Abstract The overvoltage fault protection of the frequency converter is a protective measure taken when the intermediate DC voltage of the frequency converter reaches a dangerous level. This is a major defect in the design of voltage-type AC-DC-AC frequency converters. There are many reasons for this fault in the actual operation of the frequency converter. In the case of overvoltage of the frequency converter in the tea machine, we analyzed the two causes of overvoltage and then took appropriate measures after clarifying the cause of the fault.
Keywords: frequency converter; overvoltage fault; tea machine
1. Introduction
Inverter overvoltage fault protection is a protective measure taken when the intermediate DC voltage of the inverter reaches a dangerous level. This is a major defect in the design of voltage-type AC-DC-AC inverters. There are many reasons for this fault in the actual operation of the inverter. In the case of overvoltage in a tea machine inverter, we analyzed the two causes of overvoltage, clarified the cause of the fault, and took several targeted measures to deal with it.
2. Overvoltage fault symptoms of the frequency converter in the tea machine
A tea factory user was using two Emerson EV1000-4T0022G (2.2KW) frequency converters to control two 6CBC type octagonal roasting and drying machines (as shown in Figure 1). One frequency converter operated smoothly throughout, while the other began to occasionally experience an E006 constant speed overvoltage fault after two weeks of operation. The user then increased the frequency converter's power rating to 3.7KW, but the E006 fault persisted.
Figure 1. 6CBC type star anise roasting and drying machine
3. Introduction, causes and countermeasures of inverter overvoltage
(1) The problem of overvoltage
The overvoltage fault protection of the frequency converter is a protective measure taken when the intermediate DC voltage of the frequency converter reaches a dangerous level. This is a major defect in the design of voltage-type AC-DC-AC frequency converters. There are many reasons for this fault in the actual operation of the frequency converter, and there are also many measures that can be taken. When dealing with this type of fault, it is necessary to analyze the cause of the fault clearly and take corresponding measures to deal with it.
Most general-purpose frequency converters are voltage-type AC-DC-AC converters. From the basic structure diagram of a frequency converter, we can see that three-phase AC power first passes through a diode uncontrolled rectifier bridge to obtain pulsating DC power, then is filtered and regulated by electrolytic capacitors, and finally outputs AC power with adjustable voltage and frequency through a passive inverter to power the motor. Generally speaking, the energy of a load can be divided into kinetic energy and potential energy. Kinetic energy (its magnitude is determined by the speed and weight of the load) accumulates as the object moves; when the kinetic energy decreases to zero, the object is at rest. Figure 2 shows four operating modes of motor drives. A common characteristic of loads involved in many technological applications is that the motor must not only operate in motoring mode (quadrants I and III) but also in regenerative braking mode (quadrants II and IV).
Figure 2. Four operating modes of motor drive
For frequency converters, if the output frequency decreases, the motor speed will also decrease accordingly, resulting in a braking process. The power generated by this braking will return to the frequency converter side. Since the energy transfer of the diode-controlled rectifier is irreversible, the generated regenerative energy is transferred to the DC-side filter capacitor, generating a pump-up voltage. However, fully controlled devices such as GTRs and IGBTs have low voltage withstand capabilities, and excessively high pump-up voltage may damage switching devices, electrolytic capacitors, and even the motor insulation, thus threatening the safe operation of the system. This limits the application range of general-purpose frequency converters. Therefore, this power must be dissipated, for example, by using resistors to dissipate the heat. When used for lifting loads, if the load decreases, the energy (potential energy) must also return to the frequency converter (or power supply) side. This operation method is called "regenerative braking."
If, during load deceleration or prolonged reverse driving, the power flowing from the motor side to the inverter's DC bus is not dissipated through heat, but instead returned to the inverter's power supply side or consumed by other motors in parallel with the DC bus, this method is called regenerative braking. Clearly, if it is necessary to directly return the energy to the power supply side, a special device, namely an energy feedback unit, is required.
In summary, to improve braking capacity, we cannot simply expect to solve the problem by increasing the capacity of the frequency converter. Instead, we must adopt methods to handle regenerative energy: resistive energy consumption braking and regenerative braking.
(2) The hazards of frequency converter overvoltage faults
Inverter overvoltage mainly refers to overvoltage in its intermediate DC circuit. The main hazards of overvoltage in the intermediate DC circuit are:
a. It can cause magnetic circuit saturation in the motor. For motors, excessively high voltage will inevitably increase the magnetic flux of the motor core, which may lead to magnetic circuit saturation, excessive excitation current, and consequently, excessive temperature rise in the motor.
b. Damage to motor insulation. When the intermediate DC circuit voltage increases, the pulse amplitude of the inverter's output voltage becomes excessive, significantly impacting the lifespan of the motor insulation.
b. It directly affects the lifespan of the intermediate DC circuit filter capacitor, and in severe cases, it can cause the capacitor to explode. Therefore, inverter manufacturers generally limit the overvoltage value of the intermediate DC circuit to around DC 800V. Once the voltage exceeds the limit, the inverter will trip for protection according to the specified requirements.
Due to the serious hazards of overvoltage, users must consider using braking methods in the following frequency converter applications: motors driving large inertia loads (such as centrifuges, gantry planers, tunnel trolleys, and overhead cranes) requiring rapid deceleration or stopping; motors driving potential energy loads (such as elevators, cranes, and mine hoists); and motors that are frequently in a driven state (such as centrifuge auxiliary machines, paper machine guide roller motors, and chemical fiber machinery drawing machines).
(3) Countermeasures for handling overvoltage faults
The key to handling overvoltage faults is twofold: first, how to promptly handle excess energy in the intermediate DC circuit; and second, how to avoid or reduce the feeding of excess energy into the intermediate DC circuit, thus limiting the degree of overvoltage to within permissible limits.
The following are the main countermeasures:
a. Add an absorption device on the power input side to reduce overvoltage factors.
In cases where there is a possibility of surge overvoltage, lightning-induced overvoltage, or overvoltage caused by the compensation capacitor during closing or opening on the power input side, surge absorption devices or series reactors can be connected in parallel on the input side to address the issue.
b. Find a solution from the parameters already set on the frequency converter.
c. Analyze the process flow and find solutions within it.
d. Using the method of increasing braking resistance
e. Method of adding an inverter circuit on the input side
f. Using the method of adding an appropriate capacitor to the intermediate DC circuit.
g. If conditions permit, appropriately reduce the mains frequency power supply voltage.
h. Method for multiple frequency converters to share a DC bus
i. Solve the inverter overvoltage problem by leveraging the functional advantages of the control system.
4. Overvoltage Analysis and Handling of Tea Machine Inverter
Since the frequency converter can operate normally after reset, the voltage changes during operation should be the focus of inspection. Measuring the DC bus voltage UPN of the EV1000-4T0022G frequency converter revealed occasional voltage increases during constant speed operation. When the voltage reached 760V, the frequency converter reported E0006 (constant speed overvoltage fault). This phenomenon indicates that the octagonal star anise roasting dryer experienced regenerative feedback during constant speed operation due to instability in the mechanical center of gravity.
To further confirm that the inverter itself was not the cause of the E006 fault, this faulty inverter was used to control another working dryer. The result showed that the inverter worked well, which further demonstrates that the fault was unrelated to the quality of the inverter itself. For feedback phenomena occurring during constant speed operation, an energy-consumption braking method should be adopted. Only a braking resistor needs to be added to the inverter side (because the EV1000 series inverter has a built-in braking unit). The value of the braking resistor can be obtained from experience, by referring to a table, or by calculation (as described in the previous section). Here, 200Ω/500W is selected.
The tea factory user connected the wires as shown in Figure 3.
Figure 3. Variable frequency control of an octagonal star anise roasting machine with braking resistor.
After powering on the frequency converter, modify the following parameters:
(1) F2.13: Select "1" for energy-efficient braking.
This parameter is set according to the actual situation, that is, "0" is a non-energy-consuming braking component, and "1" is an energy-consuming braking component.
(2) F2.14: Braking utilization rate is 10%.
Braking utilization rate is selected as 10% based on actual conditions.
After the parameters were modified, the inverter operated under load and functioned well, thus eliminating the fault.
5. Summary and Conclusion
When the DC bus voltage is monitored during constant speed operation of the frequency converter and meets the following condition (DC bus voltage ≥ overvoltage protection point (parameter setting)), the frequency converter will activate protection and display fault code E006. For this fault, troubleshooting can be performed step-by-step according to Table 1.
Table 1 Troubleshooting for Fault E006
For Emerson frequency converters, proper selection of braking resistors is a prerequisite for system configuration. Table 16.3 shows the selection table of braking resistors for the built-in braking unit of the EV1000/TD1000 series.
Table 2 Braking Resistor Selection List
References:
[1] Li Fangyuan. Principles and Maintenance of Frequency Converters [M]. Beijing: China Machine Press, 2010.
English:
The trouble-shooting for the overvoltage in AC inverter of Tea Machine
LI Fang-yuan
(Zhejiang Business Technology Institute, Ningbo 315012, China)
Abstract : AC Inverter overvoltage fault protection is taken after when the inverter DC voltage reaches a dangerous level. It is a major flaw for voltage source AC - DC - AC inverter design. There are many reasons for this failure in the actual operation of the inverter. In the case of the inverter overvoltage for the tea machine, by analyzing two reasons, the cause of the malfunction is found and the measure is taken.
Key words : AC inverter; Overvoltage Fault; Tea machine
