In the use of frequency converters , improper selection and use often lead to malfunctions, equipment failures, production interruptions, and unnecessary economic losses. This article uses the Fuji FRNP7/G7 frequency converter as an example to discuss several issues that should be noted when using frequency converters.
1. Selection
A feed oil-water separator pump uses variable frequency drive (VFD) control. The motor model is JR127_10, 115kW, Ue=380V, Ie=231A, and it uses an FRNll10P7-4EX VFD . During operation, it was found that sometimes, although the given frequency was high, the actual frequency could not be adjusted, and the VFD tripped frequently, with the fault indicator showing "OL1", indicating VFD overload. Inspection revealed that the VFD's rated current was 210A, while the oil-water separator pump motor's operating current fluctuated around 220A at high feed rates. The drive torque reached its limit setting, preventing the frequency from being adjusted upwards. The operating current exceeded the VFD's rated current, causing the VFD to trip due to overcurrent. The analysis concluded that the cause was an undersized VFD.
The selection of frequency converters should meet the following conditions:
(1) The voltage level is compatible with the control motor.
(2) The rated current is 1.1 to 1.5 times the rated current of the control motor.
(3) Select the type of frequency converter according to the load characteristics of the controlled equipment.
The oil separator pump is a constant torque load, so it is best to choose a G7 frequency converter with a wide driving torque limit range. The FRNl60G7_4EX frequency converter was selected, with a rated voltage of 400V, a rated output current of 304A, and a driving torque limit of 150%. After switching to the FRNl60G7.4EX, the above problems no longer occurred.
2. Installation Environment
Due to their high integration, compact structure, and significant heat dissipation, frequency converters have stringent requirements regarding the temperature, humidity, and dust levels of the installation environment. At the Shanxi Aluminum Plant, the frequency converters were installed in the control room. Because the workshop was a dry-process workshop, the operating environment for the frequency converters was poor, with high dust levels and high temperatures in summer, leading to several frequency converter failures. After sealing the control room and adding cooling facilities, the situation improved significantly. Later, however, a failure occurred due to excessive condensate from the central air conditioning system in the control room, which was too close to the control cabinet, resulting in damage to components on the frequency converter control board. This demonstrates that providing a suitable operating environment for frequency converters is crucial during installation.
3. Parameter settings
Inverters have many setting parameters, each with a certain selection range. During use, it is common to encounter situations where the inverter cannot work properly due to improper setting of individual parameters.
(1) External start/stop button and potentiometer frequency adjustment are ineffective. The inverter is factory set to be operated via the keypad panel, and external control is ineffective. Terminal FWD_CM is shorted with a jumper. When selecting external start/stop and frequency adjustment control, this jumper must be removed. The above problem may be caused by the FWD_CM jumper not being removed, or by incorrect selection of operation mode and frequency adjustment mode parameters. This part should be checked carefully.
(2) The frequency converter works normally when the motor is unloaded, but cannot start under load. This problem often occurs with constant torque loads. At a Shanxi aluminum plant, an FRN160P7.4EX frequency converter worked normally during no-load testing, but tripped immediately under load. Increasing the acceleration and deceleration times did not resolve the issue. Further checking the torque boost value and changing it from "2" to "7" improved the voltage output at low frequencies. This improved the low-frequency load characteristics, and the motor started normally under load. When encountering the above problem, the acceleration and deceleration time settings and torque boost settings should be checked.
(3) The inverter trips due to overload before the motor starts even after being put into operation. A 7.5kW 6-pole motor at a Shanxi aluminum plant uses inverter control. The inverter frequently trips during startup. Investigation revealed that the original bias frequency was set to 2H2. Before the inverter receives the running command but provides the frequency adjustment signal, the controlled motor continuously receives the low-frequency 2H2 running command and cannot start. Measurements showed that the motor's stall current reached 47A, approximately three times the motor's rated current, indicating that the inverter's overload protection was functioning normally. Changing the bias frequency to 0Hz resolved the issue, and the motor started normally.
(4) The frequency has reached a relatively high value, but the motor speed is still not high. A newly commissioned frequency inverter has a high frequency setting, but the motor speed is significantly lower than other motors at the same frequency. The frequency gain setting is checked and found to be 150%. According to the definition of frequency setting signal gain, the setting gain is the ratio of the set analog frequency signal to the output frequency. Assuming the set frequency is 30Hz, the actual output frequency is only 20Hz. The problem was solved after changing the setting gain to 100%.
(5) When the frequency rises to a certain value and continues to be adjusted upwards, the frequency remains at a certain value and keeps jumping, while the speed cannot be increased. When the frequency converter is working, it will automatically calculate the output torque and limit the output torque within the set value. If the drive torque setting value is too small, the output torque may be limited, causing the frequency converter's output frequency to fail to reach the given frequency. If you encounter the above problem, you should check whether the drive torque setting value is too small and whether the frequency converter's capacity is too small, and then try to solve the problem.
4. Fault Diagnosis
The frequency converter has strong fault diagnosis capabilities, protecting against faults in its internal rectifier and inverter components, CPU, peripheral communication systems, and motors. The frequency converter will continuously display fault codes after a protection trip and before a fault reset. Determining the cause of the fault based on the fault indication codes can narrow down the fault search area and significantly reduce troubleshooting time.
(1) When a frequency converter was started after cleaning, it tripped due to an "OH2" fault indication. OH2 indicates an external fault in the frequency converter. At the factory, the terminals "THR" and "CM" connecting the external fault signal are shorted together with a jumper. Since this frequency converter did not have external protection installed, THR_CM should still be shorted. Upon inspection, it was found that the jumper between "THR" and "CM" was loose and fell off during cleaning. After restoring the jumper, the frequency converter operated normally.
(2) The frequency converter tripped immediately upon startup, with the fault indicator "OCl". OCL indicates overcurrent during acceleration, suggesting a motor fault. The connection between the frequency converter and the motor was disconnected, and a short circuit between the motor windings was checked. After replacing the motor, the frequency converter operated normally.
(3) In summer, if the cooling and ventilation of the inverter control room are inadequate and the ambient temperature rises, the "OH1" and "OH3" overheat protection will frequently trip. At this time, check whether the fan inside the inverter is damaged and whether the temperature of the control room is too high. Take measures to force cooling to ensure that the inverter can safely get through the summer.
(4) When the frequency of the inverter is adjusted to above 15Hz, the "LU" undervoltage protection trips. The "LU" protection signal indicates insufficient rectified voltage. We checked from the rectifier section to the power input terminal of the inverter and found that a phase was missing on the power input side. Since the voltmeter takes signals from the other two phases, the voltmeter reading was normal, and the phase loss on the inverter input side was not detected in time. After the phase is missing at the input terminal, the rectified output voltage of the inverter drops. In the low-frequency range, the frequency can still be adjusted due to the effect of the charging capacitor. However, after the frequency is adjusted to a certain value, the rectified voltage drops rapidly, causing the inverter to trip with "LU".
5. Maintenance
During inverter operation, the operating status can be visually inspected from the outside of the equipment for any abnormalities. Dedicated inspectors can use the keypad on the control panel to view the inverter's operating parameters, such as output voltage, output current, output torque, and motor speed, to understand the inverter's daily operating range and promptly identify any problems with the inverter and motor. In addition, the following points should be noted:
(1) Assign a dedicated person to regularly clean and blow away dust from the inverter to keep the inside of the inverter clean and the air duct unobstructed.
(2) Keep the environment around the frequency converter clean and dry. Do not place any objects near the frequency converter.
(3) After each maintenance of the frequency converter, carefully check for any missing screws and wires to prevent small metal objects from causing short circuits in the frequency converter.
(4) When measuring the insulation of the frequency converter (including the motor), a 500V megohmmeter should be used. If only the frequency converter is being tested, all external wiring connected to the frequency converter terminals should be disconnected. After cleaning the components, short-circuit all the main circuit terminals with wires and test them with a megohmmeter to ground. If the megohmmeter reading is above 5MΩ, it indicates that it is normal. The purpose of this is to reduce the number of tests. (Yunxin Electromechanical Automation Class)
