What is an Auto Transformer ? | Advantages and Disadvantages|Electronicsinfos

What is an Auto Transformer?

An autotransformer is a type of Electrical transformer which consists of a Single winding(Primary and Secondary) called an autotransformer.In conventional transformer, In Conventional Transformer there are two windings in single phase transformer and In Three phases there are Three primary and three secondary windings. Autotransformers have fewer losses as compared to conventional transformers. Autotransformers are used in various applications including impedance matching, voltage regulation etc.

Working Principle of Auto Transformer

The working principle of an autotransformer is 'principles of electromagnetic induction'.

Why do we need an Autotransformer?

There are Several reasons of why we need auto transformer for our applications.Firstly it is used in voltage transformation purpose(Step-Up or Step-Down).Secondly autotransformer efficeicy is higher compared to conventional transformer.Another advantage of Autotransformer is its compact design that lead to smaller size.The relation of first winding to second winding is called turn ratio. The first winding is called common winding and the second winding is called series winding of the transformer. The input voltage is equal to the sum of the series winding and common winding voltage. The output voltage is equal to the common winding voltage.

Transformation Ratio(Turn Ratio)

The transformation ratio of a transformer is the ratio of the number of turns in the primary winding to the number of turns in the secondary winding.

The transformation ratio of the autotransformer is 

V2/V1 =N2/N1 = K

The K is called the transformation ratio.

Equation of Auto Transformer 

To derive the equation for an autotransformer. The voltage transformation ratio, denoted as 'K' which is the ratio of the number of turns in the secondary winding 'Ns' to the number of turns in the primary winding 'Np'.

 

The equation for the voltage transformation ratio is  

K = Ns/Np

Now, let's derive the equation step by step

Step 1

In a transformer, the voltage is directly proportional to the number of turns in the winding. Therefore, we can write

Vs = K Vp 

Where

• Vs is the secondary voltage.
• Vp is the primary voltage.
• K is the voltage transformation ratio Ns/Np 

Step 2

The voltage transformation ratio K is defined as the ratio of secondary turns to primary turns

 

K= Ns/Np

Step 3 

Substitute the expression for K from the turns ratio equation into the voltage equation

Vs = (Ns/Np). Vp

If we rearrange the equation slightly, we get 

  

Vs = 1 + (Ns/Np). Vp 

This equation shows the relationship between primary and secondary voltages in an autotransformer.

Advantages of auto transformer

1. if the transformation Ratio is minimum the conductor Material is saved in terms of quantity.
2. The I²R losses are Reduced if the transformation ratio is low.
3. The efficiency of a transformer is high.
4. The cost is reduced.
5. The voltage Regulation is better in autotransformers.

Disadvantages of Autotransformer

1. The complete winding is designed for a high voltage rating.
2. Due to low reactance, the short circuit current is high.
3. In the case of three phases, we need the same primary and secondary connection i.e Star-Star, Delta-Delta etc
4. Primary and secondary isolation is not possible in autotransformers.
5. Both windings are not electrically isolated so there is a chance of electric shock.

Applications Of Auto Transformer

1. Autotransformer is used as a booster in the feeder.
2. To start the induction and synchronous motor: an autotransformer is used.
3. Autotransformer used as arc Furnace Transformer.
4. Autotransformer is used as a balance set in different ratings of the transformer.
5. To provide a neutral in three-wire lighting system.
6. To provide step-down voltage in single-phase railway electrification.
7. To provide different voltages for testing purposes in electrical equipment.
8. To provide neutral in three wire direct current generator.
9. Autotransformers are used in long transmission lines.
10. Autotransformer is used as a variable voltage regulator.

Example:  How Autotransformers Save Copper ?

Auto Transformer: Total winding turns: 2000

Conventional Transformer: Total winding turns: 2000 (1000 for primary, 1000 for secondary)

Copper Winding Comparison

Auto Transformer Uses a single winding with 2000 turns for both primary and secondary functions. Conventional Transformer Requires separate windings, each with 1000 turns for primary and secondary, totalling 2000 turns.

Result

Both the autotransformer and the conventional transformer use 2000 turns of copper winding in total. However, the autotransformer achieves this with a single winding, while the conventional transformer requires two separate windings.

Related Posts :

What is a Transformer?

How to Reduce The Transformer Losses?

Single Phase Transformer Vs Three Phase Transformer

Transformer Oil Testing 

EMF Equation Of Electrical Transformer

What is a Step-Down Transformer?

What is a Step-Up Transformer?

3 Types of Electrical Transformers w.r.t Core

What is the Efficiency of Electrical Transformers?

Electrical Ideal Transformer?

Regulations of Electrical Transformer?