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Heating and cooling of motors 1. The heating process of motors During operation, motors generate heat as a result of total losses; this causes the motor’s temperature to rise, leading to a temperature increase, and the motor then needs to dissipate heat to its surroundings. The higher the temperature rise, the faster the heat dissipation. When the heat generated per unit time is equal to the heat dissipated, the temperature of the motor no longer rises, remaining at a constant level of temperature increase; in other words, it is in a state of balance between heat generation and heat dissipation. This process is an elevated thermal transition process, known as heating. Given the complexity of the heat generation in electric motors, for the purpose of easier analysis and study, it is assumed that the motor operates continuously, with a constant load and total losses, that the temperature across all parts of the motor remains uniform, and that the temperature of the surrounding environment stays constant. 2. Cooling process of the motor: For a motor operating under load, once the temperature rise stabilizes, if the load on the motor is reduced or the motor is stopped, then the total losses within the motor as well as the heat generation per unit time, Q, will decrease or cease to occur. This results in less heat generation than heat dissipation, disrupting the thermal equilibrium; as a consequence, the temperature of the motor drops and the temperature rise is reduced. During the cooling process, as the temperature rise decreases, the heat dissipation per unit time also decreases. When the heat generated is equal to the heat dissipated, the motor stops cooling down further, and its temperature rise stabilizes at a new value. When parked, the temperature rise will drop to zero. The process of the temperature rise decreasing is called cooling. Method for selecting motor capacity: Choosing the capacity of a motor is a complicated process that requires not only certain theoretical analyses and calculations but also verification. The basic steps are as follows: based on the load curve PL= f ( t ) provided by the production machinery’s load, as well as the temperature rise curve τ= f ( t ), and taking into account the motor’s overload capacity, a motor is initially selected. Then, a heat generation check is carried out using the load curve; the results of this check are compared with the parameters of the motor selected initially. If it is found that the capacity of the selected motor is either too large or too small, a new selection is made until an optimal capacity is achieved. Finally, it is checked whether the motor’s overload capacity and starting torque meet the required standards. Selection of motor types, models, voltage, and speed 1. Selection of motor type The principle for choosing a motor type is to, while meeting the technical requirements of the manufacturing machinery, give priority to motors that have a simple structure, reliable operation, low cost, easy maintenance, and efficient performance. In this sense, AC motors are superior to DC motors, asynchronous motors are superior to synchronous motors, and squirrel-cage asynchronous motors are superior to wound-rotor asynchronous motors. When the load on the production machinery is stable and low requirements are placed on starting, braking, and speed control performance, asynchronous motors should be given priority. For example, ordinary cage-type asynchronous motors can be used for standard machine tools, water pumps, fans, etc. For devices such as air compressors and belt conveyors that require motors with good starting performance, deep-groove or double-cage asynchronous motors can be selected. For lifting machinery such as elevators and bridge cranes, which experience frequent starting and stopping operations and have certain requirements regarding the motor’s starting, stopping, and speed control, wound-rotor asynchronous motors should be used. For production machinery with high power levels that does not require speed control, such as high-power water pumps and air compressors, synchronous motors can be used to improve the power factor of the power grid. Since the speed control range requirements are not high, and for manufacturing equipment equipped with mechanical gearboxes, such as ordinary machine tools and boiler exhaust fans, multi-speed squirrel-cage asynchronous motors can be used. For manufacturing machines that require a wide speed range and smooth speed control, such as steel rolling mills, gantry planers, large precision machine tools, and papermaking machines, separately-excited DC motors or squirrel-cage asynchronous motors with variable frequency speed control should be used. For production machinery that requires high starting torque and soft mechanical characteristics, such as trams, locomotives, heavy cranes, excavators, and portable tools, series-wound or compound-wound DC motors are generally preferred. In special locations such as mines where flammable or explosive gases are present, DC motors cannot be used; instead, asynchronous and synchronous motors should be employed. With the development of AC variable-frequency speed control technology, the application of AC motors will become increasingly widespread, gradually replacing DC motors. 2. Selection of motor voltage level: The rated voltage of an AC motor is primarily determined based on the voltage level of the power supply at the location where it will be used. Typically, the voltage in low-voltage power grids is 380 V; therefore, the rated voltages of small and medium-sized three-phase asynchronous motors are mostly 380 V (in Y or Δ connection), 220/380 V (in Δ/Y connection), and 380/660 V (in Δ/Y connection). The rated voltage of single-phase asynchronous motors is usually 220 V. For large-scale equipment in mines and steel plants, high-power motors can be used; this not only reduces the size of the motors but also saves on the amount of copper wire required. The rated voltage of the DC motor must also match the supply voltage. The rated voltage of DC motors powered by DC generators is generally 110 V or 220 V. High-power motors can operate at voltages of 600–1000 V. When the grid voltage is 380 V, and DC motors are powered by thyristor rectifier circuits, a three-phase rectification setup allows for a rated voltage of 440 V, while a single-phase rectification setup enables a rated voltage of 160 V or 180 V. 3. Selection of the motor’s rated speed: The motor’s rated speed is determined based on the requirements of the transmission system of the manufacturing machinery. At a given power level, the higher the rated speed of a motor, the smaller its size, the lighter its weight, the lower its cost, and the higher its operating efficiency. Additionally, the flywheel torque of the motor is lower; therefore, using a high-speed motor is more economical. However, if the speed required by the production machinery is low, choosing a high-speed motor will complicate the transmission mechanism. For production machinery that is frequently started, stopped, and reversed, if the transition time has a significant impact on production efficiency, the rated speed of the motor should be selected to be as low as possible, at GD2 · nN. If the time taken for the transition process has little impact on production efficiency, then the rated speed of the motor should be chosen to minimize energy loss during that transition process. 4. Selection of motor structure type: The motor structure types include open-type, protected-type, enclosed-type, and explosion-proof-type. To enable the motor to operate properly in various environments, it is necessary to select the appropriate protection type for the motor based on the working conditions, in order to protect it from damage over time. (1) Open-type: Open-type motors are inexpensive and have good cooling conditions. However, since the rotor and windings are exposed to the air, they are prone to damage from moisture, dust, iron shavings, oil contamination, etc., which can affect the motor’s normal operation and service life. Therefore, it can only be used in environments that are dry, have little dust, and are free of corrosive and explosive gases. (2) Protective type: Protective motors generally prevent external contaminants such as water droplets and metal shavings from entering the motor, but they cannot stop moisture and dust from getting in. It is only suitable for working environments that are relatively dry, have little dust, and are free of corrosive and explosive gases. This type of motor also has good ventilation and heat dissipation conditions. (3) Enclosed type: Enclosed motors come in three types: self-cooling, forced ventilation, and hermetically sealed. Self-cooling motors generally come equipped with fans. Both self-cooling and forced-ventilation motors can prevent water droplets or debris from entering the motor from any direction; moist air and dust also find it difficult to penetrate. As a result, they are suitable for harsh working environments such as those that are humid, dusty, exposed to wind and rain, or filled with corrosive gases, and they are therefore widely used. Closed-type motors, on the other hand, are suitable for manufacturing machinery that is submerged in liquids, such as submersible pumps. (4) Explosion-proof type: Explosion-proof motors are designed as flameproof, increased safety, and positive pressure types based on a sealed structure; they are suitable for work environments where there is a risk of explosion, such as mines, oil depots, gas stations, and similar locations. Furthermore, for motors used in humid and hot regions, high-altitude areas, and ships, motors with special protection requirements should also be selected. Motor temperature rise and insulation: When a motor is operating under load, the total losses within it are converted into heat, which raises the temperature of the motor. In electric motors, it is the insulation material that has poor heat resistance. If the load on the motor is too high, resulting in excessive losses that raise the temperature above the limit tolerated by the insulation material, its lifespan is significantly shortened; in severe cases, the insulation can be damaged, causing the motor to smoke and catch fire. This temperature limit is known as the allowable temperature of insulating materials. It can be seen that the allowable temperature of the insulating material is the allowable temperature of the motor ； The lifespan of the insulating material is the lifespan of the motor.