Auto Transformer

An Auto Transformer refers to a transformer that features a single winding wound around a laminated core. An autotransformer is like a two-winding transformer however contrast in the manner the primary winding and secondary winding are interrelated. A piece of the winding is common to both the primary side and secondary sides.

In this article, we will be going through Auto Transformer, First we will start our Article with What is an AutoTransformer, next, we will go through the construction and Working of an Autotransformer, Then we will go through the Autotransformer Theory and Copper Savings in an Auto Transformer, At last, we will conclude our Article with Advantages, Disadvantages, Applications and Some FAQs.

What is an AutoTransformer?

An autotransformer is an electrical transformer characterized by having a solitary winding. The “auto” prefix refers to the single coil acting alone (In Greek “self”) – not to any programmed instrument. An auto transformer is like a two-winding transformer yet changes in the manner the primary winding and secondary winding of the transformer are interrelated.

An autotransformer has a single consistent winding with a tap point between the primary winding and secondary windings. The autotransformer has a clear advantage because the tap point can be changed to get the voltage you want at the output. The fact that the secondary winding is not electrically isolated from the primary is the autotransformer’s primary drawback. On load condition, a piece of the load current is applied directly from the supply, and the remaining part is acquired by transformer action. An Autotransformer functions as a voltage regulator.

Auto Transformer

Construction and Working of AutoTransformer

In a conventional transformer, the primary and the secondary windings are electrically insulated from one another but connected magnetically as shown in the figure below. In an auto Transformer, the primary and secondary windings are linked both magnetically and electrically.

Ordinary Transformer

Auto transformers can be categorized into two types based on their construction. In the first type, a continuous winding is employed with taps strategically positioned to achieve the desired secondary voltage. Conversely, the second type features at least two distinct coils that are electrically interconnected to create a seamless winding. The schematic representation of the auto transformer construction is depicted in the figure below.

Auto Transformer

The primary winding AB from which a tapping at C is taken, that CB goes about as an optional winding. The supply voltage is applied across the AB, and the load is connected across the CB. The tapping could be constant or fluctuating. When AC voltage V1 is applied across AB, an alternating flux is set up in the core, thus, an emf E1 is induced in the winding AB. A part of this induced emf is taken in the secondary side circuit.

Let,

V1 is Primary applied voltage

V2 is Secondary voltage across the load

I1 is Primary current

I2 is load current

N1 is number of turns among A and B

N2 is number of turns among C and B

Neglecting no-load current, leakage reactance and losses,

V₁ = E₁ and V₂ = E₂

Therefore, the transformation ratio:

K = V₂ / V₁ = N₂ / N₁ = I₁ / I₂

As the secondary ampere-turns are opposite to primary ampere-turns, so the current I2 is in phase opposition to I1. The secondary voltage is less than the primary. Accordingly current I2 is more than the current I1. As a result, the current that flows through section BC is (I2–I1).

The ampere-turns because of section BC = current x turns

Ampere-turns because of section BC

= ( I₁ – I₂) N₂

= ( I₁ /K – I₁) x N₁ K

= I₁ N₁ (1 – K) – – – ( Equation 1)

Ampere-goes because of section AC

=I₁ (N₁ – N₂ )

= I₁N₁ ( 1 – N₂/N₁)

= I₁N₁(1 – K) – – – (Equation 2)

Auto-transformer-Equation (1) and (2) shows that the ampere-turns because of section BC and AC balance each other which is characteristics for the transformer action.

Case 1 : Autotransformer on No-Load

The connection diagram of a unloaded autotransformer (step-down and step-up) is shown in the figure. In this, the winding ‘AB’ is called primary winding having N1 turns and the winding ‘bc’ is called secondary winding having N2 turns.

AutoTransformer on No-Load

Here it can be noticed that the primary and secondary windings are connected magnetically as well as electrically. As a result, the primary winding’s power is transferred magnetically and conductively to the secondary winding.

Case 2: Autotransformer on Load

A load is connected to be loaded if it is connected to an autotransformer’s secondary. The figure shows the circuit diagram of a autotransformer (step-down and step-up)

Autotransformer on Load

In which the current I1 is the primary input current and the current I2 is the secondary output current or load current. The current in like common portion of the primary and secondary windings is the difference of the currents I1 and I2,whether the autotransformer is stepdown or step up.

For the step-down autotransformer the ongoing I2 > I1, thus the current flowing in the common portion is

Current in common portion = I₂ – I₁

For the step-up autotransformer the current I2 < I1, thus the current in the common portion is

Current in common portion = I₁ – I₂

Copper Savings in an Auto Transformer

Now, we’ll talk about the how much copper is saved in an auto transformer when compared to a standard two-winding transformer.

Any winding’s copper weight is known to be depends by its length and cross-sectional area. Again, the length of a conductor in a winding is inversely proportional to the number of turns it has, and the cross-sectional area varies depending on the rated current.

So weight of copper in winding is directly proportional to result of number of turns and rated current of the winding.

Hence, the weight of copper in section AC is directly proportional to,

(N₁ – N₂) I

also, similarly, weight of copper in the section BC proportional to,

N₂( I₂– I₁)

Consequently, total weight of copper in the winding of auto transformer proportional to,

(N₁ – N₂) I₁ = N₂ ( I₂ – I₁)

N1 I1 – N2 I1 + N2 I2 – N2 I1

N₁ I₁ + N₂ I₂ – 2 N₂ I₁

2N₁ I₁ – 2N₂ I₁

2(N₁ I₁ – N₂ I₁)

Similarly, it can be demonstrated that the weight of copper in a two-winding transformer is directly proportional to,

N₁ I₁ + N₂ I₂

2N₁ I₁

N₁ I₁ + N₂ I₂

2N₁ I₁ (Since, in a transformer N₁ I₁ = N₂ I₂)

Assume that, Wa and Wtw are weight of copper in auto transformer and two winding transformer separately,

Hence, W_a / W_tw = 2 ( N₁ I₁ – N₂ I₁ ) / 2( N₁ I₁ )

= N₁ I₁ – N₂ I₁ / N₁I₁ = ( 1 – N₂ I₁ / N₁ I₁ )

= ( 1 – N₂/ N₁ )

= 1-K

W_a = W_tw ( 1 – k)

W_a = W_tw – kW_tw

Saving of copper in auto transformer compared with two winding transformer,

W t_w – W_a = k_w t_w

Auto transformer employ’s just single winding per phases work as against two particularly separate windings in a conventional transformer.

Advantages of Autotransformer

The Following are Advantages of autoTransformer

• An autotransformer has smaller core and copper losses and thus higher efficiency when compared with a conventional 2-winding transformer.
• Autotransformer requires less conductor material when contrasted with the 2-winding transformer.
• Reduced voltage drops in resistance and leakage reactance result in improved voltage regulation.
• It has more smaller size and cheaper in cost than a 2-winding transformer.
• It requires more modest excitation current.
• Variable voltages can be produced by an autotransformer.

Disadvantages of Autotransformer

The Following are Disadvantages of autoTransformer

• There is an direct connection between the primary and secondary sides. The load on the secondary side would receive the full primary voltage in the event of an open circuit in the common portion bc of the windings. This might harm the hardware connected with the secondary side.
• An autotransformer has reduced inside impedance when compared with a 2-winding transformer which brings about a larger short circuit current.
• The autotransformer isn’t electrically isolated, in this manner can’t be used to electrically disconnect two circuits

Applications of Autotransformer

The Following are Applications of autoTransformer

• Power systems with varying voltage levels can be connected to one another through the use of autotransformers.
• The autotransformers with recordings are utilized for turning over enlistment engines and coordinated engines.
• When a continuous variable voltage over a wide range is required, autotransformers are used as variac (variable AC).
• It is used as a starter to surrender to 50 to 60% of full voltage to the stator of a squirrel confine enlistment engine during turning over.

Conclusion

In conclusion, autotransformers stand apart as efficient and cost-effective electrical devices with flexible applications. Their single-winding design, serving both as primary and secondary coils, empowers a more compact and economical construction contrasted with ordinary transformers. Autotransformers are especially advantageous when space is a premium, and cost contemplations are crucial.

Autotransformers are suitable for applications requiring precise voltage regulation due to their ability to adjust output voltage through tap variations. This flexibility renders them important in power conveyance, engine control, and voltage change situations. Be that as it may, it’s essential to recognize their constraint in lacking total electrical separation among essential and optional sides, which might be a worry in applications focusing on segregation for safety or hardware protection.

Autotransformers are adept at taking care of dynamic load requirements, on account of tap-changing systems in certain models. Cautious thought of working circumstances is fundamental to improve their productivity. Despite their limitations, the general advantages of proficiency, flexibility, and cost-viability go with autotransformers a favored decision in different enterprises, adding to smoothed out electrical systems and conservative answers for voltage change.

FAQs on AutoTrasnformers

What is an autotransformer?

An autotransformer is a type of transformer with a single winding that fills in as both the primary and secondary winding. It has a tap that considers voltage changes, enabling it to work as a step-up or step-down transformer.

How does an autotransformer vary from an ordinary transformer?

Not at all like traditional transformers with independent essential and optional windings, autotransformers utilize a solitary twisting for the two capabilities. This plan brings about a more minimized and savvy transformer.

Are there any safety consideration with autotransformers?

Autotransformers need total electrical separation between the primary and secondary sides. Consequently, in applications where separation is essential for safety or equipment protection, extra measures might be vital.

In which applications are autotransformers generally used?

Power distribution, motor control, and voltage regulation systems all use autotransformers. Their adaptability makes them reasonable for situations where cost proficiency and space investment funds are basic.

What main advantages do autotransformers offer?

Autotransformers are versatile, cost-effective, and take up little space. They offer voltage guideline abilities, making them reasonable for applications where exact control of voltage levels is fundamental.