Which mechanism primarily operates to open contacts when a motor starter coil is de-energized?

When a motor starter coil is de-energized, the mechanism that primarily operates to open contacts is gravity and spring tension. In most motor starters, the coil generates a magnetic field when energized, which pulls the contacts together to close the circuit and allow current to flow to the motor. Once the coil is de-energized, the magnetic field collapses, and the contacts need to return to their original position to open the circuit.

Gravity and spring tension play a crucial role in this process. The springs are designed to exert force against the contacts, pulling them open when the magnetic force is removed. Gravity also aids in the movement of the contacts back to their open position. This mechanism ensures that the system reliably returns to a safe state (contacts open) when power is removed, which is essential for the safety and functionality of electrical equipment.

Other options like magnetism, electromagnetic force, and thermal expansion do not accurately represent the mechanism at play in this situation. Magnetism and electromagnetic force are involved when the coil is energized, not when it is de-energized, while thermal expansion pertains to changes in materials due to heat, which is not relevant in the opening of motor starter contacts.