The basic contents required for motor selection are: driven load type, rated power, rated voltage, rated speed, and other conditions.

Motors can be simply divided into DC motors and AC motors , and AC is further divided into synchronous motors and asynchronous motors.

Advantages of DC Motors

The speed can be easily adjusted by changing the voltage , and a large torque can be provided . It is suitable for loads that need to adjust the speed frequently , such as rolling mills in steel mills, hoists in mines, etc. But now with the development of frequency conversion technology, the AC motor can also adjust the speed by changing the frequency. However, although the price of variable frequency motors is not much more expensive than ordinary motors, the price of frequency converters occupies a major part of the whole set of equipment, so another advantage of DC motors is that they are cheap .

Disadvantages of DC Motors

The structure is complex. As long as any equipment has a complex structure, it will inevitably lead to an increase in the failure rate. Compared with AC motors, DC motors are not only complicated in windings (excitation windings, commutation pole windings, compensation windings, armature windings), but also add slip rings, brushes and commutators.

Not only the process requirements of the manufacturer are high, but the maintenance cost in the later period is also relatively high. Therefore, in industrial applications, DC motors are in an embarrassing situation where they are gradually declining but still have a place in the transitional stage. If the user has sufficient funds, it is recommended to choose the solution of AC motor with frequency converter. After all, the use of frequency converter also brings many benefits, which will not be discussed in detail.

Advantages of asynchronous motors

Simple structure, stable performance, convenient maintenance and low price. And the manufacturing process is also the simplest. I have heard from an old technician in the workshop that it takes two synchronous motors or four asynchronous motors of similar power to assemble a DC motor. Therefore, asynchronous motors are the most widely used in industry.

Asynchronous motors are further divided into squirrel-cage motors and wound motors , the difference being the rotor. Squirrel-cage motor rotors are made of metal strips, either copper or aluminum. The price of aluminum is relatively low, and my country is a big country of aluminum ore, which is widely used in occasions with low requirements. But copper has better mechanical and electrical properties than aluminum, and most of the rotors I've come into contact with are copper rotors. After the squirrel cage motor solves the problem of disconnection in the process, the reliability is far higher than that of the winding rotor motor.

Disadvantages of asynchronous motors

The torque obtained by the metal rotor cutting the magnetic field lines in the rotating stator magnetic field is small, and the starting current is large, so it is difficult to handle the load that requires a large starting torque. Although more torque can be obtained by increasing the length of the motor core, the power is very limited.

The winding type motor energizes the rotor winding through the slip ring when it is started to form the rotor magnetic field, which moves relative to the rotating stator magnetic field, so the torque is greater. And in the starting process, the water resistance is connected in series to reduce the starting current, and the water resistance is controlled by a mature electronic control device to change the resistance value with the starting process.

For rolling mills, hoists and other loads. Compared with the squirrel cage motor, the wound asynchronous motor adds slip rings, water resistance, etc., and the overall equipment price has a certain increase. Compared with the DC motor, the speed regulation range is relatively narrow and the torque is relatively small, and the corresponding value is also low.

In addition to the overexcitation state that can compensate for reactive power, the advantages of synchronous motors include:

1) The speed of the synchronous motor strictly follows n=60f/p, which can precisely control the speed;

2) The operation stability is high. When the grid voltage suddenly drops, the excitation system will generally force the excitation to ensure the stable operation of the motor, while the torque of the asynchronous motor (proportional to the square of the voltage) will drop significantly;

3) The overload capacity is larger than the corresponding asynchronous motor;

4) High operating efficiency, especially for low-speed synchronous motors.

Synchronous motors cannot be started directly, and need to be started asynchronously or with variable frequency. Asynchronous start means that the synchronous motor is equipped with a start winding similar to the cage winding of the asynchronous motor on the rotor, and an additional resistance about 10 times the resistance value of the excitation winding is connected in series in the excitation circuit to form a closed circuit, and the stator of the synchronous motor is directly connected. The power grid, so that it is started as an asynchronous motor. When the speed reaches the sub-synchronous speed (95%), the starting method of cutting off the additional resistance; frequency conversion starting is not repeated. So one of the disadvantages of synchronous motors is the need to add additional equipment for starting .

Synchronous motors run on excitation current, and if there is no excitation, the motor is asynchronous. The excitation is a DC system applied to the rotor, and its rotation speed and polarity are consistent with the stator. If there is a problem with the excitation, the motor will lose its step and cannot be adjusted, triggering the protection "excitation fault" The motor trips.

Therefore, the second disadvantage of the synchronous motor is that it needs to increase the excitation device, which was directly supplied by the DC machine in the past, but is now mostly supplied by the thyristor rectifier. As the old saying goes, the more complex the structure and the more equipment, the more failure points and the higher the failure rate.

According to the performance characteristics of synchronous motors, their applications are mainly on loads such as hoists, mills, fans, compressors, rolling mills, and water pumps.

To sum up, the principle of selecting a motor is that on the premise that the performance of the motor meets the requirements of the production machinery, the motor with simple structure, low price, reliable operation and convenient maintenance is preferred .

The rated power of the motor refers to the output power , that is, the shaft power , also known as the capacity, which is the iconic parameter of the motor. People often ask how big the motor is. Generally, it does not refer to the size of the motor, but to the rated power . It is the most important indicator to quantify the motor's drag load capacity, and it is also the parameter requirements that must be provided when the motor is selected .

(For rated power, for rated voltage, for rated current, cosθ is power factor, η is efficiency)

The principle of correctly selecting the motor capacity should be the most economical and most reasonable decision on the power of the motor under the premise that the motor can meet the requirements of the production mechanical load . If the power is too large, the equipment investment will increase, causing waste, and the motor often runs under load, and the efficiency and power factor of the AC motor are low; on the contrary, if the power is too small, the motor will be overloaded, causing the motor to run prematurely. damage.