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EE 306

TRANSFORMERS

Instructor:

Dr. Mahamed Shwehdi

Done By:

Ahmad Saleh Al-Owied

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Transformers

Very large three-phase network transformer.

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Transformerelectromagnetic device (mutual induction)electromagnetic device (mutual induction)

1. components:

-- 2 adjacent coils

-- coils not physically connected to each other

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Transformer (cont.)

2. operation:

a) AC in primary coil produces ( B

b) Secondary coil receives a ( in I & V

N

S

N

S

N

S

N

S

N

S

N

S

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Transformers

N n

A

B

C

a

b

c

IA

Ia

VAN

VAB

VCN

VBN V

bn

Van

Vcn

Vab

Wye / Wye connection

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Transformers

LoadSupply

Network

Transformer

Y-Y

Three-phase network supplies a

load through a transformer.

Transformer LoadSupply Network

Xtr_sV

anV

load_anV Vnet_

Xnet

I Ia

N

n

a

Xtr_s

Van

Vload_an

V I IaV

net_

Xnet

Equivalent circuit of the wye-wye

Single-phase equivalent circuit for phase

of the three-phase wye-wye

transformer system

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Wye / Delta connection

A

B

C

N

VAN

VAB

VBN

VBC

VCN

IA

Iba

Icb

VCA

a

b

c

a

b

c

Ia

Ic

Ib

Vab

Vbc

Vca

Iac

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Transformers

Ia

Ib

Ic

Vab

Vbc

Vca

V

V

V

a

b

c

Icb

Iba

Iac

Transformer

I

I

I

V

V

V

upply

Iab

Ibc

Ica

oad

V

V

V

Iac

Vca

Vbc

Vab

Wye-delta connected transformer

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Transformers

Single-phase equivalent circuit of a wye-delta transformer.

Xtr_s

Van Vs_an

VI IaV

p_

Xtr_p

Rtr_p

Rtr_s

Xm

Rc

spe

-(

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Delta / Wye connection

A

B

C

n

a

b

cVBC

IaIA

Ib

Ic

VAB

VCA

Vca

Vab

Vbc

Van

Vbn

Vcn

IC

IB

ICA

IAB

IBC

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Delta / Delta connection

A

B

C

a

b

c

VAB

Ia

Iba

Iac

IA

IAB

ICA

Vab

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Transformers

Transformer

Connection

Primary and SecondaryVoltage Phase

RelationPhase Shift*

Windings Placed on the Same

Leg

Wye-WyeAN

is in phase withan

Zero A and a

Wye-Delta AN is in phase with ab 30 A and ab

Delta-WyeAB

is in phase withan

30 AB and a

Delta-DeltaAB

is in phase withab

Zero AB and ab

* Note: Phase shift lead between line-to-line voltages, AB and ab, and line-to-neutral voltages, AN and an, and line

currents, IA and Ia.

Three-phase Transformer elations

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Transformer Turns Ratio

ratio of the # of turns in the

secondary coil to the # of turns in

the primary coil P

S

N

NTR!

numerical factor indicating relative change in

PD across a transformerTR> 1 = V (step up)

TR< 1 = V (step down)

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Transformer Law(change in voltage in a transformer)

( in V is directly proportional to the ratio of the# of turns in the secondary to the # of turns inthe primary

Formula:N

N

V

V

S

P

S

P

=NS secondary coil turns

NP primary coil turns

VS secondary voltage

VP primary voltage

direct relationship

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Transformer Current

Current in a transformer will be inversely

proportional to the TR of the transformer

formula:N

N

I

IS

P

P

S

=

V

V

I

I

S

P

P

S=

IP primary coil currentIS secondary coil current

I vs. V = inverse relationship

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Types of Transformers Based on TURNS RATIO

2. step-down

turns ratio < 1

VS < VP

IS > IP

1. step-up

turns ratio > 1

VS > VP

IS < IP

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Transformer Types (cont.)Based on STRUCTURE OF COILS

1. air core

-- inside area of coil is

empty

-- inefficient induction

2. open core

-- ferromagnetic

material inside coils-- oB = oinduction

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Transformer Types (cont.)

3. closed core

-- ferromagnetic endsconnected

-- o& oinduction

4. shell

-- lazy square eight

-- overlap of coils onsame ferromagneticcore