Page 61 - Electrical Equipment Handbook _ Troubleshooting and Maintenance

TRANSFORMERS

3.18 CHAPTER THREE

I S
I P R P j X P a R S ja X S a
2
2

R j X
V P c m aV S

(a)
R P X P
aI P a 2 j a 2 R S j X S I S

R
V P c j X m V
a 2 a 2 S
a

(b)
FIGURE 3.13 (a) The transformer model referred to its primary voltage level. (b) The
transformer model referred to its secondary voltage level.

The excitation branch is moved to the front of the transformer, and the primary and secondary
impedances are added, creating equivalent circuits (Fig. 3.14a and b). In some applications,
the excitation branch is neglected entirely without causing serious error (Fig. 3.14c and d).

THE TRANSFORMER VOLTAGE REGULATION
AND EFFICIENCY

The transformer’s output voltage varies with the load even when the input voltage remains
constant. This is due to the fact that a real transformer has impedances within it. The volt-
age regulation (VR) is used to compare the voltage variations in transformers. The full-
load voltage regulation is a parameter that compares the transformer’s output voltage at no
load with the output voltage at full load.
It is given by

V S,nl V S,fl
VR 100%
V S,fl
Since the secondary voltage at no load is given by

V P
V
S
a

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