SINGLEPHASE

TRANSFORMERS

1

1. components:-- 2 adjacent coils-- coils not physically connected to each other

2

2. operation:a) AC in primary coil produces Bb) Secondary coil receives a in I & V

3

N

SN

SN

SN

SN

SN

S

N n

A

B

C

a

b

c

IA Ia

VANVAB

VCN

VBN Vbn

Van

Vcn

Vab

4

Wye / Wye connection

LoadSupplyNetwork

Transformer

Y-Y

Transformer LoadSupply Network

Xtr_s

Van Vload_anVAN

Vnet_ANXnet

IAIa

N

n

A

a

5

Three-phase network supplies a load through a transformer.

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

Wye / Delta connection

6

A

B

C

N

VANVAB

VBN

VBC

VCN

IA

Iba

Icb

VCA

a

b

c

a

b

c

Ia

Ic

IbVab

Vbc

Vca

Iac

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

7

Wye-delta connected transformer

Xtr_s

Van Vs_anVAN

IA Ia

Vp_AN

Xtr_pRtr_p Rtr_s

XmRc

Tspe

Y-

8

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

Delta / Wye connection

9

A

B

C

n

a

b

cVBC

IaIA

Ib

Ic

VAB

VCA Vca

Vab

Vbc

Van

Vbn

Vcn

IC

IBICA

IAB

IBC

Delta / Delta connection

10

A

B

C

a

b

c

VAB

Ia

Iba

Iac

IA

IAB

ICA

Vab

ratio of the # of turns in the secondary coil to the # of turns in the primary coil

11

P

S

N

NTR

numerical factor indicating relative change in PD across a transformer

TR > 1 = V (step up)TR < 1 = V (step down)

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

Formula:

12

NN

VV

S

P

S

P

=NS secondary coil turnsNP primary coil turns

VS secondary voltageVP primary voltage

direct relationship

Current in a transformer will be inversely proportional to the TR of the transformer

formula:

13

NN

II

S

P

P

S

=

VV

II

S

P

P

S

=

IP primary coil currentIS secondary coil current

I vs. V = inverse relationship

Based on TURNS RATIO

14

2. step-down turns ratio < 1VS < VP

IS > IP

1. step-up turns ratio > 1VS > VP

IS < IP

1. air core --inside area of coil is

empty-- inefficient induction

15

2. open core-- ferromagnetic

material inside coils

-- B = induction

3. closed core-- ferromagnetic

ends connected-- induction

16

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

same ferromagnetic core