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Motor Protection: Should you monitor on-line, off-line or both?


In modern industrial settings, motor protection comes in many shapes and sizes. Bi-metallic overload relays incorporated on most motor starters still represent a reliable, if imprecise, form of motor protection. In more sensitive applications where a company cannot afford to lose a motor, more expensive solid-state overload relays or solid-state motor sensing systems can provide detailed on-line performance data. Solid state motor protection has also become an integral part of most inverters, providing precise protection.

This on-line motor protection is valuable for some of the following reasons:

  • It protects the motor from overheating that results in winding insulation damage while the motor is running.
  • It provides the maintenance electrician and production personnel with valuable performance data from each motor in the process.
  • It provides maintenance personnel with a warning signal that a failure has occurred or will occur in the motor while running.

The controls industry has developed sophisticated technology to monitor every aspect of motor performance while it’s running. This is a good starting point — but this view is too limited.

Few people in the controls industry grasp the significance of monitoring the status of a motor when it isn’t running. After all, a motor that isn’t running isn’t drawing current and can’t cause or readily display mechanical problems? Dirt, dust, chemicals and moisture can, however, still build up inside a motor that isn’t running — this contamination can lead to potentially catastrophic motor failures.

Testing and monitoring the condition of a motor’s windings when it’s not running provides an increased level of protection from catastrophic motor failure. This process of "off-line testing and monitoring" protects a motor when it is most vulnerable to failure — at start-up. A motor that has been idling could have a dangerously high level of moisture built up on the motor’s windings. This condition occurs in all types of motor applications due to condensation formed inside the motor caused by environmental conditions or everyday heating and cooling. When the motor is started, the motor windings can experience six to ten times the nameplate full load current. If the winding insulation is not in good condition due to moisture or other insulation breakdown, the windings can short to ground leading to motor failure, fire, and/or explosion. This type of motor failure is a major problem in any industrial setting, creating serious safety and production problems. Off-line monitoring can detect these conditions and prevent a catastrophic motor failure.

The heart of off-line monitoring is a system that automatically and continuously monitors the condition of a motor’s winding insulation. A simple, patented technology automatically injects a current into a motor’s windings. Leakage to ground is measured and can provide a reliable indication of the condition of a motor’s winding insulation. Maintenance personnel can monitor these conditions and plan to recondition motors before they fail. They can also track the degradation of the winding insulation and predict when a failure is most likely to occur. This allows maintenance personnel to pull the motor out of service before it fails and repair the problem. Reconditioning a motor by cleaning, baking, and re-insulating the windings is far less expensive than rewinding a motor that has failed catastrophically.

Historically, the most common way to measure the condition of motor windings has been to manually meg-ohm test the windings. Maintenance electricians must shut down and lockout the motor. They then have to manually test the motor windings with a portable device.

There are several flaws with this approach. Human error and the accuracy of the portable equipment are large factors. Safety issues are raised by having electricians working around potentially live electrical systems. One mistake in the lockout procedure can result in injury or death. Another flaw in this approach is the snap shot nature of this manual testing. As soon as the measurement is completed and the motor is re-energized, the electrician’s view of the motor windings is lost. Conditions change, insulation fatigues and there is no way maintenance electricians can manually meg-ohm test a motor often enough to maintain an accurate view of the motor windings.

Automatic meg-ohm testing of a motor’s winding insulation provides electricians with a reliable ongoing report. For example, increasing moisture levels can be tracked over time and when they reach dangerous levels, interior motor heaters can be activated to dry out the windings, or the motor can be locked out to prevent it from starting until conditions improve. Common motor failures on start up can be eliminated as well as the high cost of manual testing by maintenance personnel.

When motor protection is a critical issue, a combination of both on-line protection and off-line protection is the most cost-effective solution. Together, these uniquely different approaches combine to significantly increase motor reliability and reduce costs for maintenance and production.


Rick Zelm is the founder and president of Meg-Alert, a Wisconsin-based firm specializing in motor and generator protection. You can reach him at (800)778-5689 or through www.mealert.com.