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

What is an Auto Transformer?

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

    An autotransformer is a type of Electrical transformer which consists of a Single winding(Primary and Secondary) called an autotransformer. In a conventional transformer, There are two types of winding in a single phase transformer. 

    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 why we need autotransformers for our applications. Firstly it is used for voltage transformation purposes (Step-Up or Step-Down). Secondly, autotransformer efficiency is higher compared to conventional transformers.

    Another advantage of Autotransformer is its compact design which leads to a smaller size. 

    The relation of the first winding to the second winding is called the 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' 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.

    Conclusion

    An autotransformer is a type of electrical transformer with a single winding. it acts as both the primary and secondary winding. This design allows for efficient operation.

    The autotransformer is known for its higher efficiency compared to a two-winding transformer due to reduced ohmic loss and core loss.
    which results from the reduction in transformer material.

    Frequently Asked Questions – FAQs

    What is an Auto Transformer?

    An autotransformer is a transformer with only one winding that acts as both the primary and secondary winding. 

    Are auto transformers more efficient than traditional transformers?

    Auto transformers can be more efficient than traditional transformers because they have fewer losses associated with the smaller amount of winding material and size.

    How do you determine the voltage transformation ratio of an auto transformer? 

    The voltage transformation ratio of an auto transformer is determined by the ratio of the number of turns between the input and output taps. For example, if the input is connected to a tap with 50 turns and the output is connected to a tap with 40 turns, the voltage transformation ratio would be 40/50 or 0.8.

    How does an auto transformer work? 

    An auto transformer operates on the principle of electromagnetic induction.

    it has one winding with multiple taps. By connecting the input and output at different tap points along the winding, different voltage levels can be achieved.

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