In Henderson, Nevada, P-19A is one of the most critical potable pump stations operated by the city. It supplies drinking water to the second largest city in Nevada. The station was put in service in 1999 with three vertical turbine pumps. Each pump was driven by a six-pole 700-horsepower (HP) vertical induction motor. Two additional vertical pumps, also driven by six-pole 700-HP vertical induction motors, were installed in 2006.
Routine machinery vibration surveys taken at the station in November 2014 identified possible early stages of bearing failure, but no additional mechanical or electrical issues were detected. The vibration summary concluded that all pumps were operating within acceptable limits and no maintenance was necessary.
In January 2015 the motors for pumps 1 and 5 exhibited overheating as well as current and speed fluctuations. A variety of de-energized and energized diagnostic tests were conducted—including a rotor inductance test (RIT), vibration testing, a manual rotor test and a 10-second current trace. De-energized tests showed all measurements were balanced, which meant there were no indications of winding issues. Machinery vibration data again indicated early bearing degradation, but no additional electrical or mechanical issues.
Both the manual rotor test and RIT exhibited some anomalies. The motor current had a phase unbalance of about five percent and was modulating approximately 15 amps. The rotor speed appeared to be fluctuating and was running about 12 rpm below nameplate. These symptoms suggested rotor bar issues on the affected motors.
However, since vibration tests failed to indicate rotor issues, management wanted additional confirmation or indication of rotor problems before expending limited resources on pulling and inspecting the motors. Based on these concerns, the staff had electrical signature analysis (ESA) performed on all the pumps.
Electrical Signature Analysis
ESA is a diagnostic technology that uses the motor supply voltage and operating current to identify existing and developing faults throughout the motor system. ESA performs simultaneous data acquisition of all three phases of voltage and current to create a three-phase power-quality table. Additionally, it digitizes and stores the voltage and current waveforms for additional processing and analysis using fast Fourier transforms (FFT).
During the motor tests, ESA data was taken at different times: first shortly after start-up, then again after the pump had been running for about 30 minutes. The most significant finding was the large change in rotor speed with a very small change in motor load. The rotor was running considerably below nameplate. After running 30 minutes, the rotor speed was 1,171 rpm. This was confirmed by a handheld tachometer used by the vibration analyst while taking the vibration data.
Rotor bars are copper or aluminum bars that run the length of the rotor in alternating current (AC) squirrel cage induction motor rotors. These parallel bars are connected to rings—called end rings or shorting rings—on either end of the rotor providing a path for current flow through the rotor. Current flowing through the bars creates an electromagnet. Since these bars create parallel paths, the voltage across each path is the same and current flow through each path will vary depending on the resistance of each path. If the rotor bars develop cracks, separate from the end ring or have other imperfections, the resistance of the bars will increase and the current flow through the affected bar will decrease.
Induction motors rely on the rules of mutual inductance to get power onto the rotor of a squirrel cage rotor. These rules (Faradays first law of electromagnetic inductance) require:
- a magnetic field
- a path for current flow
- relative motion between magnetic field & conductor
In three-phase induction motors, a rotating magnetic field is created by applying a three-phase voltage to the three-phase stator windings. The rotor bars act as the conductor to create the path for current flow. The relative motion is established by the difference in the rotational speeds between the rotational speed of the magnetic field (synchronous speed) and the speed of the rotor (rotor speed). The difference between them is the slip speed.
When rotor bar imperfections cause unbalanced current flow through individual rotor bars, it affects the total rotor current flow. The frequency of the current through the rotor is the difference between the rotor speed and the speed of the rotating magnetic field, which is the slip speed.