How to select the motor capacity in mechanical design

Selecting the motor capacity is a complex process that requires both theoretical analysis and verification. The basic steps are as follows:

1. Based on the load diagram PL = f (t) and the temperature rise curve τ = f (t) provided by the mechanical load, consider the motor's overload capacity and pre-select a motor.
2. Perform a thermal verification based on the load diagram. Compare the results with the parameters of the pre-selected motor. If the motor capacity is found to be too large or too small, select a new motor and repeat the verification process until the motor capacity is fully utilized.
3. Finally, verify whether the motor's overload capacity and starting torque meet the requirements.

Selection of motor type, model, voltage, and speed

1. Selection of motor type

When selecting the motor type, the priority should be to choose a motor with a simple structure, reliable operation, low price, easy maintenance, and good economy while meeting the technical performance requirements of the mechanical load. From this perspective, AC motors are preferred over DC motors, induction motors over synchronous motors, and cage rotor induction motors over wound rotor induction motors.

When the mechanical load is stable, and the requirements for starting, braking, and speed regulation are low, induction motors should be used preferentially. For example, ordinary machine tools, pumps, and fans can use standard cage induction motors. Deep groove or double cage induction motors can be used for machines that require good starting performance, such as vacuum pumps and belt conveyors. For lifting machines like elevators and bridge cranes, where frequent starting and braking are required, and there are certain requirements for starting, braking, and speed regulation, wound rotor induction motors should be selected. For large power machines that do not require speed regulation, synchronous motors can be used to improve the power grid's power factor, such as large power pumps and compressors.

For machines that require a small adjustment range and can be combined with a mechanical speed changer, such as ordinary machine tools and boiler fans, multi-speed cage induction motors can be used.

For machines that require a large adjustment range and smooth speed regulation, such as rolling mills, portal milling machines, large precision machine tools, and paper machines, DC motors with separate excitation or variable frequency cage induction motors should be used.

For machines that require large starting torque and soft mechanical characteristics, such as trains, electric trucks, heavy lifting machines, and excavators, series-excited or compound-excited DC motors are generally used. If direct current motors cannot be used in special environments with flammable and explosive gases, such as mines, asynchronous motors and synchronous motors should be used.

With the development of AC variable frequency speed regulation technology, the application of AC motors is becoming more and more extensive, gradually replacing DC motors.

2. Selection of motor voltage level

The selection of the motor voltage level is mainly based on the voltage level of the power supply at the usage location. Low voltage power grids are usually 380V, so the rated voltage of most small and medium-sized three-phase induction motors is 380V (Y or Δ connection), 220/380V (Δ/Y connection) and 380/660V (Δ/Y connection). The rated voltage of single-phase induction motors is often 220V. For large-scale equipment in mines and steel plants that use high-power motors, high voltage motors can be used to reduce the size of the motor and save copper wire usage.

The rated voltage of DC motors should also be coordinated with the power supply voltage. For DC motors powered by DC generators, the rated voltage is generally 110V or 220V. High-power motors can be increased to 600～1000V. When the power grid voltage is 380V, and the DC motor is powered by a thyristor rectifier circuit, the rated voltage can be selected as 440V for three-phase rectification or 160V or 180V for single-phase rectification.

3. Selection of motor rated speed

The motor's rated speed is selected based on the requirements of the mechanical transmission system. In a certain power range, the higher the motor's rated speed, the smaller its size, lighter weight, lower price, higher efficiency, and smaller flywheel moment. Therefore, using high-speed motors is more economical. However, if the mechanical load requires low speed, using a high-speed motor will cause the transmission system to become more complex.

For machines that frequently start, brake, and reverse, and the transition process time has a significant impact on production efficiency, the motor's rated speed should be selected based on the minimum value of GD2 · nN. If the impact of the transition process time on production efficiency is not significant, the motor's rated speed should be selected based on the minimum energy loss during the transition process.

4. Selection of motor structural form

There are four types of motor structural forms: open, protected, enclosed, and explosion-proof. To ensure that the motor can operate normally in different environments, the motor's protective form should be selected based on the working environment conditions to protect the motor from damage during long-term operation.

(1) Open type

Open-type motors are inexpensive, have good heat dissipation conditions, but are vulnerable to erosion by water vapor, dust, iron filings, oil stains, and other substances, affecting the motor's normal operation and service life. Therefore, they can only be used in dry environments with little dust and no corrosive or explosive gases.

(2) Protected type

Protected motors can generally prevent water droplets, iron filings, and other external objects from entering the motor, but they cannot prevent the intrusion of salt fog and dust. They are only suitable for use in relatively dry environments with little dust and no corrosive or explosive gases. Protected motors have good ventilation and heat dissipation conditions.

(3) Enclosed type

Enclosed motors include self-cooled, forced ventilation, and sealed types. Self-cooled and forced ventilation motors can prevent water droplets or debris from entering the motor in any direction, and salt fog and dust are not easily intruded. Therefore, they are suitable for use in humid, dusty, windy, and corrosive environments. They are widely used. Sealed motors are suitable for use in machines submerged in liquids, such as submerged pumps.

(4) Explosion-proof type

Explosion-proof motors are based on sealed structures and are divided into explosion-relief, increased safety, and positive pressure types. They are suitable for use in environments with explosive hazards, such as mines, oil depots, and gas stations.

Additionally, motors with special protective requirements should be selected for use in humid and hot regions, high altitude regions, and ships.

In conclusion, selecting the appropriate motor for mechanical design is a crucial step that requires careful consideration of various factors. To ensure optimal performance and efficiency, the motor's capacity should be carefully selected based on the load diagram and temperature rise curve, as well as the motor's overload capacity. Additionally, the type, voltage, and speed of the motor should be chosen based on the specific requirements of the mechanical load, taking into account factors such as stability, starting and braking requirements, and adjustment range. Finally, the motor's structural form should be chosen based on the working environment conditions to ensure that it can operate normally and safely over the long term. By following these guidelines, mechanical designers can select the best motor for their specific needs, improving the overall performance and reliability of their machines.