Predictive motor maintenance

Motor testing is critical to PdM, system uptime and revenue generation.

By Timothy M. Thomas, SKF Condition Monitoring Center

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In brief:

  • Predictive maintenance of motor systems is a necessity when it comes to supporting reliability objectives that in turn support business objectives.
  • Static or off-line testing is commonly performed just once in a given period of months, usually up to a year.
  • Dynamic or in-service testing, is performed while the motor is operating within its normal system or environment.

Predictive maintenance (PdM) programs are crucial to an organization’s ability to avert unplanned or even unnecessary downtime that can adversely affect its ability to produce or operate. Unlike time-based or run-to-failure approaches to maintenance management, condition-based programs are ideally geared to pay for the cost of implementation by extending the service lives of motors and rotating equipment, averting costly unplanned downtime and minimizing the costs of replacing expensive equipment. Predictive maintenance programs are most effective when all available means of measuring health and analyzing health trends of electric motors, cables, power quality, and load are rigorously implemented.

In other words, safe and continuous operation of plants and facilities drives revenue and profit and depends upon high motor reliability. Predictive maintenance of motor systems is a necessity when it comes to supporting reliability objectives that in turn support business objectives.

The power generation industry, as an example, ranks at the top of this requirement for uninterrupted operation and safe, continuous production. A number of motors run equipment that is ancillary to the production or health of a company, for example, one of a few rooftop motors for an HVAC system, which won’t have an immediate impact on the HVAC system if it stops working. Other motors, however, are critical to a company’s ability to conduct business — that is, they are motors that drive such things as conveyor systems, fluid pumps, or production-line machinery that a company relies upon to generate revenue and profit every day.

Motor systems

Motors, whether used to drive conveyors, pumps, cooling fans, or any other machinery, are best viewed as core parts of systems, such as motor systems or machine systems. These systems include the motor, the source of motor’s power, and the rotating equipment or machinery driven by the motor.

Electric motor test equipment today is generally categorized into two types: static motor test equipment and dynamic motor test equipment. The first type is capable of simulating real-world situations when motors are off-line. The second type is used for safely acquiring accurate and valuable health data across a working motor system, or a motor’s in-service environment.

Static motor test data provides visibility into the integrity and condition of a motor’s insulation and motor circuit. Modern equipment helps maintenance technicians predict or identify imminent failures before they cause costly unplanned downtime of motors and the rotating machinery they support. The most effective static test equipment is capable of testing the components of motors at voltage levels similar to those the motor will see in its normal operation without destructive currents. Static testing should include the surge test which is the most effective method of ensuring the integrity of the turn-to-turn insulation. The best static motor analyzers produce trend logs and reports, which allow technicians to track any decline or degradation to a given motor’s health.

The latest dynamic test equipment can locate and identify problems that adversely impact motor health and life that are on either side of the motor within the motor system. These are generally power-related issues and load problems, but can include vibration, or circuit condition problems within a motor while the motor is in service. Dynamic motor analyzers can often calculate speed and torque, define rotor bar problems, and measure distortion. Dynamic motor testing can also identify a number of mechanical issues, such as bearing problems or motor shaft misalignment. Dynamic testing helps isolate the mechanical (system) issues from electrical (internal to the motor) while providing valuable information to discern root causes of motor failures.

The goal of a predictive maintenance program is almost always to reduce unscheduled downtime. An effective predictive maintenance program is measured by how well it works to predict imminent failures and identify potential problem areas before they fail and create expensive recovery costs for an organization. They should also work to determine the root causes of failures and, ultimately, save money by extending the service life of motors and rotating equipment. This is why electrical testing of motors is such a critical component of predictive maintenance. Static and dynamic analysis, along with trend data acquisition and analysis, provides the information technicians need to make good decisions regarding use or maintenance of a given motor.

Off-line testing

Static or off-line testing is commonly performed just once in a given period of months, usually up to a year. It’s also performed opportunistically during outages when a motor shut is down for other reasons.

Off-line testing is often used as a quality assurance measure when receiving new, reconditioned or rewound motors from a supplier or motor shop. This is to assure they work as expected before they are stored or returned to service. Tests of these motors serve to prove the motor shop is doing its job properly, and they create new baselines for future trend analysis.

Static motor test equipment can troubleshoot motor problems or failures. Any time a problem occurs, the motor involved should first be tested for insulation integrity. Out-of-spec voltages, motor loads, and contaminates are examples of problems that can adversely impact a motor’s internal insulation.

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