Autotransformer
An autotransformer is a transformer that uses a single winding for both primary and secondary. Unlike isolation transformers, part of the winding is shared, meaning there is no electrical isolation between input and output. It is more compact, efficient, and cost-effective for many applications.
What Is an Autotransformer?
An autotransformer has one continuous winding with taps at different points.
- Single winding used for both input and output
- Part of winding is common to both sides
- No galvanic isolation
Working Principle
It works using electromagnetic induction, but with a shared winding:
- AC voltage applied to a portion of the winding
- Magnetic field is created in the core
- Voltage is induced across the entire winding
- Output is taken from a different tap
Voltage Relationship
Voltage depends on the tap position:
Vs / Vp = Ns / Np
By selecting different tap points, you can step up or step down voltage.
Key Feature: Shared Winding
- Part of power is transferred directly (electrically)
- Part is transferred magnetically
- This increases efficiency
Advantages
- Smaller and lighter than conventional transformers
- Higher efficiency
- Lower cost (less copper required)
- Better voltage regulation
Disadvantages
- No electrical isolation
- Higher risk of electric shock
- Faults can directly affect output
Applications
| Application | Purpose |
|---|---|
| Voltage Regulators (Variac) | Adjust output voltage smoothly |
| Motor Starting | Reduce starting voltage/current |
| Power Distribution | Small voltage adjustments |
| Laboratory Equipment | Variable AC supply |
Autotransformer vs Isolation Transformer
| Feature | Autotransformer | Isolation Transformer |
|---|---|---|
| Windings | Single | Separate |
| Isolation | No | Yes |
| Efficiency | Higher | Lower |
| Size | Smaller | Larger |
| Safety | Lower | Higher |
Real-Life Example
A Variac (variable autotransformer):
- Input: 230V AC
- Output: 0–250V adjustable
- Used in labs and testing
Key Points
- Uses a single shared winding.
- No electrical isolation.
- More efficient and compact.
- Can step up or step down voltage.
- Common in voltage regulation applications.