The sinusoidal waveform, also known as a sine wave, is the most commonly used AC waveform in circuit theory. An electromotive force (EMF) that alternates between positive and negative polarities at regular intervals is produced by a voltage source with a periodic AC waveform. The time taken for one complete reversal is known as the period of the waveform.
Direct Current (DC) Supply
A uni-directional supply, known as Direct Current (DC), flows in only one direction in an electrical circuit. Common DC power sources include batteries, dynamos, and solar cells. The magnitude (amplitude) and direction of DC voltage or current remain constant over time.
For example, +12V represents 12 volts flowing in the positive direction, whereas -5V represents 5 volts flowing in the negative direction. DC power sources maintain a constant, steady-state value that does not vary with time, unless the connections are physically reversed.
Alternating Current (AC) Supply
In contrast, Alternating Current (AC) is a bi-directional waveform that varies in both magnitude and direction over time. The shape of an AC waveform often resembles a mathematical sinusoid and can be represented as:
A(t) = Amax × sin(2πft)
An AC function can represent either a power source or a signal source. The term “Alternating Current” refers to a time-varying waveform, the most common of which is the sinusoidal wave or sine wave. Among all AC waveforms used in electrical engineering, the sine wave is the most significant.
What is an AC Waveform?
An AC waveform is the graphical representation of the instantaneous values of voltage or current plotted against time. A domestic mains voltage supply is a common example—it continuously changes polarity every half cycle, alternating between positive and negative maximum values with respect to time.
Thus, an AC waveform is a time-dependent signal, and the most common type is the periodic waveform. The output of a rotating electrical generator typically produces a periodic or AC waveform.
In general, any periodic waveform can be constructed by superimposing harmonic signals of various frequencies and amplitudes. However, that topic is explored further in advanced AC theory. Unlike DC, alternating voltages and currents cannot be stored in batteries or cells, but they are easier and more economical to generate using alternators or waveform generators.
All AC waveforms have a zero-voltage line that divides the waveform into two symmetrical halves, regardless of the generating device.
Characteristics of AC Waveform
- Period (T): The time taken in seconds for the waveform to complete one full cycle. For sine waves, this is called the periodic time, and for square waves, it is called the pulse width.
- Frequency (ƒ): The number of times the waveform repeats in one second. Frequency is the reciprocal of the time period (ƒ = 1/T), measured in Hertz (Hz).
- Amplitude (A): The magnitude or intensity of the waveform, measured in volts or amps.
Visual Representation of AC Waveform
Waveforms are visual representations of how voltage or current fluctuates over time. The horizontal axis generally denotes time, and the zero line represents the point of zero voltage or current.
Any region of the waveform above the zero axis indicates current or voltage flowing in one direction, while any region below the zero axis indicates flow in the opposite direction. For sinusoidal AC waveforms, the shape above and below the zero axis is typically identical.
However, in many non-power AC signals such as audio waveforms, this symmetry might not be perfect. Even though not all alternating waveforms are perfectly sinusoidal, the sine wave remains the fundamental and most commonly used form in electrical and electronic engineering.
Conclusion
The study of AC waveforms and circuit theory is essential for understanding how electrical power is generated, transmitted, and utilized. The sinusoidal waveform, with its predictable and consistent nature, forms the foundation of modern electrical systems and electronic signal processing.