Ideally, a motor should operate to its maximum designed life. Many applications, such as food or chemical processing, can’t afford to shut down a line, even for maintenance, because it is too costly or the process cannot be interrupted. Most motors are designed to operate for about 20 years, with proper maintenance, so longevity should not be a problem.

However, a large number of motors are not reaching their expected life. A 2 hp motor, for example, running a conveyor section at a major food producer’s facility suddenly failed, shutting down the line. Water had seeped into the conduit box during washdowns and eventually ate away at the cabling insulation. It cost the company hundreds of thousands of dollars in lost product and production time.

That company spends $1 million annually to monitor motor vibration on critical systems. Yet none of the complex equipment and maintenance procedures were sufficient to detect problems in that 2 hp motor. Part of the reason is due to the expense of monitoring equipment. Another part is insufficient knowledge about application effects on motor operation.

A solution is at hand. Through recently developed sensors and mathematical algorithms, several motor manufacturers are developing monitoring devices that will model, analyze, and predict the health of motors and other rotating machinery.

These devices will:
• Operate on all size motors, from 2 to over 500 hp.
• Enable users to monitor any and all motors in a facility.
• Monitor winding, bearing and lubrication, rotor, and shaft misalignment parameters simultaneously.
• Provide continuous, real-time, online monitoring of the attached motor.
• Measure and analyze parameters on environment, duty cycles, and installation — monitoring the process and not just the motor.

Sensors: A step beyond control

In addition, these monitoring devices will soon be able to predict the residual life of rotating equipment, a feature long desired by engineers. “It’s no longer sufficient to just diagnose a motor, for example, and say that’s good or that’s bad,” says consultant Dr. Carl Talbott. “Tell me how good, how bad. How long is it going to run? When do I need to relubricate the bearing? How much more time do I have?”

Sensors have been one of the hurdles to monitoring devices with such capabilities. Another hurdle has been the lack of sufficiently powerful diagnostic algorithms.

Part of the problem with sensors has been their cost. A good singleaxis accelerometer, for example, often exceeds the cost of a low-horsepower motor. Also, sensors were difficult to integrate into hardware and condition monitoring systems, until the development of device-level networks and microprocessors. In other cases, the size of a sensor or its sensitivity to noise caused problems.

Reliance Electric Div., Rockwell Automation, has been working with universities and the National Institute of Standards and Technology (NIST) on sensor and software algorithm development to create an on-line motor monitoring device. “The intent is to provide an early warning to incipient motor failure,” says Richard Schaefer, product manager, Reliance Electric. “But it could go beyond that. Eventually, the motor will become an intelligent sensor. It will inform people about application problems and help them manage the process.”

Engineers and scientists are researching several new sensors, one of which is a noise-immune optical sensor. This optical sensor offers potential benefits of low cost, high sensitivity, and low susceptibility to stray magnetic fields. These features are particularly desirable for current sensing. In development are versions to measure current, flux, and torque.

These non-contact sensors are based on the Faraday principle, which says that an external magnetic field can influence the plane of polarization of light passing through a medium. Possible mediums include fiber-optic coil, bulk optical material, or optical thin-film.

Fiber-optic coil has been an uncommon medium for a current sensor because it made such a sensor difficult to manufacture. Stress from bending the fiber into a coil resulted in bi-refringence, an effect that distorts sensor response.

NIST recently developed an annealing procedure that reduces this effect. The procedure makes it possible to manufacture a sensor with more turns. Because the output is proportional to the number of turns, the more turns a sensor has, the higher is its output. In operation, the sensor works with a polarized light source, typically a linear diode. The magnetic field produced by the motor current rotates the plane of polarization of the light source. The amount of the rotation is proportional to the current.

In other research areas, development of bulk and thin-film ferromagnetic iron garnets results in optical current sensors with even more sensitivity. These sensors also have bandwidths of hundreds of megahertz, which enables monitoring devices to sense higher frequencies.

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