10 Modern Commercial Wiring Copyright Goodheart-Willcox Co., Inc. The speed at which the generator is turned determines the frequency of the current. One com- plete turn is called a cycle, Figure 1-12. Frequency is measured with units of hertz (Hz), which represent cycles per second. NOTE In the United States, the frequency of alternat- ing current is 60 Hz. An alternator produces 60 complete turns and 120 changes of current direction per second. The frequency is so high that the reversals cannot be detected as so much as a fl icker in an electric light. 1.3.3 Instantaneous, Peak, and Eff ective Voltage Instantaneous voltage is the voltage at a par- ticular instant. For example, looking again at our sine curve, the maximum positive voltage occurs at 90° and the maximum negative volt- age occurs at 270°. These instantaneous voltage values are particularly important since they also represent the peak positive and negative voltages. Another important instantaneous volt- age occurs at 45° and 135° in the positive part of the cycle, and again at 225° and 315° during the negative part. The voltage at these points is referred to as effective voltage, or root- mean-square (RMS) voltage, and is equal to 70.7% of the peak voltage. Nominal voltage, such as 120 volts or 240 volts, refers to the effec- tive voltage of a circuit. 1.3.4 Phase Relationship In ac circuits, the voltage and current vary con- tinuously with time. This relationship is often NOTE Voltmeters read the eff ective voltage of a circuit. Equipment is rated for the eff ective voltage cal- culations and specifi cations are made using the eff ective voltage values. N S N S N S N S Rings Armature Field AC output DC source Brushes Motion Goodheart-Willcox Publisher Figure 1-11. A commercial alternator looks much different from the simple sketches you have seen. This unit uses some of the current it produces to magnetize the field. Sine curve Angle of rotation N S EMF Lines of force 90° 0° 180° 270° 360° Goodheart-Willcox Publisher Figure 1-12. Voltage produced by an alternator varies. Compare the position of the coil above with the sine curve below. Peaks in the wave represent points of highest voltage during one complete cycle.