Electromagnetic

Principles

Definition

Electromagnetic is made up of two words.

1. Electro, which has something to do

with electricity and

2. magnetic, which has something do

with magnetism.

Definition

So, electromagnetic principles deal

with the relationship between

electricity and magnetism, as well as

the relationship between magnetism

and electricity.

There are two electromagnetic

principles that we are going to

review.

The first principle that we want to review

states that electrons will flow in a

conductor when the conductor MOVES

or PASSES through a magnetic field.

Principle # 1

If it were not for this principle, we would

not have ‘electricity’ today. Let’s take a

closer look at this principle.

© 2005 Refrigeration Training Services - E1#2 AC and DC Current v1.2

Magnet

Conductor

SOUTH

NORTH

Magnet

Passing a conductor between two magnets and the magnetic field causes electrons to flow in the wire.

This produces electrical current in the wire.

Generating Current

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0º 90º 180º 270º 360º

Positive

Negative

One cycle

Magnet

SOUTH

NORTH

Magnet

Conductor

Alternating Current

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Effective voltage

• Alternating current starts at 0, reaches a peak, then returns to 0

• Peak voltage at 90° (electrical degrees)• Effective voltage is .707 times peak voltage

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0º 90º 180º

Effective Voltage

Peak Voltage 170 v

120 v

0 v

Effective voltage = .707 x Peak voltage

.707 x 170 = 120 v

Note: Meters measure effective voltage

Effective Voltage

© 2005 Refrigeration Training Services - E1#2 AC and DC Current v1.2

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0º 90º 180º

Effective Voltage

Peak Voltage 340 v

240 v

0 v

Effective voltage = .707 x Peak voltage

.707 x 340 = 240 v

Note: Meters measure effective voltage

Effective Voltage

The ‘effective voltage’ is also

known as the RMS (Root Mean

Square) value or the DC (Direct

Current) equivalent.

Many years ago, there was only

Direct Current. AC was not

invented yet. But when it was,

what do you think the reference

current would be?

If you said DC, you would be

correct. So, 120v AC will do the

same ‘work’ that 120 DC will do.

In other words, if you plug a 100w

incandescent light bulb into 120v

AC, it will glow just as bright as if

you plugged it into 120v DC, thus

the DC equivalent.

For another example, if you wired

a 240v AC electric hot water tank

to 240v DC, it will heat the water

just as quickly. Thus, the DC

equivalent.

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Generating Alternating Current (AC)

• Passing a conductor through a magnetic field• A generator uses many conductors and a large

magnetic field to produce electrical current

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Expressing AC with a Sine Wave

• A sine wave shows how alternating current flows in one direction, then reverses to flow in the opposite direction.

• A sine wave is a mathematical derivation of the trigonometric function – sine. You do not need to know trig. in order to understand AC though.

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0º 90º 180º 270º 360º

Positive

Negative

One cycle

Magnet

SOUTH

NORTH

Magnet

Conductor

Sine Wave of Alternating Current

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Cycles and Frequency• Cycle:

– One complete electrical alternation• Frequency

– Number of cycles in a second• Measurement of frequency:

– Hertz (Hz)– Cycles

• U.S. frequency is 60 hertz, or 60 cycles

The second principle that we want to

review states that a magnetic field will be

produced when electrons flow in a

conductor.

Principle # 2

If it were not for this principle, we would

not have motors, relays, contactors,

solenoids, timers, etc. Let’s take a

closer look at this principle.

For the first example, let’s take a look at

how electricity produces a magnetic field

and is used on a device known as a

‘solenoid valve.’

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Solenoid Valve

Magnetic coil energized

Plunger pulled up

Fluid flowsFluid stops

Power off

Plunger drops

Plunger

Seat

For the second example, let’s take a look

at how electricity produces a magnetic

field and is used on a device known as a

‘contactor.’

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Side View

L1 L2

T1 T2

LINE

LOAD

CONTROL CIRCUIT

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L1 L2

T1 T2

LINE

LOAD

Power to Contactor

Power to Load

CONTROL CIRCUIT

Contacts pulled in

Magnetic Coil energized

Side View

For the third example, let’s take a look at

how electricity produces a magnetic field

and is used on a device known as a

‘motor.’

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Two-pole stator motor

NN S

_+

SN S

Stator (Stationary)

When stator is energized the rotor will make a half turnwith each half of current cycle

Rotor(Rotates)

Polarity reverses (N to S) on stator

Stator repels & attracts rotor into motion

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Second half of cycle

NS

+ _

N S

The alternating current now

changes direction

NSPolarity reverses (N to S) on stator

Stator continues motion by repelling and attracting

In conclusion, if it were not for

electromagnetic principles, we would be

living in the dark ages.

T h eE n d