Topic 02: Electrical Characteristics
Understanding the electrical characteristics of linear and non-linear loads is the most important first foundational topic when attempting to understand why loads of the past acted differently in past electrical environments versus loads of today in today’s challenging and complex electrical environments. One of the interesting facts you will learn in Course No. blank, title is the building electrical systems used in past electrical environments is essentially the same as the BES used in today’s electrical environments.
However, how the linear loads and non-linear (non-solid state) loads used in past electrical environments behaved are significantly different as compared to how non-linear electronic (solid-state) loads behave in today’s electrical environments.
Voltage is the stimulus that is applied to a load, and current is the response that flows from the voltage source into the load, then back out of the load and back to the source where it came from. All current leaving a source must be returned back to it—no matter what! Another way to say it, “The current flows from the voltage source, through the load and back to the source to complete the circuit to form a closed loop.” (This will be shown in Topic 3.)
For a Linear Load: If you graph the voltage on the x-axis (horizontal) and the current on the y-axis (vertical) for a linear load (left graph), the line (red – – – – dashed) that relates the voltage to the current is straight, or constant. The line (relationship) between the voltage and current has a constant slope. This is because the proportionality constant (for now, we will call it R for resistance) is a constant number.
For a Non-Linear Load: If you graph the voltage on the x-axis (horizontal) and the current on the y-axis (vertical) for a non-linear load (right graph), the line (red – – – – dashed) that relates the voltage to the current is never straight and is never constant. The red – – – – dashed can be any shape as long as it is not straight and only has one current value for one voltage value. The relationship between the voltage and current is not proportional (for now, we will call it Z for impedance) and is not a constant number.
Definitions
A few basic definitions will help you start understanding the most basic relationship between voltage and current.
Why is it important in this course (and other courses to follow) to know what resistance is and what impedance is and the difference between them? Resistance is used in DC circuits to talk about the constant relationship between voltage and current in a circuit where the direct current (DC) is not changing. The current in a DC circuit is constant. Thus, the frequency in a DC circuit is 0 (zero), since the voltage (and current) is not changing.
Impedance is used in AC circuits to talk about the relationship between voltage and current in a circuit where the alternating current (AC) is changing.
The current in an AC circuit is changing at a fixed rate (or frequency). In standard AC electrical systems for residential, commercial, and industrial electrical environments in the US, the frequency of the AC voltage is 60 Hertz. This means the AC voltage is changing 60 times per second. The 60 Hertz frequency in an AC circuit is constant; that is, the frequency doesn’t change, or increase above 60 Hertz or decrease below 60 Hertz. (Prior to the use of the term Hertz, the frequency was referred to as cycles.)
When an AC voltage of 60 Hertz is applied to a linear load (or linear circuit), the current that flows through that circuit is also 60 Hertz. Remember, voltage is the stimulus, which is the force that causes electrons in a conductor to flow. The measure of how fast the electrons are flowing is the current, which is the response. The current responds to the voltage stimulus in a circuit.
Resistance – a measure of the opposition of electric current flow. Resistance is denoted by the letter, R and has units of ohms, denoted by the Greek symbol omega (Ω).
Impedance – (simplified definition) a measure of the opposition of AC electric current flow caused by the combined effects of resistance and reactance. Impedance is denoted by the letter, Z and also has units of ohms, and is also denoted by the Greek symbol omega (Ω).
The three most common types of linear loads in past residential electrical environments are shown in Figure 2. They are incandescent lamps, electric motors and strip heaters. All of these linear loads draw linear current. The incandescent lamp and strip heater are resistive loads. The electric motor is an inductive load.
One of the objectives of the course is to learn the three different types of linear loads. The three different types of linear loads are resistive, inductive, and capacitive. These are the only loads that draw a smooth sinewave of current—in other words, the characteristics of these loads do not impact the quality of the current. If a smooth sinewave of voltage is applied to them, then a smooth sinewave of current flows through them.