A variable-frequency drive (VFD) is a power control device that uses frequency conversion technology and microelectronics to control an AC motor by changing the frequency of the power supply. A VFD mainly consists of a rectification unit (AC to DC), a filtering unit, an inverter unit (DC to AC), a braking unit, a drive unit, a detection unit, and a microprocessor unit.
Frequency converters adjust the voltage and frequency of the output power supply by switching their internal IGBTs, providing the required power voltage according to the actual needs of the motor, thereby achieving energy saving and speed regulation. In addition, frequency converters have many protection functions, such as overcurrent, overvoltage, and overload protection. With the continuous improvement of industrial automation, frequency converters have been widely used.
The main indicators for evaluating the performance of a general-purpose frequency converter include: starting torque, control method, torque control accuracy, speed control accuracy, speed control method, control signal type, frequency jump function, carrier frequency, multi-segment speed setting, and communication interface. The correct selection of the frequency converter plays a crucial role in the normal operation of the electrical control system of mechanical equipment.
When selecting a frequency converter, we recommend first understanding the type of mechanical equipment, load torque characteristics, starting torque, speed range, static speed accuracy, and environmental requirements. Only then should you decide which control method and protection structure is most suitable. "Suitable" means meeting the actual production requirements and application conditions of the mechanical equipment, achieving the best cost-effectiveness for the frequency converter.
I. Principles for Selecting Frequency Converters
Specifically, the selection of low-voltage general-purpose frequency converters includes two aspects: the selection of the type and the selection of the capacity. The basic principles for selecting a frequency converter are twofold: the function and characteristics of the frequency converter can reliably meet the process requirements and can achieve a relatively good cost performance.
To ensure that the inverter's functional characteristics reliably meet process requirements, the following technical parameters should be closely monitored when selecting an inverter:
1. Select the frequency converter based on the actual operating current of the motor.
The actual operating current of the motor is the most critical factor in selecting a frequency converter. During long-term operation, the frequency converter's output current must be greater than the motor's actual operating current. Remember this!!!
In projects, the motor is usually selected first, and then the frequency converter is selected based on the motor. The actual operating current of the motor is not the rated current marked on the motor nameplate. When selecting a frequency converter, the operating conditions should be understood first, and the relationship between the motor's operating current and its change over time should be estimated before determining the appropriate frequency converter model.
1.1 Generally, when a frequency converter drives a constant torque load motor, the frequency converter is selected based on the motor's rated current.
1.2 Generally, when a frequency converter drives a motor that drives a fan or pump, the frequency converter should be selected based on the motor's rated current.
1.3 For motors that frequently operate under short-term overload, it is necessary to calculate the overload cycle and overload current.
For motors driving this type of load, the inverter's maximum output current (Imax) must be greater than the motor's peak current, and the inverter's parameter (I²t) must be within its allowable range. When selecting an inverter, it may be necessary to increase the current by one or several levels to meet the site requirements. Taking a 10kW, 20A rated current motor as an example: if the motor operates intermittently, with a peak current of 40A (twice the rated current) during overload operation within 1 second, and then stops for 20 seconds, the inverter's overload curve must be used for selection. First, obtain the curve of the motor current changing over time. Second, check if the inverter's output current curve covers the motor current curve (i.e., the inverter's output current exceeds the motor's actual operating current). Only inverter models whose output current curve covers the motor current curve are suitable for heavy-load motors. For heavy-load inverter selection, there are often some empirical data that can be used for reference.
Delixi frequency inverters generally have better overload capacity, typically allowing 1.6 times the short-term overload. For overload capacity information of different brands of frequency inverters, please refer to the inverter selection catalog.
II. The selection of frequency converters should fully consider the impact of the environment on the frequency converter.
1. The impact of temperature-controlled frequency converters
When selecting a frequency converter, the ambient temperature should be considered. Generally, the ambient temperature ranges from -10 to 40℃. If the ambient temperature is higher than 40℃, the frequency converter should be derated by 5% for every 1℃ increase. For every 10℃ increase in ambient temperature, the lifespan of the frequency converter will be halved. Therefore, the issues of the surrounding environment and the heat dissipation of the frequency converter must be properly addressed.
2. The effect of humidity on frequency converters
When selecting a frequency converter, if it is to operate in an environment with humidity below 90% (relative humidity less than or equal to 90%, with no condensation), then condensation is more likely to occur inside the frequency converter when the humidity is too high or fluctuates significantly. This will greatly reduce the insulation performance and may even cause a short circuit. If necessary, a desiccant or heater must be added to the enclosure.
3. The impact of altitude on frequency converters
The frequency converter can output its rated power when installed at an altitude below 1000m. When the altitude exceeds 1000m, its output power will decrease.
4. The impact of dust on frequency converters
Inverters should not be installed in environments with conductive metallic dust. This is because conductive dust can penetrate the inverter's interior, easily causing short circuits in the internal wiring, and in severe cases, burning out the inverter. This must be considered when selecting an inverter.
III. Selection of Power Supply for Inverter
Commonly used voltages are single-phase AC 220V, three-phase AC 220V, three-phase AC 380V, and three-phase AC 690V. The incoming power supply is determined by the voltage level of the existing upstream transformer and should be specified at the initial stage of inverter selection. The voltage level determines the wiring method of the motor terminal box. Star connection has higher voltage withstand capability and lower operating current than delta connection. The incoming power frequency is generally 50Hz. The inverter has a relatively high tolerance for fluctuations in the incoming frequency, and the inverter's diode rectifier bridge is not sensitive to frequency.
IV. Correctly Select the Inverter Cooling Method
Most common low-voltage frequency converters below 1000VAC are internally air-cooled. When high-power frequency converters are used in group drives, the operating noise of the converter cooling fans is quite high. In necessary cases, water-cooled frequency converters can be selected.
V. External Configuration Requirements for Inverter Selection
1. For frequency converters that do not have a fast-acting fuse to protect the silicon device before the internal rectifier circuit, a suitable fuse and disconnect switch should be installed between the frequency converter and the power supply (an air circuit breaker cannot be used to replace the fuse and disconnect switch) to avoid damage to the frequency converter's rectifier device due to internal short circuit.
2. Select the inverter's input and output cables according to the inverter's power. If economically feasible, shielded power cables should be used from the inverter to the motor, and they should be as short as possible to reduce capacitive leakage current and electromagnetic radiation. If the actual length of the power cable used on site exceeds the inverter's allowable output cable length, the inverter should be equipped with an output reactor to prevent stray capacitance from the excessively long cable from affecting the inverter's normal operation.
Shielded cables should be used for frequency converter control signals and frequency converter feedback signals, and they should be properly grounded to reduce interference from the frequency converter to other instruments and control systems.
3. Configuring an EMC filter or AC reactor at the inverter input can effectively suppress electromagnetic interference caused by the switching of the inverter's power devices, thus meeting the power grid quality requirements of other equipment operating in the same condition as the inverter. Note: An EMC filter cannot be used at the inverter input when the transformer neutral point is not grounded.
VI. Detailed Explanation of Inverter Selection Steps
When selecting a frequency converter, you can follow these six steps:
1. Clearly define the equipment's operating mode, capacity, and load type.
2. Clearly define the equipment's process, performance indicators, and control requirements.
3. Determine the internal system architecture, I/O interfaces, communication interfaces, etc. 4. Summarize the performance indicators and requirements.
5. Provide technical consultation or directly initiate bidding for the summarized results.
6. Conduct a comprehensive comparison of inverter performance, inverter lifespan, inverter price, and inverter service.
VII. Selection Considerations for Frequency Converters
1. Pay attention to the voltage.
Domestic voltage is divided into three types: low voltage, medium voltage, and high voltage. Generally speaking, low voltage includes single-phase 220V, three-phase 220V (generally applicable to foreign countries and Hong Kong, Macao and Taiwan regions), and three-phase 380V. Under normal circumstances, three-phase 380V and single-phase 220V are used. When choosing a chair, you must specify the voltage. If you choose the wrong one, it cannot be used.
2. Check if the motor has a capacitor installed.
If the motor is a capacitor-driven single-phase motor, its principle is to utilize the phase-leading characteristic of the capacitor to shift the phase of the starting winding voltage, thereby generating a rotating magnetic field in the stator to drive the motor. If a frequency converter is directly connected, the high-frequency pulses act as a circuit for the capacitor, failing to achieve the phase-shifting effect. This is equivalent to simultaneously connecting voltage to both the main and starting windings, but without generating a rotating magnetic field. The motor current will surge due to stall, potentially burning out both the frequency converter's IGBT module and the motor.
3. Are the power ratings consistent?
When selecting a frequency converter, its power rating must be greater than or equal to the motor's rated power. If the motor operates under a heavy load, a load converter or a higher-range frequency converter should be used to ensure the normal operation of the motor. In short, when choosing a frequency converter, it is crucial to consider the power supply and the motor's specifications. After all, in this industry, safety and efficiency are paramount. Therefore, if the motor's specifications are uncertain, it is advisable to purchase a higher-range or higher-performance frequency converter and consult with technicians regarding these aspects.
