Page 64 - Electrical Equipment Handbook _ Troubleshooting and Maintenance

TRANSFORMERS

TRANSFORMERS 3.21

V P
a

R eq I cos X I sin
S
eq S
V S
I
j X eq S
R eq I S
I S
V P ≈ V S + R eq I S cos + X eq I S sin

a
FIGURE 3.17 Derivation of the approximate equation V /a.
P
The voltage regulation can be determined by using the V /a term calculated from the equa-
P
tion above in the voltage regulation equation:

V P/a V S,fl
VR 100%
V S,fl

Transformer Efficiency

The efficiency of a device is normally given by

P out

100%
P in
P out
100%
Pout Ploss
Since the output power of the transformer is expressed by

P V I cos
S S
out
the efficiency of the transformer is given by
V S I S cos

100%
P P core V I cos
Cu
S S
where P copper losses and P core losses.
Cu core

Transformer Taps and Voltage Regulators
Since the transformer’s output voltage varies with the load, most distribution transformers
use taps in the windings to change the turns ratio of the transformer. Most applications
have four taps with spacings of 2.5 percent of full voltage between them in addition to

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