When a magnetic field consists of two poles, how many times does the induced voltage switch polarity per revolution?

When a magnetic field consists of two poles, how many times does the induced voltage switch polarity per revolution?

• A. Never
• B. Once per revolution
• C. Twice per revolution
• D. Three times per revolution

Answer: (B)

Induced voltage polarity follows how the magnetic flux through the coil changes as the field rotates. For a simple two-pole (dipole) field the flux through a fixed coil varies like a cosine of the rotor angle. If the rotor turns with constant speed, the induced voltage is proportional to the time rate of change of that flux, which gives a sine dependence on the angle. The sine function changes sign once during a full 360-degree rotation: positive in the first half, negative in the second half. At the halfway point the flux crosses zero and the polarity of the induced voltage reverses, and after a full turn it returns to its original state. So the polarity switches once per revolution. If you had more poles, you’d get more polarity reversals per revolution corresponding to the number of pole pairs.

Induced voltage polarity follows how the magnetic flux through the coil changes as the field rotates. For a simple two-pole (dipole) field the flux through a fixed coil varies like a cosine of the rotor angle. If the rotor turns with constant speed, the induced voltage is proportional to the time rate of change of that flux, which gives a sine dependence on the angle. The sine function changes sign once during a full 360-degree rotation: positive in the first half, negative in the second half. At the halfway point the flux crosses zero and the polarity of the induced voltage reverses, and after a full turn it returns to its original state. So the polarity switches once per revolution. If you had more poles, you’d get more polarity reversals per revolution corresponding to the number of pole pairs.