Comparison between Transformer & Auto-Transformer

Differences	Auto-Transformer	Conventional Transformer
Definition	A transformer, having only one winding a part of which acts as a primary and the other as a secondary.	It is a static machine that transfers electrical energy from one end to another without changing frequency.
Number of Windings	Auto-transformer has only one winding wound on a laminated core	It has two separate winding, i.e., primary and secondary winding.
Insulation	The primary and secondary winding are not electrically insulated.	The primary and secondary winding are electrically insulated from each other.
Induction	Self Induction	Mutual Induction
Size	Small	Large
Power Transfer	Partly by transformation and partly by direct electrical connection.	Through transformation
Voltage Regulation	Better	Good
Winding Material	Less requires	More requires
Circuit	The primary and secondary winding circuits are connected magnetically.	The primary and secondary winding circuits are connected both electrically and magnetically.
Connection	Depends upon the tapping	Connect directly to the load.
Starting current	Decreases	Decreases by 1/3 times.
Excitation current	Small	Large
Economical	More	Less
Cost	Less costly	More costly
Efficient	More	Less
Leakage flux and resistance	Low	High
Impedance	Less	High
Cost	Cheap	Very costly
Losses	Low	High
Output voltage	Variable	Constant.
Applications	Use as a starter in an induction motor, as a voltage regulator, in railways, in a laboratory.	Use in power system for step up and step down the voltage.

FAQs on Transformers vs. Autotransformers

Q1: What is the primary difference between a conventional transformer and an autotransformer?

• A conventional transformer has two separate windings (primary and secondary) that are electrically insulated, while an autotransformer has a single winding that acts as both primary and secondary.

Q2: How do the number of windings differ between a transformer and an autotransformer?

• An autotransformer has only one winding wound on a laminated core, whereas a conventional transformer has two separate windings: primary and secondary.

Q3: Why are autotransformers smaller in size compared to conventional transformers?

• Autotransformers use a single winding, which reduces the core and winding material, making them smaller and more compact.

Q4: Are the primary and secondary windings of an autotransformer electrically insulated?

• No, the primary and secondary windings in an autotransformer are not electrically insulated, unlike conventional transformers.

Q5: What type of induction is used in autotransformers and conventional transformers?

• Autotransformers use self-induction, while conventional transformers rely on mutual induction.

Q6: Why are autotransformers more efficient than conventional transformers?

• Autotransformers are more efficient because they transfer power both through transformation and direct electrical connection, resulting in lower losses.

Q7: Which transformer type has better voltage regulation?

• Autotransformers have better voltage regulation compared to conventional transformers.

Q8: What are the cost differences between autotransformers and conventional transformers?

• Autotransformers are less costly due to fewer materials used, whereas conventional transformers are more expensive due to their separate windings and larger size.

Q9: Why do autotransformers have lower impedance than conventional transformers?

• Autotransformers have fewer winding turns, leading to reduced leakage flux and resistance, which results in lower impedance.

Q10: In which applications are autotransformers commonly used?

• Autotransformers are used as starters for induction motors, voltage regulators, in railways, and laboratories.

Q11: What are the typical applications of conventional transformers?

• Conventional transformers are used in power systems to step up or step down voltage levels.

Q12: How does the excitation current of autotransformers compare to conventional transformers?

• Autotransformers require a smaller excitation current compared to conventional transformers.

Q13: Can the output voltage of an autotransformer be varied?

• Yes, the output voltage of an autotransformer can be varied based on the tapping points, unlike conventional transformers that usually provide a constant output voltage.

Q14: How does the starting current differ between autotransformers and conventional transformers?

• The starting current is reduced more significantly in autotransformers compared to conventional transformers.

Q15: What are the economic advantages of using autotransformers over conventional transformers?

• Autotransformers are more economical due to their smaller size, lower material requirements, and higher efficiency, making them cheaper to produce and operate.