A motor analysis team puts an online power analyzer to use where you might not expect it
Eastman Chemical Co., Kingsport, Tenn., started its motor analysis program in late 1998 when it purchased an online power analyzer. After several months of testing, the company found the analyzer to be limited in its ability to conduct rotor/stator testing at its plant, due in part to the low-level loads found on many of its motors. Power quality testing was also limited at the facility because it produced most of its own utilities, which resulted in good power quality levels.
In 1999 the facility started pursuing other options for condition monitoring of motors. A motor was set up under each of the following controlled conditions — shorted wires in turn, offset rotor 0.01 in., shorted stator core steel, and an open rotor bar — and a vendor representative came in to test the motors offline. After the vendor representative correctly identified all known conditions the company decided to purchase its analyzer.
After almost four years of use, the analyzer has performed extremely well. So well, in fact, that Eastman expanded its use into areas that it hadn't considered when it purchased the unit. Although most of these types of analyzers are advertised as rotor bar analysis tools, Eastman Chemical's experience proves that there are several other areas into which this technology can be put to use in a plant.
Rod mills crush the coal used in a facility's gasification process. The pinion gear transmits motor torque through the gear reducer to the rod mill's gear reducer. The motor is connected to the gearbox through a clutch, which uses an inert gas to engage the clutch disk. The clutch is coupled to the gearbox, which is coupled to the rod mill. The minimum design life of the pinion is five years.
Between 1997 and 1998, five pinions failed between the two rod mills in Eastman's gasification process. The company formed a root cause failure analysis team to determine the root cause or causes of the pinion shaft failures.
One of the verification steps during the root cause investigation was to determine the clutch engagement time. The motor analysis team (MAT) proposed monitoring the current on the motor to attempt to determine the clutch engagement time. The MAT used the current analysis in-rush test function on the analyzer to capture the motor current data. From this data they were able to calculate the engagement time for the clutch. The initial test indicated a very rapid clutch engagement time — about 2.25 sec. — similar to an across-the-line start for the motor and clutch (Fig. 1).
With the assistance of operations, maintenance, and the plant support engineering staff, the team adjusted the clutch engagement time back to the manufacturer's specification of between 5 sec. and 7 sec. (Fig. 2). Since this adjustment, the rod mills haven't experienced a shaft breakage.
Eastman now uses the in-rush current test as an annual proactive maintenance procedure for verifying the clutch engagement time remains within specification.
The facility currently has six grinders driven by 50-hp, 3-phase, 480V, 1,800-rpm motors in its polymer process plant. The MAT was called upon to evaluate the subject motors because four of the five kept blowing their 100A fuses at an excessive rate — about one per 12-hr shift (Photo 1). Each time a fuse blew, maintenance personnel replaced all three 100A fuses. Although they used more fuses, they claimed it resulted in fewer failures and less production losses in the long run.
The MAT evaluated the six motors using the in-rush current analysis tester. The results indicated that four of the motors had a very high startup acceleration rate when compared with the other two motors. The instantaneous amps were 20% greater and the acceleration time was about 2.5 times longer on these four motors when compared with the other two motors (Fig. 3 and Fig. 4).
When comparing time with current for these fuses, the MAT reps found that the motor was operating at or near the failure point of the fuse during startup. Upon closer investigation of the motors, it was determined that NEMA A design motors had been installed in the four locations that were blowing fuses excessively.
As a result, the MAT recommended replacing the NEMA A motors with NEMA C design motors. The motors were eventually replaced with TECO NEMA B design motors, which have starting torque characteristics similar to NEMA C design motors. No problems or concerns have been noted since these motors were replaced.
VFD driven agitator motor. The MAT's next job was checking horsepower loading on three crystallizer agitator drive motors powered by variable frequency drives (VFD); operations had loading on the motors that exceeded the manufacturer's specifications. The team performed power analysis testing on both the input and output of the inverter (Fig. 5). The total kW reading on the input corresponded with the kW displayed on the distributed control system (DCS) in the control room. The total kW reading on the output matched the manufacturer's estimate for horsepower loading in this application.
During analysis of the test results, the team determined that the instrumentation equipment supplying the DCS, which was connected to the input of the VFD, was creating readings that exceeded expectations. After analyzing the voltage waveforms of the VFD output, the team also discovered that one of the VFD output drivers had a distorted waveform. This was due to a failing output driver on one phase of the VFD. However, this distortion wasn't severe enough to trigger the self-diagnostics feature of the VFD.
The MAT team has also implemented a quality assurance program for repaired motors. They perform an off-line test on all motors over 100 hp to verify that repairs have been made properly and to establish a new base line for future test measurements. The following example shows how well the program is working.
The team recently performed quality assurance testing on a 125-hp, 444T frame, 1,800-rpm motor. During testing they noted a 5.1% resistive imbalance. The resistance readings were identical across all phases. The MAT retested the motor using a bridge type ohmmeter to confirm if errors might have been made during the original test. However, the off-line results compared very favorably with the bridge results. A review of the repair data sheet revealed that the motor shop had documented a 0% resistive imbalance.
The MAT asked the motor repair shop to send representatives to the facility to verify their original resistance readings. This time measurements with a bridge ohmmeter revealed a 7% resistive imbalance. However, the repair shop representatives didn't feel the imbalance was a problem because the motor had passed the surge comparison and load tests made at the shop during the repair process. After some coaxing they agreed to perform additional tests on the motor for the benefit of both parties.
The motor was torn down and 25% of the nameplate voltage and full nameplate current was applied to the stator. An infrared image taken 10 min to 15 min into the test shows that two coils of the stator were about 25°F hotter than the rest of the stator coils (Photo 2). Due to these findings, resistive imbalance limits have now been added to Eastman Chemical Co.'s Motor Repair Specification.
Although the advertised benefits of a motor analyzer may be very rewarding, it's good to step outside the box to find additional opportunities to reap the many benefits that can be realized in using the tool for purposes other than which it was initially designed.
Whittemore, Jr., is a senior mechanical engineer, and Hawkins and Aesque are MCA specialists in the rotating equipment group at Eastman Chemical Co., Kingsport, Tenn.