[Introduction] How to choose the most suitable frequency inverter?
(I) Understanding Variable Frequency Load Types
The correct selection of a frequency converter is crucial for the normal operation of a control system. When selecting a frequency converter, it is essential to fully understand the characteristics of the load it will drive. In practice, production machinery is often categorized into three types: constant torque loads, constant power loads, and fan/pump loads.
(01) Constant Torque Load: The load torque TL is independent of the speed n, and TL remains constant or essentially constant at any speed. For example, friction loads such as conveyor belts, mixers, and extruders, as well as potential energy loads such as cranes and hoists, are all constant torque loads. When a frequency converter drives a constant torque load, the torque at low speeds must be large enough, and it must have sufficient overload capacity. If it is necessary to operate at a stable speed at low speeds, the heat dissipation capacity of the standard asynchronous motor should be considered to avoid excessive temperature rise of the motor.
(02) The torque required by constant power load machine tool spindles and winding machines and uncoilers in rolling mills, paper machines, and plastic film production lines is roughly inversely proportional to the speed. This is what is called a constant power load. The constant power nature of the load should refer to a certain speed variation range. When the speed is very low, due to the limitation of mechanical strength, TL cannot increase indefinitely, and it transforms into a constant torque nature at low speeds. If the constant torque and constant power speed regulation range of the motor matches the constant torque and constant power range of the load, that is, under the so-called "matching" condition, the capacity of both the motor and the frequency converter are minimized.
(03) Fan and pump loads: In various fans, water pumps, and oil pumps, the resistance generated by air or liquid within a certain speed range as the impeller rotates is approximately proportional to the square of the speed n. As the speed decreases, the torque decreases proportionally to the square of the speed. The power required by this type of load is proportional to the cube of the speed. When the required air volume or flow rate decreases, adjusting the air volume and flow rate using a frequency converter can significantly save energy. Because the power required at high speeds increases too rapidly with the speed, being proportional to the cube of the speed, fans and pump loads should generally not be operated above the operating frequency.
(II) Familiarity with the principles of frequency converter selection
(01) In conjunction with the overall framework of the project, starting from the process characteristics and electrical control, the load type, usage environment, communication architecture and interface type must be considered, such as whether it is a serial port, DP or PN communication interface.
(02) Select the frequency converter according to the load characteristics. For example, if the load is a constant torque load, you can choose the Siemens G120 frequency converter. If the load is a fan or pump type load, you can choose the Siemens G120XA frequency converter.
(03) When selecting a frequency converter, the actual motor current value should be used as the basis for selection; the rated power of the motor can only be used as a reference. Furthermore, it should be fully considered that the frequency converter's output contains high-order harmonics, which will degrade the motor's power factor and efficiency. Therefore, compared to powering the motor from the mains grid, using a frequency converter to supply power results in a 10% increase in motor current and approximately a 20% increase in temperature rise. Therefore, this situation should be taken into account when selecting the motor and frequency converter, and an appropriate margin should be allowed to prevent excessive temperature rise, which could affect the motor's service life.
(04) If the frequency converter is to be operated with a long cable, measures should be taken to suppress the influence of the long cable to ground coupling capacitance to avoid insufficient output of the frequency converter. Therefore, the frequency converter should be selected by one level or an output reactor should be installed at the output end of the frequency converter.
(05) For some special applications, such as high ambient temperature, high switching frequency (especially when used in applications with high noise restrictions such as building automation), high altitude, etc., the inverter will be derated, and the inverter needs to be selected with a larger capacity.
(06) When a frequency converter is used to control several motors connected in parallel, the total length of the cables from the frequency converter to the motors must be within the frequency converter's allowable range. If it exceeds the specified value, the frequency converter should be selected by one or two sizes larger. In addition, in this case, the frequency converter can only be used in V/F control mode, and the frequency converter cannot protect the motors from overcurrent and overload. In this case, a fuse must be added to each motor to achieve protection.
(07) For some special applications, such as high ambient temperature, high switching frequency, high altitude, etc., the inverter will be derated, and the inverter needs to be selected with a larger capacity.
(08) When using a frequency converter to control a high-speed motor, the high-speed motor has a small reactance, and the higher harmonics also increase the output current value. Therefore, the frequency converter selected for a high-speed motor should be slightly larger than that for a regular motor.
(09) When using a frequency converter for a pole-changing motor, careful 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 the motor will run dry, which may damage the frequency converter in severe cases.
(10) When driving an explosion-proof motor, if the frequency converter does not have an explosion-proof structure, the frequency converter should be installed outside the hazardous area.
(11) When using a frequency converter to drive a geared motor, the range of application is limited by the lubrication method of the rotating gear parts. When lubricated with lubricating oil, there is no limitation in the low-speed range; however, in the high-speed range exceeding the rated speed, there is a risk of running out of lubricating oil. Therefore, do not exceed the maximum permissible speed.
(12) When a frequency converter drives a wound-rotor asynchronous motor, it mostly utilizes an existing motor. Compared to a regular squirrel-cage motor, a wound-rotor motor has lower winding impedance. Therefore, it is prone to overcurrent tripping due to ripple current, so a frequency converter with a slightly larger capacity than usual should be selected. Generally, wound-rotor motors are mostly used in applications with a large flywheel torque GD2, and more attention should be paid when setting the acceleration and deceleration times.
(13) When the frequency converter drives the synchronous motor, the output capacity is reduced by 10% to 20% compared with the power frequency power supply. The continuous output current of the frequency converter must be greater than the per-unit value of the rated current of the synchronous motor and the synchronous pull-in current.
(14) For loads with large torque fluctuations, such as compressors and vibratory machines, and for loads with peak loads, such as hydraulic pumps, if the frequency converter is selected based on the rated current or power value of the motor, overcurrent protection may be triggered due to peak current. Therefore, the operating conditions at the power frequency should be understood, and a frequency converter with a rated output current greater than its maximum current should be selected. When a frequency converter drives a submersible pump motor, because the rated current of the submersible pump motor is larger than that of a regular motor, the rated current of the frequency converter should be greater than that of the submersible pump motor.
(15) When the frequency converter controls the Roots blower, the starting current is very large, so when selecting the frequency converter, you must pay attention to whether the capacity of the frequency converter is large enough.
(16) When selecting a frequency converter, it is essential to ensure that its protection level matches the site conditions. Otherwise, dust and moisture at the site will affect the long-term operation of the frequency converter.
(17) Single-phase motors are not suitable for frequency converter drives.
(18) When the motor load is very light, even if the motor load current is within the rated current of the inverter, an inverter with a capacity much smaller than that of the motor should not 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 larger its pulsating current value, which may exceed the current allowable value of the inverter. (19) If the power supply of the inverter is a self-provided power supply, it is best to add an input line reactor.
