 8:30 a.m. on Wednesday. The very first test using de-energized equipment on the stator windings indicated a large inductive imbalance. Finally, a break in what seemed like a steady stream of failed attempts to identify a problem. Another key piece of information was that the imbalance was not varying with rotor position. This locked the focus squarely on the stator windings. Knowing the problem must be stemming from the stator windings prompted a detailed visual inspection of the stator winding connections, resulting in an eye-opening discovery. Rather than the normal wye configured connection, the #2 and #5 leads were reversed, causing a phase inversion. This error creates a reverse torque opposing the normal rotation, which explains the motor's inability to start rotating from a standstill.

The leads were corrected, and a follow-up test was performed with the MCEmax to verify the balance of the stator windings. Aha! The MCEmax was reporting a perfect balance on the de-energized stator windings. The only thing left was to start the motor with fingers crossed. With the MCEmax connected for energized testing, and following a realignment, the motor was restarted with great success. All voltage and current values were normal, balanced, and flowing in the right direction.

Ask the tired technicians, who spent four days in an effort to troubleshoot and repair not one, but two wound rotor motors, what they would do differently. You might hear that performing MCEmax quality control testing of the spare wound rotor motor would be a good start. In fact, the right approach for motor reliability using the MCEmax includes the triangulated attack of quality control, trending, and troubleshooting. Quality control verifies the manufactured or repair integrity of the motor upon receipt. Trending gives you advance notice of conditions conducive to failures so they can be corrected early, thereby extending the life of the motor. And finally there will be a time, usually a Saturday, when something fails and you need a troubleshooting tool to rapidly diagnose and isolate the root cause to minimize production losses. Without the MCEmax, use the first start rule: On the first start of a motor, always let the new guy push the start button.

Fig 2

Noah P. Bethel, CMRP, has over 18 years of broad operations and electrical systems maintenance experience in industrial, commercial, and military settings ranging from nuclear submarines to world-class amusement parks. His experience includes high and low voltage, AC and DC, power generation, power distribution, motors, and motor controllers. Noah is currently in charge of product development of new and existing PdM technology for PdMA Corporation. www.pdma.com

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