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Lesson Objectives :

Define the primary and secondary of a transformer

Describe the construction and properties of transformer

Explain the operating characteristic of transformer

Calculate the turns-voltage-current relationships of transformer

Differentiate between iron and copper losses occurring in transformer

Calculate the total losses at difference load of transformer

Describe the construction, use and operating characteristic of autotransformer

Perform basic test of transformers
2.1
Introduction to machinery
An electric machine is a device that can convert either mechanical energy to electric
energy or electric energy to mechanical energy. When such a device is used to converts
electric energy to mechanical energy, it is called a motor. When it converts mechanical
energy to electric energy, it is called a generator. Almost all practical motors and
generators convert energy from one form to another through the action of a magnetic
field.
Another closely related device is the transformer. A transformer is a device that converts
ac electric energy at one voltage level to ac electric energy at another voltage level.
Since transformers operate on the same principles as generators and motors, they are
usually studied together.
2.2
The transformer and power transmission
Figure 2-1
Transformers enable the economical transmission
of power over a large distance
Figure 2-1 shows a simple transmission system composed of a generating station, a
transmission line, and a power consumer. At the generation station, a transformer T1
raises the voltage from 4160 V to 69 kV to reduce the transmission line current and
corresponding line losses. At the other end of the line a second transformer T2 reduces
the voltage from 69 kV to a more usable, practical value such as 480 V.
2.3
Simple transformer
A transformer in its simplest form is composed of two coils that are mounted on a
laminated iron core see Fig. 2-2. The coils are carefully insulated from each other and
from the core.
Figure 2-2
Construction of two winding transformer
One of the coils is connected to the ac source; it is called the primary winding, or simply
primary. The other coil, connected to the load, is called the secondary winding, or simply
secondary. Power therefore flows from the primary to the secondary winding.
Note that the power flow through a transformer is reversible: the primary can become a
secondary, and vice versa. Thus in Fig 2-2, if terminals 1 and 2 are connected to a
source, the secondary winding automatically becomes a primary winding. Furthermore,
because terminals a and b are now connected to a load, the primary winding becomes
the secondary winding.
2.4 Transformer Construction
There are two different transformer core shapes in common use, namely, the core type,
see Fig 2-3 and the shell type see Fig 2-4. The cores of both types are fabricated of
special low-loss steel and are laminated to minimize core losses
Figure 2-3: Single phase transformer core type
Figure 2-4
Single phase transformer shell type
In the core type construction shown in Fig 2-3, the winding surround the laminated iron
core. For the sake of simplicity, the primary winding of the core-type transformer
represented in Fig 2-1 is shown on one leg of the core and the secondary on the other
hand.
Commercial transformers are not constructed in this manner because a large amount of
the flux produced by the primary winding does not cut the secondary winding, or it is
said that the transformer has a large leakage flux. To keep the leakage flux to minimum,
the winding are divided with half of each winding being placed on each leg of the core.
The completed core and coil assembly of a core type transformer is shown in Fig 2-5.
Figure 2-5
Transformer core and coil assembly, core type
In transformer shell type construction, the iron core surrounds the winding. It is shown
in Fig 2-6.
Figure 2-6
Transformer core and coil assembly, shell type
Both of these transformers are designed to be immersed in insulating oil in a steel tank.
In addition to having insulating properties, the oil conducts heat from the core and coils
to the surface of the tank where it is given off to the surrounding air. Connections from
the transformer coils to the external circuits are made through insulating bushings
usually made of porcelain.
Coils of transformers are wound with copper or aluminum wire or strap. For heavycurrent winding, several strands of conductor are paralleled to reduce eddy-current loss
in the conductors. Coil insulation materials used are cotton, cellulose, special papers,
polyester glass tapes, or other similar materials. Completed coils are dried in an oxygenfree atmosphere to remove all moisture. Coils of oil-immersed transformers are then
impregnated with dry insulating oil while they are in a vacuum.
Two basic types of transformer windings are commonly used. These are concentric and
the pancake types. Concentric windings are cylindrical in form with one winding placed
inside the other with the necessary insulation between them. The low-voltage winding is
normally placed on the inside next to but insulated from the core. Pancake windings are
built up with primary and secondary sections interleaved. In both types, spacers are
provided between adjacent coils to permit ventilation or the circulation of the cooling
liquid.
2.5
An ideal transformer
An ideal transformer is one which has no losses, ie. Its winding have no ohmic resistance
and there is no magnetic leakage. An ideal transformer consists of two coils which are
purely inductive and wound on a loss-free core. However it may be noted that it is
impossible to realize such a transformer in practice.
2.6
Theory of operation: No Load
Figure 2-7
Elementary diagram of a step-down transformer
When an alternating voltage VH is applied to the primary (H) winding of the step-down
transformer represented in Fig 2-7, with the load switch open, a small current called the
exciting current flows. As in any inductive circuit, the current is limited by the counter
emf of self-induction that is induced in the winding. Transformer windings are designed
to have an inductance high enough to make the counter emf practically equal to the
applied voltage at no load. This limits the no-load or exciting current to a very low value.
The exciting current causes an alternating flux to be set up in the core. This alternating
flux cuts across the turns of both the primary and secondary windings as it increases and
decreases in alternate directions, thereby inducing an emf in both windings. The emf
induced in the primary winding opposes the applied voltage VH. Since the turns of both
windings are cut by the same flux, the emf induced in each turn of both windings is the
same.
If EH is the emf induced in the primary winding and EX is the emf induced in the
secondary winding, then the voltage per turn in the two windings is EH/TH and EX/TX,
respectively and EH/TH = EX/TX
If the resistance of the primary winding is small then we can assume that EH will be very
nearly equal to the applied voltage VH. Neglecting this small difference and noting that
the secondary terminal voltage VX will be equal to EX since there is no current flowing,
then VH/TH = VX/TX. Cross-multiplying and dividing by VX/TX results in
VH / VX = TH / TX
2-1
This equation shows that the voltages of each of the winding of a transformer are
directly proportional to the number of turns in each winding.
Example calculation 2.1
A transformer with 200 turns on the primary winding is to be wound to step the voltage
down from 240 to 120 V. Find the number of turns required on the secondary winding
Solution:
2.7 Operation under load
When the load switch in the secondary circuit of the transformer in Fig 2-7 is closed, a
current IX, equal to VX divided by the load impedance, will flow. By Lenz’s law, any
current caused to flow by an induced emf flows in such direction as to oppose the action
that causes the emf to be induced. In the case of transformer, this means that IX will
always flow in a direction such that its magnetizing action will oppose the magnetizing
action of the primary winding. The current IX, then, tends to reduce the flux in the
transformer core. However if the flux is reduced, the counter emf EH is reduced, thereby
permitting more primary current IH, to flow, which restores the flux to its original value.
If more load is added, causing the transformer secondary current IX to increase, its
increase demagnetizing action which permits more primary current to flow. Conversely,
when the secondary load is decreased, the magnetizing action of IX decreases, causing a
decrease in primary current. Thus the magnetizing action of the primary winding adjusts
itself with each change in secondary winding current.
From the above discussion the following relations are evident:
Or
Primary ampere-turns = secondary ampere-turns
IH TH = IXTX
2-2
If both sides of eq. 2.2 are divided by IHTX, then
I X / IH = T H / T X
2-3
That is the ratio of the currents in a transformer is inversely proportional to the ratio of
the turns.
When the load current flows from the secondary winding of a transformer, there is a
small potential drop in the transformer as a result of its impedance. Thus, the terminal
voltage is slightly lower than the induced emf. However, this difference is often
neglected and VX is assumed to be equal to EX. Thus eq. 2-1 still applies. Combining eqs.
2-1 and 2-3 the results is
And
VH / V X = I X / I H
VH I H = VX I X
2-4
Equation 2-4 shows that the voltampere input of a transformer is equal to the
voltampere output.
It should be noted in connection with eqs 2-1 to 2-4 that they are approximate
equations only. They are true only for an ideal transformer, that is a transformer with no
losses. However, they are sufficiently accurate for most practical purposes since the
losses in most transformers are very small.
Example calculation 2.2
A transformer supplies a load with 30 A at 240 V. If the primary voltage is 2400 V, find
a. The secondary voltamperes
b. The primary voltampere
c. The primary current
Solution:
2.8
Transformer losses and efficiency
The efficiency of a transformer is the ratio of the useful power output to the total power
input. Since the input to a transformer is equal to its useful output plus its losses, the
efficiency equation may be written in either of the following forms:
Percent efficiency = (power output/power input) X 100 %
2-5
= [power output/(power output + losses)] X 100 %
2-6
From the above equation, we concluded that the efficiency of a transformer can be
determined for any given load by making a direct measurement of its power input and
its power output. Because of limitations of available testing facilities, it is sometimes
difficult to perform direct input-output load measurement, especially for very large
transformers. When input-output measurements are not feasible, the losses of a
transformer may be measured and calculated and its efficiency determined by using eq.
2-6
Although transformers are highly efficient, some losses are present in all transformers.
There are two classes of losses, namely
1. I2R losses in the transformer winding (load losses or copper losses)
2. Hysteresis and eddy current losses in the core ( iron losses or no-load losses)
Load or I2R losses are present because power is used whenever a current is made to
flow through a resistance. Load currents flowing through transformer windings produce a
power or I2R loss that varies with the load being supplied by the transformer. Load
losses can be calculated for any given load if the resistances of both windings are known
or can be measured. If RH and RX are the high- and low-voltage winding resistances,
then the load loss is
Load loss = IH2RH + IX2RX
2-7
No-load or iron or core losses are due to the effects of hysteresis and eddy currents in
the iron core of the transformer. These effects are similar to those occurring in
generators and motors that we will study in the next chapter. Core losses in a
transformer can be determined by energizing one winding of the transformer with the
other winding open. The power input to the open-circuited transformer as measured with
a wattmeter supplemented with a frequency meter, voltmeter, and ammeter is then the
no-load or core loss. The core loss of a transformer is essentially constant for all loads
when rated frequency and voltage are applied to the transformer.
2.9
The autotransformer
The autotransformer differs from the standard transformer in that it has only one
winding. Figures 2-8 and 2-9 show how the windings are connected.
Figure 2-8
Autotransformer
Figure 2-9
Autotransformer connected to a load
One portion of the winding serves for both the primary and the secondary. If the
transformer is being used to lower the voltage, the turns between H1 and H2 constitute
the primary winding and those between L1 and L2 constitute the secondary winding. The
ratio of the voltage, as in a two-winding transformer, is equal to the ratio of the primary
turns to the secondary turns.
If the exciting current is neglected, the primary and secondary ampere-turns in an
autotransformer are equal. Therefore, the turns ratio equation (VP/VS = NP/NS = IS/IP)
also applies to autotransformers.
At any instant, the current in the primary and secondary of a transformer flow in
opposite directions. In an autotransformer, the current in the portion of the winding that
is common to both the primary and secondary is equal to the difference between the
source current and the load current.
Autotransformers require less copper; thus; they are less expensive to manufacture than
the conventional transformer. They operate at higher efficiency and are smaller in
physical size than a two-winding transformer of the same rating. Most autotransformers
have lower noise levels for the same load and frequency than do two-winding
transformers.
One disadvantage of autotransformers is that they can present hazards if they are used
to make large voltage changes. For example, if an autotransformer is used to lower the
source voltage from 480V to 120V for lighting and small appliance loads, the difference
between the two voltages is 480V-120V = 360V. Figures 2-10 is a diagram of this
connection.
Figure 2-10
Autotransformer used as a step-down transformer
If a break should occur in the winding that is common to both the primary and
secondary, approximately 480V will appear across the load. Under ground-fault
conditions, the potential to ground will be 480V not 120V, see figure 2-11
Figure 2-11
Autotransformer: Possible voltages to ground
Autotransformers are used to reduce the voltage to ac motors during the starting period
and to increase the voltage in special situations. They are also used to compensate for
voltage drops in transmission line and to make minor changes in voltage to meet certain
load requirement, see Figure 2-12.
Figure 2-12
Autotransformer: To increase voltage for special equipment