Chapter 12 Electromagnetic Induction
229
and secondary are out of phase with one
another. This relationship would show up on
a dual trace scope.
Voltage-Current
Relationships
Transformers are used in electrical circuits
because they are very effi cient. Almost all the
power applied to the primary winding is trans-
ferred to the secondary winding, Figure 12-29.
Recall from Chapter 5 that power is equal to
the voltage times current (P = EI). We can take
this formula and use it in transformer circuit
analysis. If we say that transformers are 100%
effi cient and have no losses, then
P
P
= P
S
(12-5)
where P
P
is the primary power and P
S
is
secondary power. Since P = EI, this equation
is equal to
E
P
I
P
= E
S
I
S
(12-6)
If the power on the primary winding (P
P
)
of a transformer is 600 W and the voltage of
the secondary (E
S
) is 150 V, what is the current
of the secondary? Use the following formula:
P
P
= P
S
Cross multiply:
110 V × N
S
= 3 V × 2200
Divide both sides by 110 V:
110 V N
S
________
110 V
=
6600
______
110 V
N
S
= 60
The tap would be attached after winding
number 60.
Transformer Phase
Relationships
When the primary and secondary volt-
ages are in-phase with one another, you
have “like-wound transformers.” To signify
this, two dots are placed above the windings
(both primary and secondary) of the sche-
matic transformer symbol to show they are
in-phase. If the dots were on opposite ends
of the primary and secondary windings,
you have “unlike-wound transformers.” See
Figure 12-28. This means that the primary
Figure 12-28. Two dots are placed above the
windings of a transformer symbol to show they
are in phase. When the dots are placed at oppo-
site ends of the primary and secondary windings,
the primary and secondary windings are out of
phase.
Unlike-wound
(Out of Phase)
Like-wound
(In Phase)
Figure 12-29. Since transformers are very
effi cient, you can assume that power applied
to the primary equals power produced by the
secondary.
Power in
Secondary
Power in
Primary
Will be
equal to
In the Perfect Transformer