Motor heating and cooling
1. The heating process of the motor
During the operation of the motor, due to the heat generated by the total loss conversion, the temperature of the motor rises and there is a temperature rise, and the motor will dissipate heat to the surroundings. 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 rises, but maintains a stable and constant temperature rise, that is, it is in a state of balance between heating and heat dissipation. This process is an elevated thermal transition process, called heating.
Since the specific situation of the motor heating is more complicated, for the convenience of research and analysis, it is assumed that the motor runs for a long time, the load is unchanged, the total loss is unchanged, the temperature of each part of the motor itself is uniform, and the ambient temperature is unchanged.
2. The cooling process of the motor
For a motor running under load, if the temperature rise is stable, if the load is reduced or stopped, then the total loss in the motor and the heat value Q per unit time will be reduced or no longer generated. In this way, the heat generation is less than the heat dissipation, which destroys the thermal equilibrium state, the temperature of the motor drops, and the temperature rise decreases.
During the cooling process, as the temperature rise decreases, the heat dissipation per unit time also decreases. When the calorific value is equal to the heat dissipation, the motor no longer continues to cool down, and its temperature rise is stabilized at a new value. During parking, the temperature rise will drop to zero. The process of temperature rise and fall is called cooling.
Selection method of motor capacity
Choosing the capacity of the motor is cumbersome, not only requires a certain theoretical analysis and calculation, but also needs to be verified. The basic steps are: according to the load diagram PL = f (t) and temperature rise curve τ = f (t) provided by the production machinery load, and considering the overload capacity of the motor, pre-select a motor, and then perform heat verification according to the load diagram , Compare the verification result with the parameters of the pre-selected motor. If the capacity of the pre-selected motor is found to be too large or too small, re-select it until its capacity is fully utilized, and finally verify whether its overload capacity and starting torque meet the requirements .
Selection of motor type, type, voltage, speed
1. Selection of motor type
The principle of choosing the type of motor is that under the premise of meeting the technical performance of the production machinery, the motor with simple structure, reliable work, cheap price, convenient maintenance and economic operation is preferred. In this sense, AC motors are better than DC motors, asynchronous motors are better than synchronous motors, and cage asynchronous motors are better than 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 used first. For example, ordinary machine tools, water pumps, fans, etc. can use ordinary cage asynchronous motors. If the air compressor, belt conveyor and other motors are required to have a good starting performance, you can choose deep-slot or double-cage asynchronous motors. For hoisting machinery such as elevators and bridge cranes, where starting and braking are frequent, when there are certain requirements for starting, braking, and speed regulation of the motor, a wound rotor asynchronous motor should be selected. For production machinery with high power and no need for speed regulation, such as high-power water pumps and air compressors, in order to improve the power factor of the power grid, synchronous motors can be used.
The range of speed regulation is not large, and the production machinery that can be matched with the mechanical gearbox, such as ordinary machine tools, boiler induced draft fans, etc., can use multi-speed cage asynchronous motors.
If the speed regulation range is large, and production machinery that requires smooth speed regulation, such as rolling mills, gantry planers, large-scale precision machine tools, and paper machines, should use a separately excited DC motor or a cage-type asynchronous motor with variable frequency speed regulation.
Production machinery requiring high starting torque and soft mechanical characteristics, such as trams, electric locomotives, heavy cranes, excavators, portable tools, etc., should generally choose series or compound excitation DC motors. If there are special places such as mines with flammable and explosive gases, DC motors cannot be used, but asynchronous motors and synchronous motors should be used.
With the development of AC variable frequency speed regulation technology, the application of AC motors will become more and more extensive, and it is gradually replacing DC motors.
2. Selection of motor voltage level
The selection of the rated voltage of the AC motor is mainly based on the power supply voltage level of the place of use. Generally, the low-voltage power grid is 380 V, so the rated voltage of small and medium-sized three-phase asynchronous motors is mostly 380 V (Y or △ shape method), 220/380 V (△ / Y shape connection method) and 380/660 V (△ / Y Shape connection method). The rated voltage of single-phase asynchronous motors is mostly 220 V. High-power motors for large-scale equipment in mining and iron and steel enterprises can use high-voltage motors, which reduces the size of the motor and saves the amount of copper wire.
The rated voltage of the DC motor should also be matched with the power supply voltage. The rated voltage of a DC motor powered by a DC generator is generally 110 V or 220 V. The high-power motor can be increased to 600 ~ 1000 V. When the grid voltage is 380 V, the DC motor is powered by the thyristor rectifier circuit, the optional rated voltage is 440 V using three-phase rectification, and the optional rated voltage is 160 V or single-phase rectification. 180 V.
3. Selection of rated motor speed
The rated speed of the motor is selected according to the requirements of the production machinery transmission system. At a certain power, the higher the rated speed of the motor, the smaller the volume, the lighter the weight, the lower the price, the higher the operating efficiency, and the smaller the flywheel torque of the motor, so it is more economical to choose a high-speed motor. However, if the rotational speed required by the production machinery is low, if the high-speed motor is selected, the transmission mechanism will be complicated.
For production machinery that is frequently started, braked, and reversed, if the transition process time has a greater impact on production efficiency, the rated speed of the motor should be selected with GD2 · nN as the minimum. If the transition process time has little effect on production efficiency, the rated speed of the motor should be selected with the minimum energy loss during the transition process.
4. Selection of motor structure
The structure of the motor has four types: open type, protective type, closed type and explosion-proof type. In order to allow the motor to work normally in different environments, the motor’s protection form must be selected according to the working environment conditions to protect the motor from long-term work without being damaged.
(1) Open type
Open motors are cheap and have good heat dissipation conditions, but because the rotor and windings are exposed to the air, they are easily eroded by moisture, dust, iron filings, oil stains, etc., affecting the normal operation and service life of the motor. Therefore, it can only be used in dry, dust-free environments without corrosive and explosive gases.
(2) Protective
Protective motors can generally prevent external debris such as water droplets and iron filings from falling into the motor, but cannot prevent the intrusion of moisture and dust. It is only suitable for the working environment which is relatively dry and has little dust and no corrosive and explosive gas. The ventilation and heat dissipation conditions of this motor are also good.
(3) Closed
There are three types of enclosed motors: self-cooled, forced-ventilated and sealed. Self-cooled motors generally have their own fans. Self-cooled and forced-ventilation motors can prevent water droplets or debris from entering the motor in any direction, and humid air and dust are not easy to invade, so they are suitable for humidity, dust, and are easily eroded by wind and rain The harsh working environment such as corrosive gas is widely used. The enclosed motor is suitable for production machinery immersed in liquid, such as submersible pumps.
(4) Explosion-proof
Explosion-proof motors are made of explosion-proof type, increased safety type and positive pressure type on the basis of sealed structure, and are suitable for working environments with explosion hazards, such as mines, oil depots, gas stations and other places.
In addition, for motors used in hot and humid areas, high altitude areas and ships, motors with special protection requirements should also be selected.
Motor temperature rise and insulation
When the motor is running under load, the total internal loss is converted into heat energy to increase the temperature of the motor. The poor heat resistance in the motor is the insulating material. If the load of the motor is too large and the loss is too large to make the temperature exceed the allowable limit of the insulating material, the life of the insulating material will be shortened sharply. In severe cases, the insulation will be damaged and the motor Smoke and burn. This temperature limit is called the allowable temperature of the insulating material. It can be seen that the allowable temperature of the insulating material is the allowable temperature of the motor; the life of the insulating material is the life of the motor.
