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Electrical Motor Diagnostics for Generators Part 2 – Case Studies


In Part 1 of this 3 Part series, the basic principles of Motor Circuit Analysis (MCA) and Electrical Signature Analysis (ESA), and how they relate to turbine and salient-pole generators was discussed. In Part 2, we shall discuss case studies related to marine generators and wind turbine generators. These cases shall discuss both generators in poor condition and in good condition. Following Part 2, Part 3 shall discuss the details of how these analysis were performed, including how condemning criteria was formulated.

Case #1: Marine Salient-Pole Generator, Insulation Failure

A marine generator on board a military vessel was experiencing increasing over-air-temperature faults during operation. These occurred within 24 hours of operation and gradually worked towards occurring every 6 hours of operation during the course of eight months. The engine and cooling temperatures were evaluated and found to be operating satisfactorily.

Figure 1 - Generator

The generator had been installed on the vessel following eighteen years of storage in an uncontrolled environment. It was determined that Electrical Motor Diagnostics (EMD) would be used to evaluate the condition of the system.

Figure 2 - Switchgear Test Points

Testing was performed at the switchgear using MCA. The first set of data identified a problem in the circuit (Table 1). A second set of tests were performed in the connection box at the generator (Table 2) which identified both a winding short and poor insulation condition (unmatched Z and L).

Table 1 - MCA Test at Switchgear

As it requires removal of the generator through a hole in the hull of the ship, additional testing was performed using ESA over a period of 30 minutes.

 Figure 3: ESA at ‘0’ Minutes

Figure 3: ESA at ‘0' Minutes

Figure 4: ESA at ‘10’ Minutes

Figure 4: ESA at ‘10' Minutes

Figure 5: ESA at ‘20’ Minutes

Figure 5: ESA at ‘20' Minutes

Figure 6: ESA at ‘30’ Minutes

Figure 6: ESA at ‘30' Minutes

At the end of the ESA test period, another set of MCA data was performed while the generator was warm (Table 3).

Table 3 - MCA after ESA Test

The reduction in insulation resistance from 750 MegOhm to 55 MegOhm indicates a temperature rise somewhere in the insulation system of approximately 140 C.

The vessel was scheduled for work in three months following these tests, in which the time included a cruise overseas. The question was whether the generator would be serviceable during this time. If not, the vessel would be unable to perform its mission.

A review of loads and temperatures were performed along with the data from the EMD analysis. Based upon historical references and estimated time to failure research, it was determined that the generator could be operated in parallel with a second generator at 50% load, or less. Watch standers were given instructions to observe for variations in current unbalance as an indicator of advanced winding failure. Recommendations were provided to the vessel's shore engineering support group for the storage of rotating machines and generators.

After its mission, the generator was removed for repair (Figure 7) and shipped to the contracted repair shop.

Figure 7: Generator Removal

Repair requirements included complete rewind of the stator and rotor with approved overtime in order to meet the vessel's shipyard schedule. Acceptance inspections were performed at the repair center's site under load.

Figure 8: Acceptance ESA Test

Figure 8: Acceptance ESA Test

Several details on the repair were determined during the inspection:

1. The lead wire was reduced in size. This increases the heat related losses at the leads and restrict the maximum current capability of the generator.

2. The winding conductors were increased in size. While this allows the generator to operate slightly cooler, it affects the circuit enough that significant tuning was required to be able to synchronize the generator.

3. The rotating fields were not rewound as was determined by the ESA analysis. This results in a reduced reliability in the life of the generator.

Case #2: Marine Salient-Pole Generator, Poor Condition

In this generator, both MCA and ESA were performed as part of a routine maintenance.

Table 4 - Case 2 Generator MCA

The MCA analysis involved positioning the rotor to a set location and performing an analysis. A loose connection, low insulation to ground, impedance and inductance matching and questionable winding conditions were determined. However, as will be discussed in Part 3, the Fi and I/F primarily are used as trendable values unless specific procedures are followed.

Figure 9: Case 2 Generator Voltage Signature

Figure 9: Case 2 Generator Voltage Signature

The signature found in Figure 9 indicates a relatively good rotor with the line frequency sidebands of the number of rotor fields times the line frequency (in this case, 360 Hz +/- 60Hz) with a gradually dampening set of harmonics.

Based upon these findings, coupled with a visual inspection, the generator should be scheduled for a cleaning. It is expected that the winding conditions (L and Z) will balance out and the insulation resistance will improve.

Case #3: Wind Turbine Generators, Good

A common issue with wind turbine generators is the passage of grit or dust through the generator windings causing winding shorts, bearing problems,etc.

Figure 10: Good Readings

Figure 10: Good Readings

In figure 10, the peaks are dominant in current (upper spectra) which is related to the load. The voltage signatures identify a relatively low level of rotor frequency. This identifies a good online analysis of this system.

Case #4: Wind Turbine Generators, Loose Coils

Figure 11: Loose Coils

Figure 11: Loose Coils

During a routine analysis, a signature was determined as 60Hz sidebands around the running speed times the number of stator slots of the generator. As identified in Part 1 of this series, this indicates ‘Stator Mechanical' which is defined as loose coils or a loose stator. In this case, it was determined to be, most likely, loose coils.


The electrical motor diagnostics techniques of motor circuit analysis and electrical signature analysis are uniquely adapted for troubleshooting and trending of developing generator electrical and mechanical problems. Used together, they can also be used to detect potential warranty issues in new and repaired machines prior to bringing the generators online.

Author Howard W. Penrose Ph. D


Penrose, Howard W, Ph.D., Motor Circuit Analysis: Theory, Applications and Energy Analysis, SUCCESS by DESIGN Publishing, 2001.

Penrose, Howard W, Ph.D., Motor Diagnostics 2-Day Training Manual, ALL-TEST Pro, 2004.

"Electrical Motor Diagnostics for Generators Part 1: The Basics," ALL-TEST Pro, 2005

"Electrical Motor Diagnostics for Generators Part 2: Case Studies," ALL-TEST Pro, 2005

Howard W Penrose, Ph.D.

Howard W Penrose, Ph.D., CMRP, is the President of MotorDoc LLC, the Executive Vice President of MotorSight Corp, Treasurer of SMRP and Web Editor-in-Chief of the IEEE Dielectrics and Electrical Insulation Society. Howard has over 30 years in the electric machinery repair, design, materials, energy and systems industry and is an award-winning author.

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