The basic contents required for motor selection include: the type of load driven, rated power, rated voltage, rated speed, and other conditions.
1. The type of load drivenThis has to be said in turn from the characteristics of the motor. Motors can be simply divided into DC motors and AC motors, and AC motors are further divided into synchronous motors and asynchronous motors.1. DC motorThe advantage of the DC motor is that the speed can be easily adjusted by changing the voltage, and it can provide a large torque. 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, AC motors can also adjust the speed by changing the frequency. However, although the price of the frequency conversion motor is not much more expensive than that of the ordinary motor, the price of the frequency converter occupies the main part of the whole set of equipment, so another advantage of the DC motor is that it is cheap.The disadvantage of the DC motor is that the structure is complex. As long as the structure of any equipment is complex, it will inevitably lead to an increase in the failure rate. Compared with AC motors, DC motors have more complicated windings (excitation windings, commutation pole windings, compensation windings, armature windings), slip rings, brushes and commutators are also added.Not only does it have high requirements on the manufacturer's workmanship, but also the later maintenance cost is relatively high. Therefore, in industrial applications, DC motors are in an embarrassing situation where they are gradually declining but still useful in the transitional stage. If the user has more 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, so I won’t go into details. 2. Asynchronous motorThe advantages of asynchronous motors are simple structure, stable performance, convenient maintenance and low price. And the manufacturing process is also the simplest. I once heard an old technician in the workshop say that the man-hours required to assemble a DC motor can complete two synchronous motors or four asynchronous motors of similar power, which is evident. Therefore asynchronous motors are the most widely used in industry.Asynchronous motors are further divided into squirrel-cage motors and wound-wire 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 mines, so it is widely used in occasions with low requirements. However, the mechanical properties and electrical conductivity of copper are better than that of aluminum. Most of the rotors I have come into contact with are copper rotors. After the squirrel-cage motor solves the problem of broken rows in technology, its reliability far exceeds that of a motor with a winding rotor.The disadvantage is that the metal rotor cuts the magnetic induction line in the rotating stator magnetic field to obtain a small torque, and the starting current is large, which is difficult for loads that require a large starting torque. Although more torque can be obtained by increasing the length of the motor core, the strength is very limited.When the wound motor is started, the slip ring is used to energize the rotor winding to form a rotor magnetic field, which moves relative to the rotating stator magnetic field, so the torque is greater. And the water resistance is connected in series to reduce the starting current during the starting process, 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 wire-wound asynchronous motor has added slip rings and water resistors, so the overall equipment price has increased to a certain extent. Compared with DC motors, the speed range is narrower and the torque is relatively small, and the corresponding value is also lower.However, the asynchronous motor establishes a rotating magnetic field by energizing the stator winding, and the winding is an inductive element that does not perform work, so it needs to absorb reactive power from the grid, which has a great impact on the grid. Intuitively experience that when high-power inductive appliances are connected to the power grid, the voltage of the power grid will drop, and the brightness of the lights will decrease at once.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 mills, choose to build their own power plants to form their own independent power grids to reduce restrictions on the use of asynchronous motors. Therefore, if the asynchronous motor needs to be used for 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. The greater the power, the more obvious the advantages of the synchronous motor, resulting in synchronous Motor stage.3. Synchronous motorThe advantages of synchronous motors include:1) The speed of the synchronous motor strictly complies with n=60f/p, which can precisely control the speed;2) The operation stability is high. When the grid voltage drops suddenly, the excitation system will generally be forcibly excited to ensure the stable operation of the motor, while the torque of the asynchronous motor (proportional to the square of the voltage) will drop sharply;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 require asynchronous or variable frequency starting. Asynchronous starting means that the synchronous motor is equipped with a starting 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 exciting winding is connected in series in the excitation circuit to form a closed circuit, and the stator of the synchronous motor is directly connected to the Power grid, so that it starts as an asynchronous motor, and when the speed reaches the sub-synchronous speed (95%), the starting method of cutting off the additional resistance; the frequency conversion starting will not be repeated. So one of the disadvantages of synchronous motors is the need to add additional equipment for starting.Synchronous motors run on field current, and if there is no field, the motor is asynchronous. Excitation is a DC system applied to the rotor. Its rotation speed and polarity are consistent with those of the stator. If there is a problem with the excitation, the motor will lose its step and cannot be adjusted. It will trigger the protection "excitation failure" motor trip.Therefore, the second disadvantage of the synchronous motor is the need to increase the excitation device, which was directly supplied by the DC machine in the past, but now mostly supplied by the thyristor rectifier. As the old saying goes, the more complex the structure and the more equipment, the more points of failure and the higher the failure rate.According to the performance characteristics of synchronous motors, its applications are mainly in loads such as hoists, mills, fans, compressors, rolling mills, and water pumps.To sum up, the principle of selecting a motor is that under 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, AC motors are superior to DC motors, AC asynchronous motors are superior to AC synchronous motors, and squirrel cage asynchronous motors are superior to wound asynchronous motors.For production machinery with stable load and no special requirements for starting and braking, the continuous operation of production machinery should preferably use ordinary squirrel-cage asynchronous motors, which are widely used in machinery, water pumps, fans, etc.Starting and braking are relatively frequent, and production machinery that requires a 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 requirement for speed regulation, where constant speed is required or power factor improvement is required, synchronous motors should be used, such as medium and large capacity water pumps, air compressors, elevators, mills, etc.The speed regulation range is required to be above 1:3, and the production machinery that requires continuous, stable and smooth speed regulation should use separately excited DC motors or squirrel-cage asynchronous motors or synchronous motors with frequency conversion speed regulation, such as large precision machine tools, gantry planers, Rolling mills, elevators, etc.For production machinery that requires large starting torque and soft mechanical characteristics, use series-excited or compound-excited DC motors, such as trams, electric locomotives, and heavy cranes.2. Rated powerThe rated power of the motor refers to the output power, that is, the shaft power, also known as the capacity, which is the symbolic parameter of the motor. People often ask how big the motor is. Generally, it does not refer to the size of the motor, but the rated power. It is the most important index to quantify the motor's ability to drag the load, and it is also a parameter requirement that must be provided when selecting the motor.(Rated power, rated voltage, rated current, cosθ is power factor, η is efficiency)The principle of correct selection of motor capacity is to determine the power of the motor most economically and reasonably under the premise that the motor is capable of producing mechanical load requirements. If the power is selected too large, the equipment investment will increase, resulting in waste, and the motor is often under-loaded, and the efficiency and power factor of the AC motor are low; on the contrary, if the power is selected too small, the motor will be overloaded and run, causing the motor to start prematurely. damage.There are three factors that determine the main power of the motor:1) The heating and temperature rise of the motor, which is the most important factor determining 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 the motor pre-selects the rated power according to the load power, working system, and overload requirements. After the rated power of the motor is pre-selected, it is necessary to check the heat generation, overload capacity and starting capacity when necessary.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, it will be processed according to the most conventional S1 working system by default; motors with overload requirements also need to provide overload multiples and corresponding running time; asynchronous squirrel-cage motors drive fans and other large rotations For inertial loads, it is also necessary to provide the moment of inertia of the load and the graph of the starting resistance torque to check the starting ability.The above selection of rated power is carried out under the premise that the standard ambient temperature is 40°C. If the ambient temperature of the motor 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, areas with harsh climates also need to provide ambient temperature. For example, in India, the ambient temperature needs to be checked at 50°C. In addition, high altitude will also affect the power of the motor. The higher the altitude, the greater the temperature rise of the motor and the lower the output power. And motors used at high altitudes also need to consider the influence of corona phenomenon.Regarding the power range of electric motors currently on the market, I would like to list several data for reference.DC motor: ZD9350 (mill) 9350kWAsynchronous motor: squirrel cage YGF1120-4 (blast furnace fan) 28000kWWinding type YRKK1000-6 (raw mill) 7400kWSynchronous motor: TWS36000-4 (blast furnace fan) 36000kW (test unit reaches 40000kW)3. Rated voltageThe rated voltage of the motor refers to the line voltage under the rated working mode.The selection 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 selection of AC motor voltage level is mainly determined by 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), 380/660V (△/Y connection). When the power of the low-voltage motor increases to a certain level (such as 300KW/380V), the current is limited by the bearing capacity of the wire, so it is difficult to increase the size, 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 in foreign countries. The advantages of high-voltage motors are high power and strong impact resistance; the disadvantages are large inertia and difficulty in starting and braking.The rated voltage of the DC motor should also match the power supply voltage. Generally 110V, 220V and 440V. Among them, 220V is the common voltage level, and the high-power motor can be increased to 600-1000V. When the AC power supply is 380V and a three-phase bridge thyristor rectifier circuit is used for power supply, the rated voltage of the DC motor should be 440V. When a 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 speedThe 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 selecting the speed of the motor, it should be noted that the speed should not be selected too low, because the lower the rated speed of the motor, the more stages, the larger the volume, and the higher the price; at the same time, the speed of the motor should not be selected too high. High, as this would make the drive mechanism overly complex and difficult to maintain.In addition, when the power is constant, the motor torque is inversely proportional to the speed.Therefore, those who do not have high requirements for starting and braking can make a comprehensive comparison with several different rated speeds in terms of equipment initial investment, floor space and maintenance costs, and finally determine the rated speed; 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 based on the condition that the loss of the transition process is minimal. For example, the hoist motor requires frequent forward and reverse rotation and a large torque, but 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. The amplitude of the rotor increases with the increase of the speed. Gradually decrease and stabilize within a certain range, the speed at which the rotor amplitude is the largest is called the critical 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. By analogy, there are third-order and fourth-order critical speeds. If the rotor runs at a critical speed, it will vibrate violently, and the curvature of the shaft will increase significantly. Long-term operation will also cause serious bending deformation of the shaft, or even break it.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. Conversely, for a 2-pole high-speed motor with a rated speed close to 3000 rpm, this effect needs to be considered and the motor should not be used 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. But these basic parameters are not enough if the load requirements are to be met optimally. The parameters that need to be provided include: frequency, working system, overload requirements, insulation level, protection level, moment of inertia, load resistance moment curve, installation method, ambient temperature, altitude, outdoor requirements, etc., according to specific conditions.
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