Learn about motor selection parameters, principles and methods-Xinda Motor- electric motors and EV accessories supplier|INDEX--

Learn about motor selection parameters, principles and methods

Date：2024-07-22　　　Author：Shandong Xinda Motor Co., Ltd.

Heating and cooling of electric motors

1. The heating process of the motor

During the operation of the motor, the heat converted from the total loss is continuously generated, and the motor temperature rises. There is a temperature rise, and the motor needs to dissipate heat to the surrounding. The higher the temperature rise, the faster the heat dissipation. When the heat generated per unit time is equal to the heat dissipated, the motor temperature no longer rises, but maintains a stable temperature rise, that is, it is in a state of balance between heat generation and heat dissipation. This process is a rising heat transition process, called heating.

Since the specific situation of motor heating is relatively complicated, for the convenience of research and analysis, it is assumed that the motor runs for a long time, the load remains unchanged, the total loss remains unchanged, the temperature of each part of the motor itself is uniform, and the ambient temperature remains unchanged.

2. Cooling process of the motor

For a motor running under load, if the load is reduced or the motor is stopped after the temperature rise stabilizes, the total loss in the motor and the heat generated per unit time Q will be reduced or will no longer be generated. This will make the heat generation less than the heat dissipation, destroy the thermal balance state, reduce the temperature of the motor, and reduce the temperature rise.

During the cooling process, as the temperature rise decreases, the heat dissipation per unit time also decreases. When the heat generation is equal to the heat dissipation, the motor will no longer continue to cool down, and its temperature rise will stabilize at a new value. When the motor stops, the temperature rise will drop to zero. The process of temperature rise reduction is called cooling.

How to select motor capacity

Selecting the capacity of the motor is rather complicated, requiring not only certain theoretical analysis and calculation, but also verification. The basic steps are: based on 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, then perform a heat check according to the load diagram, and compare the check 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, reselect it until its capacity is fully utilized, and finally verify whether its overload capacity and starting torque meet the requirements.

Selection of motor type, model, voltage and speed

1. Selection of motor type

The principle of selecting the type of motor is to give priority to motors with simple structure, reliable operation, low price, convenient maintenance and economical operation, under the premise of meeting the technical performance of the production machinery. 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 has a stable load and does not require high starting, braking and speed regulation performance, asynchronous motors should be used first. For example, ordinary machine tools, water pumps, fans, etc. can use ordinary cage asynchronous motors. For air compressors, belt conveyors, etc., which require motors with good starting performance, deep groove or double cage asynchronous motors can be used. For lifting machinery such as elevators and bridge cranes, which start and brake frequently and have certain requirements for the starting, braking and speed regulation of the motor, wound rotor asynchronous motors should be used. For production machinery with large power but no speed regulation, such as high-power water pumps, air compressors, etc., in order to improve the power factor of the power grid, synchronous motors can be used.

For production machinery that does not require a large speed regulation range and can be matched with a mechanical gearbox, such as ordinary machine tools, boiler induced draft fans, etc., multi-speed cage-type asynchronous motors can be selected.

For production machinery that requires a large speed regulation range and smooth speed regulation, such as rolling mills, gantry planers, large precision machine tools, papermaking machines, etc., separately excited DC motors or cage-type asynchronous motors with variable frequency speed regulation should be used.

Production machinery that requires large starting torque and soft mechanical characteristics, such as trams, electric locomotives, heavy cranes, excavators, portable tools, etc., generally should use series-excited or compound-excited DC motors. In 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 is gradually replacing DC motors.

2. Selection of motor voltage level

The selection of rated voltage of 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 △ connection), 220/380 V (△/Y connection) and 380/660 V (△/Y connection). The rated voltage of single-phase asynchronous motors is mostly 220 V. Large equipment in mines and steel enterprises uses high-power motors, and high-voltage motors can be used, which not only reduces the size of the motor, but also saves the amount of copper wire.

The rated voltage of the DC motor should also match the power supply voltage. The rated voltage of the DC motor powered by the DC generator is generally 110 V or 220 V. The rated voltage of the high-power motor can be increased to 600-1000 V. When the grid voltage is 380 V and the DC motor is powered by a thyristor rectifier circuit, the rated voltage can be 440 V with three-phase rectification and 160 V or 180 V with single-phase rectification.

3. Selection of rated speed of motor

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 its size, the lighter its weight, the lower its price, the higher its operating efficiency, and the smaller its flywheel torque. Therefore, it is more economical to choose a high-speed motor. However, if the speed required by the production machinery is low, if a high-speed motor is selected, the transmission mechanism will be complicated.

For production machinery that is frequently started, braked, and reversed, if the transition time has a significant impact on production efficiency, the rated speed of the motor should be selected with GD2·nN as the minimum. If the transition time has little impact 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

There are four types of motor structures: open type, protective type, closed type and explosion-proof type. In order to make the motor work normally in different environments, the protection form of the motor must be selected according to the working environment conditions to protect the motor from long-term operation without damage.

(1) Open type

Open type motors are cheap and have good heat dissipation conditions, but because the rotor and winding are exposed to the air, they are easily corroded by water vapor, dust, iron filings, oil stains, etc., which affects the normal operation and service life of the motor. Therefore, it can only be used in dry environments with little dust and no corrosive and explosive gases.

(2) Protective type

Protective motors can generally prevent water droplets, iron filings and other foreign debris from falling into the motor, but cannot prevent moisture and dust from entering. They are only suitable for working environments that are relatively dry, dusty, and free of corrosive and explosive gases. The ventilation and heat dissipation conditions of this type of motor are also better.

(3) Closed

There are three types of enclosed motors: self-cooling, forced ventilation, and sealed. Self-cooling motors generally have their own fans. Self-cooling and forced ventilation motors can prevent water droplets or debris from any direction from entering the motor, and humid air and dust are not easy to enter. Therefore, they are suitable for harsh working environments such as humid, dusty, easily eroded by wind and rain, and corrosive gases, and are widely used. Enclosed motors are suitable for production machinery immersed in liquids, such as submersible pumps.

(4) Explosion-proof

Explosion-proof motors are made into flameproof, increased safety and positive pressure types based on a sealed structure. They 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.

Temperature rise and insulation of electric motors

When the motor is running under load, its internal total loss is converted into heat energy, which increases the temperature of the motor. The insulation material in the motor has the poorest heat resistance. If the load of the motor is too large, the loss is too large, and the temperature exceeds 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. This temperature limit is called the allowable temperature of the insulation material. It can be seen that the allowable temperature of the insulation material is the allowable temperature of the motor; the life of the insulation material is the life of the motor.

Learn about motor selection parameters, principles and methods

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