# Electrical Sciences AC Theory

 Generates one full sine wave when the coil rotates 2p radians Cycle The amount of AC that produces the same heating effect as an equal amount of DC. Effective value The number of cycles per second. Frequency Unit of measurement for the number of cycles per second. Hertz The time it takes for the generator to complete one cycle. Period The fraction of a cycle, in degrees, that has gone by since a voltage or current has passed through a given value. Phase angle Used to describe two different voltages, currents, or a voltage and a current that have the same frequency, which pass through zero values in the same direction at different times. Phase difference An angle that subtends an arc equal to the radius of a circle. One radian equals 57.3°. 2p radians is a full revolution. Radian Units of angular velocity. Radians per second Root-mean-square value. RMS value Given an alternating current (AC) waveform, DESCRIBE the relationship between average and RMS values of voltage and current, and the angular velocity within that waveform DESCRIBE the construction and operation of a simple AC generator The elementary AC generator consists of a conductor, or loop of wire in a magnetic field that is produced by an electromagnet. The two ends of the loop are connected to slip rings, and they are in contact with two brushes. When the loop rotates, it cuts magnetic lines of force, first in one direction and then the other. EXPLAIN the development of a sinusoidal wave output At the instant the loop is in the vertical position, the coil sides are moving parallel to the field and do not cut magnetic lines of force. In this instant, there is no voltage induced in the loop. As the coil rotates in a counter-clockwise direction, the coil sides will cut the magnetic lines of force in opposite directions. The direction of the induced voltages depends on the direction of movement of the coil. The induced voltages add in series, making slip ring X positive (+) and slip ring Y negative (–). The potential across resistor R will cause a current to flow from Y to Xthrough the resistor. This current will increase until it reaches a maximum value when the coil is horizontal to themagnetic lines of force The horizontal coil is moving perpendicular to the field and is cutting the greatest number of magnetic lines of force. As the coil continues to turn, the voltage and current induced decrease until they reach zero, where the coil is again in the vertical position. In the other half revolution, an equal voltage is produced, except thatthe polarity is reversed. The current flow through R is now from X to Y . DEFINE the following terms in relation to AC generation: a. radians/second b.hertz c.period radians/second, the velocity the loop turns; hertz, the number of cycles in one second; period, the time to complete one cycle. DEFINE effective value of an AC current relative to DC current. Effective value of AC equals value of DC that produces the same heating effect. Given 169.73vAC, CALCULATE the effective (RMS) and average values of AC voltage. RMS 120 vACAvg 108 vAC Given a diagram of two sinusoidal waves, DESCRIBE the phase relationship Phase angle is used to compare two wave forms. It references the start, or zero point, of each wave. It compares differences by degrees of rotation. Wave forms with the same start point are “in-phase” while wave forms “out-of-phase” either lead or lag Authorereim ID30067 Card SetElectrical Sciences AC Theory DescriptionAC Theory Updated2010-08-16T13:25:26Z Show Answers