Transformers: Principle and Types
Overview
A transformer is a static electrical device that transfers electrical energy between two or more circuits through electromagnetic induction without any moving parts. It works on the principle of mutual induction and is used to step up or step down AC voltage and current. Transformers are essential in power transmission and distribution systems. Understanding their construction, working principle, EMF equation, and types is critical for ITI Electrician Semester 2 NCVT examinations.
Typed Notes
Principle and Construction of Transformer
A transformer works on the principle of mutual induction. When alternating current flows through the primary winding, it creates a changing magnetic flux in the iron core. This changing flux links with the secondary winding and induces an EMF in it (Faraday's Law). A transformer has three main parts: (1) Primary winding — receives input AC supply; (2) Secondary winding — delivers output; (3) Laminated iron core — provides a low-reluctance path for magnetic flux. The core is made of silicon steel laminations to reduce eddy current losses. Laminations are insulated from each other using varnish.
EMF Equation and Turns Ratio
The EMF equation of a transformer is: E = 4.44 × f × Φm × N. Here, f is supply frequency (50 Hz in India), Φm is maximum flux in Weber, and N is number of turns. For primary: E1 = 4.44fΦmN1; for secondary: E2 = 4.44fΦmN2. Turns ratio a = N1/N2 = V1/V2 = I2/I1. Note that voltage is directly proportional to turns but current is inversely proportional to turns. An ideal transformer has 100% efficiency; in practice, efficiency ranges from 95% to 98% for large power transformers. The constant 4.44 = 4 × form factor (1.11) × π/√2.
Types of Transformers
Transformers are classified by various criteria: (1) By voltage ratio — Step-up (N2>N1, increases voltage) and Step-down (N2<N1, decreases voltage). (2) By core construction — Core type (windings surround the core, better cooling) and Shell type (core surrounds the windings, better mechanical protection). (3) By application — Power transformer (used in transmission, large capacity), Distribution transformer (used at consumer end, 11kV/415V), Instrument transformers: CT (Current Transformer, measures high current) and PT (Potential Transformer, measures high voltage). (4) Auto-transformer — single winding acts as both primary and secondary; used in starter circuits and voltage regulators.
Transformer Losses and Cooling Methods
Two main losses occur in a transformer: (1) Iron/Core losses — constant losses independent of load; include Hysteresis loss (due to repeated magnetisation of core, reduced by using silicon steel) and Eddy current loss (due to circulating currents in core, reduced by laminating the core). (2) Copper loss — variable loss, depends on load current; equal to I²R in windings. Maximum efficiency occurs when Copper loss = Iron loss. Cooling methods: ONAN (Oil Natural Air Natural) — most common for distribution transformers; ONAF (Oil Natural Air Forced); OFAF (Oil Forced Air Forced) for large power transformers. Transformer oil (mineral oil) acts as coolant and insulator.
Topics & Sub-topics
Har topic ke saath sub-topics aur example — chapter ka complete coverage.
01Transformer Principle
- Static device, mutual induction
- Primary and secondary windings
- No DC — only AC works
DC dene par winding jal jaati hai — kyunki back-EMF nahi banti.
02Transformation Ratio K = V2/V1
- Step-up: K > 1
- Step-down: K < 1
- I1/I2 = 1/K (power same)
220 V → 11 V bell transformer: K = 1/20, secondary current 20 guna (safe).
03Types of Transformers
- Step-up / step-down
- 1-phase / 3-phase, auto, isolation
- Instrument: CT and PT
Welding transformer high-current low-voltage deta hai — special type.
04Losses and Efficiency
- Copper loss (I²R), Iron loss (hysteresis + eddy)
- Efficiency 96-99%
- Cooling: oil immersed
Transformer 24×7 ON rehta hai — iron loss tab bhi hota hai jab load zero.
Key Concepts
Revision ke liye ek nazar — exam se pehle yahi padhna hai.
- 01Transformer works on the principle of MUTUAL INDUCTION — discovered by Michael Faraday; it operates ONLY on AC supply, NOT on DC.
- 02EMF Equation: E = 4.44 × f × Φm × N (where f = frequency, Φm = max flux in Weber, N = number of turns).
- 03Turns Ratio (a) = N1/N2 = V1/V2 = I2/I1; Step-up transformer: N2 > N1; Step-down transformer: N2 < N1.
- 04Core losses = Eddy current loss + Hysteresis loss; Copper loss = I²R loss in windings; these are the two main transformer losses.
- 05Transformer efficiency η = (Output Power / Input Power) × 100%; at maximum efficiency, Copper loss = Iron (Core) loss.
Questions & Solutions
Pehle khud try karo — phir solution kholo. Yahi tariqa topper banata hai.
State the working principle of a transformer and explain why it does not work on DC supply.
Write the EMF equation of a transformer and state the meaning of each term.
Differentiate between core-type and shell-type transformers.
What is an auto-transformer? State two advantages and two applications.
Explain the two main losses in a transformer and state how each is minimised.
A transformer is a static AC device based on mutual induction, transferring energy without moving parts. Key formula: E = 4.44fΦmN. Types include step-up, step-down, core-type, shell-type, power, distribution, CT, PT, and auto-transformer. Losses are iron loss (constant) and copper loss (variable). Maximum efficiency is achieved when copper loss equals iron loss. Efficiency of large transformers reaches 95–98%.
Chapter-end quiz
4 MCQsOption tap karo — turant pata chalega sahi ya galat, explanation ke saath.
Q1.A transformer has N1 = 500 turns and N2 = 100 turns. If primary voltage is 230V, what is the secondary voltage?
Q2.Which of the following losses in a transformer remains CONSTANT irrespective of load?
Q3.The constant 4.44 in the transformer EMF equation is equal to:
Q4.In which type of transformer is a single winding used as both primary and secondary?
Mark karo — dashboard par progress dikhega.