What effects could excessive heating have on a motor How can

What effects could excessive heating have on a motor? How can the effects of heating be mitigated?

Solution

Effects of exessive heating on motor:

A major consideration in both motor design and application is heat. Excessive heat will accelerate motor insulationdeteriation and cause premature insulation failure. Excessive heat may also cause a breakdown of bearing grease, thus damaging the bearing system of a motor.The total temperature a motor must withstand is the result of two factors: external, or ambient temperature; and internal, or motor temperature rise.

An understanding of how these components are measured and expressed is important for proper motor application. For a given application, the maximum sustained ambient temperature, measured in degrees Centigrade (Celsius),should be determined. The ambient temperature is the temperature that the motor sees. If the motor is in a housing or chamber, then the ambient temperature is the temperature inside the housing. Most motors are designed to operate in a maximum ambient temperature of 40°C. The temperature rise is the result of heat generated by motor losses during operation. At no-load, friction in the bearings, core losses (eddy current and hysteresis losses),and stat or I2R losses contribute to temperature rise; at full-load, additional losses which cause heating are rotor I2R losses and stray load losses. Motor current increases with an increase in load and under locked-rotor, temperature rise will be significantly higher under these conditions. Therefore, applications requiring frequent starting and/or frequent overloads may require special motors to compensate for the increase in total temperature.

MOTOR COOLING

The total temperature of a motor is greater than thesurrounding environment; heat generated during motoroperation will be transferred to the ambient air. The rateof heat transfer affects the maximum load and/or theduty cycle of a specific motor design. Factors affectingthis rate of transfer are:

1. Motor Enclosure

Different enclosures result in different airflow patternswhich alter the amount of ambient air in contract withthe motor.

2. Frame Surface Area

Increasing the area of a motor enclosure in contact

3. Airflow over motor

The velocity of air moving over the enclosure affects therate of heat transfer. Fans are provided on most totallyenclosed and some open motors to increase the velocityof air over the external parts.

4. Ambient air density

A reduction in the ambient air density will result in areduction of the rate of heat transfer from the motor.Therefore, total operating temperature increases withaltitude. Standard motors are suitable for operationsup to 3300 feet; motors with service factor may be usedat altitudes up to 9900 feet at 1.0 service factor.