A 5000/9000 horsepower, 712/886 RPM, 6600 VAC PAM motor was sent to a motor repair shop for a rebuild. The rotor was remanufactured and the motor was returned to service. The initial start-up increased from 14 to 40 seconds, with a voltage drop from 7200 to 5800 VAC. When the motor changed from low to high speed, the speed change had also jumped from 7 to 39 seconds.
When questioned, the repair facility claimed that it used new rotor laminations that would improve the motor efficiency, and that the fault must be in the application. Their calculated torque curve appeared to exceed the original motor's capabilities, but observed operation proved otherwise.
A consultant was then brought in to review the application and, after several weeks of gathering information, was able to determine the cause of the changes in operation. When the rotor was determined to have loose rotor bars and damaged laminations, the repair shop decided to rebuild the rotor.
The repair facility had assumed that the rotor bar material was the same naval brass used in similarly sized motors from the same manufacturer. In addition, newer silicone steel laminations were manufactured, the core slots moved in by 0.125 inches per side and the rotor bar slot dimensions were increased by 10%. As it turns out, the original rotor bar material was actually a low silicon bronze. The rotor lamination modification had also changed the back iron density of the rotor.
In effect, the torque curve had actually changed, so that the starting torque was significantly lower than the original, as well as the starting current being significantly higher. The breakdown torque also increased, as did the full load operating speed. If Motor Circuit Analysis (MCA) had been applied before and after, a defect would have been immediately apparent in all AC readings before re-installation, avoiding weeks of downtime.
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