What two things determine the frequency of an aircraft generator?

• A. voltage and current
• B. windage and slip
• C. number of poles, and RPM/speed
• D. ambient temperature

Answer: (C)

The frequency of an aircraft generator is set by how fast the rotor’s magnetic poles pass the stator and by how many poles there are. In other words, frequency is determined by the rotor speed (RPM) and the number of poles in the machine. The standard relationship is f = (P × N) / 120, where P is the number of poles and N is the rotor speed in RPM. This means doubling the speed doubles the frequency, and adding more poles also increases frequency for a given speed.

Voltage and current come from the excitation and load, not from the rate at which magnetic poles pass the stator, so they don’t set the frequency. Ambient temperature affects resistance and insulation, not the rotational rate. Windage and slip are related to losses and rotor speed deviation in some machines, but they don’t determine the frequency in the way rotor speed and pole count do.