First, the load must be high-power and be a fan/pump type; second, the equipment itself must have energy-saving functions (software support); third, it must operate continuously for extended periods. These are the three conditions that best demonstrate its energy-saving effect. Knowing the reasons, you'll be able to cleverly utilize it to your advantage. However, it's crucial to pay attention to the application environment and conditions for proper use.
Power factor compensation for energy saving: Reactive power not only increases line losses and equipment heating, but more importantly, the decrease in power factor leads to a decrease in the active power of the power grid. A large amount of reactive power is consumed in the lines, resulting in low equipment utilization efficiency and serious waste. After using variable frequency speed control equipment, the reactive power loss is reduced and the active power of the power grid is increased due to the effect of the filter capacitor inside the frequency converter.
Soft-start energy saving: Hard starting of motors severely impacts the power grid and places excessive demands on grid capacity. The large current and vibration generated during starting are extremely harmful to baffles and valves, severely impacting the service life of equipment and pipelines. However, by using variable frequency energy-saving equipment, the soft-start function of the inverter ensures that the starting current starts from zero, with a maximum value not exceeding the rated current. This reduces the impact on the power grid and the demand on power supply capacity, extending the service life of equipment and valves. It also saves on equipment maintenance costs.
In theory, frequency converters can be used in all mechanical equipment with electric motors. When the motor starts, the current is 5-6 times higher than normal, which not only affects the service life of the motor but also consumes more electricity. The system design will leave a certain margin in the selection of motors, and the speed of the motor is fixed. However, in actual use, sometimes it is necessary to operate at a lower or higher speed, so frequency conversion is very necessary.
Now everyone has a general understanding of the energy-saving properties of frequency converters, which will make them more convenient and simpler to use in the future.
If the frequency of the general-purpose frequency converter cannot be increased, let's first check the no-load condition and disconnect the motor.
1. For any frequency inverter, there are frequency range settings, such as the maximum output frequency and the upper limit of the operating frequency. The maximum output frequency is determined according to the structure of the frequency inverter and can be found in the manual, for example, 0~400.00Hz. The upper limit of the operating frequency is the upper limit of the frequency inverter's output frequency, which is less than or equal to the maximum output frequency. If the frequency constraint value of the frequency inverter is set too low, it will not be able to jump to the maximum frequency no matter how much it is increased.
2. Check the frequency setting method to see if the frequency adjustment method matches the frequency command. For example, if the inverter is set to communication frequency adjustment, but the program is written as analog quantity adjustment, the frequency command generally includes local adjustment (keyboard potentiometer analog quantity adjustment), external analog quantity control (current or voltage method), high-speed pulse setting, multi-speed setting, PID control, and communication control. After confirming the command, check if the wiring method is correct, such as positive and negative terminals, COM terminal, communication line AB, etc.
3. Check the program. For example, in analog control, check for errors or discrepancies between digital and analog outputs. Use a multimeter to measure the voltage or current output of the DA module to see if it matches your calculations. Check if the DA module is damaged. In communication control, check the communication indicator lights for reading and writing to see if they are flashing or constantly lit (depending on the hardware). First, ensure normal communication. Check if communication parameters such as station number, parity, and baud rate are consistent with the settings on the host computer, and whether the inverter can accurately receive signals.
4. Check if the frequency converter is damaged, such as alarms or malfunctions, or damage to the wiring terminals. If all these are normal, the frequency can be adjusted before connecting the motor. If the motor load is too high, the frequency may not be able to be adjusted.
The correctness of the wiring of a general-purpose frequency converter directly affects whether the converter can start normally and whether it can input and output normally, which plays an important role in the accuracy of the results. So let's take a look at the precautions for frequency converter wiring.
I. Main circuit wiring:
1. The inverter inputs (R, S, T) and outputs (U, V, W) must never be connected incorrectly.
2. Main power path selection: The wiring method between the power supply and the frequency converter is the same as the wire diameter selection method for motors of the same capacity; the wiring between the frequency converter and the motor should take into account the line voltage drop ΔU, and the general requirement is: ΔU≤(2～3)Un Where: Imn-motor rated current (A), R0-resistance of conductor per unit length (mΩ/m).
II. Wiring of the control circuit:
1. Shielded wires should be used for analog control lines. Connect one shielded end to the common terminal (COM) of the inverter control circuit, and do not connect it to the inverter ground terminal (E) or earth. Leave the other end floating.
2. Unshielded wires are allowed for switch control lines, but the two wires of the same signal must be twisted together.
III. Grounding of the frequency converter:
When multiple frequency converters are grounded, each frequency converter should be connected to the earth separately. It is not allowed to connect the grounding terminals of one frequency converter to the grounding terminals of another frequency converter before grounding.
① A circuit breaker with grounding and leakage protection should be installed on the input side of the power supply and frequency converter. It is highly sensitive to frequency converter current. Additionally, an air switch and an AC electromagnetic contactor should be installed. The air switch itself has overcurrent protection and can automatically reset; it can also be operated manually under fault conditions. The AC electromagnetic contactor is controlled by contact input and can connect the fault output of the frequency converter and the output of the motor overheat protection relay, thereby cutting off the power supply from the input side of the entire system in case of a fault, achieving timely protection. If both the AC electromagnetic contactor and the leakage protection switch fail simultaneously, the air switch can also provide reliable protection.
② A thermal relay should be installed between the frequency converter and the motor, especially when the frequency converter drives a high-power motor. Although the frequency converter has built-in thermal protection, this may not be sufficient for protecting the external motor. This is because users often select a frequency converter with a capacity greater than the motor's rated capacity. If the user-set protection value is inadequate, the frequency converter may not have time to activate before the motor burns out; or, if the frequency converter's protection fails, the motor will require external thermal relay protection. This is especially important when driving older motors, as rust and aging can reduce their load capacity. Considering these factors, an external thermal relay allows for intuitive and quick setting of protection values. Thermal relay protection is particularly necessary in systems with multiple motors operating or with power frequency/frequency conversion switching.
③ When the connection line between the frequency converter and the motor is too long, the thermal relay may malfunction due to the effect of high-order harmonics. Therefore, it is necessary to install an AC reactor between the frequency converter and the motor, or use a current sensor in conjunction with a relay for thermal protection to replace the thermal relay.
④ The primary purpose of grounding the frequency converter system is to prevent leakage current, interference, and external radiation. The circuit must be grounded in accordance with electrical equipment technical standards and regulations, and a valid and secure grounding stake must be selected.
For unit-type frequency converters, the grounding wire can be directly connected to the grounding terminal of the frequency converter; when the frequency converter is inside a distribution cabinet, it should be connected to the grounding terminal or grounding busbar of the distribution cabinet. In either case, it must be directly connected to the grounding electrode or grounding busbar, not through the grounding terminals or grounding busbar of other equipment. According to electrical equipment technical standards, the grounding wire must be a soft copper wire with a diameter of 1.6mm or more.
⑤ In order to enhance the reliability of the transmission system, the design principle of maintenance measures is generally multiple redundancy. Although a single maintenance plan can save money, it will reduce the overall safety factor of the system.
