Heating and cooling of electric motors

The heating process of an electric motor

During the operation of the electric motor, due to the continuous generation of heat converted from total losses, the temperature of the motor increases, resulting in a temperature rise, and the motor needs to dissipate heat to the surrounding area. The higher the temperature rise, the faster the heat dissipation. When the amount of heat emitted per unit time is equal to the amount of heat dissipated, the temperature of the motor no longer increases, but maintains a stable and constant temperature rise, that is, in a state of balance between heating and heat dissipation. This process is an elevated thermal transition process called heating.

Due to the complexity of the specific situation of electric motor heating, for the convenience of research and analysis, it is assumed that the electric motor will operate for a long time, the load remains unchanged, the total loss remains unchanged, the temperature of each part of the electric motor itself is uniform, and the ambient temperature remains unchanged.

Cooling process of electric motor

After the temperature rise stabilizes, if the load of the motor is reduced or stopped, the total loss and unit time heat generation Q inside the motor will decrease or no longer continue to be generated. This results in less heat generation than heat dissipation, disrupting the thermal equilibrium state, causing the temperature of the motor to decrease and the temperature rise to decrease.

During the cooling process, as the temperature rise decreases, the heat dissipation per unit time also decreases. When the heat generation equals the heat dissipation, the motor no longer continues to cool down, and its temperature rise stabilizes at a new value. When parking, the temperature rise will decrease to zero. The process of temperature rise and fall is called cooling.

Selection of types of electric motors

The principle of selecting the type of electric motor is to prioritize the use of motors with simple structure, reliable operation, low price, easy maintenance, and economical operation, while meeting the technical performance requirements of production machinery. In this sense, AC motors are superior to DC motors, asynchronous motors are superior to synchronous motors, and cage type asynchronous motors are superior to wound rotor asynchronous motors.

When the production machinery load is stable and the requirements for starting, braking, and speed regulation performance are not high, asynchronous motors should be prioritized. For example, ordinary machine tools, water pumps, fans, etc. can use ordinary cage type asynchronous motors. If air compressors, belt conveyors, and other motors require good starting performance, deep groove or double cage asynchronous motors can be selected. When lifting machinery such as elevators and bridge cranes have frequent starting and braking, and have certain requirements for the starting, braking, and speed regulation of the electric motor, wound rotor asynchronous motors should be selected. For production machinery with high power and no need for speed regulation, such as high-power water pumps, air compressors, etc., synchronous motors can be used to improve the power factor of the power grid.

Production machinery that does not require a large speed range and can be matched with mechanical gearboxes, such as ordinary machine tools, boiler induced draft fans, etc., can use multi speed cage asynchronous motors.

Production machinery that requires a large speed range and smooth speed regulation, such as steel mills, gantry planers, large precision machine tools, paper machines, etc., should use separately excited DC motors or cage asynchronous motors with variable frequency speed regulation.

Production machinery with high starting torque and soft mechanical characteristics, such as electric cars, electric locomotives, heavy-duty cranes, excavators, portable tools, etc., should generally use series excited or compound excited DC motors. In special places such as mines with flammable and explosive gases, DC motors cannot be used, and asynchronous and synchronous motors should be used instead.

With the development of AC variable frequency speed regulation technology, the application of AC motors will become increasingly widespread, gradually replacing DC motors.

Temperature rise and insulation of electric motors

When the electric motor is running under load, its total internal losses are converted into heat energy, causing the temperature of the motor to rise. The insulation material in electric motors has poor heat resistance. If the load on the motor is too large and the loss is too high, causing the temperature to exceed the allowable limit of the insulation material, the life of the insulation material will be sharply shortened. In severe cases, the insulation will be damaged, and the motor will smoke and burn out. This temperature limit is called the allowable temperature of the insulation material. From this, it can be seen that the allowable temperature of the insulation material is the allowable temperature of the motor; The lifespan of insulation materials is the lifespan of electric motors.