Four steps for motor selection

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　I. Type of load driven This needs to be explained by referring to the characteristics of the motor. Electric motors can be simply divided into DC motors and AC motors, with AC motors further classified into synchronous motors and asynchronous motors. 　　1. DC Motor The advantage of a DC motor is that its speed can be easily adjusted by changing the voltage, and it is capable of generating high torque. Suitable for loads that require frequent adjustment of speed, such as rolling mills in steel plants and hoists in mines. But now, with the development of frequency conversion technology, AC motors can also have their speed adjusted by changing the frequency. However, although variable-frequency motors are not much more expensive than ordinary motors, the price of the inverter accounts for a large portion of the total cost of the entire system; therefore, another advantage of DC motors is their lower cost. 　　The disadvantage of DC motors is their complex structure; any device with a complex structure inevitably has an increased failure rate. Compared to AC motors, DC motors have more complex windings (excitation windings, commutator windings, compensation windings, armature windings), in addition to which they also include slip rings, brushes, and commutators. It not only requires high standards in terms of manufacturing processes but also incurs relatively high maintenance costs later on. Therefore, DC motors find themselves in an awkward position in industrial applications: they are gradually falling out of use, but still have a role to play during the transition period. If the user has sufficient funds, it is recommended to opt for a solution that uses an AC motor paired with an inverter; after all, using an inverter brings many benefits, which need not be elaborated on here. 　　2. Asynchronous motor The advantages of asynchronous motors include simple structure, stable performance, easy maintenance, and low cost. Moreover, its manufacturing process is also the simplest. I once heard from an experienced technician in the workshop that the time required to assemble one DC motor is enough to assemble two synchronous motors or four asynchronous motors with similar power levels, which illustrates this point. Therefore, asynchronous motors are most widely used in industry. 　　Asynchronous motors are further divided into squirrel-cage motors and wound-rotor motors, with the difference lying in the rotor. The rotor of a squirrel-cage motor is made of metal strips, either copper or aluminum. Aluminum is relatively inexpensive, and China is a major producer of aluminum ore; it is widely used in applications where high requirements are not necessary. However, copper has better mechanical and electrical conductivity than aluminum; in most of the cases I’ve encountered, the rotors are made of copper. After the issue of broken windings was resolved in terms of manufacturing, cage-type motors exhibit far greater reliability than motors with wound rotors. Its drawback is that the metal rotor generates a low torque by cutting magnetic flux lines in the rotating stator magnetic field, and it has a high starting current, making it unsuitable for loads that require a high starting torque. Although increasing the length of the motor core can yield more torque, the improvement is quite limited. During startup, a wound-rotor motor supplies electricity to the rotor windings through slip rings, creating a rotor magnetic field that moves relative to the rotating stator magnetic field, thereby generating greater torque. Moreover, a water resistor is connected in series during startup to reduce the starting current; this water resistor is controlled by a sophisticated electronic control unit that adjusts its resistance value as the startup process progresses. Suitable for loads such as rolling mills and hoists. Due to the addition of components such as slip rings and water resistors in wound-rotor asynchronous motors compared to squirrel-cage motors, the overall cost of these devices is somewhat higher. Compared to DC motors, it has a narrower speed control range and relatively lower torque, resulting in a lower value as well. 　　However, since an asynchronous motor generates a rotating magnetic field by energizing its stator windings, and these windings are inductive components that do not perform work, it must draw reactive power from the power grid, causing significant strain on the grid. It is intuitive to see that when high-power inductive devices are connected to the power grid, the grid voltage drops, causing the brightness of the lights to decrease immediately. Therefore, the power supply authorities impose restrictions on the use of asynchronous motors, and this is something that many factories must take into account. Some large electricity consumers, such as steel mills and aluminum plants, choose to build their own power plants and create independent power grids, in order to bypass the restrictions on the use of asynchronous motors. Therefore, if asynchronous motors are to be used for high-power loads, they need to be equipped with reactive power compensation devices, whereas synchronous motors can supply reactive power to the power grid through their excitation systems. The greater the power level, the more significant the advantages of synchronous motors become, which is why they play such an important role. 　　3. Synchronous motor The advantages of synchronous motors include, in addition to the ability to compensate for reactive power in an over-excited state, 1) the fact that their speed adheres strictly to the formula n=60f/p, allowing for precise control of the speed ； 2) It has high operational stability; when the grid voltage drops suddenly, its excitation system typically provides forced excitation to ensure stable operation of the motor, whereas the torque of an asynchronous motor (which is proportional to the square of the voltage) decreases significantly ； 3) The overload capacity is greater than that of the corresponding asynchronous motor ； 4) High operating efficiency, especially for low-speed synchronous motors. 　　Synchronous motors cannot be started directly; they require asynchronous starting or variable-frequency starting. Asynchronous starting refers to a method in which a synchronous motor is equipped with a starting winding on its rotor that resembles the cage winding of an asynchronous motor. An additional resistor, whose value is approximately 10 times that of the resistance of the excitation winding, is connected in series in the excitation circuit to form a closed loop. The stator of the synchronous motor is then connected directly to the power grid, allowing it to start in a manner similar to that of an asynchronous motor. Once the speed reaches sub-synchronous levels (95%), the additional resistor is removed ； Variable-frequency starting requires no further explanation. Therefore, one of the disadvantages of synchronous motors is the need for additional equipment to facilitate starting. 　　A synchronous motor operates relying on an excitation current; without excitation, the motor becomes 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 synchronization and be unable to adjust, which triggers the protection mechanism for an \"excitation fault\" and causes the motor to trip. So the second drawback of synchronous motors is the need for an excitation device; previously, this was provided directly by a DC motor, but now it is mostly supplied by thyristor rectifiers. As the old saying goes, the more complex the structure and the more equipment there is, the more failure points there are, and the higher the failure rate. 　　(Reference materials on synchronous motors: Baidu Wenku > Professional Materials > Engineering Technology > Electricity/Water Resources, “Characteristics of Synchronous Motors”) Based on the performance characteristics of synchronous motors, they are primarily used in applications such as elevators, grinders, fans, compressors, rolling mills, and water pumps. 　　In summary, the principle for selecting a motor is to give priority to those with a simple structure, low cost, reliable operation, and easy maintenance, provided that their performance meets the requirements of the manufacturing machinery. In this regard, AC motors are superior to DC motors; AC asynchronous motors are better than AC synchronous motors; and squirrel-cage asynchronous motors are superior to wound-rotor asynchronous motors. For production machinery that operates continuously with a stable load and no special requirements regarding starting and braking, ordinary squirrel-cage asynchronous motors are the preferred choice; they are widely used in machinery, water pumps, fans, and other applications. 　　For production machinery that requires frequent starting and braking and thus needs a high starting and braking torque, such as bridge cranes, mine hoists, air compressors, and irreversible rolling mills, wound-rotor asynchronous motors should be used. 　　In applications where there is no requirement for speed control, and a constant speed is needed or the power factor needs to be improved, synchronous motors should be used, such as in medium and large-capacity water pumps, air compressors, elevators, grinders, etc. 　　For production machinery that requires a speed control range of more than 1∶3, as well as continuous, stable, and smooth speed regulation, separately-excited DC motors, or squirrel-cage asynchronous motors or synchronous motors with frequency conversion speed control, are suitable options; examples include large precision machine tools, gantry planers, rolling mills, hoists, etc. 　　Production machinery that requires high starting torque and soft mechanical characteristics uses series-wound or compound-wound DC motors, such as trams, locomotives, and heavy cranes. 　　II. Rated Power The rated power of a motor refers to the output power, that is, the shaft power; it is also known as the capacity, and constitutes a key parameter of the motor. People often ask about the size of a motor; generally, this does not refer to the motor’s physical dimensions, but rather to its rated power. It is the most important indicator for quantifying a motor’s ability to drive loads, and it is also a parameter that must be provided when selecting a motor. 　　(Where is the rated power, is the rated voltage, is the rated current, cosθ is the power factor, and η is the efficiency.) The principle for correctly selecting the capacity of a motor is to determine the motor’s power in the most economical and reasonable way, provided that the motor can meet the requirements of the mechanical load it is intended to drive. If the power rating is set too high, it increases the equipment investment, leading to waste; moreover, the motor often operates under partial load, resulting in lower efficiency and a lower power factor for the AC motor ； Conversely, if the power is chosen to be too low, the motor will operate under overload, leading to premature damage of the motor. 　　There are three factors that determine the main power of an electric motor: 1) The heating and temperature rise of the motor, which is the most important factor in determining its power ； 2) Allowable short-term overload capacity ； 3) For asynchronous squirrel-cage motors, the starting capacity also needs to be considered. 　　First, for the specific manufacturing machinery, the load power is calculated and selected based on its heat generation, temperature rise, and load requirements; thereafter, the motor has its rated power predetermined according to the load power, operating mode, and overload requirements. After the rated power of the motor is selected, it is also necessary to verify its heat generation, overload capacity, and starting capacity if required. If *** one item fails, the motor must be selected again and rechecked until all items pass. Therefore, the working schedule is also one of the required specifications; if not specified, it will be handled according to the standard S1 working schedule by default ； Motors with overload requirements also need to specify the overload factor and the corresponding operating time ； When an asynchronous squirrel-cage motor is used to drive loads with a large moment of inertia such as fans, it is also necessary to provide curves showing the load’s moment of inertia and starting torque in order to assess its starting capacity. 　　The above selection of rated power is based on a standard ambient temperature of 40°C. If the ambient temperature in which the motor operates changes, the motor’s rated power must be adjusted. Based on theoretical calculations and practical experience, the power of the motor can be roughly increased or decreased according to the table below, depending on the ambient temperature. 　　Therefore, in areas with harsh climates, the ambient temperature also needs to be taken into account; for example, in India, the ambient temperature must be considered at 50°C for calibration purposes. Furthermore, high altitude also has an impact on the motor’s power; the higher the altitude, the greater the temperature rise of the motor and the lower its output power. Moreover, motors used at high altitudes also need to take into account the effects of corona discharge. 　　Regarding the power range of motors currently available on the market, I would like to provide the performance data from my company for your reference. 　　DC motor: ZD9350 (grinder), 9350 kW. Asynchronous motor: Squirrel-cage type YGF1120-4 (blast furnace fan), 28000 kW; Wound-type YRKK1000-6 (raw material grinder), 7400 kW. Synchronous motor: TWS36000-4 (blast furnace fan), 36000 kW (the test unit can reach 40000 kW). III. Rated voltage The rated voltage of a motor refers to the line voltage under normal operating conditions. 　　The selection of the motor’s rated voltage depends on the supply voltage provided by the power system to the enterprise and the size of the motor’s capacity. 　　The selection of the voltage class for AC motors is primarily determined by the voltage class of the power supply at the location of use. Typically, the low-voltage grid voltage is 380V; therefore, the rated voltages are 380V (Y or Δ connection), 220/380V (Δ/Y connection), and 380/660V (Δ/Y connection). When the power of low-voltage motors increases to a certain level (such as 300KW/380V), it becomes difficult to increase the current due to the limitations imposed by the capacity of the wires, or the cost becomes too high. High power output requires increasing the voltage. The supply voltage in high-voltage power grids is generally 6000V or 10000V; abroad, voltage levels of 3300V, 6600V, and 11000V are also used. The advantage of high-voltage motors is their high power output and strong ability to withstand shocks ； The downside is high inertia, making starting and braking difficult. 　　The rated voltage of the DC motor must also match the supply voltage. It is generally 110V, 220V, and 440V. Among them, 220V is the common voltage level, while high-power motors can operate at 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 supply is used, the rated voltage of the DC motor should be 220V. 　　IV. Rated speed The rated speed of a motor refers to the speed it operates at under rated operating conditions. 　　Both the electric motor and the working machine it drives have their own rated speeds. When selecting the speed of an electric motor, it should be noted that the speed should not be set too low, as the lower the rated speed of the motor, the more stages it will have, resulting in a larger size and a higher price ； At the same time, the speed of the motor should not be set too high, as this will make the transmission mechanism overly complex and difficult to maintain. 　　Furthermore, when the power is constant, the motor torque is inversely proportional to the speed. 　　Therefore, for applications where high starting and braking requirements are not necessary, a comprehensive comparison can be made across several different rated speeds in terms of initial equipment investment, floor space required, and maintenance costs, before finally deciding on the appropriate rated speed ； For those that are frequently started, stopped, and reversed, but for which the duration of the transition process has little impact on productivity, in addition to considering the initial investment, the speed ratio and the rated speed of the motor are chosen primarily with the aim of minimizing losses during the transition process. For example, hoist motors require frequent forward and reverse rotation, deliver high torque, operate at low speeds, are large in size, and are expensive. 　　When the motor speed is high, the motor’s critical speed also needs to be taken into consideration. The motor rotor vibrates during operation, and the amplitude of these vibrations increases as the speed increases. At a certain speed, the amplitude reaches its maximum value (what is commonly referred to as resonance). Beyond this speed, the amplitude gradually decreases as the speed keeps rising, and it stabilizes within a certain range. The speed at which the rotor amplitude is at its maximum is known as the rotor’s critical speed. This rotational speed is equal to the natural frequency of the rotor. As the rotational speed continues to increase and approaches 2 times the natural frequency, the amplitude increases again. When the rotational speed equals 2 times the natural frequency, it is referred to as the second-order critical speed; similarly, there are third-order, fourth-order, and so on critical speeds. If the rotor operates at its critical speed, severe vibrations will occur, and the bending of the shaft will increase significantly. Prolonged operation can lead to serious bending deformation of the shaft, or even its breakage. The first-order critical speed of motors is generally above 1500 revolutions per minute; therefore, the effect of critical speed is usually not taken into account in conventional low-speed motors. Conversely, for 2-pole high-speed motors with a rated speed approaching 3,000 rpm, this effect must be taken into account; it is necessary to prevent the motor from operating for extended periods within the critical speed range. 　　Generally speaking, by providing the type of load driven, the motor’s rated power, rated voltage, and rated speed, it is possible to determine the motor roughly. But to optimize the fulfillment of load requirements, these basic parameters are far from sufficient. Other parameters that need to be provided include: frequency, operating mode, overload requirements, insulation class, protection rating, moment of inertia, load torque curve, installation method, ambient temperature, altitude, outdoor requirements, etc., to be specified according to the specific circumstances.