Which statement about transformer polarity is supported by the material?

• A. Additive when currents flow in the same direction
• B. Subtractive when currents flow in the same direction
• C. Subtractive when currents flow in opposite directions
• D. Additive when currents flow in opposite directions

Answer: (B)

Polarity tells us whether the windings’ voltages and the resulting magnetomotive forces (mmf) in the core reinforce each other or oppose each other. The dotted ends mark instantaneous polarity, so the direction of current flow relative to those dots determines if the two mmfs subtract or add.

When currents flow in the same direction through the two windings (as seen with respect to their polarity markings), the mmfs produced by the windings tend to oppose one another in the core, producing subtractive polarity. That’s why subtractive is the correct description for currents in the same direction. If the currents were to flow in opposite directions, the mmfs would reinforce each other, giving additive polarity.