(b) A 10 KVA, 500/100 V transformer has the primary referred parameters are Rieq
ohm, Xieq=5.2 ohm. When supplying power to a lagging load the current, power and
voltage measured on primary side were 20 A, 8 kW and 500 V respectively. Calculate
the voltage on secondary terminals. Also find volta- regulation at power factor of
0.85 lagging.
Answers
Answer:
Rating of transformer =10\: KVA=10*10^{3}\: VA=10KVA=10∗10
3
VA
Power factor on primary side,
cos\: \phi _{1}=\frac{P}{V_{1}I_{1}}=\frac{8*10^{3}}{500*20}=0.8\: kgcosϕ
1
=
V
1
I
1
P
=
500∗20
8∗10
3
=0.8kg
\therefore∴ sin\: \phi _{1}=sin\: cos^{-1}\: 0.8=0.6sinϕ
1
=sincos
−1
0.8=0.6
Primary induced emf, E_{1}=V_{1}-I_{1}R_{ep}\: cos\: \phi _{1}-I_{1}\: X_{ep}\: sin\: \phi _{1}E
1
=V
1
−I
1
R
ep
cosϕ
1
−I
1
X
ep
sinϕ
1
=sin\: 500-20*0.3*0.8-20*5.2*0.6=432.8\: V=sin500−20∗0.3∗0.8−20∗5.2∗0.6=432.8V
Transformation ratio, K=\frac{100}{500}=0.2K=
500
100
=0.2
Secondary induced emf, E_{2}=E_{1}*K=432.8*0.2=86.56\: VE
2
=E
1
∗K=432.8∗0.2=86.56V
Secondary terminal voltage on load,
V_{2}=E_{2}=86.56\: VV
2
=E
2
=86.56V (since all the parameters are referred to primary side)
The equivalent circuit and phasor diagram is shown in Fig. 2.42(b
