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Basic content of motor selection

Date:2022-08-12   Author:XINDA MOTOR
The basic contents required for motor selection are: driven load type rated power rated voltage rated speed , and other conditions.

First, the type of load driven
This has to be reversed from the characteristics of the motor. Motors can be simply divided into DC motors and AC motors , and AC is further divided into synchronous motors and asynchronous motors .

1. DC motor

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.

2. Asynchronous motor

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. This is evident. 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 more than that of the motor with the winding rotor.
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.
It is suitable for loads such as rolling mills and hoists. 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.
However, the asynchronous motor establishes a rotating magnetic field because the stator winding is energized, and the winding is an inductive element that does not do work. It needs to absorb reactive power from the power grid, which has a great impact on the power grid. Intuitive experience when high-power inductive electrical appliances are connected to the grid, the grid voltage drops, and the brightness of the lights decreases.The public account "Mechanical Engineering Literature", the gas station for engineers!
Therefore, the power supply bureau will limit the use of asynchronous motors, which is also something that many factories must consider. Some large power consumers, such as steel mills and aluminum plants, choose to build their own power plants to form their own independent power grids to reduce the restrictions on the use of asynchronous motors. Therefore, if the asynchronous motor needs to meet the use of high-power loads, it needs to be equipped with a reactive power compensation device, while the synchronous motor can provide reactive power to the grid through the excitation device. Motor stage.

3. Synchronous motor

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 that of 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 steps 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 In this respect, the AC motor is better than the DC motor, the AC asynchronous motor is better than the AC synchronous motor, and the squirrel-cage asynchronous motor is better than the wound asynchronous motor.
For production machinery that has stable load and no special requirements for starting and braking, ordinary squirrel-cage asynchronous motors should be preferred, which are widely used in machinery, water pumps, fans, etc.
Starting and braking are frequent, and production machinery that requires large starting and braking torque, such as bridge cranes, mine hoists, air compressors, irreversible rolling mills, etc., should use wound asynchronous motors.
Where there is no speed regulation requirement, the rotational speed is required to be constant or the power factor is required to be improved, synchronous motors should be used, such as medium and large capacity water pumps, air compressors, hoists, mills, etc.
For production machinery that requires a speed regulation range of more than 1:3, and requires continuous, stable and smooth speed regulation, it is advisable to use separately excited DC motors or squirrel-cage asynchronous motors or synchronous motors with variable frequency speed regulation, such as large precision machine tools, gantry planers, Rolling mills, hoists, etc.
Production machinery that requires a large starting torque and soft mechanical characteristics, uses series or compound excitation DC motors, such as trams, electric locomotives, heavy cranes, etc.

2. Rated power

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, resulting in 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.
There are three factors that determine the main power of the motor:
1) The heating and temperature rise of the motor are the most important factors that determine the power of the motor ;
2) Allow short-time overload capability;
3) For asynchronous squirrel-cage motors, the starting ability should also be considered.
First, the specific production machinery calculates and selects the load power according to its heat generation, temperature rise and load requirements, and then pre-selects the rated power of the motor according to the load power duty system and overload requirements After the rated power of the motor is preselected, the heating, overload capacity and if necessary, the starting capacity should be checked.
If one of them is unqualified, the motor must be re-selected and checked again until all items are qualified. Therefore , the working system is also one of the requirements that must be provided. If there is no requirement, the most conventional S1 working system will be handled by default; motors with overload requirements also need to provide overload multiples and corresponding running time; asynchronous squirrel-cage motors drive large rotation such as fans In the case of inertia load, it is also necessary to provide the load moment of inertia and starting resistance torque curve to check the starting ability.The public account "Mechanical Engineering Literature", the gas station for engineers!
The above selection of rated power is made under the premise that the standard ambient temperature is 40°C. If the ambient temperature in which the motor works changes, the rated power of the motor must be corrected. According to theoretical calculation and practice, when the ambient temperature is different, the power of the motor can be roughly increased or decreased according to the table below.
Therefore, it is necessary to provide the ambient temperature in areas with harsh climates, such as India, where the ambient temperature needs to be checked at 50°C. In addition, high altitude will also affect the motor power. The higher the altitude, the greater the temperature rise of the motor and the smaller the output power. And the motor used at high altitude also needs to consider the influence of the corona phenomenon.
For the power range of electric motors currently on the market, I would like to list a few data for reference.
DC motor: ZD9350 (mill) 9350kW
Asynchronous motor: squirrel cage type YGF1120-4 (blast furnace fan) 28000kW
Winding type YRKK1000-6 (raw mill) 7400kW
Synchronous motor: TWS36000-4 (blast furnace fan) 36000kW (test unit reaches 40000kW)

3. Rated voltage

The rated voltage of the motor refers to the line voltage in the rated working mode .
The choice of the rated voltage of the motor depends on the power supply voltage of the power system to the enterprise and the size of the motor capacity.
The choice of AC motor voltage level mainly depends on the power supply voltage level of the place of use. Generally, the low voltage network is 380V, so the rated voltage is 380V (Y or △ connection), 220/380V (△/Y connection), and 380/660V (△/Y connection). When the power of the low-voltage motor increases to a certain extent (such as 300KW/380V), it is difficult to increase the current due to the limitation of the carrying capacity of the wire, or the cost is too high.
It is necessary to achieve high power output by increasing the voltage. The power supply voltage of the high-voltage grid is generally 6000V or 10000V, and there are also voltage levels of 3300V, 6600V and 11000V abroad. The advantage of the high-voltage motor is that it has high power and strong impact resistance; the disadvantage is that the inertia is large, and it is difficult to start and brake.
The rated voltage of the DC motor should also match the power supply voltage. Generally 110V, 220V and 440V. Among them, 220V is a common voltage level, and high-power motors can be increased to 600-1000V. When the AC power supply is 380V and the three-phase bridge thyristor rectifier circuit is used for power supply, the rated voltage of the DC motor should be 440V. When the three-phase half-wave thyristor rectifier power supply is used for power supply, the rated voltage of the DC motor should be 220V.

4. Rated speed

The rated speed of the motor refers to the speed in the rated working mode.
The motor and the working machinery driven by it have their own rated speed. When choosing the speed of the motor, it should be noted that the speed should not be too low, because the lower the rated speed of the motor, the more series, the larger the volume, and the higher the price; at the same time, the speed of the motor should not be too high. high, as this would make the transmission too complex and difficult to maintain.
In addition , when the power is constant, the motor torque is inversely proportional to the speed .
Therefore, those with low requirements for starting and braking can comprehensively compare several different rated speeds from the aspects of initial investment, floor space and maintenance costs of the equipment, and finally determine the rated speed; while frequent starting, braking and reversing, However, if the duration of the transition process has little effect on the productivity, in addition to considering the initial investment, the speed ratio and the rated speed of the motor are mainly selected on the condition of the minimum amount of loss in the transition process. For example, the hoist motor needs frequent forward and reverse rotation and has a large torque, so the speed is very low, the motor is bulky and expensive.
When the motor speed is high, the critical speed of the motor also needs to be considered The rotor of the motor will vibrate during operation, and the amplitude of the rotor increases with the increase of the speed. When it reaches a certain speed, the amplitude reaches the maximum value (that is, the resonance is usually called). After this speed, the amplitude increases with the speed. It gradually decreases and stabilizes within a certain range. The rotational speed with the largest rotor amplitude is called the critical rotational speed of the rotor.
This rotational speed is equal to the natural frequency of the rotor. When the speed continues to increase, the amplitude will increase again when it is close to 2 times the natural frequency. When the speed is equal to 2 times the natural frequency, it is called the second-order critical speed, and by analogy, there are third-order and fourth-order critical speeds. If the rotor runs at a critical speed, severe vibration will occur, and the bending degree of the shaft will increase significantly. Long-term operation will also cause serious bending deformation of the shaft, or even breakage.
The first-order critical speed of the motor is generally above 1500 rpm, so conventional low-speed motors generally do not consider the influence of the critical speed. On the other hand, for a 2-pole high-speed motor, the rated speed is close to 3000 rpm, and this influence needs to be considered, and it is necessary to avoid using the motor in the critical speed range for a long time.
Generally speaking, the motor can be roughly determined by providing the type of load driven, the rated power, rated voltage, and rated speed of the motor. However, these basic parameters are not enough if the load requirements are to be optimally met. Parameters that also need to be provided include: frequency, working system, overload requirements, insulation class, protection class, moment of inertia, load resistance torque curve, installation method, ambient temperature, altitude, outdoor requirements, etc., which are provided according to specific conditions.