Foshan Yongkun Electric Machinery Co., Ltd. shares the basic contents of motor selection
The basic contents that need to be known before motor selection are: type of load driven, rated power, rated voltage, rated speed and other conditions.
1： Type of load driven
The principle of selecting motor is that on the premise that the motor performance meets the requirements of production machinery, the motor with simple structure, low price, reliable operation and convenient maintenance shall be selected preferentially. In this respect, AC motor is better than DC motor, AC asynchronous motor is better than AC synchronous motor, squirrel cage asynchronous motor is better than wound type asynchronous motor.
For the production machinery with stable load and continuous operation without special requirements for starting and braking, the ordinary squirrel cage asynchronous motor should be preferred, which is widely used in machinery, water pump, fan, etc.
For the production machinery with frequent starting and braking, which requires large starting and braking torque, such as bridge crane, mine hoist, air compressor, irreversible rolling mill, etc., wound asynchronous motor shall be used.
Where there is no requirement for speed regulation, constant speed or improvement of power factor is required, synchronous motors shall be used, such as medium and large capacity water pumps, air compressors, hoists, mills, etc.
For the production machinery whose speed range is required to be more than 1:3 and which needs continuous, stable and smooth speed regulation, it is advisable to adopt the separately excited DC motor or the squirrel cage asynchronous motor or synchronous motor with frequency conversion speed regulation, such as large-scale precision machine tool, gantry planer, rolling mill, elevator, etc.
It is required to start production machinery with large turning distance and soft mechanical characteristics, using series or compound excitation DC motor, such as tramcar, electric locomotive, heavy crane, etc.
Therefore, it can be seen that different types of driving loads are more suitable for different motors.
2、 Rated power
The rated power of the motor refers to the output power, that is, the shaft power, also known as the capacity, which is the landmark parameter of the motor. It is often asked how big the motor is, generally not the size of the motor, but the rated power. It is not only the most important index to quantify the load capacity of motor, but also the parameter requirement that must be provided when selecting the motor.
The principle of selecting the motor capacity correctly should determine the motor power most economically and reasonably on the premise that the motor can meet the requirements of production mechanical load. If the power is too large, the equipment investment will increase, resulting in waste, and the motor often runs under load, the efficiency and the power factor of the AC motor are low; otherwise, if the power is too small, the motor will overload operation, resulting in premature damage of the motor.
There are three factors that determine the main power of the motor:
1) The heating and temperature rise of the motor are the main factors that determine the power of the motor;
2) Allowable short-time overload capacity;
3) The starting ability of asynchronous squirrel cage motor should be considered.
Firstly, the load power should be calculated and selected according to the heating, temperature rise and load requirements of the specific production machinery, and then the appropriate rated power of the motor should be pre selected according to the load power, working system and overload requirements. After the rated power of the motor is preselected, it is necessary to check the heating, overload capacity and starting capacity if necessary. If one of the items is unqualified, the motor shall be re selected and 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 most conventional S1 working system will be used by default; the motor with overload requirement also needs to provide overload multiple and corresponding operation time; when the asynchronous squirrel cage motor drives the fan and other large moment of inertia loads, the curve chart of the load moment of inertia and starting resistance moment is also needed to check the starting ability.
The above selection of rated power is carried out under the premise that the standard ambient temperature is 40 ℃. If the ambient temperature of the motor changes, the rated power of the motor must be corrected. According to the theoretical calculation and practice, when the ambient temperature is different, the motor power can be roughly increased or decreased according to the table below.
Therefore, it is also necessary to provide environmental temperature in severe climate areas. For example, in India, the environmental temperature needs to be checked at 50 ℃. In addition, high altitude has an impact on the motor power. The higher the altitude is, the greater the motor temperature rise is, and the smaller the output power is. In addition, the influence of corona phenomenon should be considered for the motor used at high altitude.
3、 Rated voltage
Rated voltage of motor refers to line voltage under rated working mode. The selection of rated voltage of motor depends on the power supply voltage and motor capacity of the enterprise.
The selection of voltage level of AC motor mainly depends on the voltage level of power supply in the place of use. Generally, the low-voltage network is 380V, so the rated voltage is 380V (Y or delta connection), 220 / 380V (delta / Y connection), 380 / 660V (delta / Y connection). When the power of low-voltage motor increases to a certain extent (such as 300kW / 380V), the current is difficult to be increased or the cost is too high due to the limit of conductor bearing capacity. High power output needs to be achieved by increasing the voltage. The power supply voltage of high-voltage power grid is generally 6000V or 10000v, and there are 3300v, 6600v and 11000v voltage levels in foreign countries. The advantages of high-voltage motor are high power, strong ability to withstand impact; the disadvantages are large inertia, starting and braking are difficult.
The rated voltage of DC motor shall also match with the power supply voltage. Generally 110V, 220V and 440V. Among them, 220V is the 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 silicon controlled rectifier circuit is used for power supply, the rated voltage of its DC motor shall be 440V, and the three-phase is used for power supply
The basic contents that need to be known before motor selection are: type of load driven, rated power, rated voltage, rated speed and other conditions.
1： Type of load driven
The principle of selecting motor is that on the premise that the motor performance meets the requirements of production machinery, the motor with simple structure, low price, reliable operation and convenient maintenance shall be selected preferentially. In this respect, AC motor is better than DC motor, AC asynchronous motor is better than AC synchronous motor, squirrel cage asynchronous motor is better than wound type asynchronous motor.
For the production machinery with stable load and continuous operation without special requirements for starting and braking, the ordinary squirrel cage asynchronous motor should be preferred, which is widely used in machinery, water pump, fan, etc.
For the production machinery with frequent starting and braking, which requires large starting and braking torque, such as bridge crane, mine hoist, air compressor, irreversible rolling mill, etc., wound asynchronous motor shall be used.
Where there is no requirement for speed regulation, constant speed or improvement of power factor is required, synchronous motors shall be used, such as medium and large capacity water pumps, air compressors, hoists, mills, etc.
For the production machinery whose speed range is required to be more than 1:3 and which needs continuous, stable and smooth speed regulation, it is advisable to adopt the separately excited DC motor or the squirrel cage asynchronous motor or synchronous motor with frequency conversion speed regulation, such as large-scale precision machine tool, gantry planer, rolling mill, elevator, etc.
It is required to start production machinery with large turning distance and soft mechanical characteristics, using series or compound excitation DC motor, such as tramcar, electric locomotive, heavy crane, etc.
Therefore, it can be seen that different types of driving loads are more suitable for different motors.
2、 Rated power
The rated power of the motor refers to the output power, that is, the shaft power, also known as the capacity, which is the landmark parameter of the motor. It is often asked how big the motor is, generally not the size of the motor, but the rated power. It is not only the most important index to quantify the load capacity of motor, but also the parameter requirement that must be provided when selecting the motor.
The principle of selecting the motor capacity correctly should determine the motor power most economically and reasonably on the premise that the motor can meet the requirements of production mechanical load. If the power is too large, the equipment investment will increase, resulting in waste, and the motor often runs under load, the efficiency and the power factor of the AC motor are low; otherwise, if the power is too small, the motor will overload operation, resulting in premature damage of the motor.
There are three factors that determine the main power of the motor:
1) The heating and temperature rise of the motor are the main factors that determine the power of the motor;
2) Allowable short-time overload capacity;
3) The starting ability of asynchronous squirrel cage motor should be considered.
Firstly, the load power should be calculated and selected according to the heating, temperature rise and load requirements of the specific production machinery, and then the appropriate rated power of the motor should be pre selected according to the load power, working system and overload requirements. After the rated power of the motor is preselected, it is necessary to check the heating, overload capacity and starting capacity if necessary. If one of the items is unqualified, the motor shall be re selected and 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 most conventional S1 working system will be used by default; the motor with overload requirement also needs to provide overload multiple and corresponding operation time; when the asynchronous squirrel cage motor drives the fan and other large moment of inertia loads, the curve chart of the load moment of inertia and starting resistance moment is also needed to check the starting ability.
The above selection of rated power is carried out under the premise that the standard ambient temperature is 40 ℃. If the ambient temperature of the motor changes, the rated power of the motor must be corrected. According to the theoretical calculation and practice, when the ambient temperature is different, the motor power can be roughly increased or decreased according to the table below.
Therefore, it is also necessary to provide environmental temperature in severe climate areas. For example, in India, the environmental temperature needs to be checked at 50 ℃. In addition, high altitude has an impact on the motor power. The higher the altitude is, the greater the motor temperature rise is, and the smaller the output power is. In addition, the influence of corona phenomenon should be considered for the motor used at high altitude.
3、 Rated voltage
Rated voltage of motor refers to line voltage under rated working mode. The selection of rated voltage of motor depends on the power supply voltage and motor capacity of the enterprise.
The selection of voltage level of AC motor mainly depends on the voltage level of power supply in the place of use. Generally, the low-voltage network is 380V, so the rated voltage is 380V (Y or delta connection), 220 / 380V (delta / Y connection), 380 / 660V (delta / Y connection). When the power of low-voltage motor increases to a certain extent (such as 300kW / 380V), the current is difficult to be increased or the cost is too high due to the limit of conductor bearing capacity. High power output needs to be achieved by increasing the voltage. The power supply voltage of high-voltage power grid is generally 6000V or 10000v, and there are 3300v, 6600v and 11000v voltage levels in foreign countries. The advantages of high-voltage motor are high power, strong ability to withstand impact; the disadvantages are large inertia, starting and braking are difficult.
The rated voltage of DC motor shall also match with the power supply voltage. Generally 110V, 220V and 440V. Among them, 220V is the 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 silicon controlled rectifier circuit is used for power supply, the rated voltage of its DC motor shall be 440V, and the three-phase is used for power supply
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