Classification of variable frequency load
The correct selection of the frequency converter is very critical to the normal operation of the control system. When selecting an inverter, it is necessary to fully understand the characteristics of the load driven by the inverter. People often divide production machinery into three types in practice: constant torque load, constant power load and fan and water pump load.
- Constant torque load
Load torque TL has nothing to do with speed n, TL always remains constant or almost constant at any speed. For example, frictional loads such as conveyor belts, mixers, extruders, and potential loads such as cranes and hoists are all constant torque loads. When the inverter drives a load with constant torque, the torque at low speed must be large enough and have enough overload capacity. If it is necessary to run at a steady speed at a low speed, the heat dissipation capacity of a standard asynchronous motor should be considered to avoid excessive temperature rise of the motor.
- Constant power load
The torque required by machine tool spindles, rolling mills, paper machines, and coilers and uncoilers in plastic film production lines is roughly inversely proportional to the speed. This is the so-called constant power load. The constant power nature of the load should be in terms of a certain speed range. When the speed is very low, limited by the mechanical strength, TL cannot increase indefinitely, and it becomes a constant torque property at low speed. If the constant torque and constant power speed regulation range of the motor is consistent with the constant torque and constant power range of the load, that is, in the so-called “matching” case, the capacity of the motor and the inverter are the smallest.
- Fan and pump load
In various fans, water pumps and oil pumps, with the rotation of the impeller, the resistance of air or liquid within a certain speed range is roughly proportional to the second power of speed n. As the speed decreases, the torque decreases according to the second power of the speed. The power required by this load is proportional to the third power of the speed. When the required air volume and flow are reduced, the frequency converter can be used to adjust the air volume and flow through speed regulation, which can greatly save electric energy. Because the required power increases with the speed too fast at high speed, and is proportional to the third power of the speed, it is usually not allowed to make the fan and pump load exceed the power frequency.
Frequency conversion selection principle
- When selecting the inverter, the actual motor current value should be used as the basis for selecting the inverter. The rated power of the motor can only be used as a reference. In addition, it should be fully considered that the output of the inverter contains high-order harmonics, which will cause the power factor and efficiency of the motor to deteriorate. Therefore, compared with the power supply of the power frequency grid with the frequency converter, the current of the motor increases by 10% and the temperature rise increases by about 20%. Therefore, when selecting a motor and a frequency converter, this situation should be considered, and a margin should be properly reserved to prevent excessive temperature rise and affect the service life of the motor.
- If the inverter needs to run with a long cable, measures should be taken at this time to suppress the influence of the long cable to the ground coupling capacitance to avoid insufficient output of the inverter, so the inverter should be selected by amplifying the first gear or installing output reactance at the output of the inverter Device.
- For some special applications, such as high ambient temperature, high switching frequency (especially when using in building automation and other applications with high noise restrictions), high altitude, etc., this will cause the inverter to For derating, the inverter needs to be enlarged by one gear for selection.
- When the inverter is used to control several motors connected in parallel, it must be considered that the total length of the cable from the inverter to the motor is within the allowable range of the inverter. If it exceeds the specified value, zoom in one or two gears to select the inverter. In addition, in this case, the control mode of the frequency converter can only be the V/F control mode, and the frequency converter cannot protect the overcurrent and overload protection of the motor. At this time, a fuse must be added to each motor to realize the protection.
- For some special applications, such as high ambient temperature, high switching frequency, high altitude, etc., this will cause the inverter to degrade, and the inverter needs to be enlarged by one gear.
- When using a frequency converter to control a high-speed motor, due to the low reactance of the high-speed motor, higher harmonics also increase the output current value. Therefore, when selecting the frequency converter for high-speed motors, it should be slightly larger than that of ordinary motors.
- When the frequency converter is used in a pole-changing motor, full attention should be paid to selecting the capacity of the frequency converter so that its maximum rated current is below the rated output current of the frequency converter. In addition, when changing the number of poles during operation, the motor should be stopped first, otherwise it will cause the motor to run idly and damage the inverter in severe cases.
- When driving an explosion-proof motor, the inverter does not have an explosion-proof structure, and the inverter should be installed outside the hazardous area.
- When using a frequency converter to drive a gear reduction motor, the scope of use is restricted by the lubrication method of the gear rotating part. When lubricating oil, there is no limit in the low-speed range; in the high-speed range above the rated speed, the risk of running out of lubricant may occur. Therefore, do not exceed the maximum speed allowable value.
- When the frequency converter drives the wound rotor asynchronous motor, most of the existing motors are used. Compared with ordinary squirrel cage motors, the winding resistance of the wound motor is small. Therefore, it is prone to overcurrent tripping caused by ripple current, so you should choose an inverter with a slightly larger capacity than usual. Generally, winding motors are mostly used in occasions where the flywheel torque GD2 is large, so pay more attention when setting the acceleration and deceleration time.
- When the frequency converter drives the synchronous motor, compared with the industrial frequency power supply, the output capacity is reduced by 10%-20%. The continuous output current of the frequency converter should be greater than the product of the nominal value of the synchronous motor rated current and the synchronous pull-in current.
- For compressors, vibration machines and other loads with large torque fluctuations and peak loads such as hydraulic pumps, if the inverter is selected according to the rated current or power value of the motor, overcurrent protection due to peak current may occur Action phenomenon. Therefore, you should understand the power frequency operation and choose an inverter with a rated output current greater than its maximum current. When the frequency converter drives the submersible pump motor, because the rated current of the submersible pump motor is larger than the rated current of the normal motor, when selecting the frequency converter, its rated current should be greater than the rated current of the submersible pump motor.
- When the frequency converter controls the Roots blower, because its starting current is very large, you must pay attention to whether the capacity of the frequency converter is large enough when choosing the frequency converter.
- When selecting the inverter, be sure to pay attention to whether its protection level matches the situation on site. Otherwise, dust and water vapor on site will affect the long-term operation of the inverter.
- Single-phase motors are not suitable for frequency converter drive.
- When the motor load is very light, even if the motor load current is within the rated current of the inverter, the inverter with a much smaller capacity than the motor cannot be used. This is because the reactance of the motor varies with the capacity of the motor. Even if the motor load is the same, the larger the motor capacity, the greater the ripple current value, which may exceed the current allowable value of the inverter.
- If the inverter’s power supply is self-provided, it is best to add an incoming line reactor.
