Transformers enable efficient power transmission and safe utilization by changing AC voltage levels without changing frequency. This guide explains everything in simple language with formulas, quick comparisons, and exam-focused tips.
- Definition & Key Features
- Why Transformers Matter
- Working Principle (Mutual Induction & Faraday’s Law)
- Construction & Core Types
- EMF Equation (Derivation & Meaning)
- Turns Ratio, Polarity & Voltage Transformation
- Ideal vs Practical Transformer
- Losses, Efficiency & Regulation
- Open-Circuit & Short-Circuit Tests
- Types of Transformers
- Applications
- Selection Tips & Safety
- FAQs
- Key Takeaways
1) Definition & Key Features
A transformer is a static electrical device that transfers energy between circuits using electromagnetic induction. It changes AC voltage levels (step-up or step-down) while maintaining the same frequency and providing galvanic isolation (except in autotransformers).
Core Features
- No moving parts: High reliability and low maintenance.
- AC only: With DC, flux is steady ⇒ no induced EMF.
- Magnetic coupling: Energy transfer via a shared magnetic core linking the windings.
2) Why Transformers Matter
High-voltage transmission reduces I2R losses. Transformers step up generator voltages for long lines and step down near loads for safe use. They also provide isolation, noise reduction, and precise measurement (CTs/PTs).
3) Working Principle (Mutual Induction & Faraday’s Law)
Faraday’s law (induced EMF in a coil):
Applying AC to the primary winding creates a time-varying core flux (Φ). This changing flux links the secondary winding, inducing EMF as above.
Concept: AC in primary → alternating core flux → induced secondary voltage.
4) Construction & Core Types
Main parts: primary winding, magnetic core, and secondary winding, enclosed in a tank (oil-filled) or cabinet (dry-type).
Core & Lamination
- Silicon steel laminations: High permeability; laminations reduce eddy currents.
- CRGO steel: Grain-oriented; minimizes hysteresis for power units.
Core Arrangements
- Core-type: Windings around core limbs; good cooling and insulation access.
- Shell-type: Core surrounds windings; tighter leakage control.
Cooling & Insulation
- Oil-immersed: Mineral oil insulation & cooling; radiators/fans for heat removal.
- Dry-type: Air-natural/air-forced; epoxy cast resin for indoor/critical spaces.
5) EMF Equation (Derivation & Meaning)
For sinusoidal flux, RMS induced EMF per winding is:
- f = frequency (Hz)
- N = turns
- Φmax = maximum flux (Wb)
The constant 4.44 arises from integrating sinusoidal flux over a cycle and converting to RMS.
6) Turns Ratio, Polarity & Voltage Transformation
Ideal voltage and current relations:
- Step-up: Ns > Np ⇒ Vs > Vp ⇒ Is < Ip
- Step-down: Ns < Np ⇒ Vs < Vp ⇒ Is > Ip
Polarity
Dot convention marks terminals that are in-phase. Essential for paralleling and protection circuits.
7) Ideal vs Practical Transformer
| Aspect | Ideal | Practical |
|---|---|---|
| Winding resistance | Zero | Finite → copper loss (I2R) |
| Core losses | None | Hysteresis & eddy current losses |
| Leakage flux | Zero | Non-zero → leakage reactance |
| Efficiency | 100% | ~95–98% (good designs) |
8) Losses, Efficiency & Regulation
8.1 Core (Iron) Losses
- Hysteresis: Repeated magnetization cycles; reduced by low-loss steels (CRGO).
- Eddy currents: Circulating currents in core; minimized by thin laminations and higher resistivity.
8.2 Copper (Winding) Loss
8.3 Other Losses
- Stray load loss: Eddy in tank/structure due to leakage flux.
- Dielectric loss: Heating in insulation under AC stress.
8.4 Efficiency
Max efficiency occurs when iron loss ≈ copper loss.
8.5 Voltage Regulation
9) Open-Circuit & Short-Circuit Tests
Open-Circuit (OC) Test
- Secondary open; apply rated voltage to LV side.
- Measure V, I0, W → core loss & shunt branch (Rc, Xm).
Short-Circuit (SC) Test
- Secondary shorted; apply reduced voltage on HV side until rated current flows.
- Measure V, I, W → copper loss & series branch (Req, Xeq).
10) Types of Transformers
By Function
- Step-up: Raises voltage (power stations → transmission).
- Step-down: Lowers voltage (distribution to consumers).
- Isolation: 1:1 ratio for safety/noise reduction/ground-loop control.
- Autotransformer: Single winding with tap(s); compact, efficient, no galvanic isolation.
- Instrument: CT (current scaling) & PT/VT (voltage scaling) for metering/protection.
By Phases
- Single-phase: Small loads, appliances, electronics.
- Three-phase: Industrial power systems (single 3-φ unit or bank of single-φ units).
By Application/Design
- Power vs Distribution: Power transformers optimized at high load; distribution for low no-load loss.
- Dry-type vs Oil-filled: Dry for indoor/critical spaces; oil-filled for higher ratings/outdoor.
- Core vs Shell type: Leakage control and insulation requirements drive selection.
Quick Comparison
| Type | What it does | Common Uses |
|---|---|---|
| Step-Up | Increases voltage | Transmission |
| Step-Down | Decreases voltage | Distribution, appliances |
| Isolation | Galvanic isolation | Medical, labs, audio |
| Autotransformer | Tap-based regulation | Motor starting, AVR |
| Three-Phase | 3-φ AC conversion | Industrial systems |
11) Applications
Power System
- Generator step-up (GSU) at plants
- Transmission interconnections
- Substation step-down for feeders
- Distribution near consumer premises
Industrial & Commercial
- Motor control (autotransformers, soft starters)
- Welding, furnaces, UPS
- HVAC, building services
Electronics & Instrumentation
- SMPS isolation (line or HF ferrite)
- Audio isolation, impedance matching
- CTs/PTs for metering & relays
Healthcare & Safety
- Medical isolation for patient safety
- Noise suppression & ground-loop mitigation
12) Selection Tips & Safety
How to Choose
- kVA rating: Match load with headroom.
- Voltages/taps: Primary/secondary ratings, frequency, OLTC/Off-load taps.
- Impedance: Affects fault level & regulation.
- Environment: Indoor/outdoor, oil/dry, ambient, altitude.
- Efficiency class: Consider lifecycle energy cost.
- Standards: BIS/IEC/IEEE compliance.
Operation & Safety
- Ensure proper earthing before energizing.
- Verify polarity/vector group before paralleling.
- Monitor oil level/temperature (oil-filled) and ventilation (dry-type).
- Respect nameplate limits (kVA, temp rise, tap position).
- Follow LOTO procedures during maintenance.
13) FAQs
Q1) How does a transformer work step-by-step?
- Apply AC to primary → alternating current flows.
- Alternating core flux links both windings.
- Faraday’s law induces EMF in secondary.
- With load, secondary current flows; power transfers magnetically.
Q2) What is the voltage transformation equation?
Vp / Vs = Np / Ns (ideal); current ratio is inverse.
Q3) Why iron/steel cores?
High permeability lowers magnetizing current; laminations cut eddy currents; CRGO reduces hysteresis loss.
Q4) Isolation vs step-down?
Isolation focuses on galvanic isolation (often 1:1). Step-down reduces voltage and isolates. Autotransformers regulate voltage without isolation.
Q5) Which losses vary with load?
Copper loss varies with I2; core loss ~ constant with V and f.
Q6) EMF equation—what is 4.44?
E = 4.44 · f · N · Φmax; 4.44 comes from sinusoidal wave/RMS conversion.
Q7) What is voltage regulation?
%Reg = [(Vno-load − Vfull-load)/Vfull-load] × 100.
Q8) Can a transformer work on DC?
No. Pure DC gives steady flux → no induced EMF; primary overheats due to resistive current.
14) Key Takeaways (Quick Revision)
- Static device: Transfers AC power via magnetic coupling; no frequency change.
- EMF law: E = 4.44 · f · N · Φmax
- Turns ratio: Vp/Vs = Np/Ns, Ip/Is = Ns/Np (ideal).
- Losses: Iron + copper (+ stray & dielectric); η ≈ 95–98% in good designs.
- OC/SC tests: Estimate parameters, efficiency, regulation.
- Types: Step-up/down, isolation, autotransformer, CT/PT, single/three-phase, dry/oil.
- Applications: Grid, industry, electronics, healthcare, metering & protection.