114 Chapter 8 • Transformers EXAMPLE 8.2 Example 8.2
A single-phase transformer delivers a full load secondary current of 85 amperes at 24 volts with a power factor of 0,8. Determine;
(a) the rating of the transformer, (b) the power, and (c) the reactive power.
Solution: (a) S = V2
I2
= 24 × 85 = 2 040 VA (or 2,04 kVA)
(b) P = V2I2 cos ϕ
= 24 × 85 × 0,8 = 1 632 W (1,62 kW)
(c) Q = VI sin ϕ But ϕ = cos-1
0,8 = 24 × 85 × sin 36,87˚
ϕ = 36,87˚ = 1 224 VAr
(1,224 kVAr
or Q = S × sin ϕ = 2 040 × sin 36,87˚ = 1,224 kVAr
)
__ V2
V1 I2
Cos ϕ2 V1
= N1
__ N2
= I2
__ I1
= 85 A V2 = 0,8
___ ___ ___ V2
=
N1 N2
=
I2 I1
ϕ = 36,87˚ = 24 V
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