1

ElectricitySection 11

Unit 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:

1. Electromechanical

2. Electrochemical

3. Thermoelectrical

4. 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.

6

Electrochemical

Electrochemical reactions can either produce electricity, or use electricity.

Chemical reaction causes a voltage

A voltage causes a chemical reaction

7

Thermoelectrical

Thermoelectricity has to forms:

1. the production of electricity from temperature differentials and

2. the development of temperature differences using electricity.

A thermocouple uses a difference in electricity to produce electricity.

An electric heater produces heat using electricity.

8

Photoelectrical

Photoelectricity is the emission of electrons from matter upon absorption of electromagnetic radiation.

Photovoltaic Cell (PV Cell)

http://www.srpnet.

9

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.

11

Principles of Electricity

12

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.

13

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.

14

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.

15

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.

16

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.

17

Amperes, Volts and Watts-cont.

• Watts: the measure of electrical energy (work) that can be done.

• Watt Hour: the measure of electrical energy used.

€

Watts = Amperes x Volts

€

Volts =Watts

Amperes

€

Amperes =WattsVolts

18

Ohm’s 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)

19

Ohm’s Law Example

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

€

E = IR

I =ER

=120 V0.23

= 521.7 A

20

Two Types of Current

21

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

22

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.

23

Alternating Current

• The amperage and voltage varies over time and periodically reverses direction (cycle).

• Standard domestic current.• Standard domestic electrical service is 60 cycle.

24

Power & Energy

25

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:

€

P = IE

P = Watts

I = Amps

E = Volts

• Examples of resistance loads are heaters and incandescent lamps.

26

Power example

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

€

P = IE =2.1 12 A x V =25.2 W

27

Electrical wheel

• The electrical wheel Illustrates Ohm’s 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

€

P • R P

I

I • R

28

Electrical Energy

• Electrical energy is measured in units of kilowatt-hours (kWh)

• Electricity is sold in units of $/kWh.

29

Energy Example

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

€

Energy= Power x Time

=100 8 watts x hour

=800 wh

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

€

$ =0.12 $

kWh 800 x W x

1 kW

1, 000 W 8 x h=0.77 $

30

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:

€

P = ( ) IE PF

31

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.

32

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?

€

P = ( ) IE PF PF=P

IE=

1, 100 W15.0 120 A x V

=0.61

• 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.

33

Efficiency

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

€

% Effeciency= Output Power

Input Power 100 x

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)

34

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.

€

( )Energy kWh =(120 1.5 ) V x A W x8 hday

x30 day

month x

1 kW

1, 000 W=43.2 kWh

35

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

36

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 @ $0.07/kWh– Next 1,000 kwh @ $0.065/kWh– Over 1500 kWh @ $0.057/kWh

€

Service charge= 4.50$

500 kWh x 0.07$

kWh= 35.00

1, 000 kwh x0.065 $

kWh= 65.00

1, 000 kWh x0.057 $

kWh= 56.00

160.50$

ICA

37

Magnetism & Electricity

38

Magnetism and Electricity

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

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

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

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

39

Circuits

40

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

41

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.

42

Closed (Complete) Circuit

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

43

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.

44

Electrical Safety

45

Five criteria for wiring systems

1. Safe

2. Adequately sized

3. Expandable

4. Convenient

5. Neat

46

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

47

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 GFCI’s 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.

48

Textbook Safety Recommendations-cont.

10. Do not increase size of circuit over load protection.

11. Do not leave heat producing devices unattended.

12. Place all heaters and lamps away from combustible materials.

13. Insure metal cabinets of electrical devices are grounded.

14. Do not use any switches, outlets, fixtures, or extension cords that are cracked or damaged.

49

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

50

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.

51

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.

52

1. Safety-Metal Case Tools and Grounding

53

1. Safety-Equipment Grounding

• Stationary equipment must also be grounded.

• May use electrical system ground or be grounded at the site.

54

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.

55

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.

56

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.

57

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.

58

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

• GFCI’s 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.

• GFCI’s are designed to provide protection to the user(s) of the electrical circuit.

59

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.

60

1. Safety--Breakers

Breakers are mechanical overcurrent devices. Three different tripping mechanisms:

Thermal Magnetic Combination thermal and magnetic

61

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.

62

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.

63

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.

64

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.

Minimum Allowable Length of Run (feet)

Cable, Conduit, Earth Overhead in air

Compare size shown below with size shown to the left of the double line. Use larger size.

Load (amps)

Types R, T, TW

Types RH, RWH, THW

Bare & Covered

30

40

50

60

75

100

125

150

175

200

225

250

300

350

400

5 12 12 10 12 12 12 12 12 12 12 10 10 10 10 8 8 8 6

7 12 12 10 12 12 12 12 12 12 10 10 8 8 8 8 6 6 6

10 12 12 10 12 12 12 12 10 10 8 8 8 6 6 6 4 4 4

15 12 12 10 12 12 10 10 10 8 6 6 6 4 4 4 4 3 2

20 12 12 10 12 12 10 8 8 6 6 4 4 4 4 3 2 2 1

25 10 10 10 10 10 8 8 6 6 4 4 4 3 3 2 1 1 0

30 10 10 10 10 8 8 8 6 4 4 4 3 2 2 1 1 0 00

35 8 8 10 10 8 8 6 6 4 4 3 2 2 1 1 0 00 00

40 8 8 10 8 8 6 6 4 4 3 2 2 1 1 0 00 00 000

65

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.

66

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.

67

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.

68

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

69

5. Neat

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

70

Electrical System

71

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.

72

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.

73

Service Entrance-Meter

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

(kWhr)

74

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.

75

Three Ways of Wiring Circuits

76

Three Ways of Wiring Circuits

• The loads and electrical components in a circuit can be

connected in three different ways:

– Series

– Parallel

– Series-parallel.

77

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.

€

Rt =R1 + R2 + . . . Rn

For these calculations assume no resistance in the conductors or connections.

Determine the total resistance for the circuit in the illustration.

€

Rt =5.0 + 8.2 =13.2

78

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.

79

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.

€

1Rt

=1R1

+1

R2+ . . .

1Rn

80

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.

€

Rt =R1 x R2R1 + R2

81

Parallel Circuit Resistance

Determine the total resistance for the circuit in the illustration.

€

1Rt

=1R1

+1R2

+1R3

= 1

2.5+

14.0

+1

5.2 =

2152

+1352

+1052

= 4452

= 0.846

Rt =1

0.846=1.18

€

1Rt

=1R1

+1R2

+1R3

= 1

2.5+

14.0

+1

5.2 = 0.4 + 0.25 + 0.192 = 0.842

Rt =1

0.842=1.19

€

Rt =R1 x R2R1 + R2

=2.5 4.0x

2.5 + 4.0=

106.5

=1.54

1 .54 x5 .21.54 + 5.2

=8 .06 .74

=1 .19

€

1Rt

=1R1

+1

R2+ . . .

1Rn

€

Rt =R1 x R2R1 + R2

or

or

82

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.

€

Rtb =R1 x R2R1 +R2

=1.2 5.8x

1.2 + 5.8=

6.967

=0.99

€

Rtc =0.99 +2.3 =3.29

83

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.

84

Calculating Voltage In A Series Circuit

• What would V1 read in the illustration?

• Ohm’s Law states: • Therefore:

€

R=E

I

€

E = IR

• At this point there is insufficient data because I (amp) is unknown.

• Using Ohm’s Law to solve for the current in the circuit:

• Knowing the amount of current we can calculate the voltage drop.

€

E = IR I=E

R=

120 V13.2

=9.09 A

€

E= IR=9 .09 5.0 A x =45.4 V

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.

€

E = IR I=ER

=12 V

1.5 + 4.2 + 6.3 = 1.0 A

• 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:

€

Rt =R1 x R2R1 + R2

=1.5 4.2x

1.5 + 4.2=

6.35.7

=1.10

1.10 6.3x

1.10 + 6.3=

6.967.40

=0.94

88

Determining Amperage in a Parallel Circuit--cont.

When the total resistance is known, the circuit current (Amp’s) can be calculated.

Branch current is:

€

I =E

R=

12 V6.3

=1.9 A A2 = 1.9 A

A1= 12.76 A

€

I =E

R=

12 V0.94

=12.76 ATotal current is:

When the circuit current (Amp’s) is known, the current for each branch circuit can be calculated.

89

Determining Voltage in a Series-Parallel Circuit

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

€

R1 x R2

R1 + R2

=1.2 V x 5.8 V

1.2 V +5.8 V=

6.96

7.0= 0.99 Ω + 2.3 Ω = 3.29 Ω

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

90

Determining Voltage in a Series-Parallel Circuit

Circuit current is:

€

E = IR = 3.65 2.3 x = 8.39 V€

E = IR I =E

R=

12 V

3.29 Ω= 3.65 A

Volt meter 1 will read source voltage: 12 V

Volt meter 2 will read 3.6 V.

The voltage drop caused by the 2.3 Ohm resistance is:

€

12 V - 8.39 V = 3.61 VThe voltage remaining in the circuit is:

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.

€

I =VR

=3.61 V5.8

= 0.62 A

A2 = 0.62 A

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

92

Transformers

• Domestic and commercial service is alternating current.

• Alternating current can be transformed.

• 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.

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

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.

96

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.

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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.

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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.

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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.

WyeDelta

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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.

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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