How does applying AC to a coil produce motion in a relay?

• A. DC energizes the coil to push the contacts apart.
• B. AC reduces coil resistance, causing contact movement.
• C. AC heats the coil causing expansion to move the contacts.
• D. AC energizes the coil, creating a fluctuating magnetic field that pulls the contacts together, switching the circuit.

Answer: (D)

When AC is applied to a relay coil, the current creates a magnetic field that continuously fluctuates in strength. That magnetic field produces a magnetic force on the movable armature. The force pulls the armature toward the coil’s core, which moves the relay’s contact set and switches the circuit. The motion is due to magnetic attraction from the energized coil, not heating or a change in resistance. With AC, the field reverses polarity each half cycle, but the armature is drawn toward the magnetic pole each time the current is present, so the contacts stay switched as long as the coil is energized. When the current stops, the return spring or mechanism pushes the armature back, opening the contacts.