What could cause the relay coil to burn?
Overvoltage may have been applied to the relay coil. If an overvoltage is applied to the coil of the relay, the relay coil will generate abnormal heat, causing a layer short of the core wire and resulting in disconnection.
Externally, if the condition is mild, the discoloration of the coil wire surface and the melting of the coil sheath tape may occur. But if the condition is severe, the coating of the coil wire will melt, the spool around the coil will melt or deform, or the case will swell.
Layer short of coil
When the relay coil is energized, the coil wire generates heat by Joule heat and the temperature rises.
Applying an overvoltage generates excessive heat, causing the coating (typically polyurethane) to melt and short-circuit between the core wires, resulting in a decrease in coil resistance.
The decrease in coil resistance increases the current flowing through the coil wire, causing further heat generation. The cycle of melting of the coating, short circuit of the core wire, decrease of the coil resistance, and increase of heat generation accelerates, and finally the coil wire is burned and melted.
The maximum allowable voltage of the coil is set for each relay. If a voltage exceeding this is applied, abnormal heat generation and melting of the coating will occur, leading to a layer short.
Heat-resistant temperature of polyurethane-coated copper wire
The heat resistance of polyurethane-coated copper wires commonly used in relays is 120°C, and the relay coil is designed to generate less heat than this when the rated voltage is applied.
Regarding to malfunction examples and countermeasure for relays, refer to The SOLUTIONS [General-purpose Relay Edition].
- An abnormal overvoltage should not be applied to the relay coil due to a circuit failure in the relay drive circuit.
- When a relay is used in a circuit that is continuously energized for a long period of time without switching operation (emergency light or alarm equipment or error check circuit that releases only when an error occurs and issues an alarm at the b-side contact), a de-energized design is desired.
- Consider using latching relays in the relay drive circuit and sequence circuits that require continuous energization.
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