MOTOR OVER TEMPERATURE THERMAL PROTECTIVE DEVICE (49) INFORMATION SHEET

There are two main classes of overtemperature thermal protective devices. One is a line break type, which interrupts load current directly. The second is a control circuit system using detector devices, which interrupts the motor current through its controller.

Thermal protectors are intended to limit motor-winding temperature and motor current to predetermined values during abnormal motor operating conditions. This prevents premature motor insulation failure.

Abnormal conditions that can result in overheating include overload, stalling, failure to start, high ambient temperature, restricted motor ventilation, reduced speed operation, frequent starting or jogging, high or low line voltage or frequency, mechanical failure of the driven load, improper installation, and unbalanced line voltage or single phasing.

Current sensing alone cannot detect some of these conditions, such as restricted ventilation. Temperature sensing alone may be inadequate, for example, with frequent starting or jogging.

For some conditions, a coordinated arrangement of current and temperature sensing may be required. The temperature sensing capability of thermal protectors depends on their location with respect to the motor windings.

The protectors should be installed within or on the motor frame in such a manner that the temperature at the device changes in proportion to the winding temperature, and they should be matched to the motor's insulation class.

The advent of microprocessor-based protection systems has made it possible to provide a more refined thermal protection than was possible with electromechanical relays. Since no attempt is made with electromechanical relays to match the thermal time constant of the motor, thermal coordination is achieved under limited operating conditions.

Conversely, microprocessor-based protection systems make it possible to match thermal properties identified by motor data and, as a result, tracks the motor temperature more accurately over a wider range of operating conditions (Zocholl [B115]).

Microprocessor systems can also monitor resistance temperature detectors (RTDs) embedded in the motor stator windings to detect temperature rise caused by impaired or lost ventilation not indicated by current. In addition, these systems can monitor temperature detection devices for motor bearings.