Category: Industrial control technology

In the gypsum board production industry, precision control of cutting, water supply, and belt conveyors is crucial. To achieve this, many facilities install frequency converters (VFDs) like those from the Comron brand. However, issues such as fluctuating frequency values and unexplained shutdowns can disrupt operations and reduce efficiency. This article delves into these problems, their causes, and effective solutions, ensuring your VFDs operate smoothly.

The Challenge: Frequency Fluctuations and False Shutdowns

During the installation of four low-power Comron VFDs in a control cabinet for synchronous speed control, significant fluctuations in displayed speed values were observed. These fluctuations were as high as ±30 revolutions, raising concerns about speed instability. Initially, grounding the main circuit’s G terminal and shielding the speed control signal line improved the situation but did not eliminate the problem entirely.

Further investigation revealed that the issue persisted even when running a single VFD unit, with fluctuations increasing when multiple units were operational. Despite attempts to resolve the issue, including consultations with the manufacturer, a definitive solution was not found. However, the problem was temporarily deemed tolerable and left unresolved.

A Similar Issue: Repeated False Shutdowns

In another gypsum board factory, a 3.7kW water supply VFD experienced repeated shutdowns, sometimes exceeding ten times a day. On-site observations showed erratic speed display values, with all three digits flashing, and the FWD indicator light also flashing intermittently. The VFD would either recover and continue operating or require a restart. Occasionally, it would display F000 and enter the parameter setting state inexplicably, suggesting external interference with the CPU’s operation.

Identifying the Root Cause: Signal Interference

Given the symptoms, it was evident that signal interference was the culprit. The VFD’s carrier wave, generated during operation, was entering the CPU’s I/O port via the operation panel’s connection cable. This interference caused fluctuations in input frequency and display values and, in severe cases, triggered random shutdowns.

Step-by-Step Solution: Combating Interference

1. Conventional Grounding Treatment:
   • As a first step, ensure proper grounding of the main circuit and shield the speed control signal line. While this may offer some improvement, it often does not completely resolve the issue.
2. Adjusting Carrier Frequency:
   • Lowering the VFD’s carrier frequency to the minimum setting (e.g., 2kHz) can reduce interference. This step may provide slight improvements but is usually not sufficient on its own.
3. Shielding the Connection Cable:
   • Wrapping a layer of tin foil around the cable connecting the control panel to the VFD can further reduce interference. This method is effective but may not be the most elegant or long-term solution.
4. Using Magnetic Rings:
   • The most effective solution involves installing magnetic rings on the signal line. Purchase magnetic rings with a moderate diameter and wind 2-3 turns at each end of the connecting wire. This simple yet powerful method can eliminate speed value fluctuations and stabilize VFD operation.

Results: Stable and Reliable Operation

After implementing the above steps, particularly the use of magnetic rings, the displayed speed value stabilized, and the flashing FWD indicator light ceased. Even the smallest fluctuations disappeared, ensuring consistent and reliable VFD performance.

Conclusion: Ensuring Smooth VFD Operation

Frequency fluctuations and false shutdowns in Kemron VFDs can disrupt production and reduce efficiency. By understanding the root cause—signal interference—and implementing targeted solutions, you can ensure your VFDs operate smoothly and reliably. Grounding treatments, adjusting carrier frequencies, and shielding connection cables are helpful steps, but using magnetic rings is the most effective solution. With these measures in place, you can maintain precise control over your gypsum board production process, enhancing productivity and quality.

By addressing these issues proactively and implementing the recommended solutions, you can avoid costly downtime and ensure your VFDs contribute to the success of your gypsum board production operations.

Repair Process and Analysis of a TECO 7200GA-22kW VSD after a Lightning Strike

In the realm of industrial equipment maintenance, dealing with the aftermath of a lightning strike on sensitive electronic devices such as variable speed drives (VSDs) can be particularly challenging. This case study delves into the repair process of a TECO 7200GA-22kW (41kVA) VSD that suffered damage due to a lightning fault. By dissecting the diagnostic steps, repairs, and underlying issues, we aim to provide insights that can aid technicians in similar situations.

Initial Assessment and Repairs

Upon receiving the damaged VSD for repair, the initial inspection revealed that the input rectifier module, switch tube, and shunt tube of the switching power supply had been compromised by the lightning strike. These components were promptly replaced. Following the replacements, the operation panel screen lit up normally, suggesting that the core functionality of the VSD had not been severely impacted.

Subsequent tests on the negative pressure and optocoupler drive input signals for the six channels returned normal results, further reassuring that the repair was progressing positively. However, during the assembly test of the entire unit, an Over Current (OC) fault was triggered immediately upon power-up. Interestingly, the VSD could be reset and started, with the screen displaying normal frequency output values. But during actual testing, no three-phase voltage output was observed at the U, V, and W terminals.

Investigating the Drive Circuit

The local driving ICs employed in the VSD were optocouplers PC923 and PC929, which worked in conjunction with the SN0357 to relay OC signals. An examination of the power amplifier circuit on the output side of the driving ICs and the IGBT tube’s detection circuit revealed no abnormalities. The focus then shifted to the pulse input pin of the PC923 optocoupler, where an anomaly was detected.

Specifically, the level of pin 3 was unexpectedly high while pin 2 was low. This raised the question of whether the driver power supply had been reversed. Normally, in optocoupler circuits, the two pins (anode of the photodiode) are powered by +5V and regulated to provide an excitation power of around 4V, while the third pin (cathode of the photodiode) connects to the pulse output terminal of the CPU. A low-level output is effective, meaning that when the CPU outputs a signal, current is drawn from pin 3 of the PC923, causing the diode to conduct.

Diagnosing the Power Supply Issue

Further investigation revealed that the power supply to pin 2 of the PC923 was a simple series-connected voltage regulator comprising a transistor and a voltage regulator. A faulty base bias resistance in this circuit was identified as the culprit, resulting in a zero supply voltage to pin 2. Replacing the bias resistor restored the voltage levels on pins 2 and 3 of the PC923 to normal. Consequently, upon receiving an operation command, the VSD began outputting voltage from the U, V, and W terminals.

Addressing the OC Fault

With the output issue resolved, attention turned to the persistent OC fault upon power-up. Measurements of the SN0357 optocoupler, responsible for transmitting the OC signal, showed that the voltage value of the two pins on the input side was zero, indicating no OC signal was being input. However, the voltage on the output side of the three optocouplers was 0.5V, which was abnormal. In the absence of an OC signal, the voltage between the two pins should have been 5V (one pin connected to a 5V ground level).

This anomaly pointed to a problem with the 5V pull-up resistance on the signal output pin, which had either changed or become open-circuit. As a result, the CPU erroneously interpreted the situation as receiving an OC signal from the drive circuit, triggering the alarm. Connecting a 10k resistor between the signal output pin and the 5V power supply resolved the issue. Subsequent power-ups showed the signal output pin at 5V, and the OC fault no longer occurred.

Root Cause Analysis

Both faults—the lack of output voltage and the OC fault upon power-up—stemmed from a common issue: power loss. The input pin of the pulse signal and the output pin of the OC signal were directly connected to the CPU pin. When the pull-up high level disappeared, the CPU pin was left with only a low level of 0.5V, insufficient to drive the optocoupler or trigger the inverter module. This low level also led to the detection of OC signals and the jumping of the OC fault code during power-up.

Additional Insights

The motherboards of various Dongyuan VSD series exhibit good replacement characteristics, requiring only the adjustment of the VSD’s capacity value after replacement. This adjustment automatically modifies the relevant parameters for checking the motor’s rated current value. It is worth noting that Dongyuan VSDs’ capacity labeling is often based on kVA rather than kW. For instance, a 22kW capacity is labeled as 41kVA with a rated current of approximately 48A.

The relationship between kW, kVA, and HP values can be confusing for users. In simple terms, 1HP equals 0.75kW, indicating that HP is less than the kW value. kW represents active power, closely approximating the actual power value, while kVA is the apparent power, which includes both active and reactive power components. When selecting a VSD, the kVA value should be chosen based on the rated current value, as it accounts for the reactive power consumption of inductive loads like motors.

Conclusion

This case study underscores the importance of thorough diagnostics and a methodical approach in repairing VSDs damaged by lightning strikes. By understanding the intricacies of the drive circuit, power supply, and signaling mechanisms, technicians can effectively pinpoint and resolve issues, ensuring the reliable operation of industrial equipment. The repair process not only restored the VSD to working condition but also provided valuable insights into the underlying causes of the faults, contributing to a deeper understanding of VSD technology.

In the realm of industrial machinery, inverters play a crucial role in converting direct current (DC) to alternating current (AC), enabling the efficient operation of various electrical devices. However, like any complex electronic system, they are susceptible to faults, especially after encountering events such as lightning strikes. This article delves into the diagnostic process and subsequent repair of a TECO 7200GA-30kW inverter that exhibited random shutdowns and starting difficulties following a lightning-related repair.

Initial Symptoms and Preliminary Inspections

The inverter, after being repaired from lightning damage, operated inconsistently for over a month. It experienced random shutdowns, sometimes occurring every few days or hours, and encountered difficulties during the starting process. Specifically, the capacitor charging short circuit contactor would jump, leading to failed starts, yet the operation panel did not display any fault codes. Despite these challenges, the inverter could occasionally start and run for a period before shutting down again.

To begin the diagnostic process, the control board was removed from the site. To bypass potential thermal protection issues, the terminals of the thermal relay were short-circuited. Similarly, the contact detection terminal of the capacitor charging contactor was short-circuited to prevent low voltage protection states. However, these measures did not resolve the issue, prompting a more comprehensive inspection. Upon detailed examination, no visible abnormalities were found, indicating that the problem was likely more complex and required a deeper investigation.

Identifying the Root Cause

With the control board reinstalled, testing revealed that the contactor jumped during startup, preventing the inverter from initiating. Interestingly, unplugging the 12CN plug cooling fan significantly improved the starting success rate. Observing the display panel during startup showed a decrease in brightness, suggesting a problem with the control power supply’s load capacity.

Further testing under load conditions revealed that while the power supply outputs were normal when unloaded, connecting resistive loads caused voltage drops. Specifically, when a 24V load was connected to the cooling fan and relay, the +5V line dropped to +4.7V. Although the screen display and other operations remained normal at this voltage, attempting to start the inverter resulted in the relay jumping and occasional fault codes such as “low DC voltage” and “communication interruption between CPU and operation panel.” Measurements indicated that when the +5V dropped below +4.5V, the inverter would automatically switch from the starting state to standby mode.

Analysis and Solution

The diagnosis of poor load capacity in the control power supply was confirmed. The CPU, with its strict power supply requirements, could barely function at voltages no less than 4.7V but would enter standby mode at voltages below 4.5V. The challenge lay in the fact that despite thorough checks, no components of the switch power supply were found to be damaged.

In an attempt to adjust the output voltage, a parallel resistance test was conducted on R1, one of the reference voltage divider resistors of the U1 (KA431AZ) voltage regulator. While this slightly increased the output voltage, the load capacity remained insufficient. Closer inspection of the circuit board revealed welding marks on the diversion adjustment tube Q1, indicating potential previous modifications. However, replacing Q1 with a similar component from another machine did not resolve the issue.

Given the complexity of the circuit and the high specificity of the transistors involved, particularly the bipolar transistor Q2 with high back pressure and amplification, a more nuanced approach was required. Analysis suggested that the working point of the shunt adjustment tube Q1 had shifted, causing excessive shunting of the base current of Q2. This, in turn, compromised the power supply’s load capacity.

To address this, a resistor (R6, 330 ohms) was series-connected with a voltage feedback optocoupler and a 47-ohm resistor. This modification aimed to reduce the base current of Q1, thereby decreasing its shunting capability towards Q2 and enhancing the power supply’s load capacity. Upon testing, the +5V line stably maintained 5V output, both under load and during startup operations, effectively resolving the issue.

Fault Inference and Conclusion

The fault was inferred to be due to the aging of the Q1 switch tube, leading to a decrease in its amplification ability. This resulted in insufficient base current (Ib) after shunting, causing Q1 to become fully conductive and increasing its conduction resistance. Additionally, a characteristic deviation in the shunt branch led to excessive shunting and poor driving of the switching tube, further reducing the power supply’s load capacity.

In summary, the repair process of the TECO 7200GA-30kW inverter involved meticulous diagnostic steps, from initial symptom observation to component testing and eventual circuit modification. The successful resolution highlights the importance of understanding the intricate workings of electronic systems and the creative application of engineering principles to overcome unexpected challenges. This case study serves as a valuable resource for technicians and engineers facing similar issues in industrial electronics repair.

When dealing with complex electronic devices like the Jialing VSD JP6C-9, encountering issues such as a malfunctioning switch power supply can be both frustrating and challenging. This article details a step-by-step diagnostic and repair process for a specific case where the operation panel failed to display any information upon powering on, despite the input and output terminals of the main circuit exhibiting normal resistance levels. The root cause was identified as a power failure on the control board switch, accompanied by tell-tale signs of an abnormal power load.

Initial Observations and Symptom Analysis

Upon initial inspection, the most glaring symptom was the complete lack of display on the operation panel. Given that the resistance checks on the main circuit’s terminals were within acceptable ranges, it became evident that the issue was not with the primary power delivery but rather with the control circuitry. The presence of intermittent clicking sounds, which indicated a struggle in initiating the power supply, further narrowed down the potential culprits. Based on experience, such sounds often suggest issues related to power load abnormalities.

Systematic Diagnosis

The first step in the diagnostic process involved examining the rectification, filtering, and load circuits of each power supply segment. These components are crucial in ensuring smooth and stable power delivery. After thorough checks, no irregularities were found in these circuits, ruling out common failure points.

Next, an attempt was made to isolate and disconnect high-current power supply branches, including the cooling fan power supply, inverter drive power supply, and operation panel display power supply. This isolation technique is often effective in pinpointing which branch might be causing the overload. However, even after disconnecting these branches, the problem persisted, indicating that the issue was more deeply ingrained within the core power supply circuitry.

Deep Dive into the Switch Transformer Circuit

With the basic circuits ruled out, attention turned to the more intricate components of the power supply, specifically the peak voltage absorption network associated with the primary winding of the switch transformer. This network, typically comprising a resistor and capacitor in parallel, connected in series with a diode, serves a critical role in protecting the switching tube from abnormal peak voltages during its cut-off period.

Using a pointer multimeter, the forward and reverse resistance of the diode was measured, revealing an unusual reading of 15 ohms in both directions. This anomaly prompted a closer examination of the diode itself. Upon disassembly and individual testing, the diodes were found to be functional, suggesting that the issue lay elsewhere in the network.

A meticulous visual inspection revealed subtle cracks in the capacitor within the peak voltage absorption network. Further testing confirmed that the capacitor, rated at 2kV 103, had indeed suffered a breakdown and short-circuited. This discovery was crucial, as the capacitor’s failure had significant implications on the operation of the switch transformer.

Understanding the Impact of the Capacitor Failure

In a properly functioning system, the peak voltage absorption network absorbs energy during the switching tube’s conduction period, which is then safely discharged through the diode during the tube’s cut-off period. This mechanism prevents excessive voltage spikes from damaging the switching tube. However, with the capacitor short-circuited, the primary winding of the switch transformer was effectively paralleled with the diode. As a result, the energy accumulated during the conduction period was rapidly discharged, preventing the necessary oscillation energy from being stored.

Moreover, the diode acted as an excessive load on the switch transformer, impeding its ability to initiate the power supply’s startup sequence. This unusual manifestation of a capacitor short-circuit causing startup difficulties is relatively rare and underscores the importance of meticulous component inspection in troubleshooting such issues.

Repair and Restoration

With the faulty capacitor identified, the repair process involved replacing it with a new one of the same specifications. After the replacement, the machine was powered on once again, and this time, the operation panel lit up as expected, indicating a successful repair. The switch power supply was now functioning correctly, free from the previous startup difficulties.

Conclusion

This case study highlights the intricacies involved in diagnosing and repairing switch power supply malfunctions, particularly in sophisticated devices like the Jialing VSD JP6C-9. It underscores the importance of a systematic approach, thorough component inspection, and the use of appropriate diagnostic tools. By understanding the functionality of each component and its role in the overall system, technicians can effectively pinpoint and resolve even the most perplexing issues, ensuring the reliable operation of electronic equipment.