Electrical Machines-I

ECE-2107

Transforer-SL2

Fariya Tabassum
Assistant Professor, Dept. of Electrical & Computer Engineering
Rajshahi University of Engineering & Technology, Rajshahi-6204

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[Quranic Inscription]

“Guide us to the straight path—”.

[Sura Fatihah]

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Transformer Construction

Go through the article 32.2, 32.3, 32.4 of the book written by B. L. Theraza to have a look at the transformer construction.

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Transformer Phasor Diagram

Two cases are consider

• When transformer is on no load
• When it is loaded

When an actual transformer is put on load, there is iron loss in the core and copper loss in the windings (both primary and secondary) and these losses are not entirely negligible. Even when the transformer is on no-load, the primary input current is not wholly reactive.

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Transformer Phasor Diagram

No load condition

$I_0=\text{Energizing current/ primary input current}$
$I_w=\text{Iron loss current}$
$I_{\mu }=\text{magnetizing current}$
${\theta }_0=\text{No load power factor angle}$

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Transformer Phasor Diagram

No load condition

No-load input power $W_0=V_1{I}_0\cos {\theta }_0$
Iron loss current $I_w=I_0\cos {\theta }_0$
magnetizing current $I_{\mu }=I_0\sin {\theta }_0$

For preparing your answer you can go through the article 14.5 of the book written by “Rosenblatt”

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Transformer Phasor Diagram

No load condition

The followings should be noted:

• The no-load primary current $I_0$ is very small as compared to the full-load primary current. It is about 1 % of the full-load current.
• As the permeability of the core varies with the instantaneous value of the exciting current, the wave of the exciting or magnetizing current is not truly sinusoidal and should not be represented by a vector.

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Transformer Phasor Diagram

No load condition

• As $I_0$ is very small, the no-load primary Cu loss is negligibly small which means that no-load primary input is practically equal to the iron loss in the transformer.
• As it is principally the core-loss which is responsible for shift in the current vector, angle ${\theta }_0$ is known as hysteresis angle of advance.

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Problems

Practice example 14.4 of Rosenblatt and 32.9 of B. L. Theraza and also the related tutorial problem.

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Transformer Phasor Diagram

Loaded condition

• When load is applied to the secondary terminals, in accordance with the Lenz’s law, the current that flows through the windings must act in such a direction as to oppose the flux set up by the primary current.
• When the flux is momentarily reduced, the induced emf in the primary winding is also reduced and as a result more current flows in the primary winding.
• The increased primary current will cause the flux to increase to its original value. When more load current flows in the secondary, the process is repeated and the primary current will again increase.

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Transformer Phasor Diagram

Loaded condition

• The load current $I_2$ is shown lagging the secondary induced voltage $E_2$. $I_1^{\prime }$ is the current that flows in the primary winding to balance the demagnetizing effect of $I_2$.
• Since flux $\phi$ remains constant, $I_0$ must be the same current that energizes the transformer at no-load.
• $I_1$, the actual current that flows in the primary, is therefore the phasor sum of currents $I_1^{\prime }$ and $I_0$

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Transformer Phasor Diagram

Loaded condition

When the load is purely resistive

For preparing your answer you can go through the article 32.10 of the book written by “B. L. Theraza”

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Problems

Practice example 32.12, 32.13 and 32.14 of B. L. Theraza and also the related tutorial problem.