What are the causes of burnt-out opening and closing coils in high-voltage vacuum circuit breakers?

Burnout of the opening and closing coils in a high-voltage vacuum circuit breaker is a common fault, and its causes can be categorized into the following main types:

Core cause: Coil overheats and burns out due to prolonged energization.

Under normal circumstances, the opening or closing coil operates on a short-time duty cycle, energized only for a brief moment (usually no more than a few hundred milliseconds) upon receiving the command. After the electromagnet completes its action, the circuit automatically disconnects. If the coil is energized for too long, it will overheat and burn out.

The following are the specific reasons why the coil might be energized for an extended period:

I. Mechanical Transmission Failure (Most Common Cause)

 

This is the primary factor leading to coil burnout. When the mechanical mechanism is jammed or the resistance is too high, the electromagnet cannot complete its action within the normal time, and the auxiliary switch cannot cut off the coil circuit, resulting in continuous energization of the coil.

Mechanism Jamming/Refusal to Move:

Poor Lubrication: Lack of lubrication or sludge buildup in rotating parts such as shafts and connecting rods of the operating mechanism (e.g., spring mechanisms, electromagnetic mechanisms) leads to increased friction.

Component Deformation/Damage: Components such as connecting rods and cranks in the mechanism may slightly deform due to long-term stress or aging, increasing resistance to movement.

Foreign Object Intrusion: Dust, metal shavings, and other foreign objects enter the mechanism, causing jamming.

 

Auxiliary switch (microswitch) improper adjustment or malfunction:

Improper adjustment: The switching position of the auxiliary switch does not match the mating position of the circuit breaker's mechanical spindle. After the circuit breaker completes opening/closing, the auxiliary switch fails to disconnect the coil circuit in a timely manner.

Contact sticking or jamming: The auxiliary switch's contacts are burned together, or its transmission rod is jammed, preventing effective circuit disconnection.

 

II. Electrical Control Circuit Faults

 

Problems with the control circuit will directly cause the coil to fail to receive the correct energizing command or to fail to de-energize in time.

Power Supply Voltage Issues:

Low Voltage: If the operating power supply (DC or AC) voltage is too low, the attraction force generated by the electromagnet will be insufficient to overcome the spring force or mechanical resistance, preventing the core from being pulled to complete the action. The coil will remain energized and burn out.

High Voltage: Although higher voltage makes operation easier, long-term operation under overvoltage will accelerate the aging of the coil insulation.

 

Control circuit component failures:

Contactor (relay) failure:Contactor contacts may become stuck, remaining closed even after the command pulse has ended, continuously supplying power to the coil.

Secondary wiring failure:Loose wiring, insulation damage leading to a short circuit, etc., can all cause abnormal energization.

Anti-pump circuit failure:A malfunctioning anti-pump relay cannot effectively lock the circuit during continuous opening and closing commands.

 

III. Coil Quality Issues

 

Insulation Aging: Long-term exposure to the working environment can lead to decreased insulation performance due to moisture, overheating, and corrosion, resulting in inter-turn short circuits or short circuits to ground. After a short circuit, resistance decreases, current increases sharply, and the coil burns out quickly.

Mismatched Wire Diameter or Number of Turns: Non-original or inferior coils may have wire diameters or number of turns that do not match the design specifications, causing changes in resistance and inductance parameters and making them prone to overheating.

Poor Manufacturing Processes: Loose winding and inadequate impregnation processes can both lead to hidden problems within the coil.

 

IV. Environmental and Other Factors

 

Harsh Environment: Installing the circuit breaker in high-temperature, high-humidity, corrosive gas, or dusty environments will accelerate mechanism jamming and coil insulation aging.

Excessive Operation: Exceeding the circuit breaker's specified mechanical life or operating frequency will exacerbate mechanism wear and increase the accumulated heat generation of the coil.

Improper Core Stroke Adjustment: Excessive or insufficient core stroke of the electromagnet will affect its attraction effect and the switching timing of the auxiliary switch.

 

Summary and Troubleshooting Steps

 

When a burnt-out opening/closing coil occurs, do not simply replace the coil. The root cause must be thoroughly investigated; otherwise, the new coil will burn out again.

Suggested Troubleshooting Steps:

♦Manually test the mechanical mechanism: Under safe conditions (with the operating power disconnected), perform slow manual opening and closing operations to check the smoothness and absence of any jamming points in the entire transmission mechanism. This is the first and most important step.

♦Check the auxiliary switch: Check if its installation position is correct, if the contacts operate flexibly and reliably, and if its on/off state in the opening/closing position meets the drawing requirements.

♦Measure the operating voltage: When issuing the operating command, use a multimeter to measure the actual voltage across the coil to see if it is within the rated range.

♦Check the control circuit: Check if the opening/closing contactor, relays, and other components operate normally, and if the contacts are stuck. Check for loose or damaged wiring.

♦Replace the coil: After confirming and eliminating all the above mechanical and electrical faults, finally replace the coil with a new, qualified opening/closing coil.

Only through systematic analysis and investigation can the problem be fundamentally solved, ensuring the safe and reliable operation of high-voltage vacuum circuit breakers.

 

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