Concept Notes (Deep Explanation + Examples)
🔹 Introduction
An autotransformer is a type of transformer where a single winding acts as both the primary and secondary.
Unlike a two-winding transformer (which has two separate windings), an autotransformer has a common winding — part of the winding is shared between input and output.
Think of it like this:
A two-winding transformer is like two separate water tanks connected through a pipe.
An autotransformer is like a single tank with two outlets at different levels — one for input and another for output.
🔹 Basic Construction
An autotransformer consists of:
- Single continuous winding on a laminated magnetic core.
- Tapping points along the winding for various voltage levels.
- Core made of silicon steel laminations to reduce eddy current losses.
Parts:
- AB = Total winding
- AC = Common winding (shared)
- BC = Series winding
🔹 Working Principle
It works on the same electromagnetic induction principle as a regular transformer:
“When an alternating current flows through a winding, it creates a changing magnetic flux that induces an emf in the same or another winding.”
In an autotransformer:
- A portion of the same winding is common to both input and output.
- The primary (supply) and secondary (load) voltages are taken from different taps on the same winding.
If input voltage = 230 V and output tap = 115 V, the autotransformer steps down voltage.
🔹 Voltage Relationship
Let:
- V1 = Primary voltage (AB)
- V2 = Secondary voltage (AC)
- N1 = Primary turns
- N2 = Secondary turns
Then,
This is the same as a conventional transformer.
🔹 Current Relationship
But, remember — part of the current is common to both circuits, so copper is saved.
🔹 Efficiency
Because copper losses are reduced, efficiency is higher:
Autotransformers typically have efficiency around 95–99%.
🔹 Applications (Real-World Examples)
✅ Used as starting transformers for induction motors in industries (to reduce starting current).
✅ Voltage regulators in laboratories or control panels.
✅ Distribution where small voltage changes (e.g., 230 V ↔ 250 V) are needed.
✅ Railway traction systems, testing benches, and power plants.
🔹 Advantages
✔ Smaller size and lighter weight than a two-winding transformer
✔ Higher efficiency (less copper loss)
✔ Lower cost
✔ Better voltage regulation
🔹 Disadvantages
❌ No electrical isolation between primary and secondary
❌ Faults may directly transfer from primary to secondary
❌ Not suitable for large voltage differences (dangerous for >1000 V systems)
🔹 Analogy
Think of a two-winding transformer as two friends talking through a phone (electrically isolated).
An autotransformer is like talking face-to-face — direct contact, faster, but risky!
🔹 Important ECET Points
⭐ Often asked question: “Why is copper saving more in autotransformer?”
👉 Because the portion of the winding common to both sides carries less current, reducing total copper usage.
⭐ ECET also asks: “Why is there no isolation in autotransformer?”
👉 Because both circuits share a common winding.
⚙️ Formulas (Plain LaTeX Only)
🔟 10 MCQs (GATE + ECET Mixed)
1️⃣ The main advantage of an autotransformer over a two-winding transformer is:
A) Better isolation
B) Smaller size and higher efficiency
C) Higher leakage reactance
D) Poor voltage regulation
2️⃣ In an autotransformer, electrical isolation between primary and secondary is:
A) Perfect
B) Partial
C) Absent
D) Very high
3️⃣ For an autotransformer with a voltage ratio of 400/200 V, the transformation ratio is:
A) 0.5
B) 1
C) 2
D) 4
4️⃣ The efficiency of an autotransformer compared to a two-winding transformer of the same rating is generally:
A) Lower
B) Same
C) Higher
D) Constant
5️⃣ Autotransformers are not used for:
A) Starting of induction motors
B) Step-down of small voltages
C) High-voltage isolation
D) Voltage regulation
6️⃣ The portion of the winding common to both primary and secondary is called:
A) Leakage winding
B) Common winding
C) Series winding
D) Tap winding
7️⃣ In an autotransformer, copper saving is proportional to:
A) Turn ratio
B) Fractional voltage
C) Square of voltage ratio
D) Core losses
8️⃣ Which of the following is not an application of autotransformer?
A) Variable voltage supply
B) Motor starter
C) Isolation transformer
D) Voltage regulator
9️⃣ When transformation ratio approaches unity, copper saving is:
A) Minimum
B) Maximum
C) Zero
D) Constant
10️⃣ An autotransformer is most economical when voltage ratio is:
A) Very high
B) Very low
C) Unity
D) 1:10
✅ Answer Key
Q.No Answer
1 B
2 C
3 A
4 C
5 C
6 B
7 B
8 C
9 B
10 B
🧠 MCQ Explanations (Step-by-Step)
1️⃣ B — Smaller size and higher efficiency
→ Because only part of the winding carries current, saving copper → smaller and more efficient.
2️⃣ C — Absent
→ Both windings share a common part; hence no electrical isolation exists.
3️⃣ A — 0.5
→ 
4️⃣ C — Higher
→ Less copper = lower losses = higher efficiency.
5️⃣ C — High-voltage isolation
→ Isolation is poor, so unsafe for high-voltage use.
6️⃣ B — Common winding
→ The shared section between input and output circuits.
7️⃣ B — Fractional voltage
→ Copper saving = 1 − (fraction of secondary voltage).
8️⃣ C — Isolation transformer
→ Autotransformer cannot isolate circuits.
9️⃣ B — Maximum
→ When ratio = unity → both windings overlap almost fully → max saving.
10️⃣ B — Very low
→ Economical when the voltage difference is small (like 230↔250 V).
🎯 Motivation / Why Practice Matters (ECET 2026 EEE)
“Auto Transformers” are a top-repeat topic in ECET and GATE because they combine core transformer theory with practical industrial usage.
In every EEE student’s life — from lab experiments to panel boards — autotransformers appear frequently.
⚡ Mastering this concept strengthens understanding of:
- Transformer principles
- Efficiency and copper losses
- Motor starting and regulation systems
Remember, concept clarity = marks + confidence.
Consistency in small daily topics builds mastery before the exam.
💪 Keep winding your efforts like a transformer coil — layer by layer, until you step up to success!
📲 CTA
Join our ECET 2026 EEE WhatsApp Group for daily quizzes & study notes:
👉 https://chat.whatsapp.com/GniYuv3CYVDKjPWEN086X9
