I. Type of load driven
This should be indicated in reverse order, starting with the engine types.
Motor can be classified into two main categories: DC motor and AC motor. The AC motor can be further divided into two types: synchronous motor and asynchronous motor.
1. DC Motor
The advantage of a DC motor is its ability to easily adjust speed by changing voltage, providing greater torque and making it suitable for loads that require frequent speed adjustments, such as mills in steel mills and winches in mines.
However, with the development of frequency conversion technology, AC motors can now also adjust speed by changing frequency. Although the cost of a variable frequency motor is not significantly higher than a traditional motor, it still represents a significant portion of the total equipment cost, giving DC motors an advantage in being cost-effective.
The main disadvantage of a DC motor lies in its complex structure, which increases the probability of failures. DC motors have complex windings such as excitation winding, commutation pole winding, compensation winding and armature winding, as well as additional components such as slip rings, brushes and commutators. This results in high manufacturing requirements and relatively high maintenance costs.
As a result, DC motors are in decline in industrial applications, but they still have a place in the transition phase. If the user has enough resources, it is recommended to opt for the AC motor with frequency converter scheme as it brings many benefits.
2. Asynchronous motor
The advantages of asynchronous motors include simple structure, stable performance, ease of maintenance and low cost.
Furthermore, the manufacturing process is simple. According to a former workshop technician, the hours of work required to assemble a DC motor can complete the assembly of two synchronous motors or four asynchronous motors of similar power. This is why asynchronous motors are widely used in industry.
Asynchronous motors are divided into squirrel cage motors and wound motors, the main difference being in the rotor. The rotor of a squirrel cage motor is made of strips of metal, such as copper or aluminum.
Aluminum is relatively cheap and widely used in low-demand applications because China is a large aluminum mining country.
Copper, on the other hand, has better mechanical and electrical properties and is more commonly used in rotors. After addressing the issue of line breaking in technology, the reliability of squirrel cage motors has greatly improved and is now better than that of wound rotor motors.
However, squirrel cage motors have a low torque output and a large starting current, making them unsuitable for loads that require high starting torque. Increasing the length of the engine core can increase torque, but the increase is limited.
Conversely, wound motors energize the rotor winding through slip rings to form a rotor magnetic field that moves relative to the stator's rotating magnetic field, resulting in greater torque output.
The water resistance is connected in series to reduce the starting current during the starting process, and the resistance value is controlled by an electrical control device. Wound motors are suitable for applications such as rolling mills and hoists.
Compared to squirrel cage motors, wound asynchronous motors have additional components such as slip rings, water resistance, and electrical resistance, leading to a higher overall equipment cost. Asynchronous motors also have a relatively narrow speed regulation range and low torque compared to DC motors.
However, they have a significant impact on the electrical grid because they require reactive power from the grid to energize the stator winding, which is an inductive element. This can be seen as a drop in mains voltage and a decrease in light brightness when high-power inductive appliances are connected to the mains.
To mitigate this impact, power supply agencies can restrict the use of asynchronous motors. Some large energy users, such as steel and aluminum factories, have their own power plants to form their own independent electrical grids and reduce restrictions on the use of asynchronous motors.
Asynchronous motors need reactive power compensation devices to meet the requirements of high power loads, while synchronous motors can supply reactive power to the grid through excitation devices. The higher the power, the more pronounced the advantages of synchronous motors, leading to a shift to the use of synchronous motors.
3. Synchronous motor
The advantages of synchronous motors include the ability to compensate reactive power in overexcited states, as well as the following:
- Precise speed control as the speed of a synchronous motor is in strict accordance with n = 60f/p.
- High operating stability. In the event of a sudden drop in grid voltage, the synchronous motor's excitation system will generally force the excitation to maintain stability, while an asynchronous motor's torque (proportional to the square of the voltage) would drop significantly.
- Greater overload capacity compared to corresponding asynchronous motors.
- High efficiency, especially for low-speed synchronous motors.
However, synchronous motors cannot be started directly and require asynchronous or variable frequency starting methods. Asynchronous starting involves installing a starting winding similar to a cage winding of an asynchronous motor on the rotor of a synchronous motor and connecting additional resistance (about 10 times the resistance value of the excitation winding) to the starting circuit. excitation to form a closed circuit. When the speed reaches subsynchronous speed (95%), additional resistance is cut off. Variable frequency starting is not detailed.
Synchronous motors require excitation current to operate and, without it, the motor becomes asynchronous. The excitation is a DC system added to the rotor and its rotational speed and polarity are synchronized with the stator. If there is a problem with the excitation, the motor will be out of step and cannot be adjusted, resulting in an “excitation failure” protection trip.
Adding excitation devices is the second disadvantage of synchronous motors. In the past, excitation was supplied directly by DC machines, but now it is mainly supplied by silicon-controlled rectifiers. The more complex the structure and equipment, the more points of failure and the higher the failure rate.
Synchronous motors are mainly used in applications such as hoists, mills, fans, compressors, rolling mills and water pumps. The principle of engine selection is to prioritize engines with simple structures, low prices, reliable operation and convenient maintenance, as long as the engine performance meets the requirements of production machines.
In this sense, AC motors are better than DC motors, AC asynchronous motors are better than AC synchronous motors, and squirrel cage asynchronous motors are better than wound asynchronous motors. Squirrel cage asynchronous motors are preferred for continuously operating production machines with stable loads and no special requirements for starting and braking, and are widely used in machines, water pumps and fans. Wound asynchronous motors are recommended for production machines with frequent starts and braking and that require large starting and braking torques, such as overhead cranes, mine winches, air compressors and irreversible rolling mills.
Synchronous motors are ideal for applications without the need for speed regulation, constant speed or power factor improvement, such as medium to large capacity water pumps, air compressors, winches and mills.
For production machines with a speed regulation range greater than 1:3 and requiring continuous, stable and smooth speed regulation, it is recommended to use separately excited DC motors, squirrel cage asynchronous motors or synchronous motors with speed regulation. variable frequency speed, such as large precision machine tools, gantry planers, steel rolling mills and hoists.
Production machines that require large starting torque and smooth mechanical characteristics should use series or compound excitation DC motors, such as trams, electric locomotives, and heavy cranes.
II. Rated power
The rated power of a motor refers to its power output, also known as horsepower or shaft capacity. It is the key parameter that quantifies the motor drive load capacity and must be provided when selecting a motor. Other important factors in motor selection include rated voltage, rated current, power factor (cos θ), and efficiency (η).
The purpose of correctly selecting engine capacity is to determine the engine power economically and reasonably, ensuring that it meets the load requirements of production machinery. If the power is too large, investment in equipment increases, leading to waste and low efficiency and power factor of the AC motor. On the other hand, if the power is too small, the engine will overheat and suffer premature damage.
The main factors that determine engine power include:
- Engine heating and temperature rise,
- The allowable short-term overload capacity and
- The starting ability of squirrel cage asynchronous motors.
To select the rated power, the load power is first calculated based on the machinery heating, temperature rise and load requirements. Then the rated power is pre-selected based on the load power, working system and overload requirements. Heating, overload capacity and starting capacity must be checked to ensure they are qualified.
Otherwise, the engine must be selected again until all criteria are met. The operating system is also a mandatory factor, with the conventional S1 operating system being adopted by default if not specified. Motors with overload requirements must also provide a corresponding multiple of overload and operating time.
When a squirrel cage asynchronous motor drives a fan or other high moment of inertia load, the load moment of inertia and starting moment of resistance curve must be provided to verify the starting ability.
Rated power selection assumes a standard ambient temperature of 40℃. If the ambient temperature changes, the rated power must be corrected. The ambient temperature should be checked in areas with extreme weather conditions, such as India, where the ambient temperature can reach 50℃.
High altitude can also affect engine power, with a higher altitude resulting in a greater increase in engine temperature and lower power output. The corona phenomenon must also be considered for engines used at high altitudes.
For reference, the following are some examples of engine power ranges on the market today:
- DC motor: ZD9350 (mill) 9350kW
- Asynchronous motor: Squirrel cage YGF1120-4 (blast furnace fan) 28000kW
- Winding type yrkk1000-6 (raw mill) 7400kw
- Synchronous motor: TWS36000-4 (blast furnace fan) 36,000 kW (test unit reaches 40,000 kW)
III. Rated voltage
The rated voltage of a motor refers to the line voltage under its rated operating conditions.
The choice of motor nominal voltage depends on the power system supply voltage and motor capacity.
The selection of voltage level for an AC motor depends mainly on the voltage level of the power supply at the location of use.
Typically, the low voltage network operates at 380 V, so the nominal voltage can be 380 V (Y or Δ connection), 220/380 V (Δ/Y connection) or 380/660 V (Δ/Y connection).
When the power of a low voltage motor reaches a certain level (such as 300KW/380V), it becomes difficult to increase the current due to limitations in the conductor's carrying capacity, or it would be very expensive to do so.
Higher output power is obtained by increasing the voltage.
The supply voltage of high voltage electrical networks is typically 6,000 V or 10,000 V, although there are also voltage levels of 3,300 V, 6,600 V and 11,000 V used in other countries.
High voltage motors have the advantages of high power and strong impact resistance, but the disadvantage is that they have large inertia and are difficult to start and stop.
The rated voltage of a DC motor must also match the voltage of the power supply.
Common voltage levels for DC motors are 110V, 220V, and 440V.
220V is the most commonly used voltage level and high power motors can be boosted to 600 to 1000V.
When the AC power supply voltage is 380 V and a three-phase bridge silicon-controlled rectifier circuit is used for power supply, the nominal voltage of the DC motor should be 440 V.
When a half-wave silicon-controlled three-phase rectifier power supply is used for power supply, the nominal voltage of the DC motor should be 220V.
4. Rated speed
The rated speed of the engine refers to the speed at which it operates under normal conditions. Both the engine and the machinery it drives have a rated speed.
When choosing the engine speed, it is important to keep in mind that it should not be too low, as this will result in a larger engine, with more stages and a higher price. On the other hand, the speed should not be too high as it can make the transmission mechanism complicated and difficult to maintain.
It is also important to note that when power is constant, engine torque is inversely proportional to speed. As a result, those with low starting and braking requirements can compare different rated speeds in terms of initial investment, equipment footprint and maintenance cost before determining the ideal rated speed.
For applications that require frequent starting, braking and reversing, the speed ratio and rated speed of the motor should be selected based on minimizing losses during the transition process, rather than considering only the initial investment. For example, elevator motors require frequent back and forth rotation with high torque, so they have low speed and are bulky and expensive.
When the engine speed is high, it is crucial to consider the critical speed of the engine. During operation, the rotor may vibrate and its amplitude will increase with speed. At a given speed, the amplitude reaches a maximum value (known as resonance), and the amplitude will decrease and stabilize in a certain range when the speed increases further.
This speed with maximum amplitude is called the critical speed of the rotor and is equal to its natural frequency. If the rotor operates at its critical speed, this can result in violent vibrations and significant bending of the shaft, leading to long-term deformation or even fracture.
Generally, the critical first-order speed of the engine is above 1,500 RPM, so it is not a concern for conventional low-speed engines. However, for high-speed 2-pole motors, if the rated speed is close to 3000 RPM, the impact of critical speed must be considered and the motor must not operate at its critical speed for long periods.
Wrap it up
Generally, a motor's specifications can be estimated by providing information about the type of load it will drive, its rated power, voltage and speed. However, these basic parameters are not sufficient to fully satisfy the load requirements.
Additional parameters that need to be considered include frequency, operating system, overload requirements, degrees of insulation and protection, moment of inertia, load resistance moment curve, installation method, ambient temperature, altitude, and external requirements, among others. These parameters must be specified based on the specific application.
V. Principles for Engine Selection
The main criteria for engine selection include:
- Motor type, voltage and speed;
- The variety of engine types;
- Choice of type of engine protection;
- Motor voltage and speed.
Engine selection should be based on the following conditions:
- The type of motor power supply, such as single-phase, three-phase, direct current, etc.
- The operating environment of the engine. If there are particularities in the operating environment, such as humidity, low temperature, chemical corrosion, dust, etc.
- The method of engine operation. Whether it operates continuously, intermittently or by some other method.
- The mounting method of the motor, such as vertical mounting, horizontal mounting, etc.
- Engine power and speed. Power and speed must meet load requirements.
- Other factors, such as whether speed regulation is required, whether there are special control requirements, the type of load, etc.
1. Choosing the type, voltage and speed of the motor
When selecting the motor type, voltage and speed, the power transmission requirements of the production machine, such as starting and stopping frequency, whether speed regulation is required, etc., should be considered first. This will determine the type of motor current, that is, whether to choose an alternating current motor or a direct current motor.
Then, the rated voltage size of the motor should be selected based on the power supply environment. Then, its rated speed should be selected based on the speed required by the production machine and the requirements of the transmission equipment.
After that, the structure and protection type of the engine should be determined based on the engine mounting location and the surrounding environment.
Finally, the rated power (capacity) of the engine must be determined by the power required by the production machine.
After considering all these factors, select an engine from the product catalog that meets these requirements. If the engines listed in the catalog do not meet the special requirements of the production machine, you can place a custom order with the engine manufacturer.
2. Choosing the type of engine
The choice of motor is considered from the aspects of AC and DC, machine characteristics, speed regulation and starting capacity, protection and price. Therefore, the following guidelines must be observed when choosing:
(1) First, consider selecting a three-phase squirrel cage induction motor.
This is due to its simplicity, durability, reliability, low cost and easy maintenance. However, its disadvantages are difficult speed regulation, low power factor, high starting current and small starting torque. Therefore, it is mainly suitable for general production machines and drives with relatively rigid machine characteristics and no special speed regulation requirements, such as general machine tools and production machines such as water pumps or fans with a power of less than 100KW.
(2) The price of wound rotor motors is higher than that of cage motors.
However, the characteristics of your machine can be adjusted by adding resistance to the rotor, thereby limiting the starting current and increasing the starting torque. Therefore, it is suitable for situations where the power supply capacity is small, the motor power is large, or speed regulation is required, such as certain lifting equipment, lifting elevators, forging presses, and cross-beam movement of machines- heavy tools.
(3) When the speed regulation range is less than 1:10 and smooth speed regulation is required, a sliding motor can be selected first.
This motor can be divided into horizontal and vertical type according to its mounting position. The shaft of a horizontal engine is mounted horizontally, while the shaft of a vertical engine is mounted vertically at high altitudes, so the two types of engines cannot be used interchangeably. Under normal circumstances, a horizontal motor should be chosen whenever possible, and a vertical motor should only be considered when vertical operation is required (such as vertical deep well pumps and drilling rigs) to simplify transmission mounting (because it is more Dear).
3. Selection of engine protection types
There are several types of protection for motors, and the appropriate type should be selected based on different operating environments. Types of protection for motors include open, protective, closed, explosion-proof, submersible, and several others. An open type is typically chosen for everyday environments due to its affordability, but is only suitable for dry, clean conditions.
For humid, corrosion-prone, dusty, flammable or corrosive environments, a closed type should be selected. If the environment is dusty and harmful to the motor insulation, but can be cleaned with compressed air, a type of protection may be chosen. For submersible pump motors, a fully sealed type must be chosen to ensure that moisture does not enter during underwater operation. In environments with fire or explosion risks, an explosion-proof type must be selected.
4. Selection of motor voltage and speed
When choosing a motor for existing production machines in an industrial environment, the nominal voltage of the motor must be equivalent to the factory distribution voltage. For new factories, motor voltage selection must be considered according to the chosen distribution voltage.
The decision must be made based on the most economically viable option after comparing different voltage levels. The low voltage standard in our country is 220/380V, while the high voltage is mainly 10KV. Most motors with smaller capacities are high voltage, with nominal voltages of 220/380V (D/Y connection method) and 380/660V (D/Y connection method). When the motor capacity exceeds approximately 200KW, it is recommended to choose 3KV, 6KV or 10KV high voltage motors.
The selection of (nominal) engine speed should be considered based on the requirements of the driven production machinery and the condition of the transmission assembly. The number of engine revolutions per minute generally includes 3000, 1500, 1000, 750, and 600.
The rated speed of an asynchronous motor is typically 2% to 5% lower than these speeds due to the slip rate. From a manufacturing point of view, if a motor of the same power has a higher rated speed, its electromagnetic torque form will be lower, thus reducing its cost and weight.
Additionally, high-speed motors have higher power factors and efficiency than low-speed motors.
Choosing a motor with a higher speed is more economical. However, if this results in a significant difference in speed between the motor and the driven machine, more speed-increasing transmission stages will be required, increasing equipment costs and energy consumption. The ideal choice should be made after careful comparison.
Most of the motors we typically use are 1500 r/min 4-pole motors because these motors have a wide range of applications and superior power factors and operating efficiency.
