Joseph Henry and Michael Faraday independently discovered that a
changing current in an inductor would induce a current in a nearby
inductor. Credit for this property, called mutual induction, has been
given to Faraday. Modern transformers are constructed by winding two
coils of wire, either next to each other or one on top of the other on
a core made of either soft iron^{[1]} or ferrite^{[2]}.
The soft iron
concentrates the magnetic lines of flux in the
transformer. This increases the efficiency.
Trandformers
Schematic
Symbol
How Transformers Work
If you have electrical current flowing through an inductor (coil of wire,
seeInductors in DC Circuits), there will be a magnetic field surrounding that inductor. Now
put another inductor inside that magnetic field. If the current is DC, the
magnetic field will be unchanging, and nothing special will happen. However,
if the current changes, the magnetic field will become larger or smaller, so
if you put AC in the inductor, you will get a constantly changing magnetic
field. While the current is increasing, the magnetic field gets larger. As
this field passes the second inductor, it will induce a current in that
inductor. While the current is decreasing, the magnetic field will get
smaller. As this field passes the second inductor (now moving in the
opposite direction), it will induce a current in that conductor moving in
the opposite direction. As the alternating current in the first inductor
repeatedly reverses direction, another alternating current is induced in the
second inductor. Even though the inductors are not in physical contact, the
Alternating current in the first inductor induces AC in the second inductor.
This isn't easy to illustrate with static pictures. However, the short video
at the bottom of this page, though a bit dated, clearly explains mutual
induction in transformers.,
The output voltage of a transformer is primarily determined by the
ratio of turns
of wire between the two inductors. For example, if the first
inductor (the primary) has
1,000 turns and the second inductor (the secondary) has 100 turns, the
voltage induced in the
secondary will be ^{1}/_{10 }of the voltage in the
primary. In this
case, if the primary voltage is 120 volts, the secondary voltage will
be 12 volts.
The output voltage of a
transformer is primarily determined by the turns ratio. This
transformer has a 10:1 turns ratio so the output voltage is ^{1}/_{10
}of the input voltage.
A transformer that has a secondary voltage that is less than the
primary voltage is called a step-down transformer. The drop in voltage
is not a loss of energy. While there is a step-down in voltage, there
is a step-up in current. For example, if the secondary circuit has 12
volts and a
load impedance of 100 ohms the current in the secondary will be 120
milliamps. However, the current in the primary will be
only 12 milliamps. If you step up the voltage you step down the
current; if you step down the voltage you step up the current. Notice
that the load produces 1.44 watts of power (calculated by multiplying
12 volts by 0.012 amps). Also notice that the product of
the voltage and current in the primary is also 1.44. However, it is
improper to say that there are 1.44 watts of power in the primary
because
no power is produced by the transformer (except for undesirable
losses). For this reason the term volt-amp is used instead of power to
describe the product of volts and amps in the transformer. It is only
the
load of the circuit that produces power.
The voltamp
calculation is the same for either side of the transformer. Gaining
voltage from the primary to the secondary means you loose current to
maintain the same voltamp quantity. Power is not produced until it is
produced in the load (P = I^{2}R), so it is improper to use
watts to refer to the product of volts and amps in the windings.
The voltage in the secondary is determined by the combination of the
voltage in the primary and the turns ratio. However, the current in the
primary is determined by the current in the secondary. The effect of
voltage goes from primary to secondary, but the effect of current goes
from secondary to primary. In the above example the load is 100 ohms,
so the secondary current is 120 mA. Since this is a 10:1 step down
transformer the current in the primary is 12 mA (step down the voltage,
step up the current). Now let's change the load to 200 ohms. The
current in the secondary decreases to 60 mA. Therefore, the current in
the primary drops to 6 mA (still 10 times the secondary current). If we
remove the load entirely, making an open circuit on the secondary there
will be no current in the secondary. Consequently there will be no
current in the primary (except for losses, see Efficiency below). How can this be?
Recall from Inductors in DC
Circuits that inductors resist changes in current. Since AC is
constantly changing current the inductor will essentially block the
current. A transformer primary, being an inductor will tend to have
only a tiny current flow through it. When we complete the circuit on
the secondary (by adding a load) we now have current in the secondary
of the transformer. When you have current through an inductor you get a
magnetic field around that inductor. Therefore, current in the primary
causes a magnetic field around the primary which induces a current in
the secondary which causes a magnetic field around the secondary. Get
it? This magnetic field around the secondary inductor has the opposite
polarity to the magnetic field in the primary, so it tends to cancel
the primary's magnetic field. It's a complex interaction. The bottom
line is that current in the secondary decreases the inductance of the
primary causing the primary to conduct more current. To repeat,
increasing the current in the secondary increases the current in the
primary, so the effect of voltage goes from primary to secondary and
the effect of current goes from secondary to primary (whew). In a well
designed transformer there will be very little current flowing in the
primary when there is no load on the secondary. When you load the
secondary the primary current increases along with the secondary
current.
Types of Transformers
Step-up / step-down
In a step-up transformer the secondary voltage is higher than the
primary voltage; in a step-down transformer the secondary voltage is
lower than the primary voltage. A step-up or step-down transformer can
be reversed to perform the opposite function. Take the 10:1 step down
transformer in the above example. If you put 12 volts AC into the
secondary you will get 120 volts AC at the primary. However a
transformer is
usually optimized for only one operation, either step up or step down.
Isolation
An isolation transformer has the same voltage on the secondary as on
the primary. The primary and secondary may be optimized for the
circuits they are intended to couple, but otherwise the primary and
secondary are identical.
The a common purpose of an isolation transformer
is to block direct current. Let's say you have a voltage that is
repeatedly changing from 10 volts to 20 volts and back. It's a
constantly changing voltage like AC, but never changes direction. What
is it? It's a mixture of AC and DC. It's actually 15 volts DC with 10
volts peak-to-peak AC mixed with it. If you put this on the primary of
an isolation transformer only the 10 volts of AC will be seen on the
secondary. Can you get induction through a transformer if the primary
current doesn't change direction? Sure you can. The magnetic field
builds and collapses repeatedly so you still get a magnetic field that
moves back and forth across the secondary. However, it is only the
changes in the magnetic field that induce current in the secondary.
Therefore, if the primary voltage oscillates between 10 and 20 volts
the outcome is the same as if the primary voltage oscillated between -5
and +5 volts. Still a swing of 10 volts peak-to-peak.
Isolation transformers may also be used to isolate equipment
from the power grid. This may be done to limit the current from the
receptacle to the circuit. Let's say you have an isolation transformer
connected to an 120 volt power outlet. Since the turns ratio is 1:1 the
secondary will also have 120 volts. Now let's assume this is a fairly
lightweight transformer and has an output impedance of 100 ohms ^{[3]}
Now let's short circuit the
secondary. If we were connected directly to the outlet we would get 15
to 20 amps of current, sparks, smoke and a tripped circuit breaker.
With this isolation transformer we get 1.2 amps (120 V divided by 100
Ω). We have protected the building power delivery circuit from our
circuit.
Isolation transformers that connect to the power grid may have a
variable output. However, don’t mistake a Variac^{®} (see
autotransformer
below) for an isolation transformer. A Variac^{®}has a single
inductor and does not provide isolation from the power grid.
Impedance-matching
A matching transformer is used to match the impedance between two
circuits. This maximizes power transfer.
Tapped transformers
A tapped transformer
Transformers often have taps. These are connections to the coils at
places other than at the ends of the coils. The secondary may be
tapped to provide multiple voltages. These may be center-tapped to give
two equal
voltages or may have various taps to give various voltages.
Transformers may also be tapped on the primary to allow connection to
different power grid standards. For example, the 120 volt standard used
across North America may actually be anywhere from 114 volts to 126
volts, depending on local standards. It is not common for equipment to
need to be so-closely matched to the local standards but such
transformers are available for this situation. To match a transformer
to widely varied standards, such as North America and Europe, multiple
winding primary transformers (see below) are more common.
Multiple winding transformers
Some transformers have multiple primary or secondary windings. A
typical multiple winding transformer may be designed to be used with
either North American or European voltage standards. Here is an example:
A transformer with multiple primary windings.
This transformer can be wired in the following ways.
Primaries connected in series for the European standard.
Primaries connected in parallel for the North American standard.
When connecting multiple windings be sure to connect them with the
proper polarity. There will be some indication on the transformer as to
which end of each coil is which. They may be labeled with plus and
minus signs, something like 0 and 120 (for 120 volt windings) or may
just have color-coded wires with a diagram on the transformer
indicating which color is which. You may have to go by the positions of
the terminals as in the above diagrams. When connecting in parallel be
sure to put plus to plus and minus to minus or equivalent. When
connecting in series be sure to put plus to minus or equivalent. If you
connect the windings incorrectly the magnetic fields will cancel each
other and the transformer won't work. The transformer can also be
damaged.
Transformers also come with multiple secondary windings that can be
used the same way. Some multiple winding transformers are also
combination step-up-step-down transformers. These were common in the
days of vacuum tubes. Tubes often need 6 volts or 12 volts for the
filaments plus 300 or more volts for the plates. Such transformers
accommodate these needs. Tapped secondary windings (see above) are also
used in such situations.
Autotransformer
An autotransformer is a single coil that is tapped in such a way that
the primary and secondary are different parts of the same coil. A
Variac^{®} is a brand name for an autotransformer with variable output.
The
ignition coil of a car is usually an autotransformer.
An
autotransformer
Tesla coil
A Tesla coil, invented by Nicola Tesla, is a type of autotransformer
that is driven by a resonant oscillator. Very high voltages and
currents can be achieved with a Tesla coil.
Efficiency
Transformers are extremely efficient. However there are some losses.
Core losses
Core losses are caused by power consumed by the
transformer core. These losses are caused by:
Hysteresis
Hysteresis is the resistance of a
material to change the polarity of
its magnetism. With each oscillation of the transformer current, the
polarity of the magnetic domains in the core are reversed. The
hysteresis consumes power and produces heat.
Eddy currents
The transformer core is made of soft
iron, which conducts electricity.
Since this core is within the oscillating magnetic field, electric
current will be induced in it. This current does nothing useful and
produces heat. Transformer cores are usually made of laminated plates
coated with enamel.
A
laminated transformer core.
The enamel electrically insulates the layers of the
core from each other, reducing the eddy currents.
Copper losses
Copper losses are mainly caused by the
resistance of the wire that
makes up the transformer. Copper losses can be reduced by increasing
the diameter of the wire. If a transformer needs to transfer a high
current, larger wire must be used to reduce copper losses. Consequently
a larger iron core must also be used. Copper losses determine the
output impedance and, therefore, the maximum output current of a
transformer. When the current rating of a transformer is exceeded,
copper losses cause a drop in voltage.
Specifications
When discussing transformer theory we emphasize the turns ratio in
determining the output voltage of a transformer. When buying
transformers the turns ratio is not part of the specification.
Transformers are specified by the input voltage and the output voltage.
The maximum load is specified either by the secondary current
or the voltamp (VA) capacity. For example, a common step-down power
transformer for use in North America would be specified something like:
Primary voltage: 117 V Secondary voltage: 12 V Secondary current: 2 A
However, an identical transformer may be specified like:
Primary voltage: 117 V Secondary voltage: 12 V Maximum load: 24 VA
So, what's the difference? There is none. It is just a different way of
specifying the current rating. The VA rating is simply the product of
the secondary voltage and current, e.g. 12 volts times 2 amps equals 24
voltamps. You will get the same answer if you multiply the primary
voltage by the primary current. This is useful if you need to know the
primary current. This transformer will have a primary current of 205 mA
(24 VA divided by 117 volts). Be careful not to mix your primary and
secondary voltages and currents.
Transformers
Isolation Transformers - Answers to Questions
Does Ohm's Law Apply to Transformers - Answers to Questions