To understand the basic principle of
transformer
To obtain the equivalent circuit
parameters
To estimate efficiency & voltage
regulation at various load
 A WATTMETER , VOLTMETER ,AN AMMETER ARE
CONNECTED IN HIGH VOLTAGE SIDE .
 THE LOW VOLTAGE SIDE OF TRANSFORMER IS
SHORT CIRCUITED.
 THE APPLIED VOLTAGE IS A SMALL PERCENTAGE
NORMALLY 5-7 % OF THE RATED VOLTAGE .
 HENCE THE CORE LOSSES ARE VERY SMALL WITH
THE RESULT OF THE WATTMETER READING
REPRESENTS THE FULL LOAD Cu LOSS OR sq(I)*R
LOSS FOR THE WHOLE TRNSFORMER.
 RATED CURRENT IS PROVIDED.
Consider a 1 MVA 415V/ 11,000 V
transformer
If you were to perform SC test on LV side the current would
be (1 x 10^6)/(1.732 x 415) = 1391.2 A
Whereas if you were to perform the Test on HV side the
Current Would be (1 x 10^6)/ (1.732 x 11000) =52.48 A
How to find the parameters of the Transformer ?
KVA Rating of transformer= KVA
FOR OPEN CIRCUIT TEST,
Measured Power per Phase=Wo
Line Voltage
=Vo
Line Current
=Io
FOR SHORT CIRCUIT TEST,
Measured Power per Phase=Wsc
Line Voltage
=Vsc
Line Current
=Isc
CALCULATION:FOR OPEN CIRCUIT TEST:
cos A = Wo/Io*Vo
Iw = Io*cos A
Im = Io*sin A
Ro = Vo/Iw
Xo = Vo/Im
FOR SHORT CIRCUIT TEST:
Zsc=Vsc/Isc
Rsc=Wsc/Isc˄2
Xsc˄2= (Zsc˄2 – Rsc˄2)
R1=R2=Rsc/2
X1=X2=Xsc/2
APPROX. EQUIVALENT CIRCUIT
VOLTAGE REGULATION
The secondary terminal voltage changes with the change in the load due
to the change in the voltage droop across the winding resistance and leakage
reactances. The voltage regulation of transformer is defined as the net change in
the secondary terminal voltage from no load to full load expressed as a
percentage of its rated volage, for the same primary voltage. Percentage voltage
regulation is given by,
% Voltage Regulation = (Vsnl – Vsfl)/Vsfl * 100
*Where Vsnl and Vsfl are the effective values across secondary terminal
under no load and full load condition respectively.
For given OC & SC test the equetion of Voltage Regulation is given by,
% Votlage Regulation = (Isc*Rsc*cos A + Isc*Xsc*sin A)/V2(rated) * 100
EFFICIENCY OF TRANSFORMER
Efficiency of transformer is defined as the ratio of output of the
transformer to its input. The efficiency of transformer is much higher than that of
a rotating electrical machine, because there is no friction and windage losses.
thus, efficiency of transformer, η = Output power/Input power
input power = output power + total losses
total losses = iron losses + copper losses
= Wi +(Ip^2*Rp + Is^2*Rs)
Now,
η = Vs*Is*cos A / (Vs*Is*cos A + Wi +(Ip^2*Rp + Is^2*Rs))
CONDITION OF MAXIMUM EFFICIENCY
totale copper losses = Is^2*Rs
Now,
η = Vs*Is*cos A / (Vs*Is*cos A + Wi + Is^2*Rs)
By
using open circuit test we can
measure the loss of no-load losses means
that iron losses.
By using short circuit test we can
measure the loss of Full-load losses
means that Cu-losses
We can find the efficiency by using of
iron losses and cu-losses
No
require of extra load
This test are very economical and
convenient because they furnish the
required information without actually
loading the transformer
No much calculation is needed
Efficiency can be found for any desired
load without connecting it to full load
It’s
not economical to do this test on large
rating machines as, large amount of energy
is wasted
You can’t get information about the share
of different types of losses
It’s difficult task to perform different kinds
of load (R,L,C,)form no load to full load to
study performance at different situations.