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about Step up and step down transformers
More information
about Step up and step down transformers
Equality of voltage and current between
the primary and secondary sides of a transformer, however,
is not the standard for all transformers. If the inductances
of the two windings are not equal, something exciting
happens:
transformer
v1 1 0 ac 10 sin
rbogus1 1 2 1e-12
rbogus2 5 0 9e12
l1 2 0 10000
l2 3 5 100
k l1 l2 0.999
vi1 3 4 ac 0
rload 4 5 1k
.ac lin 1 60 60
.print ac v(2,0) i(v1)
.print ac v(3,5) i(vi1)
.end
freq v(2) i(v1)
6.000E+01 1.000E+01 9.975E-05 Primary winding
freq v(3,5) i(vi1)
6.000E+01 9.962E-01 9.962E-04 Secondary winding
What we cover here is a device that
steps voltage down by a factor of ten and current up
by a factor of ten:
Turns ratio of 10:1 yields 10:1 primary:secondary
voltage ratio and 1:10 primary:secondary current ratio.
This is a very useful device, definitely.
With it, we can simply multiply or divide voltage and
current in AC circuits. Certainly, the transformer
has made long-distance transmission of electric power
a practical truth, as AC voltage can be "stepped up"
and current "stepped down" for reduced wire resistance
power losses along power lines connecting generating
stations with loads. At either end (both the generator
and at the loads), voltage levels are reduced by transformers
for safer operation and less pricey equipment. A transformer
that increase voltage from primary to secondary (more
secondary winding turns than primary winding turns)
is called a step up transformer. On the other hand,
a transformer designed to do just the opposite is called
a step down transformer.
Let's revisit a photograph shown in
the previous section:
Transformer example showing primary
and secondary windings is a few inches tall (approximately
10 cm).
This is a step-down
transformer, as evidence by the high turn count
of the primary winding and the low turn count of the
secondary. As a step down unit, these transformers convert
high voltage, low current power into low voltage, high current
power. The larger gauge wire used in the secondary winding
is required due to the increase in current. The primary
winding, which doesn't have to conduct as much current,
can be made of smaller-gauge wire.
In case you were wondering, it is probable
to operate either of these transformer types backwards
(powering the secondary winding with an AC source and
letting the primary winding power a load) to make the
opposite function: a step up can function as a step down
and visa versa. Though, as we know the efficient operation
of a transformer requires that the individual winding
inductances be engineered for specific operating ranges
of voltage and current, so if a transformer is to be
used "backwards" like this it must be employed within
the original design parameters of voltage and current
for each winding, lest it prove to be inefficient (or
lest it be damaged by excessive voltage or current!).
Transformers are often constructed
in such a way that it is not clear which wires lead
to the primary winding and which lead to the secondary.
One principle used in the electric power industry to
help alleviate confusion is the use of "H" designations
for the higher voltage winding (the primary winding
in a step-down unit; the secondary winding in a step up)
and "X" designations for the lower-voltage winding.
Thus, a simple power
transformer will have wires labeled "H1",
"H2", "X1", and "X2".
There is generally significance to the numbering of
the wires (H1 versus H2, etc.)
The fact that voltage and current
get "stepped" in opposite directions (one up, the other
down) makes perfect sense when you recall that power
is equal to voltage times current, and recognize that
transformers cannot produce power, only convert it.
Any device that could output more power than it took
in would violate the Law of Energy Conservation in physics,
that is the energy cannot be created or destroyed, only
converted. As with the initial transformer example we
looked at, power transfer efficiency is very good from
the primary to the secondary sides of the device.
The practical meaning of this is made
more apparent when an alternative is considered: before
the advent of efficient transformers, voltage/current
level conversion could only be achieved during the use
of motor/generator sets. A drawing of a motor/generator
set reveals the essential principle involved:
Motor generator illustrates the fundamental
principle of the transformer.
In such a machine, a motor is automatically
coupled to a generator, the generator designed to produce
the desired levels of voltage and current at the rotating
speed of the motor. While both motors and generators
are quite efficient devices, the use of both in this
fashion compounds their inefficiencies so that the overall
efficiency is in the range of 90% or less. Moreover,
because motor/generator sets obviously require moving
parts, mechanical wear and balance are factors influencing
both service life and performance. Transformers, on
the other hand, are able to convert levels of AC
voltage and current at very high efficiencies with
no moving parts, making possible the extensive distribution
and use of electric power we take for granted.
In all fairness it should be noted
that motor/generator sets have not necessarily been
obsoletes by transformers for all applications. While
transformers are obviously superior over motor/generator
sets for AC voltage and current level conversion, they
cannot convert one frequency of AC power to another,
or (by themselves) convert DC to AC or visa versa. Motor/generator
sets can do all these things with relative simplicity,
albeit with the limitations of efficiency and mechanical
factors previously described. Motor/generator sets also
have the distinctive property of kinetic energy storage:
that is, if the motor's power supply is momentarily
interrupted for any reason, its angular momentum (the
inertia of that rotating mass) will maintain rotation
of the generator for a short duration, thus isolating
any loads powered by the generator from "glitches" in
the main power system.
Looking closely at the numbers in the
SPICE analysis, we should see a connection between the
transformer's ratio and the two inductances. Observe
how the primary inductor (l1) has 100 times more inductance
than the secondary inductor (10000 H versus 100 H),
and that the measured voltage step-down ratio was 10
to 1. The winding with other inductance will have higher voltage and less current than the other. Since the
two inductors are wound around the same core material
in the transformer (for the most well-organized magnetic
coupling between the two), the parameters affecting
inductance for the two coils are the same except for
the number of turns in each coil. If we take one more
look at our inductance formula, we see that inductance
is proportional to the square of the number of coil
turns:
So, it should be obvious that our two
inductors in the last SPICE transformer example circuit
-- with inductance ratios of 100:1 -- should have coil
turn ratios of 10:1, because 10 squared equals 100.
This works out to be the same ratio we initiate between
primary and secondary voltages and currents (10:1),
so we can say as a rule that the voltage and current
transformation ratio is equal to the ratio of winding
turns between primary and secondary.
The step up/step down result of coil
turn ratios in a transformer (Figure above) is corresponding
to gear tooth ratios in mechanical gear systems, transforming
values of speed and torque in much the same way: (Figure
below)
Torque reducing gear train steps torque
down, when stepping speed up.
Step up and step down transformers
for power distribution purposes can be gigantic in proportion
to the power transformers before shown, some units standing
as tall as a home. The subsequent photograph shows a
substation transformer standing about twelve feet tall:
(Figure below)
Substation transformer.
EXAMINATION
Transformers “step up” or “step down”
voltage according to the ratios of primary to secondary
wire turns.
A transformer designed to raise voltage from primary to secondary is called a step up transformer. A transformer designed to decrease voltage from primary to secondary is called a step down transformer.
The transformation ratio of a transformer will be equivalent to the square root of its primary to secondary inductance (L) ratio.
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