Transformer Construction
ECE 441
1
Three-Phase Transformer
ECE 441
2
Transformer Action -- DC
ECE 441
3
d
e2  N 2
dt
d
e1  N 1
dt
Opposes battery voltage
Opposes flux buildup
ECE 441
4
Transformer Action -- AC
ECE 441
5
EP  4.44 N P f  max
Opposes VT
ES  4.44 N S f max
Opposes ΦM
EP 4.44 N P f  max N P


ES 4.44 N S f  max N S
ECE 441
6
“No-Load” Condition
ECE 441
7
“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
8
IO  I fe  I M
N P I O  N P I fe  N P I M
No-Load Excitation mmf
Magnetizing mmf
No-Load Core Loss mmf
NP IM
M 
Rcore
ECE 441
9
VT  I P RP  EP
VT  EP
IP 
 IO
RP
ECE 441
10
Close the load switch
Secondary current will set up an mmf in OPPOSITION
to the primary mmf. The core flux will DECREASE to
M 
N P iM  N S iS
Rcore
ECE 441
11
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
12
M 
N PiM  N PiP ,load  N S iS
Rcore
Primary 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
13
Final steady – state primary current under
loaded conditions is
I P  I fe  I M  I P ,load
I P  I 0  I P ,load
ECE 441
14
Component Fluxes – Loaded Transformer
ECE 441
15
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
ECE 441
16