Transformers, efficiency Study Notes

By Mona Kumari|Updated : June 11th, 2021

1. INTRODUCTION

1.1.   Basic Concepts

1.1.1 Lenz law: According to Lenz law, the direction of induced EMF is such that if it is allowed to cause a current (by short-circuiting required) then the current so produced would have an affects that opposes the cause which produces it.

1. INTRODUCTION

1.1.   Basic Concepts

1.1.1 Lenz law: According to Lenz law, the direction of induced EMF is such that if it is allowed to cause a current (by short-circuiting required) then the current so produced would have an affects that opposes the cause which produces it.

Thus,   E=±dφ/dt

Here, E is the induced EMF and φ is total flux linkage.

Also, E=±dφ/dt = E=±N(d∅)/dt

Where N is the Number of turn and Φ is the flux.

1.1.2. :

Figure: Current flow in different windings across Iron core

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  • If the current enters or leave the dots simultaneously then the fluxes are additive.
  • Only the first dot needs to be assigned while the remaining dots follow automatically, depending upon the sense of the winding.
  • if the current enters through the dot in the primary winding then it should leave through the dot from the secondary winding to satisfy Lenz law.
  • The dots have same instantaneous polarity in case of transformers.

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Note: - In case of transformer, the flux Ф = Фm sin(ωt) has a sinusoidal waveform.

1.2.   Features of Single-phase Transformers

Transformer is a static device which performs:

  • Transfers electrical energy from one electrical circuit to another without changing of frequency.
  • It is done by the principle of electromagnetic induction.
  • It has electric circuit that are linked by a common magnetic circuit.
  • Transformer can be treated as a phase shifting device, since it offers a displacement of approx. 180 degrees between the two circuits.
  • The energy transfer usually takes place with change in voltage, although this is not always necessary.

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  • When the transformer raises the voltage, i.e. when output voltage of a transformer is higher than its input voltage, it is called step-up transformer and when output voltage is lowered, it is called step-down transformer.
  • Transformer doesn’t change the frequency of the system, Hence it can be treated as constant frequency device, it transfers almost same amount of power from one circuit to another so it can be treated as constant power device and the amount of flux in its core remains constant so it is also known as constant flux device.

1.3.   Construction and Working Principle:

The transformer works based on “faraday’s law of electromagnetic induction”.

According to faraday’s law, whenever there is a relative space or time variation between magnetic field and set of conductors an EMF will be in the conductors”.

1.3.1 Basic construction:

An elementary transformer consists of a soft iron or silicon steel core and two windings, placed on it. The windings are insulated from both the core by each other. The core is built up of soft iron or silicon steel laminations to provide a path of low reluctance to the magnetic flux. The winding connected to the supply main is called primary and the winding connected to the load circuit is called secondary winding.

Although in actual construction the two windings are usually wound over the other but for the sake of clarity, the figures for analysing transformer theory show the windings on opposite sides of the core as shown in the Figure below:

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Figure: Transformer with windings

When the primary is connected to an alternating source, an alternating current Iϕ, starts to flow through N1 winding. This current produce flux Φ which induces an EMF in the primary winding E1 in opposition to applied voltage. This flux Ф also links with the secondary winding and induces E2 EMF in this circuit

TRANSFORMER EFFICIENCY

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