Transformers are electromagnetic devices that transfer electrical
energy from one circuit to another by mutual induction. Mutual
induction is the coupling of inductances by their mutual
magnetic fields. In a single-phase transformer there are two
coils, a primary and a secondary coil. The following circuit
illustrates mutual induction. The AC generator provides electrical
power to the primary coil. The magnetic field produced by
the primary induces a voltage into the secondary coil, which
supplies power to a load
Transformers are used to step a voltage up to a higher level,
or down to a lower level. Transformers are used extensively
in power distribution systems, allowing power companies
to transfer electrical energy many miles. Power generators
typically generate high voltages. This voltage varies, depending
on the generator, but a typical voltage might be 15 KV. The
voltage is stepped up through a transformer to higher levels
for transmission to substations. Typical voltages range from
115 KV to 765 KV. The electrical power is received at substation
transformers many miles away where it is stepped down.
Typical voltage might be 34 KV or 69 KV. From here, electrical
power is fed to a distribution substation. It can also be fed
directly to factory locations. If the power is fed to a factory,
transformers at the factory site reduce the voltage to usable
levels. The power fed to a distribution substation is reduced by
transformers at the substation for factory and home use.
Coefficient of Coupling
Mutual inductance between two coils depends on their flux
linkage. Maximum coupling occurs when all the lines of flux
from the primary coil cut through the secondary winding.
The amount of coupling which takes place is referred to as
coefficient of coupling. To maximize coefficient of coupling, both
coils are often wound on an iron core which is used to provide
a path for the lines of flux. The following discussion of step-up
and step-down transformers applies to transfers with an iron core
There is a direct relationship between voltage, impedance,
current, and the number of coil turns in a transformer. This
relationship can be used to find either primary or secondary
voltage, current, and the number of turns in each coil. It is the
number of turns which determine if a transformer is a step up
or step down transformer. The following “rules-of-thumb” apply
to transformers:
• If the primary coil has fewer turns than the secondary coil,
it is a step-up transformer.
• If the primary coil has more turns than the secondary coil,
it is a step-down transformer.
When the number of turns on the primary and secondary coils of
a transformer are equal, input voltage, impedance, and current
are equal to output voltage, impedance, and current.
Step-Up Transformer
A step-up transformer is used when it is desirable to step
voltage up in value. The following circuit illustrates a stepup
transformer. The primary coil has fewer turns than the
secondary coil. The number of turns in a transformer is given as
a ratio. When the primary has fewer turns than the secondary,
voltage and impedance are stepped up. In the circuit illustrated,
voltage is stepped up from 120 VAC to 240 VAC. Because
impedance is also stepped up, current is stepped down from 10
amps to 5 amps
Step-Down Transformer
A step-down transformer is used when it is desirable to
step voltage down in value. The following circuit illustrates a
step-down transformer. The primary coil has more turns than
the secondary coil. The step-down ratio is 2:1. voltage and
impedance are stepped down, current is stepped up
Single-Phase Transformer
120 or 240 VAC single-phase transformers are used to supply
lighting, receptacle, and small appliance loads. A transformer
with a 240 VAC secondary can be used to supply 240 VAC to
larger appliances such as stoves, air conditioners and heaters.
A 240 VAC secondary can be tapped in the center to provide
two sources of 120 VAC power
Formulas for Calculating the Number of Primary and Secondary Turns of a Transformer
There are a number of useful formulas for calculating, voltage,
current, and the number of turns between the primary and
a secondary of a transformer. These formulas can be used with
either step-up or step-down transformers. The following legend
applies to the transformer formulas
ES = secondary voltage
EP = primary voltage
IS = secondary current
IP = primary current
NS = turns in the secondary coil
NP = turns in the primary coil
Using the values for the step-down transformer in the example
of the previous page, the secondary voltage can be verified
Transformer Ratings
Transformers are rated in kVA (kilovolt-amps). This rating is
used rather than watts because loads are not purely resistive.
Only resistive loads are measured in watts. The kVA rating
determines the current a transformer can deliver to its load
without overheating. Given volts and amps, kVA can be
calculated. Given kVA and volts, amps can be calculated
Using the illustrated step-down transformer, the kVA rating can
be calculated. The kVA rating of a transformer is the same for
both the primary and the secondary
