Electricity Section 11 Unit 32

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ElectricitySection 11Unit 32
2
Introduction
3
Electricity Theory

• Whenever an abundance of electrons (-) develops
  on one end of a material and a scarcity of
  electrons () is present on the on either end,
  electrons will flow from atom to atom from the
  abundant end to the scarce end.

4
Electrical Theory--cont.

• Electron flow can be caused by four methods
• Electromechanical
• Electrochemical
• Thermoelectrical
• Photoelectrical

5
Electromechanical

• Generators and alternators are electromechanical
  devices.
• An electromechanical device produces electricity
  when it rotates.
• Generators and alternators can be driven by
  several difference sources of power
• Wind
• Water
• Engine
• Generators produce electricity through
  electromagnetic induction.

Note Either current, a magnetic field or motion
can be produced as long as the other two are
present.
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Electrochemical

• Electrochemical reactions can either produce
  electricity,

or use electricity.
Chemical reaction causes a voltage
A voltage causes a chemical reaction
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Thermoelectrical

• Thermoelectricity has to forms
• 1. the production of electricity from
  temperature differentials and
• 2. the development of temperature differences
  using electricity.

An electric heater produces heat using
electricity.
A thermocouple uses a difference in electricity
to produce electricity.
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Photoelectrical

• Photoelectricity is the emission of electrons
  from matter upon absorption of electromagnetic
  radiation.
• Photovoltaic Cell (PV Cell)

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Unit 32Electrical Principles and Wiring Materials
10
Introduction

• Electricity is the primary source of power for
  stationary equipment.
• A basic understanding of the principles of
  electricity is a requirement for using electrical
  powered equipment efficiently and safely.

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Principles of Electricity
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Heat and Light

• Electricity a form of energy that can produce
  light, heat, magnetism, or chemical changes.
• Light occurs when electricity passes through a
  filament.
• Heat is produced when electricity flows through a
  resistance.
• A magnet field forms around any conductor
  carrying electricity.
• Electricity passing through water causes the
  hydrogen and oxygen to split.

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Heat and Light-cont

• Resistance a measure of the difficulty
  encountered by the electrons as they flow through
  a conductor.
• Resistance is present in all electrical
  conductors, devices, etc.
• Electricity passing through a resistance heat
  voltage drop.
• For building wiring, resistance increases as the
  temperature increases.
• Resistance is measured in units of Ohms (?)

An Ohm is defined as the resistance between two
points of a conductor when a constant potential
difference of 1 volt, applied to these points,
produces in the conductor a current of 1 ampere.
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Heat Light-cont.

• Conductor an material that has a low resistance
  to the flow of electricity.
• Insulator any material that provides a high
  resistance to the flow of electricity.

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Amperes, Volts and Watts

• Amperes the measure of the rate of current flow.
• 6.24 1018 electrons passing a point per second
  is equal to one amp.
• A current occurs whenever there is a source of
  electricity, conductors and a complete circuit.

An Amp meter must be wired in series to measure
current.
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Amperes, Volts and Watts-cont.

• Voltage (E or V) the electromotive force
  (potential) available to cause electrons to flow.
• Measured in units of volts (V).
• A volt is defined as the potential difference
  across a conductor when a current of one ampere
  dissipates one watt of power.

• Always measured by comparing the difference
  between two points.

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Amperes, Volts and Watts-cont.

• Watts the measure of electrical energy (work)
  that can be done.
• Watt Hour the measure of electrical energy used.

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Ohms Law

• The flow of electricity through a conductor is
  directly proportional to the electromotive force
  that produces it.
• E I R
• E electromotive force (volts)
• I current intensity (amps)
• R resistance (Ohms)

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Ohms Law Example

• What is the current flow in a circuit with a
  voltage of 120 volts and a resistance of 0.23 ??

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Two Types of Current
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Two Currents--Intro

• Electrical circuits can be classified according
  to how the current varies with time.
• The two common currents are called
• Direct current
• Alternating current

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Direct Current

• Direct current
• The electrons move in one direction only.
• Amperage is constant.
• Voltage is constant
• Must be used to store electricity in batteries.

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Alternating Current

• The amperage and voltage varies over time and
  periodically reverses direction (cycle).
• Standard domestic current.
• Standard domestic electrical service is 60 cycle.

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Power Energy
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Power

• Power is the rate of doing work.
• Electrical power is usually expressed in watts or
  kilowatts

• In DC and AC circuits, with resistance loads,
  power can be determined by

• Examples of resistance loads are heaters and
  incandescent lamps.

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Power example

• Determine the power consumed by a resistor in a
  12 volt system when the current is 2.1 amps.

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Electrical wheel

• The electrical wheel Illustrates Ohms law and
  the electrical power equation.
• The value at the point of the 14 pie slice can be
  found using any one of the three equations on the
  rim of the pie slice.
• Example E (Volts) can be determined by

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Electrical Energy

• Electrical energy is measured in units of
  kilowatt-hours (kWh)
• Electricity is sold in units of /kWh.

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Energy Example

• Determine the amount of energy a 100 Watt light
  bulb will use when operated for 8 hours.

• What will it cost to operate the light bulb if
  the electrical energy costs 0.12 /kWh?

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AC loads (non resistant)

• Non resistant AC loads are called reactance
  loads.
• Examples of reactance loads are motors and
  fluorescent lights.
• To determine the power and energy of an AC
  circuit with reactance loads the power factor
  must be included.
• Power Factor
• Reactance loads do not use all of the electricity
  that is sent to them. They store part of it for
  a short period of time and then pass it back to
  the generator.
• Power Factor will always be between 0 and 1.
• In AC circuits with reactance loads, the power is
  determined by

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Power Factor in Resistive and non Resistive Loads

• In AC both voltage and current vary with time.
• If the load is resistive, the voltage and current
  peaks occur at the same time.
• If the load is reactive, the current lags the
  voltage.
• The current and voltage peaks do not occur at the
  same time.
• During a short part of the cycle (phase shift)
  the instantaneous voltage and instantaneous
  current have different signs (polarity).

• Since the product of two numbers with different
  signs is negative, this means that for this
  portion of the cycle power is negative,
  indicating that power is flowing back to the
  source.

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Power Factor Example

• What is the power factor for a load which
  consumes 1,100 watts at 15.0 amps when connected
  to a 120 volt circuit?

• A power factor less than one means that more
  current is flowing to the load than is required
  to supply the actual power used by the load.
• If the power factor of a load is improved, with
  no other changes, the power used by the load
  stays the same, but the current to the load is
  reduced.

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Efficiency

• Efficiency indicates how effective a machine is
  at converting electrical power to some other form.

Use Efficiency ()
Electric motor 75
Light bulb 80
Resistance heater 100

• Efficiency and wattage use data can be used to
  determine energy uses by electrical machines.
  (Appendix B)

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Energy Use Calculations

• How much electrical energy will an electric
  blanket use per month if it is used 8 hours a
  day? The blanket is on a 120 V circuit and draws
  1.5 amp.

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Buying Electrical Energy

• Electrical suppliers sell energy on a dollars per
  kilowatt-hour basis.
• The charges may be based on a flat rate, but many
  agricultural production units and small
  manufacturing businesses may be on a staggered
  rate.
• Additional charges may also be included
• Hook up fee
• Energy charge

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Energy Cost Problem

• Determine the charge for 2500 kWh of electricity
  using the following rate structure. The utility
  charges a 4.50 /mth service charge.
• First 500 kwh _at_ 0.07/kWh
• Next 1,000 kwh _at_ 0.065/kWh
• Over 1500 kWh _at_ 0.057/kWh

ICA
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Magnetism Electricity
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Magnetism and Electricity

• Electricity flowing through a conductor results
  in a magnetic field developing around the
  conductor.

• When iron and steel are exposed to magnet forces
  a residue remains.
• They become what is called a permanent magnet.

• When a conductor passes through a magnetic field,
  a current is induced in the conductor.

• With a conductor, either current, a magnetic
  field or motion can be produced as long as the
  other two are present.

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Circuits
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Circuits

• Circuit a continuous conducting material
  connecting an area of an abundance of electricity
  to an area of a scarcity of electrons.
• Three circuit conditions
• Open
• Closed (complete)
• Short

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Open Circuit

• An open circuit is incomplete, therefore
  electricity will not flow.
• It may be incomplete because a switch is open, a
  conductor is broken, a conductor has been
  disconnected or many other reasons.

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Closed (Complete) Circuit

• A closed circuit is a complete circuit and if
  there is a source of electricity, electricity
  will flow through the circuit.

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Short Circuit

• A short circuit occurs when the electricity has a
  low resistance path to ground.
• Low resistance high current flow.
• If the circuit is not protected by over current
  protection devices the conductors may overheat,
  burn through or some other failure may occur.

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Electrical Safety
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Five criteria for wiring systems

1. Safe
2. Adequately sized
3. Expandable
4. Convenient
5. Neat

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1. Safety

• Safety is freedom from accidents.
• Accidents are caused by hazards.
• All work and living spaces contain hazards.
• Each hazard has a probability of causing an
  accident.
• The probability of a hazard causing an accident
  is called risk.
• Safety is managing the risks associated with
  hazards to maintain it at an acceptable level.
• Strategies for managing risk of electricity.
• Avoidance
• PPE
• Work procedures
• Work standards
• Etc.

• Factors which influence acceptable level of risk
• Age
• Experiences
• Training
• Others

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1. Safety-Textbook Safety Recommendations

1. Never disconnect or damage any safety device that
   is provided by the manufacturer or specified by
   electrical codes.
2. Do not touch electrical devices with wet hands or
   wet feet.
3. Do not remove the ground prong from three prong
   plugs.
4. Use GFCIs where recommended.
5. Immediately disconnect any extension cord that
   feels warm or smells like burning rubber.
6. Do not place extension cords under carpeting.
7. Install all electrical wiring according to NEC.
8. Use only double insulated power tools or tools
   with three-wire cords.
9. Determine the cause of a blown fuse or circuit
   breaker trip before reenergizing the circuit.

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Textbook Safety Recommendations-cont.

1. Do not increase size of circuit over load
   protection.
2. Do not leave heat producing devices unattended.
3. Place all heaters and lamps away from combustible
   materials.
4. Insure metal cabinets of electrical devices are
   grounded.
5. Do not use any switches, outlets, fixtures, or
   extension cords that are cracked or damaged.

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1. Safety-- Grounding

• Ground a low resistant circuit to the earth
• Grounding an electrical tool means establishing a
  low resistance path to earth.
• Enhances safety by providing a low resistance
  path, which limits voltage imposed by lightning,
  line surges or unintentional contact.
• Two different ground circuits are used.
• Equipment
• System

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1. Safety--Equipment Grounding

• The most common electrical service is 3-wire,
  120/240 V single phase.
• The transformer secondary winding center point is
  grounded.
• This grounded neutral is then extended on through
  the system.
• When an internal short occurs in the machine, the
  low resistant circuit to earth causes a high
  current flow and trips the breaker.
• If the machine is not grounded, the frame has
  system potential to ground and a person or animal
  touching the frame will complete the circuit.

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1. Safety--Equipment Grounding Rules

• Exposed non-current carrying metal parts of
  portable power tools, all metallic electrical
  devices and permanently wired electrical
  equipment must be grounded.
• The equipment ground is usually a bare wire or a
  wire with green insulation.
• A grounding type plug should be used with all
  metal cased electrical portable power tools.
• For the equipment grounding system to work as
  designed there must be a low resistant circuit
  from the metal parts of the tool to earth.

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1. Safety-Metal Case Tools and Grounding
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1. Safety-Equipment Grounding

• Stationary equipment must also be grounded.
• May use electrical system ground or be grounded
  at the site.

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Equipment Grounding-cont.

• This is un safe
• As long as the equipment functions as designed,
  there should not be any potential between the
  case of the motor and the earth.

• If the motor develops an internal short, and it
  does not have an equipment ground, there can be
  as much as 120 Volts between the case of the
  motor and the earth.
• Any body or any thing touching the case of the
  motor could receive a fatal shock.

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1. Safety--Double Insulated

• An alternative equipment grounding system.
• A double insulated tool has only two conductors
  in its cord.
• To be classified as double insulated the tool
  must have superior insulation.
• All electrical parts are surrounded by additional
  insulation or air space.
• Exposed parts are either non-conducting or if
  conducting, are isolated from electrical parts by
  a non-conducting link.
• Double insulated tools are usually air cooled,
  this means they are a hazard when used around
  water, because the water can enter through the
  air vents and contact energized parts.

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1. Safety--System Grounding

• The service entrance panel is connected to a
  earth ground.
• A different means of determining the size of this
  conductor is used.

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1. Safety--Service Entrance-Over Current
Protection

• Over current protection devices are used to limit
  the maximum amount of current in a circuit.
• Current passing through a conductor heat
• Excessive current excessive heat.
• Two overcurrent protection devices are fuses and
  breakers.

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1. Safety--Ground Fault Circuit Interrupt (GFCI)

• GFCIs are electrical devices that are designed
  to trip (open the circuit) when a 5 milliamp or
  more difference is measured between the hot and
  neutral conductor.
• GFCIs are designed to provide protection to the
  user(s) of the electrical circuit.

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1. Safety--Fuses

• Plug fuses have a fusible link that is designed
  to fail.
• Sudden failure due to short dark window
• Failure due to overload melted link
• Cartridge type fuses are also used.
• When a uses is used in a circuit that has an
  electric motor or other hard to start load a time
  delay fuse should be used.

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1. Safety--Breakers

• Breakers are mechanical overcurrent devices.
• Three different tripping mechanisms
• Thermal
• Magnetic
• Combination thermal and magnetic

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1. Safety--Thermal Breaker

• As the current increases beyond the designed
  level the bimetallic strip heats up.
• Each metal has a different coefficient of
  expansion.
• As it heats it bends.
• When it bends sufficiently to stop supporting the
  contact, the spring opens the contacts.
• Once the bimetallic strip cools, the breaker can
  be reset.

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1. Safety--Magnetic Breakers

• As the current increases in the solenoid the
  electromagnetic force on the moveable contact
  increases.
• When the force on the moveable contact reaches
  the design point, the points open, breaking the
  circuit.
• The breaker can be reset once the overload is
  removed from the circuit.
• Tends to operate too fast when overloaded.

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1. Safety--Managing Electrical Hazards

• Best achieved by compliance with the National
  Electric Code (NEC)
• The goal of the NEC is to have the safest system,
  not just a system that works.
• Following recommended procedures.
• Using recommended tools.

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2. Adequately Sized

• Circuits must be designed with the correct size
  of conductor for the anticipated load.
• Conductor size can be calculated or sized using
  tables. (pg 453)
• Each building should have sufficient circuits so
  that extension cords do not need to be used on a
  regular basis.

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2. Adequately Sized--Voltage Drop

• Voltage drop occurs because when electricity
  passes through a resistance heat is generated.
• Heat represents loss energy
• The energy loss is expressed as less voltage.
• Using a conductor that is too small for the load
  causes excessive voltage drop.

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Voltage Drop--Cont.

• When there is no current flow, there is no
  voltage at the load.
• A 2 voltage drop is considered normal.
• 3 under some conditions.
• A voltage drop of more than 2 is excessive and
  the circuit will not function properly.

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3. Expandable

• A good farm electrical plan will have included
  the options for additional circuits in buildings.
• Over sizing a service entrance panel during
  construction is less expensive that replacing it
  later on.
• When installed the electrical service should have
  enough reserve capacity to allow the addition of
  more load with out requiring replacement.

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4. Convenient

• A convenient electrical system is designed to
  make it easy to work with and around it.
• Location of service entrance panel
• Location of receptacles
• Necessary lighting
• Sufficient branch circuits to reduce need of
  extension cords
• Meter located in an easy to read location

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5. Neat

• A neat (orderly) electrical system is organized
  and laid out according to a plan.

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Electrical System
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Electrical Transmission System

• Electricity is produced at power plant or hydro
  dam.
• Transformers at power plant stepped up the
  voltage to 250,000 V.
• Electricity is transmitted to a sub station.
• Voltage is stepped down to 7,000 V
• Electricity is transmitted to transformer at
  user.
• Voltage is stepped down to 120/240 V.

• From the transformer the electricity passes
  through the electric meter and into the building
  service entrance panel.
• This is called the service entrance drop.

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Service Entrance

• The service entrance panel divides the electrical
  service into different types of branch circuits.
• Each branch circuit has an overcurrent protection
  device.
• Branch circuits may be 120 or 240 volts.
• The grounding bar in the service entrance panel
  must be connected to an approved earth ground.

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Service Entrance-Meter

• Electrical service will include a meter.
• The meter records the amount of electricity that
  has been used. (kWhr)

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Service Entrance-Branch Circuits

• Branch circuits get their name because they
  branch out from the service entrance panel.
• Branch circuits can be general purpose or
  special.
• General purpose
• Lighting
• Receptacles
• Special purpose
• Air conditioner
• Air compressor
• All branch circuits must have the correct size of
  conductor and correct over current protection.

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Three Ways of Wiring Circuits
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Three Ways of Wiring Circuits

• The loads and electrical components in a circuit
  can be connected in three different ways
• Series
• Parallel
• Series-parallel.

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Series Circuit

• In a series circuit the electricity has no
  alternative paths, all of the electricity must
  pass through all of the components.
• The total circuit resistance is the sum of the
  individual resistances.

For these calculations assume no resistance in
the conductors or connections.
Determine the total resistance for the circuit in
the illustration.
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Series Circuit-cont.

• To the power source, the a series circuit appears
  as one resistance.

• A characteristic of all circuits is that there is
  a voltage drop across each resistance in the
  circuit.
• The method for calculating voltage drop in series
  circuits is different than the method for
  parallel circuits.

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Parallel Circuits

• In parallel circuits the electricity has
  alternative paths.
• The amount of current in each path is determined
  by the resistance of that path. Electricity
  follows the path of least resistance
• Because there are alternative paths, the total
  resistance of the circuit is not the sum of the
  individual resistances.
• In a parallel circuit The inverse of the total
  resistance is equal to sum of the inverse of each
  individual resistance.

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Parallel Circuits--cont.

• An alternative equation is

When a circuit has more than two resistors,
select any two and reduce them to their
equivalent resistance and then combine that
resistance with another one in the circuit until
all of the resistors have been combined.
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Parallel Circuit Resistance

• Determine the total resistance for the circuit in
  the illustration.

or
or
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Series-Parallel Circuits

• Series-Parallel circuits have loads in both
  series and parallel.
• In the illustration the 1.2 and 5.8 Ohm resistors
  are in parallel, but they are in series with the
  2.3 Ohm resistor.

To determine total circuit resistance the
equivalent value of the resistors in parallel
must be calculated first, and then that value can
be combined with the resistors that are in series.
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Circuits Summary

• When the source voltage, and the total resistance
  of the circuit is known, amperages and voltages
  can be determine for any part of a circuit.
• In a series circuit the amperage is the same at
  all points in the circuit, but the voltage
  changes with the resistance.
• In a parallel circuit the amperage changes with
  the resistance, but the voltage is the same
  throughout the circuit.

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Calculating Voltage In A Series Circuit

• What would V1 read in the illustration?
• Ohms Law states
• Therefore

• At this point there is insufficient data because
  I (amp) is unknown.
• Using Ohms Law to solve for the current in the
  circuit
• Knowing the amount of current we can calculate
  the voltage drop.

Note circuit conductors behave like resistors in
series.
85
Determining Voltage In A Parallel Circuit

• Assuming no resistance in the conductors, the two
  volt meters in the illustration will have the
  same value--source voltage.

86
Determining Amperage In A Series Circuit

• Determine the readings for A1 and A2 in the
  illustration.
• In a series circuit the electricity has no
  alternative paths, therefore the amperage is the
  same at every point in the circuit.

• The current in the circuit is determined by
  dividing the voltage by the circuit resistance.

87
Determining Amperage in a Parallel Circuit

• Determine the readings for amp meters A1 and A2
  in the circuit.

• In a parallel circuit the amperage varies with
  the resistance.
• In the illustration, A1 will measure the total
  circuit amperage, but A2 will only measure the
  amperage flowing through the 6.3 Ohm resistor.
• To determine circuit amperage the total
  resistance of the circuit must be calculated

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Determining Amperage in a Parallel Circuit--cont.

• When the total resistance is known, the circuit
  current (Amps) can be calculated.

Total current is
A1 12.76 A
When the circuit current (Amps) is known, the
current for each branch circuit can be calculated.
Branch current is
A2 1.9 A
89
Determining Voltage in a Series-Parallel Circuit
Problem Determine the readings for the two volt
meters in the illustration.

• Volt meter one (V1) will read source voltage
• Volt meter two (V2) will read the voltage in the
  circuit after the 2.3 ? resistor.
• To determine this reading, the voltage drop
  across the resistor must be calculated.
• Before the voltage drop across the resistor can
  be calculated, the circuit current must be
  determined.

12 V

• To calculate the circuit current the total
  circuit resistance must be known.

90
Determining Voltage in a Series-Parallel Circuit

• Circuit current is

The voltage drop caused by the 2.3 Ohm resistance
is
The voltage remaining in the circuit is
Volt meter 1 will read source voltage 12 V Volt
meter 2 will read 3.6 V.
91
Determining Amperage in a Combination Circuits

• Determine the readings for the amp meters in the
  illustration.
• The first amp meter will read circuit amps and
  the second one will measure the current in that
  branch of the circuit.
• To determine circuit amperage, the total circuit
  resistance must be known. This was calculated in
  the previous slide as 3.29 ?.

• Total circuit amperage was also calculated as
  3.65 A.
• A1 3.65 A

• To determine the reading for A2, the current
  flowing in that part of the circuit must be
  calculated.

• In the previous slide the voltage left after the
  2.3 ? resistor was 3.61 V.

A2 0.62 A
92
Transformers

• Domestic and commercial service is alternating
  current.
• Alternating current can be transformed.

• A transformer is a device with no moving parts
  which transfers energy from one circuit to
  another by electromagnetic induction.

• Transformer has two windings
• Line (L)--connected to the power source
• Secondary (S)--output current
• When an alternating current is applied to the
  primary side the current sets up an alternating
  magnetic flux which induces an alternating
  current in the secondary windings.
• The ratio of turns between the two windings
  determines the ratio between the primary and
  secondary current.

93
Three Wire Circuits

• The most common electrical service is called
  3-wire 120/240.
• The center lead (white) is grounded and the two
  hot leads (black and red) have 120 V to neutral.
• Connecting to the two hot leads provides 240V.

94
Three Phase Circuits

• Bern Olson Electricity for Agricultural
  Applications

95
Comparing Single Phase to Three Phase

• Three phase circuits use three simultaneously
  energized conductors to carry power to a load.

Single phase has just one conductor and the power
is constantly changing.
Three simultaneous circuits provide more
electrical power in the same amount of time.
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Advantages of Three Phase Circuits

• More effective than single phase
• More economical than single phase
• Power flow to a load is more constant.
• Single phase power pulsates.
• Three phase motors are smaller, simpler and less
  expensive for same horsepower.
• Circuit conductors for three phase power can be
  smaller.
• 25 less conductor material for the same load.

97
Disadvantages of Three Phase Circuits

• Requires a separate delivery system.
• Not available at all locations.
• Very expensive to install if new service must
  come from any distance.

98
Single Phase Service

• A single phase AC generator can be constructed by
  placing a conductor loop on a shaft revolving in
  a magnetic field.

• This type of electrical current does not supply
  consistent power.
• Circuits and electrical devices must be designed
  for this type of power.

99
Three Phase Service

• A three phase generator has sets of thee
  conductor loops rotating on one shaft.
• Spaced an equal distance apart, they produce
  almost continuous power.
• The two ways the individual loops are wired
  together produces the two common types of three
  phase power.
• The type of circuit is determined by the
  transformer that steps down the voltage.

Wye
Delta
100
Wye Three Phase

• In the Wye configuration one end of each loop is
  attached together.
• The voltage between the end of any loop and the
  connection point is 120 V
• The voltage between the open ends of any two
  loops is 208 V.

• The neutral conductor only carries current when
  120 V circuits are used or if the 3-phase is out
  of balance.

101
Delta Three Phase

• In the delta configuration the three loops are
  attached in series.
• The neutral is attached in the center of one of
  the windings.
• No load can be attached between the connection
  opposite the neutral and the neutral because the
  voltage will be greater than 120 V.
• The service voltages are determined by the way
  the step down transformer is wired.

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Q u e s t i o n s