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Electrical Maintenance Fundamentals

Note: presentation best viewed as Notes Page

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Objectives Define the unit of electricity and current

flow Define the three electrical qualities

present in electrical circuits. State and apply Ohm’s Law Measurement basics

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Definitions Current (I): flow of electric charges per unit time,

measured in amperes or amps (A) Electromotive Force (emf) (E): a potential difference or

“electric pressure” which drives the flow of charges, measured in volts (V)

Resistance (R): an electrical circuit’s opposition to current flow, measured in ohms (Ω)

Conductor: a material which offers little resistance to current flow, e.g. silver, copper, iron, etc…

Insulator: a material which offers high resistance to current flow, e.g. wood, paper, plastic, etc...

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Types of Electricity Static Electricity - no motion of free

charges

Current Electricity - motion of free charges Direct Current (DC) Alternating Current (AC)

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t -Time

DC Current

Current ia(t)

2

Current Ib(t)=2 cos 2πt

0.5 1.0

AC Current

Time (secs)

ond

CCoulombAAmp

sec_1

)(_1)(_1 =

DC vs. AC Current

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Direct Current (DC)

Current flow is unidirectional and of constant magnitude.

Conventional Current Flow

Electron Flow

+ --+

E

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Alternating Current (AC)

Current is constantly changing in magnitude and direction at regular intervals.

Current is a function of time and usually varies as a sine function.

I

t

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Voltage – emf (electromotive force)Voltage (E) – electrical pressure or force with which electrons move

Measured in volts

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Electromotive force - Emf

Water Pump

Shutoff Valve

High Pressure

Low Pressure

Switch

BatteryR -

resistor

Low Potential

High Potential

E

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Resistance/Impedance Resistance / Impedance (R / Z or Ω) - opposition that an element or material has to the flow of electrons

Ohm’s Law states that one volt (E) will push one amp of current (I) through one ohm (Ω) of resistance (R).

Resistance (DC circuit); Impedance (AC circuit) Ohm’s Law formula: E=IR

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Ohm’s Law The amount of current flowing in an

electrical circuit(I - Measured in amperage) is dependent upon the value of electrical pressure (E - measured in volts) and the amount of opposition to the flow of current (R - measured in ohms).

R

EI =

R

EI =

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The Ohms Law Triangle

RIE ×=

R

EI =

I

ER =

E

I RI

ER =

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Ohm’s Law

I = E/R 20 amps = 120 volts / ? ohmsIn simpler terms; One volt (E) will push one amp of current (I) through one ohm (Ω) of resistance (R) E = IR 120 volts = 15 amps x ? ohms R = E/I 6 ohms = 120 volts / ? amps P = IE 60 watts = 20 amps / ? volts

Ohm’s Law – In an electrical circuit, the current passing through a conductor between two points is proportional to the potential difference (i.e. voltage drop or voltage) across the two points, and inversely proportional to the resistance between them.

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Power Electric power (P) is defined as the

amount of work done by an electric current. Measured in watts P = I x V

P is the power (watt or W) I is the current (ampere or A) V is the potential difference (volt or V)

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Fundamental Concepts & Terms Kirchoff’s Voltage and Current Laws

Kirchoff’s Voltage Law:The algebraic sum of the voltage (potential) differences in any loop must equal zero.

Kirchoff’s Current Law:The sum of current into a junction equals the sum of current out of the junction.

R1

R2

R3

R5

v2

v1

v4

v5

v3

a b

d c

+

+

vg

i3

i4

i1

R1i2

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Resistance in Series .

R1

R2 v2

v1

v4

-a b

c

+

+

+

--

.......321 +++= RRRRtotal

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Resistance in Parallel

R1 R2 v2v1v4

-

b

+

+ +

--

a

I1 I2 I3

......1111

321

+++=RRRRtotal

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Measuring Electrical Performance First and Foremost - follow proper safety

rules Common Electrical instruments

Voltmeters Ammeters Ohmmeters Megohmmeters Wattmeters

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Basic Instruments- Multimeters Combines reading of:

Voltages Resistance Current

Digital Multimeter

Analog Multimeter

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Digital Multimeters

Measurement Device Circuit Symbol

Voltage Voltmeter

Current Ammeter

Resistance Ohmmeter

V

A

Ω

“Through”

“Across”

“Across”(and Not in circuit)

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GALVANOMETER

PointerN

S

Moving coil

Coiled spring

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Measuring Voltage

+

Battery

-

V

V

A COM

12.000

A

OFFA

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Measuring Voltage

+

Battery

-

Conductor Resistance

V

V

A COM

11.500

A

OFFA

V

V

A COM

7.500

A

OFFA

Voltage Drop

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Measuring Current

+

Battery

-

A

V

A COM

.5000

V

AOFF

Break circuit to connect meter. Note: meter leads are moved to different inputs for current testing.

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Measuring Current Cont’d

Never clamp two wires at once!

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Measuring Resistance

A

V

A COM

5000

V

AOFF

A

V

A COM

0000

V

AOFF

Verify zero setting of meter

Reading Resistance

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Megohmmeter (Megger)

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Megger Testing

GL E

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Proof Testing & Procedure

Metal conduit

Insulation

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Testing Generators/Motors When testing

generators, motors, or transformers each winding/phase should be tested in sequence and separately while all the other windings are grounded. Testing this way, the insulation between phases is also tested.

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Power Electrical power is defined as the rate at

which electrical energy is supplied to a circuit or consumed by a load.

The watt (w) is the unit of power

IEtime

workP ×==

Where E = volts and I = current

PI E

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Measuring Power

Typical single-phase wattmeter connection.S

OU

RC

E

A

LOA

D

A

V

Voltage

Current

Ammeter

±

±

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TH

RE

E P

HA

SE

SO

UR

CE

TH

RE

E P

HA

SE

LO

ADA

V

±±

A

V

±

±

Measuring Power

Measuring Power Cont’d

Typical single-phase wattmeter connection.

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Summary

Review Objectives Question and Answer Session