Electric motor tester diagnoses problems, prevents downtime

• Electricians prepare to start the tester's 2.5 min standard test (Fig. 1). [9,636 bytes]
• The MCE tester is a lightweight (17 lb), battery-powered electric-motor analyzer. Tests are non destructive and are conducted at low voltage and low current (Fig. 2). [11,532 bytes]

The motor circuit evaluator (MCE) tester, manufactured by PdMA Corp., conducts tests to troubleshoot problems and provides diagnostic results and data trends on electric motors.

Use of the tester at ARCO Products Co.'s Los Angeles refinery saved $342,000 for one application. Designed for quality assurance, the MCE tester tracks and troubleshoots electric motor problems by providing data on the insulation system, power circuit, stator, rotor, and air gap.

MCE features

The tester requires 2.5 min for a standard test on an electric motor. Before the test, the electricians build the motor's nameplate into the MCE's database, verify that the motor is de-energized, and connect the motor's leads to the motor control center. Fig. 1 shows the joining of the tester to the motor's leads in the control center.

The MCE provides objective data results through a laptop interface (Figs. 2 and 3). Although most other motor-testing technologies provide immediate data access, the data are not always objective. Also, most other technologies act only as data-collection devices and require the user to go to a desktop computer to analyze the data; the MCE tester inputs the data directly to a computer.

Another benefit of the MCE tester is its ability to evaluate the condition of the motor with an initial group of tests. Other testers require a series of baseline tests to make a determination of the motor condition.

The tester has no limitation on the number of motors to be tested. Other technologies limit the number of motors that can be tested consecutively. For instance, surge-comparison testing, which detects faults such as grounds or turn-to-turn short circuits, restricts the user to 10 motors before the data must be downloaded.

The largest advantage of the MCE tester is the ability to store historical data on the laptop. This allows comparison of the motor data with previous results and performs trend analyses in the field. Almost all other technologies require the user to return to his or her desk to access the information.

Selection of tester

In 1994, the ARCO refinery management expanded its existing inspection program by establishing a reliability group to evaluate and support the implementation of several defined reliability strategies. One of these strategies was to reduce the cost of the maintenance and operation of its 2,500 motors.

Robert Yontz, conditioning monitoring coordinator at ARCO Products Co.'s Los Angeles refinery, researched and chose the MCE electric motor testing technology to reach the company's reliability goals. Yontz arranged for an independent test with the MCE device. After a 2-month test period, he calculated the economics of the tester. He justified the purchase of the tester over a 10-year project life with an 18-month payback and 16% return. The following application shows that the savings exceed these calculations. Reduced downtime, more-accurate repair scopes, and cost avoidances account for a 4-month payback period rather than the projected 18 months.

In September 1995, Yontz began implementing the tester at the refinery. Today, the electrical department uses the tester mostly for diagnostics.

Application

In December 1995, the MCE tester had its first true test.

A 2,300 VAC (voltage ac), 1,250 hp synchronous motor tripped 11 months after being placed in service following a rewind. When the unit operating personnel conducted a test operation of the fire suppression deluge system, the motor shut down. The first electricians on the scene determined that the two main line fuses had opened and appeared aged. Analog megger testing indicated 12 mega-ohms to the ground, which is acceptable for this voltage application.

The normal approach would have been to replace the fuses, start the motor, and see what happened. Instead, to determine the root cause, the condition-monitoring team conducted an MCE analysis to determine the motor's health.

Motor circuit analysis indicated that the temperature-corrected insulation resistance was 3.7 mega-ohms, borderline IEEE (Institute of Electrical and Electronics Engineers) minimums and significantly below preferred reliable numbers of a healthy circuit. In addition, circuit capacitance was more than three times the value of any other motor tested at the site. These two parameters, low megger and high capacitance, when correlated, indicated a loss of insulative abilities and imminent failure, which may have permanently damaged the motor.

With this knowledge, operations supervision agreed to further investigate the motor. On removal of the motor connection box, five of the six feeder leads had cracked insulation through to the conductor, at least halfway around the cable. Signs of arcing were both aged and fresh. Water from the deluge had entered the pre-existing cable insulation cracks and allowed A and C phases to ground, resulting in a phase-to-phase short.

Finding this failure in the primary state allowed the rewind vendor to conduct onsite repairs in minimal time and under warranty. If the motor had been started and failed, a rewind would have required several crafts as well as time for removal and reinstallation of the motor. Moreover, identification of the root cause may never have been possible at that point.

The impact of the tester is evident. The downtime lasted 60 hr; the previous rewind required 35 days. Also, 22 craftsman hr were required to repair the motor this time; the previous rewind used 550 craftsman hours. A total of $342,000 was saved from avoiding maintenance cost ($82,000) and gaining process opportunity ($260,000).

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