230
Electricity and Basic Electronics
Since P
S
= E
S
I
S
, you can replace P
S
with E
S
I
S
:
P
P
= E
S
I
S
You know P
P
and E
S
, so you can solve for I
S
:
600 W = 150 V × I
S
600 W
______
150 V
=
150 V × I
S
_________
150 V
4 A = I
S
The current in the secondary windings is 4 A.
Example 12-7:
Suppose the power in the primary windings
of a transformer (P
P
) is 850 W and the current
in the secondary (I
S
) is 5 A. What is the voltage
in the secondary windings (E
S
)? Use the power
formula:
P
P
= P
S
Since P
S
= E
S
I
S
, you can replace P
S
with E
S
I
S
:
P
P
= E
S
I
S
You know P
P
and I
S
, so you can solve for E
S
:
850 W = E
S
× 5 A
850 W
______
5 A
=
E
S
× 5 A
________
5 A
170 V = E
S
The voltage in the secondary windings (E
S
)
is 170 V.
High-Voltage Transformers
High-voltage transformers have ratings
of over 24,000 volt-amperes (VA) of electricity.
The volt-ampere is the result of multiplying
the volts and amps on the primary of the
transformer. If the primary was 12,000 volts
at 2 amps, you would have 24,000 VA. This
particular unit is called a 24 kVA transformer.
This type of transformer would not be used
in school power supplies or around your
home. High-voltage transformers are found in
industries where large amounts of power are
needed.
High-voltage transformers are also found
in substations, Figure 12-30. The transformers
on power poles, Figure 12-31, are used to step
down the voltage from 1200 V to 240 V and
120 V for use in businesses and homes.
Figure 12-30. The transformers in this substation
supply the power needs of a community.
Figure 12-31. Three power transformers are
mounted on a pole. The transformer windings
are placed inside a protective container fi lled with
cooling oil.