Electrical Power Systems Chapter 8 Electrical Power Systems

Objectives Explain different types of electrical circuits and their uses. Understand the purpose of common electrical components.

Atomic Structure Atoms are made up of protons (+), neutrons, and electrons (-). Electrons in the outermost ring can be gained or lost. A material made of atoms that transfer electrons easily is called a conductor (copper). A material made of atoms that does not transfer electrons easily is called an insulator (rubber). Some materials sometimes act as both conductors or insulators. They are called semiconductors.

Electron theory and current In direct current (DC) electrons move only in one direction. Batteries are common devices that produce DC. Alternating current (AC) involves electrons flowing first in one direction and then reversing and flowing in the other direction. AC is the type of current used in houses and businesses for lighting, appliances, and machinery.

Electrical Circuits An electrical circuit is made up of several components: a power source, a load, and conductors (fig 8-9, page 167). Electricity flows from the negative terminal, through the circuit, to the positive terminal. A component that uses electricity is called a load (light bulb, motor). Schematic drawings are used to represent an electrical circuit graphically (fig 8-10, page 167). A closed circuit is a properly functioning circuit in which all loads are energized. Open circuits may occur accidently when connections are not made properly. Accidental open circuits cause problems and can sometimes be difficult to troubleshoot. A short circuit occurs when the load is bypassed. When short circuits happen, components may burn out, and circuit breakers will trip and result in an open circuit.

Laws that describe electricity Ohm’s law states that voltage (E) can be determined by multiplying current (I) by resistance (R). Voltage = current x resistance (E = I x R) Watt’s Law states that power (P) equals current (I) times voltage (E). Power = current x voltage (P = I x E)

Calculating Kilowatt-Hours (kWh) Electric consumption for billing purposes is calculated in kilo watt hours. Kilo is a thousand so: 1 kW = 1000 W Ten 100 watt light bulbs on for 10 hours a day at a cost of $.10 / kWh would cost: 10 x 100 x 10 hours = 10,000 watt hours = 10 kWh per day = $.10 x 10 = $ 1.00 per day.

Series, Parallel, & Series-Parallel Circuits Series circuits have a continuous path for electrical current (fig 8-19, page 175). Total resistance RT = R1 + R2 + R3… Parallel circuits allow more than one path for electrical current (fig 8-20, page 175). Total resistance: RT = 1 / (1/R1 + 1/R2 + 1/R3…) Series Parallel circuits are a combination of series and parallel circuits. First calculate the parallel resistance and then add to the series resistance in the circuit (fig 8-25, page 178).

Magnetism Electrical current produces magnetism. Magnets can induce, or cause, electrical current in conductors. An electromagnet consists of a conductor wrapped around an iron core. When a current is passed through the conductor, the iron core becomes magnetized. Electromagnetic Induction: Electricity can be produced in conductors with the use of magnets. Most of our electricity is produced by electromagnetic induction.

Electrical Power Sources Some examples of power sources are batteries and generators. A battery is made up of one or more cells. Cells and batteries change chemical energy to electrical energy. An AC generator (alternator) is a device that converts mechanical energy into electrical energy. Power plants produce energy with the aid of AC generators (fig 8-33 , page 182).

Basic Control Elements for Electricity Switches: are used to turn loads on and off. The simplest is the single pole single throw (SPST) switch (fig 8-35, page 185). The single pole double throw (SPDT) switch is used to control a load from two different locations (same as three way switch). A double pole double throw (DPDT) switch can control one or more loads. Diodes: are control elements that allow electricity to flow in one direction only. Diodes are used in power supplies to convert AC to DC. Transformers: When we need to reduce the amount of voltage we use a step down transformer. When we need to increase the amount of voltage we use a step up transformer.

Protecting Electrical Circuitry A fuse is made of a filament that breaks the circuit if too much electrical current passes through it. A circuit breaker performs the same function as a fuse, except that a circuit breaker is resettable. A continuity checker can be used to check if a circuit breaker or fuse is working properly (fig 8-41, page 189). A ground fault circuit interrupter (GFCI) can trip open a circuit in the case of an overload. GFCI is designed to protect people.

Sizing wire and component according to load Wires and other components must be sized according to load. If they are undersized, they will generate excess heat, and trip the circuit breaker, or blow the fuse. Oversizing can result in inefficient use of materials and may create unsafe conditions.

Electrical Safety The National Safety Council reports that approximately 20% of electricity related deaths each year are the result of faulty household wiring. Use the appropriate size fuse or circuit breaker for the given application. Trouble shoot circuits in a de enegized state. Ensure that GFCI protection is installed in outdoor electrical receptacles and receptacles near water sources.

Using Instrumentation Safely and Properly The primary instrument used when measuring electricity is the multimeter which can measure voltage, resistance and amperage. It is important to ensure that the meter you choose has the capability to measure the quantity you desire to measure. Observe the placement of the meters in fig 8-46, page 194. Be certain you are not grounded when working with live electricity. Resistance is always measured with the circuit deenegized. Always start at the highest range when measuring unknown quantities. Use surge suppressors to protect delicate equipment such as computers and stereo equipment from excessive voltages.

Summary A material made of atoms that transfer electrons easily is called a conductor (copper). A material made of atoms that does not transfer electrons easily is called an insulator (rubber). In direct current (DC) electrons move only in one direction. Batteries are common devices that produce DC. Alternating current (AC) involves electrons flowing first in one direction and then reversing and flowing in the other direction. AC is the type of current used in houses and businesses for lighting, appliances, and machinery. An electrical circuit is made up of several components: a power source, a load, and conductors. An AC generator (alternator) is a device that converts mechanical energy into electrical energy. Switches: are used to turn loads on and off. Diodes: are control elements that allow electricity to flow in one direction only. Diodes are used in power supplies to convert AC to DC. Transformers: When we need to reduce the amount of voltage we use a step down transformer. Use surge suppressors to protect delicate equipment such as computers and stereo equipment from excessive voltages.

Home Work 1. What is the difference between direct current and alternating current? Give examples. 2. What are the components of an electrical circuit? 3. What equipment can you use to protect delicate equipment from excessive voltages?