What is the output frequency of an alternator with a 12-pole rotor turning 3000 rpm?

What is the output frequency of an alternator with a 12-pole rotor turning 3000 rpm?

• A. 50 Hz
• B. 150 Hz
• C. 600 Hz
• D. 300 Hz

Answer: (D)

Output frequency depends on how fast the rotor creates electrical cycles and how many poles the machine has. A machine with P poles produces P/2 electrical cycles per mechanical revolution, so at N rpm the frequency is f = (P × N) / 120 (cycles per second). With a 12-pole rotor turning at 3000 rpm, you get f = (12 × 3000) / 120 = 300 Hz. Put simply, there are six electrical cycles per revolution, and at 3000 revolutions per minute that’s 6 × 3000 = 18000 cycles per minute, which is 18000 ÷ 60 = 300 Hz. If you check the other options, you’d need different pole counts to reach those frequencies at the same speed (for example, a 2-pole machine would yield 50 Hz at 3000 rpm, a 6-pole machine would yield 150 Hz, and a 24-pole machine would yield 600 Hz).

Output frequency depends on how fast the rotor creates electrical cycles and how many poles the machine has. A machine with P poles produces P/2 electrical cycles per mechanical revolution, so at N rpm the frequency is f = (P × N) / 120 (cycles per second).

With a 12-pole rotor turning at 3000 rpm, you get f = (12 × 3000) / 120 = 300 Hz. Put simply, there are six electrical cycles per revolution, and at 3000 revolutions per minute that’s 6 × 3000 = 18000 cycles per minute, which is 18000 ÷ 60 = 300 Hz.

If you check the other options, you’d need different pole counts to reach those frequencies at the same speed (for example, a 2-pole machine would yield 50 Hz at 3000 rpm, a 6-pole machine would yield 150 Hz, and a 24-pole machine would yield 600 Hz).