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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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Transformer electromagnetic 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

SN

SN

SN

SN

SN

S

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Transformers

N n

A

B

C

a

b

c

IA Ia

VANVAB

VCN

VBN Vbn

Van

Vcn

Vab

Wye / Wye connection

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Transformers

LoadSupplyNetwork

Transformer

Y-Y

Three-phase network supplies a load through a transformer.

Transformer LoadSupply Network

Xtr_s

Van Vload_anVAN

Vnet_ANXnet

IAIa

N

n

A

a

Xtr_s

Van Vload_anVAN

IAIaVnet_AN

Xnet

Equivalent circuit of the wye-wye

Single-phase equivalent circuit for phase A of the three-phase wye-wye transformer system

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Wye / Delta connection

A

B

C

N

VANVAB

VBN

VBC

VCN

IA

Iba

Icb

VCA

a

b

c

a

b

c

Ia

Ic

IbVab

Vbc

Vca

Iac

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Transformers

Ia

Ib

Ic

Vab

Vbc

VcaVCN

VBN

VAN

A

B

C

a

b

c

Icb

Iba

Iac

Transformer

AC

AC

AC

IC

IB

IA

VAN

VBN

VCN

Supply

Iab

Ibc

Ica

Load

VAB

VBC

VCA

Iac

Vca

Vbc

Vab

N

Wye-delta connected transformer

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Transformers

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

Xtr_s

Van Vs_anVAN

IA Ia

Vp_AN

Xtr_pRtr_p Rtr_s

XmRc

Tspe

Y-

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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

IBICA

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 Secondary Voltage Phase Relation

Phase Shift*Windings Placed on the Same

Leg

Wye-Wye VAN is in phase with Van Zero A and a

Wye-Delta VAN is in phase with Vab 30° A and ab

Delta-Wye VAB is in phase with Van –30° AB and a

Delta-Delta VAB is in phase with Vab Zero AB and ab

* Note: Phase shift lead between line-to-line voltages, VAB and Vab, and line-to-neutral voltages, VAN and Van,

and line currents, IA and Ia.

Three-phase Transformer Relations

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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 transformer

TR > 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 in the primary

Formula: NN

VV

S

P

S

P

=NS secondary coil turnsNP primary coil turns

VS secondary voltageVP 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: NN

II

S

P

P

S

=

VV

II

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 < 1VS < VP

IS > IP

1. step-up turns ratio > 1VS > 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

-- B = induction

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Transformer Types (cont.)

3. closed core-- ferromagnetic ends

connected-- induction

4. shell -- lazy square eight-- overlap of coils on

same ferromagnetic core