Go Beyond Vibration Analysis in Diagnosing Rotating Equipment Problems

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.

Overheating Motors

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

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.

Symptoms of Rotor Bar Issues

When the rotor has unbalanced current flow through the rotor bars, it causes the rotor current to vary as the affected bars pass under the magnetic fields on the stator. Due to the difference of the rotational speeds between the rotor and the rotating magnetic field, each rotor bar will pass under each of the magnetic field poles each revolution of slip. For example, in a six-pole motor induction motor, there will be three north poles and three south poles. Therefore, when squirrel cage induction motors have rotor bar issues, the motor’s current modulates at a frequency equal to the number on poles on the motor times the frequency of the current flow through the rotor. This frequency is commonly referred to as Pole Pass Frequency (PPF), which is equal to the number of poles in the motor times slip speed.

ESA FFT Results & Analysis

The results of the FFTs on pump motors 1 and 5 provided clear indication of rotor bar damage. The ESA automatic analysis software applies a proprietary algorithm that uses five criteria of the ESA analysis to identify rotor bar issues and to determine the severity of the problems. The software provides the charts, tables and graphs of the motor data, as well as detailed analysis report with actual recommendations.

1. Amplitude of PPF sidebands in current spectrum
2. Motor load
3. PPF amplitude in demodulated spectrum
4. PPF harmonics in demodulated spectrum
5. PPF sidebands around running speed spectral peaks

Based on the ESA data taken and applying the above criteria, pump 5 was given a severity level of seven out of seven, and pump 1 was rated six out of seven. Though the results of the vibration analysis did not show any indications of rotor problems, the reduced speed of the rotor and the analyzed ESA data showed several rotor bars were broken. When the maintenance team pulled motor 5, it found 30 percent of the rotor bars broken.

Rebuilding the Rotor with Different Metallurgy

After discussing the issues with the motor manufacturer and a third party motor repair company, the maintenance team chose to have both motor 1 and motor 5 rotors rebuilt with copper rotor bars. Pumps 2, 3 and 4 had copper rotor bars and did not have the same issues as pumps 1 and 5. The original rotor bars in pumps 1 and 5 had been made of aluminum. This was believed to be the root cause of the problem. Gone uncorrected, the elevated operating temperature of the motor could have caused shorted stator winding and would have required a rewind or scrapping the motor as well as the repair or replacement of the rotor. Having the motors rebuilt with copper rotor bars enabled the team to avoid catastrophic failure of their vertical turbine pump motors. Motor 5 was rebuilt and installed in May. Another round of MVA and ESA testing was performed immediately after installation to ensure proper operation. The electrical signatures reported excellent rotor condition (see Figure 2). It also showed that rotor speed increased, while overall current dropped. The newly rebuilt motor 1 was installed in a few months later and a third round of testing was performed.

Lessons Learned

Although machinery vibration analysis has been used for many years, it is limited in its ability to detect and evaluate problems in electric motors. Therefore, diagnostic tools available to determine the condition of rotating equipment are invaluable when trying to identify faults in electrical equipment.