The basic content required for motor selection is: the type of load driven, rated power, rated voltage, rated speed, and other conditions.
The type of load driven
This has to be reversed from the motor characteristics. The motor can be simply divided into DC motor and AC motor, AC is divided into synchronous motor and asynchronous motor.
1. DC motor
The advantage of the DC motor is that it can easily adjust the speed by changing the voltage, and can provide a larger torque. It is suitable for loads that require frequent adjustment of speed, such as rolling mills in steel mills and hoists in mines. 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 frequency conversion motors is not much more expensive than ordinary motors, the price of frequency converters occupies a major part of the entire set of equipment, so DC motors also have the advantage of being cheap.
The disadvantage of DC motors is their complicated structure. As long as the structure of any equipment is complicated, it will inevitably lead to an increase in the failure rate. Compared with AC motors, DC motors have more complicated windings (excitation windings, commutating pole windings, compensation windings, and armature windings), and also include slip rings, brushes, and commutators. Not only the manufacturer’s process requirements are high, but also the later maintenance costs are relatively high. Therefore, in industrial applications, DC motors are in an awkward situation where they are gradually declining, but the transition stage is still useful. If the user’s funds are relatively abundant, it is recommended to choose the solution of AC motor with inverter. After all, the use of inverter also brings many benefits, this will not be elaborated.
2. Asynchronous motor
The advantages of asynchronous motors are simple structure, stable performance, easy maintenance, and low price. And the manufacturing process is also the simplest. The old technician who once heard the workshop said that the time spent in assembling a DC motor can complete two synchronous motors or four asynchronous motors of almost power, which can be seen. Therefore, asynchronous motors are the most widely used in industry.
Asynchronous motors are divided into squirrel-cage motors and wire-wound motors, the difference being the rotor.
Squirrel-cage motor rotors are made of metal bars, copper or aluminum. The price of aluminum is relatively low, and China is also a big country of aluminum ore, which is widely used in occasions where the requirements are not high. But copper’s mechanical properties and electrical conductivity are better than aluminum, so most of my contacts are copper rotors. After the squirrel-cage motor solves the problem of broken row in the process, the reliability is far more than that of the winding-type rotor motor. The disadvantage is that the torque obtained by the metal rotor cutting the magnetic induction wire in the rotating stator magnetic field is small, and the starting current is large, and the load that requires a large starting torque is difficult to handle. Although increasing the length of the motor core can obtain more torque, but the strength is very limited. When the winding motor is started, the rotor winding is energized through the slip ring to form the rotor magnetic field, which moves relative to the rotating stator magnetic field, so the torque is greater. In addition, a water resistance is connected in series during the startup process to reduce the startup current. The water resistance is controlled by a mature electronic control device to change the resistance value with the startup process. Suitable for loads such as rolling mills and hoists. Compared with the squirrel-cage motor, the winding type asynchronous motor has increased slip ring and water resistance, so the overall equipment price has increased. Compared with DC motors, the speed range is narrower and the torque is relatively small, and the corresponding value is also low.
However, the induction motor establishes a rotating magnetic field due to the energization of the stator winding, and the winding is an inductive component that does not do work. It has to absorb reactive power from the grid, which has a great impact on the grid. Intuitive experience: When a high-power inductive electrical appliance is connected to the power grid, the voltage of the power grid drops, and the brightness of the light decreases at a time. Therefore, the power supply bureau will have restrictions on the use of asynchronous motors, which is also a place that many factories must consider. Some large power consumers, such as steel mills and aluminum plants, choose to build their own power plants and form their own independent power grids to reduce or limit the use of asynchronous motors. Therefore, if the asynchronous motor needs to meet the high power load, it needs to be equipped with a reactive power compensation device, and the synchronous motor can provide reactive power to the grid through the excitation device. The greater the power, the more obvious the advantage of the synchronous motor, resulting in synchronization Motor stage.
3. Synchronous motor
The advantages of synchronous motors include not only over-excited state to compensate reactive power, but also 1) the speed of synchronous motor strictly abides by n = 60f / p, which can accurately control the speed; 2) high operating stability, when the grid voltage suddenly drops, its The excitation system will generally force excitation to ensure the stable operation of the motor, and the torque of the asynchronous motor (proportional to the square of the voltage) will be greatly reduced; 3) The overload capacity is greater than the corresponding asynchronous motor; 4) The operating efficiency is high, especially the low-speed synchronous motor .
The synchronous motor cannot be started directly, and it needs asynchronous start or variable frequency start. Asynchronous start means that the synchronous motor is equipped with a starter winding similar to the cage winding of the asynchronous motor on the rotor, and an additional resistance of 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 is started according to the asynchronous motor. When the speed reaches the sub-synchronous speed (95%), the starting method of the additional resistance is cut off; the frequency conversion start is not described in detail. So one of the disadvantages of synchronous motors is the need to add additional equipment for starting.
Synchronous motors run on excitation current. If there is no excitation, the motor is asynchronous. Excitation is a DC system that is added to the rotor. Its rotation speed and polarity are consistent with the stator. If there is a problem with the excitation, the motor will be out of step and cannot be adjusted to trigger the protection “excitation failure” motor trip. Therefore, the second shortcoming of the synchronous motor is the need to add an excitation device, which used to be directly supplied by a DC machine, but now most of them are supplied by thyristors. 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 on loads such as hoist, mill, fan, compressor, rolling mill, water pump and so on.
In summary, the principle of choosing a motor is that the motor performance meets the requirements of the production machinery, and the motor with simple structure, cheap price, reliable work 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 wire-wound asynchronous motors.
If the load is stable and the continuous running production machinery has no special requirements for starting and braking, the ordinary squirrel cage asynchronous motor should be preferred, which is widely used in machinery, water pumps, fans, etc.
Starting and braking are relatively frequent, and production machinery requiring large starting and braking torques, such as bridge cranes, mine hoists, air compressors, and irreversible rolling mills, should use wound asynchronous motors.
When 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, hoists, 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, it is advisable to use a separately excited DC motor or a squirrel-cage asynchronous motor or synchronous motor with frequency conversion speed regulation, such as large precision machine tools, gantry planers Rolling mills, hoists, etc.
Production machinery with large starting torque and soft mechanical characteristics are required to use series or compound excitation DC motors, such as trams, locomotives, and heavy cranes.
Rated power
The rated power of the motor refers to the output power, that is, the shaft power, also known as the size of the capacity, which is the iconic parameter of the motor. It is often asked how big the motor is, generally not referring to the size of the motor, but to the rated power. It is the most important index to quantify the dragging capacity of the motor, and it is also the parameter requirement that must be provided when the motor is selected.
The principle of correctly selecting the capacity of the motor should determine the power of the motor most economically and reasonably under the premise that the motor can meet the requirements of the production mechanical load. If the power is selected too large, the equipment investment increases, causing waste, and the motor is often underloaded, and the efficiency and power factor of the AC motor are low; otherwise, if the power is selected too small, the motor will overload and cause the motor to be premature damage.
There are three factors that determine the main power of the motor:
1) The heat and temperature rise of the motor are the most important factors that determine the power of the motor;
2) Allow short-term overload capability;
3) For asynchronous squirrel-cage motors, the starting capacity must also be considered.
First of all, the specific production machinery calculates and selects the load power according to its heat generation, temperature rise and its load requirements, and 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, the heating, overload capacity and the starting capacity must be verified. If one of them is unqualified, the motor must be reselected and then checked 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 default is the most conventional S1 working system; motors with overload requirements also need to provide overload multiples and corresponding operating times; asynchronous squirrel cage motors drive fans and other large rotation In the case of inertia load, it is also necessary to provide the load inertia and starting resistance torque curve to check the starting ability.
The above selection of rated power is made under the premise that the standard ambient temperature is 40 ° C. If the ambient temperature of the motor changes, the motor’s rated power 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 following table.
Therefore, the ambient temperature also needs to provide the ambient temperature, for example, India, the ambient temperature needs to be checked at 50 ℃. 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 effect of corona phenomenon.
For the power range of motors currently on the market, I would like to list the data of the company’s performance table for my 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)
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 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 method), 220 / 380V (△ / Y connection method), 380 / 660V (△ / Y connection method). The power of the low-voltage motor is increased to a certain degree (such as 300KW / 380V), and the current is limited by the bearing capacity of the wire and it is difficult to make it large, or the cost is too high. Need to increase the voltage to achieve high power output. The supply voltage of the high-voltage power grid is generally 6000V or 10000V, and there are also voltage levels of 3300V, 6600V and 11000V abroad. The advantages of high-voltage motors are 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 be matched with the power supply voltage. Generally 110V, 220V and 440V. Among them, 220V is a common voltage level, and the high-power motor 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, the rated voltage of the DC motor should be 220V.
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 the number of stages, the larger the volume, and the higher the price; at the same time, the speed of the motor is also not suitable. High, because it will make the transmission mechanism too complicated and difficult to maintain.
In addition, when the power is constant, the motor torque is inversely proportional to the speed.
Therefore, those with low starting and braking requirements can make a comprehensive comparison with several different rated speeds from the aspects of initial investment of equipment, floor space and maintenance costs, and finally determine the rated speed; and often start, brake and reverse, However, if the duration of the transition process has little effect on productivity, in addition to considering the initial investment, the speed ratio and the rated motor speed are mainly selected on the condition that the amount of loss in the transition process is minimum. For example, the hoist motor needs frequent forward and reverse rotation and the torque is very large, the speed is very low, the motor is bulky and expensive.
When the motor speed is higher, 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 the speed reaches a certain speed, the amplitude reaches the maximum value (also known as resonance). After exceeding this speed, the amplitude increases with the speed It gradually decreases and stabilizes within a certain range. The speed with the largest rotor amplitude is called the critical speed of the rotor. This speed is equal to the natural frequency of the rotor. When the speed continues to increase, the amplitude will increase when it approaches 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 in turn there are third-order, fourth-order and other critical speeds. If the rotor is operated at a critical speed, severe vibration will occur, and the bending of the shaft will increase significantly. Long-term operation will also cause serious bending deformation of the shaft, or even break. 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 it is necessary to avoid using the motor for a long time in the critical speed range.
In general, the type of load, the rated power of the motor, the rated voltage, and the rated speed of the motor can be roughly determined. But if you want to optimally meet the load requirements, these basic parameters are far from enough. The parameters that need to be provided include: frequency, working system, overload requirements, insulation level, protection level, moment of inertia, load resistance torque curve, installation method, ambient temperature, altitude, outdoor requirements, etc., provided according to specific circumstances.