When performing regular inspections of frequency converters , the power supply should be disconnected, the frequency converter stopped, and the outer cover of the frequency converter removed. The main focus should be on inspecting areas that cannot be checked without stopping the converter, areas where problems are difficult to detect during routine inspections, as well as checking and adjusting electrical characteristics.
After the inverter is powered off, there is still a high charging voltage on the filter capacitor in the main circuit. Discharging takes a certain amount of time, generally 5 to 10 minutes. You must wait for the charging indicator light to go out and use a voltmeter to test and confirm that the voltage is below the safe value of 25VDC before you can start the inspection work.
1. Check if the cooling system is working properly and clean the dust from the air filter.
First, clean all parts inside the inverter . Use a vacuum cleaner to remove internal dust, and wipe away any residue with a soft cloth. Since dust and foreign objects may fall in during operation, cleaning should proceed from top to bottom. Carefully wipe the leads, insulated terminals, and capacitor ends of the main circuit components with a soft cloth. The cooling fan system and ventilation ducts should also be thoroughly cleaned to ensure the inverter's internal cleanliness and unobstructed airflow.
2. During inverter operation, factors such as temperature rise and vibration often cause loosening of main circuit components, control circuit terminals, and leads, leading to corrosion, oxidation, poor contact, and wire breaks. Therefore, it is necessary to check whether screws, bolts, and other fasteners are loose and tighten them as necessary. For parts with solder and crimped terminals, check for corrosion, discoloration, cracks, damage, etc. Also check whether the frame structure is loose and whether conductors and wires are damaged.
3. Check the control circuit board for loose connections, leaking capacitors, and rust or breakage in the wiring. The actual capacitance of capacitors in the control circuit is generally unmeasurable; their performance and lifespan can only be inferred from their surface condition, operating status, and surface temperature rise. If no abnormalities are observed on the surface, it can be considered normal. For resistors, inductors, relays, and contactors in the control circuit, check for loose connections, broken wires, damaged terminal blocks, and rough contacts.
4. Check the filter capacitors for leakage and capacitance reduction. High-performance inverters have an automatic filter capacitor capacitance indication function, displaying the capacitance and initial factory value on the panel, along with the capacitance reduction rate and estimated capacitor life. If the inverter lacks this function, a capacitance meter is needed to measure the capacitance. The measured capacitance should be greater than 85% of the initial capacitance; otherwise, it must be replaced. For surge absorption circuits, check the surge absorption capacitors and resistors for abnormalities, and check the diode limiters and nonlinear resistors for discoloration or deformation.
5. Check if the insulation resistance is within the normal range. Inverters undergo insulation testing at the factory, and users generally do not need to perform further insulation testing. However, if insulation resistance measurement is required during maintenance after a period of operation, the following steps should be followed; otherwise, the inverter may be damaged. All inverter leads should be disconnected before measurement.
(1) Measurement of main circuit insulation resistance. When measuring the main circuit insulation resistance, ensure the main power supply is disconnected and all main circuit terminals, including incoming terminals (R, S, T or L1, L2, L3) and outgoing terminals (U, V, W) as well as external resistor terminals, are short-circuited to prevent high voltage from entering the control circuit. Connect a 500V megohmmeter between the common line and ground (PE terminal). A megohmmeter reading greater than 5MΩ is normal. The method for measuring motor cable insulation is to disconnect the motor cable from the inverter's U, V, W terminals and the motor, and measure the phase-to-phase and phase-to-ground insulation resistance. The insulation resistance should be greater than 5MΩ. The method for testing power cable insulation is to separate the power cable from the inverter's R, S, T or L1, L2, L3 terminals and the power supply, and measure the phase-to-phase and phase-to-ground insulation resistance. The insulation resistance should be greater than 5MΩ. The method for testing the insulation of an electric motor is to disconnect the motor from the cable and measure the insulation resistance of each winding of the motor between the terminals of the motor junction box. The measurement voltage should not be greater than 1000V, nor less than the power supply voltage, and the insulation resistance should be greater than 1MΩ.
(2) Measurement of insulation resistance of control circuit. To prevent high voltage from damaging electronic components, do not use a megohmmeter or other instruments with high voltage for measurement. Instead, use the high range of a multimeter to measure the insulation resistance of the control circuit. A measured value greater than 1 MΩ is normal.
(3) Measurement of insulation resistance of external lines. In order to prevent the high voltage of the megohmmeter from being applied to the frequency converter, the external line to be measured must be disconnected from the frequency converter before measuring the insulation resistance of the external line. It should also be checked whether the high voltage of the megohmmeter may be applied to the frequency converter through other circuits. If so, all relevant connections should be disconnected.
6. After completing the above checks, the protection circuit operation should be checked. It is crucial to ensure the protection circuit is always in a safe operating condition. Therefore, the reliability of the protection function under given values must be checked. The main protection functions to be checked are typically as follows.
(1) Testing the overcurrent protection function. Overcurrent protection is the circuit that most frequently malfunctions in the inverter control system, and it is also the most important circuit for protecting the main circuit components and devices. Generally, it is tested by simulating overload, adjusting the action value, and verifying that it can reliably operate and cut off the output under the set overcurrent value.
(2) Phase loss and undervoltage protection function detection. When a phase is lost or the voltage drops abnormally, it will cause the power unit to fail to commutate, resulting in overcurrent faults, etc. It is necessary to detect the phase loss and undervoltage signals instantaneously and cut off the control trigger signal for protection. The inverter can be powered through a voltage regulator at the power input terminal to simulate phase loss and undervoltage faults, and observe whether the inverter's phase loss and undervoltage protection functions operate correctly.
