Electrical Machines EE-260 Lecture-04: Instructor: DR Mehmood Alam
Electrical Machines EE-260 Lecture-04: Instructor: DR Mehmood Alam
Electrical Machines EE-260 Lecture-04: Instructor: DR Mehmood Alam
EE-260
Lecture-04
Instructor: Dr Mehmood Alam
Department of Electrical Engineering, Military College of Signals
National University of Sciences & Technology (NUST)
Outline
• Single phase Transformers
• Magnetization current In real transformer
• The Dot convention
Real Single Phase Transformer
When the voltage is applied on the primary side of the transformer then the
average flux produced by this voltage is:
__
1
NP v P (t )dt
P M LP
8
1. The Magnetization Current
v p (t ) VM cos t
1
Np VM cos t dt
VM
sin t
Np The magnetization current in
the transformer core 9
1. The Magnetization Current in Real Transformer (cont…
10
http://en.wikipedia.org/wiki/File:Power_Transformer_Over-Excitation.gif
Observations:
11
2.The core-loss current
• Since the eddy currents are proportional to the rate of change of flux.
Therefore, the eddy currents are largest when the flux is passing
through the 0.
iex im ihe
IM
Io
The Current Ratio & Dot Convention
• Connect a load on the secondary side of the transformer.
• The dots help to identify the polarity of the voltages and currents
The Current Ratio & Dot Convention
(cont…
• A current flowing into the dotted end produces +ve mmf, while
a current flowing into the undotted end produce a –ve mmf.
Fp N p i p & Fs N s is
Fnet Fp Fs N pi p N s is
Fnet N pi p N s is
• If both currents are entering the dotted end, the mmf will add to each other.
• If one current enters and the other one leaves then the mmf will subtract
from each other.
The Current Ratio & Dot Convention
(cont…
• For a good designed transformer, should be very small nearly
zero, as long as the core is operating in unsaturated region.
Fnet N p i p N s is 0
N p i p N s is
ip Ns 1
is N p a
• In order for mmf to be nearly zero, current must flow in to the one dotted
end and out of the other dotted end.