ECE 441 1

Transformer Construction

ECE 441 2

Three-Phase Transformer

ECE 441 3

Transformer Action -- DC

ECE 441 4

11

de N

dt

2 2

de N

dt

Opposes battery voltage Opposes flux buildup

ECE 441 5

Transformer Action -- AC

ECE 441 6

max4.44P PE N f max4.44S SE N f

Opposes VT Opposes ΦM

max

max

4.44

4.44PP P

S S S

N fE N

E N f N

ECE 441 7

“No-Load” Condition

ECE 441 8

“No load” condition continued

Io = Ife + IM

Io = exciting current

Io provides the “magnetizing flux” and the “core loss”

Ife = core-loss current Ife = VT / Rfe

IM = magnetizing current IM = VT / jXM

ECE 441 9

O fe M

P O P fe P M

I I I

N I N I N I

No-Load Excitation mmf

No-Load Core Loss mmf

Magnetizing mmf

P MM

core

N I

R

ECE 441 10

T P P P

T PP O

P

V I R E

V EI I

R

ECE 441 11

Close the load switch

Secondary current will set up an mmf in OPPOSITION to the primary mmf. The core flux will DECREASE to

P M S SM

core

N i N i

R

ECE 441 12

The decrease in flux causes a decrease in the counter-emf EP, and the primary current will increase by an amount known as IP,load, the load component of the primary current. Additional mmf due to this current adds to the magnetizing flux.

ECE 441 13

,P M P P load S SM

core

N i N i N i

RPrimary current increases until NPIP,load = NSIS. The flux ΦM and primary emf EP return to the same values as before the switch was closed.

ECE 441 14

Final steady – state primary current under loaded conditions is

,

0 ,

P fe M P load

P P load

I I I I

I I I

ECE 441 15

Component Fluxes – Loaded Transformer

ECE 441 16

ΦP = net flux in window of primary ΦS = net flux in window of secondary

Φlp = leakage flux of primary Φls = leakage flux of secondary

ΦM = mutual flux

ΦP = ΦM + Φlp

ΦS = ΦM – Φls