The basic contents required for motor selection are driven load type, rated power, rated voltage, rated speed, and other conditions.
1. Driven load type
This should be discussed based on the motor characteristics. Motors can be divided into DC motors and AC motors, and AC motors are divided into synchronous motors and asynchronous motors.
1. DC motor
Advantages of DC motor
It is capable of adjusting its speed easily by changing the voltage and providing a large torque. This makes it suitable for loads that require frequent speed adjustments, such as rolling mills in steel mills and hoists in mines. With the advancement of frequency conversion technology, AC motors can now also adjust their speed by changing the frequency. However, despite the fact that the price of variable frequency motors is not much higher than that of ordinary motors, the cost of frequency converters makes up a significant portion of the entire equipment set. Consequently, DC motors still have the advantage of being inexpensive.
Disadvantages of DC motors
Complex structures: Any equipment with a complex structure will inevitably lead to an increase in failure rate. DC motors, in comparison with AC motors, have complex windings (excitation windings, commutation windings, compensation windings, armature windings), as well as slip rings, brushes, and commutators.
Not only does it have high process requirements for manufacturers, but the later maintenance costs are also relatively high. Therefore, DC motors are in a challenging situation in industrial applications. They are gradually declining but still have a place in the transition stage. If the user has sufficient funds, it is recommended to choose an AC motor with a frequency converter. After all, using a frequency converter also brings many benefits, which I will not elaborate on.
2. Asynchronous motor
Advantages of asynchronous motor
The advantages of asynchronous motors lie in their simple structure, stable performance, easy maintenance, and low price. Additionally, the manufacturing process for these motors is also uncomplicated. An experienced technician in the workshop once mentioned that the time taken to assemble a DC motor can complete two synchronous motors or four asynchronous motors of similar power. Asynchronous motors, especially squirrel cage and winding motors, are widely used in the industry due to their reliability and efficiency.
The rotor of a squirrel cage motor is typically made of metal bars, often using copper or aluminum. While aluminum is more affordable and widely used in applications with lower requirements, copper offers superior mechanical and electrical properties. For this reason, copper rotors are commonly used. Furthermore, the reliability of squirrel cage motors surpasses that of motors with winding rotors, making them a popular choice in various industrial applications.
Disadvantages of asynchronous motor
The torque generated by the metal rotor, as it cuts through the magnetic flux lines in the rotating stator magnetic field, is relatively low, and the initial current required is high, making it challenging to handle applications with substantial starting torque needs. While increasing the length of the motor core can produce more torque, the increase is quite limited.
During the start-up of the wound-rotor motor, the rotor winding is energized through the slip ring to create a rotor magnetic field. This field moves in relation to the rotating stator magnetic field, resulting in a larger torque. Additionally, a water resistor is connected in series during the starting process to reduce the starting current. The water resistor is regulated by a sophisticated electronic control device that adjusts the resistance value throughout the starting cnc manufacturing process.
This type of motor is suitable for applications such as rolling mills and hoists. As the wound-rotor asynchronous motor has more slip rings and water resistors than the squirrel cage motor, the overall equipment cost has increased to some extent. Compared to a DC motor, its speed regulation range is relatively narrow, and the torque and the corresponding values are also low.
When an asynchronous motor is used, it energizes the stator winding to create a rotating magnetic field. However, since the winding is an inductive element that does not perform work, it needs to absorb reactive power from the power grid. This can have a significant impact on the power grid, leading to a drop in grid voltage and decreased brightness of lights when high-power inductive appliances are connected.
As a result, power supply authorities may impose restrictions on the use of asynchronous motors, which is a concern for many factories. Some large electricity users, such as steel and aluminum mills, have opted to build their own power plants to create independent power grids and reduce these restrictions. Asynchronous motors must be equipped with reactive power compensation devices to handle high-power loads. In contrast, synchronous motors can provide reactive power to the power grid through their excitation devices. This makes synchronous motors advantageous, especially for high-power applications and has led to their widespread use.
3. Synchronous motor
In addition to the ability to compensate for reactive power in an overexcited state, the advantages of synchronous motors also include:1) The speed of the synchronous motor strictly complies with n=60f/p, and the speed can be accurately controlled;
2) High operating stability. When the grid voltage suddenly drops, its excitation system will generally force excitation to ensure stable motor operation, while the torque of the asynchronous motor (proportional to the square of the voltage) will drop significantly;
3) The overload capacity is greater than that of the corresponding asynchronous motor;
4) High operating efficiency, especially for low-speed synchronous motors.
The starting of synchronous motors requires either asynchronous starting or variable frequency starting. Asynchronous starting involves equipping the synchronous motor with a starting winding on the rotor, similar to the cage winding of an asynchronous motor. Additionally, an extra resistor, about 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 directly connected to the power grid to start it as an asynchronous motor. Once the speed reaches the subsynchronous speed (95%), the additional resistor is disconnected. Variable frequency starting is not detailed in this text. One of the drawbacks of synchronous motors is that additional equipment is required for starting.
Synchronous motors rely on excitation current for operation. Without excitation, the motor behaves like an asynchronous motor. Excitation is a DC system added to the rotor, with a rotation speed and polarity that are consistent with the stator. If there is an issue with the excitation, the motor will lose step and cannot be adjusted, leading to the protection mechanism triggering a "excitation fault" motor trip.
Therefore, the second disadvantage of synchronous motors is the need to add excitation devices. In the past, excitation was directly supplied by DC machines, but now it is mostly supplied by thyristor rectifiers. As the saying goes, the more complex the structure and the more equipment, the more potential fault points and the higher the failure rate.
According to the performance characteristics of synchronous motors, they are primarily used in loads such as hoists, mills, fans, compressors, rolling mills, and water pumps.
In summary, when selecting motors, it is preferred to choose motors with a simple structure, low price, reliable operation, and convenient maintenance, provided that the motor performance meets the requirements of production machinery. AC motors are better than DC motors, AC asynchronous motors are better than AC synchronous motors, and squirrel cage asynchronous motors are better than winding asynchronous motors.
For continuous production machinery with stable loads and no special requirements for starting and braking, ordinary squirrel cage asynchronous motors should be preferred, as they are widely used in custom precision machining, water pumps, fans, etc.
For production machinery with frequent starting and braking and requiring large starting and braking torques, such as bridge cranes, mine hoists, air compressors, irreversible steel rolling mills, etc., winding asynchronous motors should be used.
Synchronous motors are suitable for situations where there is no speed regulation requirement, constant speed is necessary, or the power factor needs to be improved, 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 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.
For production machinery that requires a large starting torque and soft mechanical characteristics, series-excited or compound-excited DC motors should be used, such as trams, electric locomotives, heavy cranes, etc.
2. Rated power
The rated power of a motor refers to its output power, also known as shaft power or capacity. When people ask how big a motor is, they are generally referring to its rated power rather than its physical size. The rated power is the most important indicator for quantifying the load-carrying capacity of the motor and is a necessary parameter for motor selection.
In addition to rated power, other important parameters for motor selection include rated voltage, rated current, power factor (cosθ), and efficiency (η).
Choosing the right motor capacity involves determining the most economical and reasonable power for the motor to meet the requirements of the production precision machinery load. Selecting a motor with too much power can lead to increased equipment investment and wasted energy, as the motor may run at low efficiency and power factor. On the other hand, choosing a motor with too little power can cause it to run overloaded, potentially leading to premature damage.
There are three factors that determine the main power of the motor: 1) The heat and temperature rise of the motor, which is the most important factor in determining the power of the motor; 2) Allowable short-term overload capacity; 3) For asynchronous squirrel cage motors, starting capacity must also be considered.
The specific production machinery initially calculates and selects the load power based on heat generation, temperature rise, and load requirements. The motor then pre-selects the rated power according to the load power, working system, and overload requirements. Once the rated power of the motor is pre-selected, verification of heat generation, overload capacity, and starting capacity (if necessary) must be conducted.
If any of these criteria are not met, the motor must be re-selected and verified until all the requirements are satisfied. The working system is an essential consideration in this process. In the absence of specific requirements, the standard S1 working system is used by default. Motors with overload requirements should also provide overload multiples and corresponding running times.
For asynchronous squirrel cage motors driving large rotational inertia loads such as fans, verification of the rotational inertia of the load and the starting resistance torque curve is also required to ensure starting capacity. The selection of rated power is based on a standard ambient temperature of 40°C.
If the ambient temperature changes, the rated power of the motor must be adjusted. The theoretical calculations and practice suggest that the motor power can be increased or decreased based on the following table when the ambient temperature is different.
Therefore, it is important to maintain suitable ambient temperatures, especially in harsh climates. For example, in India, the ambient temperature needs to be calibrated at 50°C. Additionally, high altitude will affect the motor power, with higher altitudes resulting in greater motor temperature rise and reduced output power. Motors used at high altitudes must also consider the impact of the corona phenomenon.
For the power range of motors currently on the market, here are a few data for reference.DC motor: ZD9350 (mill) 9350kWAsynchronous motor: squirrel cage type YGF1120-4 (blast furnace fan) 28000kWWound type YRKK1000-6 (raw mill) 7400kWSynchronous motor: TWS36000-4 (blast furnace fan) 36000kW (test unit reaches 40000kW)
3. Rated voltage
The rated voltage of a motor is the line voltage when it's operating under its rated working mode. The selection of the rated voltage depends on the power supply voltage of the enterprise's power system and the motor's capacity. For AC motors, the voltage level is mainly determined by the power supply voltage of the location where it will be used.
Typically, the low voltage network is 380V, which corresponds to rated voltages of 380V for Y or △ connection, 220/380V for △/Y connection, and 380/660V for △/Y connection. When the power of a low-voltage motor, such as 300KW/380V, increases to a certain extent, the current is limited by the wire's carrying capacity, making it difficult and costly to further increase.
To achieve higher power output, the voltage must be increased. High-voltage power grids generally have a power supply voltage of 6000V or other specific voltage levels, such as 3300V, 6600V, or 10000V. High-voltage motors offer advantages such as high power and strong impact resistance, but they also have disadvantages like large inertia and challenging starting and braking.
For DC motors, the rated voltage must match the power supply voltage, typically 110V, 220V, and 440V. Among these, 220V is the commonly used voltage level, and high-power motors can have a rated voltage of 600-1000V. When the AC power supply is 380V and powered by a three-phase bridge thyristor rectifier circuit, the rated voltage should be 440V. When powered by a three-phase half-wave thyristor rectifier power supply, the rated voltage should be 220V.
4. Rated speed
The rated speed of the motor refers to the speed under the rated working mode.The motor and the working machine 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 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 too high, because this 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 who have low requirements for starting and braking can make a comprehensive comparison of several different rated speeds from the aspects of initial investment, floor space, and maintenance costs, and finally determine the rated speed; and those who often start, brake and reverse, but the duration of the transition process has little effect on productivity, in addition to considering the initial investment, mainly select the speed ratio and the rated speed of the motor based on the minimum loss of the transition process. For example, the hoist motor hollow shaft needs frequent forward and reverse rotation and large torque, so the speed is very low, and the motor is large and expensive. When the motor speed is high, the critical speed of the motor must also be considered. The motor rotor will vibrate during operation. The amplitude of the rotor increases with the increase in speed. When it reaches a certain speed, the amplitude reaches the maximum value (which is usually called resonance). After exceeding this speed, the amplitude gradually decreases with the increase in speed and stabilizes within a certain range. The speed with the maximum 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 and approaches two times the natural frequency, the amplitude will increase again. When the speed is equal to 2 times the natural frequency, it is called the second-order critical speed. By analogy, there are critical speeds for the third-order and fourth-order orders. If the rotor runs at a critical speed, it will vibrate violently, and the bending of the shaft will increase significantly. The long-term operation will also cause serious bending and 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 contrary, for 2-pole high-speed motors, the rated speed is close to 3000 rpm, so this influence needs to be considered, and the motor should be avoided from being used in the critical speed range for a long time.
Generally speaking, the motor can be determined by providing the driven load type, rated power, rated voltage, and rated speed of the motor. However, if you want to optimize the load requirements, more than these basic parameters are needed. Other 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., which should be provided according to specific circumstances.
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