Analysis and Treatment of Electrical Faults of Centrifuge

Centrifuge is a kind of solid-liquid separation equipment, which is used to separate the solid particles and liquid with different densities under the action of centrifugal force generated by high speed rotation. It is mainly used for concentration, dewatering, clarification, purification and solid particle classification. It has been widely used in chemical industry, municipal wastewater treatment, tap water purification, tobacco reconstruction, pharmaceutical, food, metallurgy, mineral processing and other fields.

At present, in order to ensure the stable, automatic and continuous operation of the centrifuge, PLC and touch screen are commonly used. If there is an electrical fault in the operation process, it will affect the normal use of the equipment. Therefore, when there is an electrical fault, it is necessary to analyze the cause of the fault and deal with it in the shortest time.

1. Methods of Fault Treatment

1.1 Field Investigation

(1) Inquiry: ask the workers about the process of the fault, the changes of operation parameters, the fault phenomenon and the fault result.

(2) Observation: carefully check the appearance of the electrical equipment and analyze the factors leading to the fault.

(3) Listening: identify whether the working voice of the equipment is normal.

1.2 Logical Reasoning

According to the fault phenomenon, combined with the control flow of the circuit schematic diagram, analyze and determine the area to be checked, finally find out the fault point and solve it.

1.3 Exclusion Method

If the fault point is not clear or difficult to judge, it can be checked when the electrical components and equipment are allowed to be powered on, so as to analyze the cause of the fault.

2. Fault Cases and Troubleshooting Process

2.1 Fault One

The fault is that the auxiliary motor of the centrifuge can not start.

Troubleshooting process:

(1) According to the control principle, the control signal is transmitted to the programmable interrupt controller (PIC) through the touch screen on the control cabinet, and the PLC output port is connected with the intermediate relay (controlling the start and stop of the frequency converter). According to the current, the state of the motor can be known. A megger with a test voltage of 500V is used to detect the insulation between the cable and the motor, so as to judge the state of the cable and the motor.

(2) Monitor the program and check whether the output signal of the auxiliary motor is normal.

(3) Disassemble the control panel of the frequency converter for inspection, and open the auxiliary frequency converter through the short-circuit terminal. If the auxiliary frequency converter operates normally, it means that the frequency converter is in normal state, but the control signal was not transmitted to the frequency converter, so the frequency converter was not started. The problem must be that the output signal was lost during the transmission from PLC to the frequency converter.

(4) Check whether the intermediate relay connected to the PLC outlet is working normally. After inspection, it is concluded that the intermediate relay was damaged, the electromagnetic armature of the intermediate relay is rusted, resulting in poor contact, and the PLC control signal was not transmitted to the frequency converter. After polishing the rusty part of the armature, the intermediate relay returns to normal operation.

2.2 Fault Two

The fault is that: after the centrifuge is started, the vibration value displayed on the touch screen suddenly increases, resulting in a vibration alarm. However, when the centrifuge is checked with a hand-held vibrometer, the vibration value is normal.

Troubleshooting process:

(1) The vibration value is calculated by the vibration transmitter which converts the vibration intensity of 0-20mm/s into the current signal of 4-20mA and sends it into the PLC.

(2) Replace the transmitter with the exclusion method and check the quality of the vibration transmitter.

(3) Connect the multimeter in series to the output current signal circuit of the vibration transmitter, and check whether the current signal is normal.

(4) Monitor the program and observe whether the value of the vibration signal collected by analog input (AI) module in PLC is normal.

According to the results of the multimeter measurement and program monitoring, the vibration signal suddenly increases after the frequency converter is started. The reason is that a number of control cabinets are newly added in the control room, and there are high-power frequency converters in the cabinet. The electromagnetic radiation generated by the frequency converter interferes with the analog signal, at the same time, the protective grounding and signal grounding in the control cabinet are not separated, so the anti-interference ability is insufficient. After changing the analog signal to signal ground, the interference problem is solved and the vibration value display of the equipment returns to normal.

2.3 Fault Three

The fault is that: the flocculation preparation device did not stop dispensing at high liquid level, resulting in overflow of the liquid.

Troubleshooting process:

(1) Measure whether the liquid level sensor works normally with a multimeter.

(2) Check whether the automatic dispensing program works normally through the monitoring program.

After inspection, due to the looseness of the high liquid level circuit of the liquid level gauge, the liquid level relay is not closed, and the high liquid level signal is not sent to the PLC. So when the liquid level of the medicine tank is high, the dispensing is not stopped in time, resulting in the overflow of the liquid. After fastening the circuit, the device returns to normal operation.

3. Conclusion

(1) The troubleshooting of electrical equipment is based on the understanding of the working principle of the mechanical and electrical system of the centrifuge. Only in this way can the faults be found and solved quickly and correctly.

(2) When troubleshooting electrical equipment faults, it is necessary to combine with the drawings, procedures and the actual situation on the site, so as to determine the fault area.

(3) When troubleshooting electrical equipment faults, we should follow the principle of “seeing” before “thinking” and then “doing”. In case of equipment fault, it is necessary to see where the fault occurs, what the fault is, and whether it is caused by mechanical or other factors. It is not allowed to operate blindly, so as to prevent affecting the maintenance, or even to enlarge the problem. It is necessary to find the source of the problem before starting. This can often achieve twice the result with half the effort.

Correct operation and careful maintenance are the guarantee to prevent common fault. A complete equipment operation management system should be established, which including equipment operation records, maintenance records. When solving the electrical fault of the centrifuge, the cause of the fault can be analyzed in combination with the change of the equipment parameters in the operation record, greatly reducing the maintenance time.