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Specifications for the selection, installation, measurement, and wiring of general-purpose frequency converters 1. Selection of frequency converters The proper selection of frequency converters is crucial for the proper operation of control systems. When selecting an inverter, it is essential to fully understand the load characteristics driven by the inverter. In practice, people often classify production machinery into three types: constant torque loads, constant power loads, and fan or pump loads. Constant torque load: The load torque TL is independent of the speed n; it remains constant or essentially constant at any speed. Frictional loads such as conveyors, mixers, and extruders, as well as potential energy loads such as cranes and hoists, all belong to constant torque loads. When a frequency converter drives a load with a constant torque characteristic, the torque at low speeds must be sufficient, and it must have adequate overload capacity. If steady operation at low speeds is required, the heat dissipation capacity of a standard asynchronous motor should be taken into account to prevent excessive temperature rise in the motor. Constant power load: The torque required by machine tool spindles, as well as rollers in rolling mills, papermaking machines, and plastic film production lines, and unwinding machines, is generally inversely proportional to the speed – this is what is known as a constant power load. The constant power characteristic of the load should be considered within a certain range of speed variations. When the speed is very low, due to limitations in mechanical strength, TL cannot increase indefinitely; at low speeds it exhibits a constant torque characteristic. The constant power region and constant torque region of the load have a significant impact on the selection of the transmission scheme. When the motor is controlled at a constant magnetic flux, its maximum allowable output torque remains unchanged; this constitutes constant-torque speed control ； During weak-magnetic speed control, the maximum allowable output torque is inversely proportional to the speed, which constitutes constant-power speed control. When the constant torque and constant power speed control ranges of the motor coincide with the constant torque and constant power ranges of the load, that is, in the so-called \"matched\" condition, both the capacity of the motor and that of the inverter are minimized. Fan and pump loads: In various fans, water pumps, and oil pumps, as the impeller rotates, the resistance generated by air or liquid within a certain speed range is roughly proportional to the square of the speed n. As the speed decreases, it drops in proportion to the square of the speed. The power required for such a load is proportional to the cube of the speed. When the required air volume and flow rate decrease, using a frequency converter to adjust the speed can help regulate these values, thereby saving a significant amount of electrical energy. Since the power required at high speeds increases too rapidly, proportional to the cube of the speed, fans and pump loads should generally not be operated above their rated frequency. Siemens can provide various types of frequency converters, allowing users to choose the appropriate type based on their specific process requirements and application scenarios. When selecting an inverter, the following points should be taken into consideration: 1. Choose the inverter based on the load characteristics; for example, if the load is a constant-torque load, a Siemens MMV/MDV inverter should be selected, while for loads such as fans and pumps, a Siemens ECO inverter is appropriate. 2. When selecting an inverter, the actual motor current value should be used as the basis for making the choice; the motor’s rated power can only serve as a reference. Furthermore, it should be fully considered that the output of the inverter contains high-order harmonics, which will degrade both the power factor and efficiency of the motor. Therefore, when powering the motor with an inverter compared to using a mains power supply, the motor’s current increases by 10% while the temperature rise increases by about 20%. Therefore, when selecting motors and frequency converters, this situation should be taken into account, and an appropriate margin should be reserved to prevent excessive temperature rise from affecting the motor’s service life. 3. When the inverter is to operate over long cables, measures should be taken to mitigate the effect of the coupling capacitance between the cables and ground, thereby preventing insufficient output from the inverter. Therefore, the frequency converter should be set to a higher gear, or an output reactor should be installed at the output terminal of the frequency converter. 4. When the inverter is used to control several motors connected in parallel, it is essential to ensure that the total length of the cables from the inverter to the motors remains within the inverter’s allowable range. If it exceeds the specified value, increase the gear by one or two levels to select the frequency converter. Furthermore, in such cases, the control mode of the inverter can only be V/F control, and the inverter is unable to provide overcurrent or overload protection for the motor; therefore, fuses must be installed on each motor to achieve protection. 5. For some special application scenarios, such as high ambient temperatures, high switching frequencies, or high altitudes, these conditions can result in a reduced capacity of the inverter; in such cases, it is necessary to select an inverter with a higher capacity rating. 6. When using an inverter to control a high-speed motor, due to the low reactance of the high-speed motor, higher harmonics also increase the value of the output current. Therefore, when selecting an inverter for a high-speed motor, it should be slightly larger than that for a regular motor. 7. When using an inverter with a pole-changing motor, care must be taken to select the appropriate capacity for the inverter, ensuring that its maximum rated current is below the inverter’s rated output current. Furthermore, when performing the pole number conversion during operation, the motor must be stopped first; otherwise, the motor will run idle, and in severe cases, this can damage the frequency converter. 8. When driving an explosion-proof motor, if the frequency converter does not have an explosion-proof design, it should be placed outside the hazardous area. 9. When using an inverter to drive a gear reduction motor, its application range is limited by the lubrication method for the rotating parts of the gears. When lubricated with lubricating oil, there is no limitation in the low-speed range ； In the high-speed range above the rated speed, there is a risk of the lubricating oil running out. Therefore, do not exceed the maximum allowable speed. 10. When driving a wound-rotor asynchronous motor with an inverter, existing motors are mostly used. Compared to ordinary squirrel-cage motors, wound-rotor motors have a lower impedance in their windings. Therefore, overcurrent tripping caused by ripple current is likely to occur, so an inverter with a capacity slightly larger than normal should be selected. Generally, wound-rotor motors are used in applications where the flywheel torque GD2 is high, so special attention should be paid when setting the acceleration and deceleration times. 11. When a variable-frequency drive is used to drive a synchronous motor, the output capacity is reduced by 10% to 20% compared to that with a power supply at line frequency. The continuous output current of the variable-frequency drive must be greater than the product of the rated current of the synchronous motor and the per-unit value of the synchronous pull-in current. 12. For loads with large torque fluctuations such as compressors and vibration machines, as well as those with peak loads like oil pressure pumps, if the inverter is selected based on the motor’s rated current or power value, overcurrent protection may be triggered due to the peak currents. Therefore, it is necessary to understand the operating conditions at line frequency and select an inverter with a rated output current that is higher than its maximum current. When using an inverter to drive a submersible pump motor, since the rated current of such a motor is higher than that of a regular motor, the inverter selected must have a rated current that is greater than that of the submersible pump motor. 13. When using an inverter to control a Roots blower, its starting current is very high; therefore, when selecting an inverter, it is essential to ensure that its capacity is sufficient. 14. When selecting an inverter, be sure to check that its protection rating matches the conditions at the installation site. Otherwise, dust and moisture on site will affect the long-term operation of the frequency converter. 15. Single-phase motors are not suitable for drive by frequency converters. 2 Installation and wiring specifications for frequency converters: Installation environment: To ensure the stable operation of the frequency converter, it is necessary to guarantee that its operating environment meets the specified allowable conditions. 1. Installation location: 1? The electrical room should have low humidity and no water damage. 2? There should be no explosive, flammable, or corrosive gases and liquids, as well as minimal dust. 3? Maintenance and inspection should be easy to carry out. 4? Ventilation openings or exhaust systems should be available to remove the heat generated by the inverter. 2. Operating conditions: 1? The operating temperature range for inverters is usually 0–40°C or –10–50°C; attention must be paid to proper ventilation within the inverter cabinet. 2? The relative humidity around the snow frequency converter should be below 90%. High humidity in the surrounding area can lead to reduced electrical insulation and corrosion of metal parts. If installation location constraints force the inverter to be installed in a humid environment, its enclosure should preferably have a sealed design. To prevent dew formation when the inverter stops, a convection heater is sometimes required for the device. 3? There should be no corrosive, explosive, or flammable gases, as well as no dust or oil mist, around the snow frequency converter. If there are explosive or flammable gases in the area where the frequency converter is installed, fires or explosions may occur sometimes, as the frequency converter contains relays and contactors that can generate sparks. In the presence of corrosive gases, the metal components corrode, affecting the long-term operation of the inverter. If there is dust and oil mist around the inverter, these gases will adhere to and accumulate inside the inverter, resulting in a decrease in insulation ； In variable-frequency drives with forced air cooling, clogged filters can cause the temperature inside the drive to rise abnormally, preventing it from operating stably. 4. The vibration resistance of frequency converters varies depending on the model. When the vibration exceeds the converter’s allowable limit, it can cause the fastening parts of the components to loosen, as well as misoperation of the moving parts such as relays and contactors, which often results in the converter not being able to operate stably. For vibration scenarios such as machine tools and ships that can be anticipated in advance, the vibration issues of frequency converters should be taken into consideration. 5? The elevation for snow frequency converters is usually specified to be below 1000 m. If the gauge pressure is high, the air pressure will be low, making it easy for the bond to be damaged. Additionally, the cooling effect at high temperatures decreases, so attention must be paid to temperature rise. Inverter wiring: When variable frequency speed control is used in various factories and equipment, harmonic interference is generated on both the power supply side and the motor side of the inverter, which can cause EMC interference to the power supply grid as well as other electrical devices surrounding the inverter. Furthermore, to ensure the long-term reliable operation of the inverter, its wiring is very important. 1. What is EMC? EMC stands for “Electromagnetic Compatibility”. It refers to the ability of electrical equipment to operate properly in an electromagnetic environment, without generating electromagnetic interference that is unacceptable for other devices operating in that same environment. file:///C:/DOCUME%7E1/ADMINI%7E1/LOCALS%7E1/Temp/msohtml1/01/clip_image001.gif 2. Noise emission and immunity EMC is determined by two characteristics related to electrical equipment – noise emission and immunity. The specified limit values for noise emission and immunity depend on the environment in which the electrical equipment is used. It is generally divided into Category 1 environments (civilian environments) and Category 2 environments (industrial environments). In the civilian environment, strict regulations exist regarding noise emission when electrical equipment is connected to public power supply systems, but lower immunity levels may be required ； On the contrary, in industrial environments, high requirements are placed on the immunity of electrical equipment, while lower requirements are imposed on noise emission. file:///C:/DOCUME%7E1/ADMINI%7E1/LOCALS%7E1/Temp/msohtml1/01/clip_image002.gif If the electrical equipment is a component of the system, it does not need to meet any requirements regarding emissions and immunity from the outset; however, the entire system must comply with the relevant electromagnetic compatibility requirements. Generally, electrical equipment must have the ability to suppress both high-frequency and low-frequency interference. High-frequency interferences mainly include electrostatic discharge (ESD), pulse interference, and electromagnetic fields of radiative frequencies, etc ； Low-frequency interference mainly refers to power supply voltage fluctuations, under-voltage, and frequency instability. file:///C:/DOCUME%7E1/ADMINI%7E1/LOCALS%7E1/Temp/msohtml1/01/clip_image003.gif 3. Inverters and their electromagnetic compatibility: Typically, inverters can operate in industrial environments where there is high levels of electromagnetic interference (EMI); in such situations, they can act both as sources of noise and as receivers of noise. file:///C:/DOCUME%7E1/ADMINI%7E1/LOCALS%7E1/Temp/msohtml1/01/clip_image004.gif (1) As a noise source, the inductive capacitance Cp exists in the motor cables and within the motor itself. Therefore, the switching edges of the PWM output voltage waveform from the inverter generate a high-frequency pulsed noise current Is through this parasitic capacitance, thereby turning the inverter into a noise source. Since the source of the noise current Is is the inverter, it must flow back to the inverter. In the figure, Ze represents the earth impedance, while Zn represents the impedance between the power cable and the ground. The voltage drop caused by the noise current flowing through these two impedances will affect other devices on the same power grid, resulting in interference. Furthermore, the rectifier section of the inverter also generates low-frequency harmonics, causing distortion in the grid voltage. file:///C:/DOCUME%7E1/ADMINI%7E1/LOCALS%7E1/Temp/msohtml1/01/clip_image005.gif If the high-frequency noise current Is has a proper path, then the high-frequency noise can be suppressed. If unshielded motor cables are used, the high-frequency noise current Is flows back to the inverter along an uncertain path, generating high-frequency voltage drops in this circuit and affecting other devices. To allow the high-frequency noise current Is to flow back to the inverter along a defined path, it is necessary to use shielded motor cables. The cable shield must be connected to the inverter housing and the motor housing. When the high-frequency noise current Is must flow back to the inverter, the shielding layer provides the most effective path. Although noise current does not cause a voltage drop across ZE, it still causes a voltage drop across the power supply impedance ZN, affecting other electrical devices. file:///C:/DOCUME%7E1/ADMINI%7E1/LOCALS%7E1/Temp/msohtml1/01/clip_image006.gif For this reason, a radio interference suppression filter should be installed at the input side of the frequency converter; this way, the noise currents in the power supply will be reduced. file:///C:/DOCUME%7E1/ADMINI%7E1/LOCALS%7E1/Temp/msohtml1/01/clip_image007.gif (2) The frequency converter as a noise receiver. file:///C:/DOCUME%7E1/ADMINI%7E1/LOCALS%7E1/Temp/msohtml1/01/clip_image008.gif 3. Measures to minimize the impact of EMC. All Siemens frequency converters are designed to operate in industrial environments where there may be high levels of electromagnetic interference (EMI). Generally, good installation experience can ensure the safe and trouble-free operation of the frequency converter. However, if you encounter problems, please refer to the following suggestions and relevant measures. (1) Ensure that all equipment in the drive cabinet is properly grounded, using short and thick grounding wires to connect to the common ground point or grounding busbar. It is particularly important that any control device connected to the inverter (such as a PLC) share the same ground with it, and short and thick wires should also be used for grounding. It is best to use flat conductors (such as metal mesh), as they have a lower impedance at high frequencies. The ground wire of the motor cable should be connected directly to the grounding terminal (PE) of the corresponding inverter. (2) When installing the inverter, it is recommended to use paint-free galvanized steel plates for the mounting plate, in order to ensure a good electrical connection between the inverter’s radiator and the mounting plate. (3) To effectively suppress the radiation and conduction of electromagnetic waves, the motor cable of the inverter must be a shielded cable, and the conductivity of the shielding layer must be at least 1/10 of the conductivity of the conductor core per phase. file:///C:/DOCUME%7E1/ADMINI%7E1/LOCALS%7E1/Temp/msohtml1/01/clip_image009.giffile:///C:/DOCUME%7E1/ADMINI%7E1/LOCALS%7E1/Temp/msohtml1/01/clip_image011.gif (4) It is advisable to use shielded cables for control cables. Generally, the shielding layer of the control cable should be grounded directly inside the inverter, while the other end is grounded through a small high-frequency capacitor (such as 3.3 nF/3000 V). When the differential voltage across the shield is not high and it is connected to the same ground, the ends of the shield can also be grounded directly. Twisting the signal line with its return line can reduce interference caused by inductive coupling. The stranding should be as close to the terminal as possible. Double-shielded twisted pair should be used for the transmission lines of analog signals. Different analog signal lines should be routed separately, with their own shielding layers, to reduce coupling between the lines. Do not place different analog signals on the same common return line. For low-voltage digital signal lines, it is preferable to use twisted pair with dual shielding; single-shielded twisted pair can also be used. For low-voltage digital signal lines, it is preferable to use twisted pair with dual shielding; single-shielded twisted pair can also be used. file:///C:/DOCUME%7E1/ADMINI%7E1/LOCALS%7E1/Temp/msohtml1/01/clip_image012.gif The transmission cables for analog and digital signals should be shielded and routed separately. Do not use the same cable for 24V DC and 115/230V AC signals! (5) Wiring: Motor cables should be routed separately from other cables, with a minimum distance of 500 mm. At the same time, it is necessary to avoid running the motor cable alongside other cables over long distances, in order to reduce electromagnetic interference caused by rapid changes in the output voltage of the inverter. If the control cable and power cable intersect, they should cross at a 90-degree angle as much as possible. file:///C:/DOCUME%7E1/ADMINI%7E1/LOCALS%7E1/Temp/msohtml1/01/clip_image013.gif The shielding layers of the motor cables and control cables must also be secured to the mounting plate using appropriate clips. file:///C:/DOCUME%7E1/ADMINI%7E1/LOCALS%7E1/Temp/msohtml1/01/clip_image014.gif (6) If the inverter operates in an environment sensitive to noise, RFI filters can be used to reduce the conductive and radiative interference generated by the inverter. To achieve the best results, it is also essential to ensure good contact between the filter and the mounting plate. (7) The incoming line reactor is used to reduce the harmonics generated by the inverter; it can also be used to increase the impedance of the power supply and to help absorb the surge voltages generated when nearby equipment starts operating, as well as voltage spikes in the main power supply. The incoming line reactor is connected in series between the power supply and the power input terminal of the inverter. If an RFI filter is also used, the incoming line reactor should be connected in series between the RFI filter and the inverter. file:///C:/DOCUME%7E1/ADMINI%7E1/LOCALS%7E1/Temp/msohtml1/01/clip_image016.gif Ensure that the contactors in the distribution cabinet have arc-quenching capabilities; AC contactors should use R-C suppressors, while DC contactors should utilize \"flywheel\" diodes incorporated into their windings. Varistor suppressors are also very effective. file:///C:/DOCUME%7E1/ADMINI%7E1/LOCALS%7E1/Temp/msohtml1/01/clip_image017.gif When switching inductive components, the transient voltages that can be generated can sometimes reach up to 4 KV, with frequencies as high as 200 MHz. Arc-suppression measures must be taken for the contactors controlled by relays on the inverter. file:///C:/DOCUME%7E1/ADMINI%7E1/LOCALS%7E1/Temp/msohtml1/01/clip_image019.gif Methods for measuring the inverter: Methods for measuring voltage and current in various parts of the inverter: file:///C:/DOCUME%7E1/ADMINI%7E1/LOCALS%7E1/Temp/msohtml1/01/clip_image021.gif Measurement locations and wiring of the measuring instruments. Since the voltage and current at the power input and output of the inverter contain harmonic components, different measuring instruments and circuits are used, resulting in different measurement values. Please use the instruments specified in the table below to measure the circuit shown in the diagram. Location and measuring instrument for measurement (available upon request from the author): Daily maintenance and repair of frequency converters: Frequency converters are stationary devices composed primarily of semiconductor components. Due to the effects of operating conditions such as temperature, humidity, dust, and vibration, as well as the gradual changes in its components over time, regular and routine inspections of the inverter are necessary to ensure its proper operation.