Motor Protection Overload and Overtemperature
Insulation breakdown is a common reason for motor failure. Windings in the motor are insulated with organic materials including epoxy and paper. Insulation degradation occurs when winding temperature exceeds its rating. The National Electrical Manufacturers Association (NEMA) states that the time-to-failure of organic insulation is halved for each 8 to 10°C rise above the motor insulation class rating. This point is illustrated in Figure 11.
FIGURE 11
Solution: An I2t Thermal Model provides thermal-overload protection of motor windings during all phases of operation. By integrating the square of the current over time, a thermal model can predict motor temperature and react much quicker than embedded temperature devices. A thermal model takes into consideration the motor service factor, full-load current and class. A dynamic thermal model adjusts the time-to-trip depending on how much motor thermal capacity has been used. Figure 12 illustrates the adjustment in trip time for different current levels at different levels of used thermal capacity.
FIGURE 12
A dynamic thermal model allows conservative protection of a motor and allows operations to get the maximum work out of a motor without sacrificing available life. If the motor is hot (high % used thermal capacity) it will trip more rapidly during an overload than if the motor is cold (0% used thermal capacity). In the event of a stall condition, when available motor torque is lower than the torque required by the load, the motor can be de-energized before it overheats.
Many old-technology electronic thermal overloads do not take into consideration the values of load current below the full-load current (FLA) pick-up value. Modern overload relays should model currents above and below the FLA pick-up current to achieve maximum output of the motor and maximum life of insulation.
On larger induction motors, blockage or loss of ventilation can cause motor hot spots that current-based protection cannot detect without the use of temperature sensors. Resistance temperature detectors (RTDs) are an inexpensive device installed between the stator windings during manufacturing and may be included on motor-end bearings. An RTD has a linear change in resistance over its rated temperature range. Using information from an RTD, motor protection relays can provide protection for loss-of-ventilation, loss-of-cooling, or high-ambient-temperature.
The RTD temperature reading can also be used as input to the thermal model to improve protection.
When hot-motor compensation is enabled, the maximum stator-RTD temperature is used to bias the thermal model by increasing used I2t when the RTD temperature is greater than the thermal-model temperature.
