The basic information required for motor selection includes: the type of load being driven, rated power, rated voltage, rated speed, and other conditions.
I. Type of load being driven
This needs to be explained from the perspective of the characteristics of motors. 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 motor
The advantages of DC motors are that their speed can be easily adjusted by changing the voltage, and they can provide a large torque. They are suitable for loads that require frequent speed adjustments, such as rolling mills in steel plants and hoists in mines. However, with the development of frequency conversion technology, AC motors can also achieve speed adjustment by changing the frequency. Although the price of a frequency conversion motor is not much higher than that of a regular motor, the price of the frequency converter accounts for a major portion of the overall equipment cost, so DC motors also have the advantage of being cheaper.
The disadvantage of DC motors lies in their complex structure. Any device with a complex structure inevitably experiences an increased failure rate. Compared to AC motors, DC motors, besides having more complex windings (excitation winding, commutating pole winding, compensating winding, armature winding), also incorporate slip rings, brushes, and a commutator. This not only places higher demands on the manufacturer's manufacturing processes but also results in relatively higher maintenance costs. Therefore, DC motors occupy an awkward position in industrial applications, gradually declining but still having a place in the transitional phase. If the user has sufficient funds, it is recommended to choose an AC motor paired with a frequency converter, as using a frequency converter brings many advantages, which will not be elaborated upon here.
2. Asynchronous motor
The advantages of asynchronous motors lie in their simple structure, stable performance, convenient maintenance, and low price. Furthermore, their manufacturing process is the simplest. I once heard an experienced technician in the workshop say that the man-hours required to assemble one DC motor could equal those required to assemble two synchronous motors or four asynchronous motors of roughly the same power, which speaks volumes. Therefore, asynchronous motors have found the most widespread application in industry.
Asynchronous motors are further divided into squirrel-cage motors and wound-rotor motors, the difference being in the rotor. Squirrel-cage motor rotors are made of metal strips, either copper or aluminum. Aluminum is relatively inexpensive, and my country is a major aluminum-producing country, so it is widely used in applications with less stringent requirements. However, copper has better mechanical and electrical properties than aluminum, and most of the motors I've encountered have copper rotors.
After solving the problem of broken rotor windings in the manufacturing process, squirrel-cage motors have a much higher reliability than wound rotor motors. However, their disadvantages are that the torque obtained by the metal rotor cutting magnetic lines of force in the rotating stator magnetic field is relatively small, and the starting current is relatively large, making them unsuitable for loads requiring high starting torque.
While increasing the length of the motor core can yield more torque, the effect is quite limited. Wound-rotor motors, during startup, energize the rotor windings via slip rings, creating a rotor magnetic field that moves relative to the rotating stator magnetic field, thus generating greater torque. Furthermore, a water resistor is connected in series during startup to reduce the starting current; the water resistor's resistance is controlled by a sophisticated electronic control device, changing its value as the startup process progresses. This makes them suitable for loads such as rolling mills and hoists. Because wound-rotor asynchronous motors add slip rings and water resistors compared to squirrel-cage motors, the overall equipment price is somewhat higher. Compared to DC motors, their speed range is narrower and their torque is relatively smaller, resulting in a lower overall value.
However, asynchronous motors, by energizing the stator windings to create a rotating magnetic field, and since the windings are inductive elements and do no work, absorb reactive power from the power grid, causing a significant impact on the grid. A direct observation is that when a high-power inductive appliance is connected to the grid, the grid voltage drops, and the brightness of lights decreases sharply. Therefore, power companies impose restrictions on the use of asynchronous motors, a factor that many factories must consider.
Some large electricity consumers, such as steel mills and aluminum plants, choose to build their own power plants to form their own independent power grids, thereby reducing restrictions on the use of asynchronous motors. Therefore, if asynchronous motors are to meet the needs of high-power loads, they need to be equipped with reactive power compensation devices, while synchronous motors can provide reactive power to the grid through excitation devices. The greater the power, the more obvious the advantages of synchronous motors become, thus creating the stage for synchronous motors.
3. Synchronous motor
Besides the ability to compensate for reactive power in overexcitation, the advantages of synchronous motors include: 1) The speed of a synchronous motor strictly follows n=60f/p, allowing for precise speed control; 2) High operational stability, as its excitation system will generally force excitation when the grid voltage suddenly drops, ensuring stable motor operation, while the torque of an asynchronous motor (which is proportional to the square of the voltage) will drop significantly; 3) Greater overload capacity than a corresponding asynchronous motor; 4) High operating efficiency, especially for low-speed synchronous motors.
Synchronous motors cannot be started directly; they require asynchronous or frequency converter starting. Asynchronous starting involves installing a starting winding on the rotor of the synchronous motor, similar to the starting winding of an asynchronous motor's squirrel cage. An additional resistor, approximately 10 times the resistance of the excitation winding, is connected in series in the excitation circuit to form a closed circuit. The stator of the synchronous motor is then directly connected to the power grid, allowing it to start as an asynchronous motor. When the speed reaches sub-synchronous speed (95%), the additional resistor is disconnected. Frequency converter starting will not be discussed in detail. Therefore, one of the disadvantages of synchronous motors is the need for additional equipment for starting.
Synchronous motors operate on excitation current; without excitation, the motor is asynchronous. The excitation is a DC system applied to the rotor, and its rotational speed and polarity are consistent with the stator. If there is a problem with the excitation, the motor will lose synchronization, cannot adjust, and trigger the "excitation fault" protection, causing the motor to trip. Therefore, a second disadvantage of synchronous motors is the need for an additional excitation device. Previously, this was directly supplied by a DC motor, but now it is mostly supplied by a thyristor rectifier. As the saying goes, the more complex the structure and the more devices involved, the more potential points of failure and the higher the failure rate.
Based on the performance characteristics of synchronous motors, their applications are mainly in loads such as hoists, mills, fans, compressors, rolling mills, and water pumps. In summary, the principle for selecting an electric motor is to prioritize motors with simple structure, low price, reliable operation, and easy maintenance, provided that the motor's performance meets the requirements of the production machinery. In this regard, 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-rotor asynchronous motors.
For production machinery operating continuously under stable loads with no special requirements for starting and braking, ordinary squirrel-cage asynchronous motors are preferred. These are widely used in machinery, pumps, and fans. For production machinery with frequent starting and braking, requiring large starting and braking torques, such as bridge cranes, mine hoists, air compressors, and irreversible rolling mills, wound-rotor asynchronous motors should be used. In applications where speed regulation is not required, constant speed is needed, or improved power factor is required, synchronous motors should be used, such as in medium and large capacity pumps, air compressors, hoists, and mills.
For production machinery requiring a speed range of 1:3 or higher and continuous, stable, and smooth speed regulation, separately excited DC motors, or squirrel-cage asynchronous motors or synchronous motors with variable frequency speed control are suitable, such as large precision machine tools, gantry planers, rolling mills, and hoists. For production machinery requiring large starting torque and soft mechanical characteristics, series-wound or compound-wound DC motors are used, such as trams, locomotives, and heavy-duty cranes.
II. Rated Power
The rated power of an electric motor refers to its output power, also known as shaft power or capacity, and is a key parameter of the motor. When people ask about the size of a motor, they are generally referring to its rated power, not its dimensions. It is the most important indicator for quantifying the load-driving capacity of an electric motor and is a crucial parameter that must be provided when selecting a motor.
The principle for correctly selecting the capacity of an electric motor is to determine its power in the most economical and reasonable way, provided that the motor can meet the load requirements of the production machinery. If the power is too large, the equipment investment will increase, resulting in waste, and the motor will often operate under load, leading to lower efficiency and a lower power factor for AC motors. Conversely, if the power is too small, the motor will operate under load, causing premature damage.
There are three main factors that determine the power of an electric motor:
1) The heat generation and temperature rise of the motor are the most important factors that determine the power of the motor; 2) Allowable short-term overload capacity; 3) For asynchronous squirrel-cage motors, starting capacity must also be considered.
First, the specific production machinery is calculated and the load power is selected based on its heat generation, temperature rise, and load requirements. Then, the motor's rated power is pre-selected based on the load power, duty cycle, and overload requirements. After the motor's rated power is pre-selected, its heat generation, overload capacity, and, if necessary, starting capacity must be verified. If any of these fail, a new motor must be selected, and the verification process repeated until all requirements are met. Therefore, the duty cycle is also a necessary requirement; if no specific requirement is specified, the most common S1 duty cycle is used by default. For motors with overload requirements, the overload multiple and corresponding operating time must be provided. When an asynchronous squirrel-cage motor drives a large moment of inertia load such as a fan, a curve showing the load's moment of inertia and starting resistance torque is also required to verify the starting capacity.
The above selection of rated power is based on a standard ambient temperature of 40℃. If the ambient temperature of the motor changes, the rated power must be adjusted. Based on theoretical calculations and practice, the motor power can be roughly increased or decreased according to the table below when the ambient temperature varies. Therefore, for regions with harsh climates, the ambient temperature also needs to be provided; for example, in India, the ambient temperature needs to be checked at 50℃. Furthermore, high altitude also affects motor power; the higher the altitude, the greater the temperature rise of the motor, and the lower the output power. Additionally, the corona effect must be considered for motors used at high altitudes.
Regarding the current power range of electric motors on the market, I have listed the performance data from my company for reference.
DC motor: ZD9350 (mill) 9350kW; Asynchronous motor: Squirrel-cage type YGF1120-4 (blast furnace blower) 28000kW; Wound-rotor type YRKK1000-6 (raw material mill) 7400kW; Synchronous motor: TWS36000-4 (blast furnace blower) 36000kW (test unit reached 40000kW).
III. Rated Voltage
The rated voltage of an electric motor refers to the line voltage under rated operating conditions. The selection of the rated voltage of an electric motor depends on the power supply voltage to the enterprise and the capacity of the motor.
The selection of the voltage level for an AC motor mainly depends on the power supply voltage level of the application environment. Generally, low-voltage grids are 380V, so rated voltages are 380V (Y or Δ connection), 220/380V (Δ/Y connection), and 380/660V (Δ/Y connection). When the power of a low-voltage motor increases to a certain level (e.g., 300KW/380V), the current is limited by the conductor's carrying capacity, making it difficult to increase the output or resulting in excessive cost. It is necessary to increase the voltage to achieve higher power output. High-voltage grids typically supply 6000V or 10000V, although 3300V, 6600V, and 11000V voltage levels are also available internationally. The advantages of high-voltage motors are high power and strong impact resistance; the disadvantages are high inertia and difficulty in starting and braking.
The rated voltage of a DC motor must match the power supply voltage. Commonly used voltages are 110V, 220V, and 440V. 220V is the most common voltage level, but high-power motors can be equipped with 600-1000V. When the AC power supply is 380V and a three-phase bridge thyristor rectifier circuit is used, the rated voltage of the DC motor should be 440V. When a three-phase half-wave thyristor rectifier power supply is used, the rated voltage of the DC motor should be 220V.
IV. Rated Speed
The rated speed of an electric motor refers to its speed under rated operating conditions. Both the electric motor and the machinery it drives have their own rated speeds. When selecting the speed of an electric motor, care should be taken not to choose too low a speed, because the lower the rated speed of the motor, the more poles it has, the larger its size, and the higher its price; at the same time, the speed of the motor should not be chosen too high either, because this will make the transmission mechanism too complex and difficult to maintain.
Furthermore, for a given power, motor torque is inversely proportional to speed. Therefore, for applications with low starting and braking requirements, a comprehensive comparison of several different rated speeds can be made, considering initial investment, floor space, and maintenance costs, to determine the final rated speed. However, for applications with frequent starting, braking, and reversing, but where the duration of the transition process has little impact on productivity, in addition to considering initial investment, the speed ratio and rated motor speed should be selected primarily based on minimizing losses during the transition process. For example, hoist motors require frequent forward and reverse rotation and have high torque, resulting in very low speeds, large motor sizes, and high prices.
When the motor speed is high, the critical speed of the motor must also be considered. The motor rotor vibrates during operation, and the amplitude of the rotor vibration increases with the speed. At a certain speed, the amplitude reaches its maximum value (commonly known as resonance). Beyond this speed, the amplitude gradually decreases with increasing speed and stabilizes within a certain range. This speed at which the rotor amplitude is maximum is called the rotor's critical speed. This speed is equal to the rotor's natural frequency. As the speed continues to increase, approaching twice the natural frequency, the amplitude will increase again. When the speed equals twice the natural frequency, it is called the second-order critical speed, and so on for the third, fourth, and so on. If the rotor operates at the critical speed, it will experience severe vibration, and the shaft bending will increase significantly. Prolonged operation can cause severe bending deformation of the shaft, and even breakage.
The first-order critical speed of an electric motor is generally above 1500 rpm, so the influence of the critical speed is generally not considered for conventional low-speed motors. Conversely, for 2-pole high-speed motors with a rated speed close to 3000 rpm, this influence must be considered, and the motor should be avoided from being used within the critical speed range for extended periods.
Generally, providing the type of load being driven, the motor's rated power, rated voltage, and rated speed is sufficient to roughly determine the type of motor. However, to optimally meet load requirements, these basic parameters are far from enough. Additional parameters include: frequency, duty cycle, overload requirements, insulation class, protection class, moment of inertia, load resistance torque curve, installation method, ambient temperature, altitude, and outdoor requirements, provided according to specific circumstances.
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