In this article, we will discuss the application of the autotransformer. As an autotransformer is small in size, cost, and weight, and also can provide variable voltage. So both the step type and variac type of the autotransformer find use in the home, industry, and high voltage labs. So let’s discuss, “What is an Auto Transformer used for?”
The main application of the autotransformer is in voltage stabilizer, lab testing, high voltage lab, testing equipment, and in industry.
A few applications of the autotransformer are as follows:
- Voltage stabilizer
- Testing
- Starter for motors
- Speed control of fans
- Control of voltage in the power system
- Generate variable voltage in high voltage lab
- Harmonics filter, generating neutral and grounding
- Convert the fixed inductor into the variable inductor
The contents of this post are as follows:
The main application of the autotransformer
Now we will discuss the above applications in detail.
1 – Application of autotransformer in voltage stabilizer
The autotransformer is used in the voltage stabilizer for home applications. The autotransformer finds use in all three types of voltage stabilizers. These three types of voltage stabilizers are
- Manual type
- Automatic type
- Automatic type with over-voltage protection
We use a step-type autotransformer in the voltage stabilizer.
In the application of the automatic type voltage stabilizer, the autotransformer has one more isolated winding as secondary. This isolated low-voltage winding is required to generate a power supply for the electronics. It is also required for voltage measurements.
2 – Application of autotransformer for testing purposes
Autotransformer is used for testing purposes like in:
- practical lab
- development
- Industry
- high-voltage lab
3 – Starting of motors
We use the autotransformer to limit the starting current in the motor. But generally, we use a star-delta type motor starter to start the motor.
4 – Application of autotransformer in speed control fan
Another application of the autotransformer is in the speed of the fan can be controlled using a step type of autotransformer. Any series resistor will not be necessary for this method of speed control of the fan.
But this is an old method, and nowadays we use Triac to control the speed of the fan.
5 – Application of autotransformer in voltage regulation in power system
Autotransformers also can be used to set the desired voltage in the power supply to the consumer depending on the load to compensate for the voltage drop.
6 – Generate variable voltage in high voltage lab
Whenever we test some equipment for high voltage, then we do not apply the high voltage suddenly. We raise the voltage across the equipment slowly for safety reasons. In such cases, a variac autotransformer is necessary to increase the voltage at the low voltage input side. The step-up of the voltage takes place after this only.
This voltage rise and lower is done using two separate switches. One switch is for raising the voltage and another switch is for lowering the voltage.
The special application of the autotransformer
I will explain here two special applications of the autotransformer (Auto-transformer)
Application of autotransformer for Harmonics filter, generating neutral and grounding
The zigzag transformer is also a kind of autotransformer, The application of an autotransformer as a zigzag design can be used:
- to filter 3rd harmonics
- to generate a neutral point (in the three-phase supply without neutral wire) and then grounding this neutral (earthing of the neutral of the transformer)
Application of autotransformer to make the variable inductor
We can convert a fixed inductor into a variable inductor. Figure 1 shows the circuit diagram for the use of the Variac autotransformer to convert the fixed inductors into variable inductors.
Simply connect the inductor across the output of the variac autotransformer. Now it behaves like a variable inductor across terminals “A” and “B”.
You can also convert the fixed resistor and fixed capacitor into the variable resistor and variable inductor. But this method works for high-rating components only, like 2 amperes inductors.
Further, with this method, you can increase the value of the inductor and resistor, and decrease the value of the capacitor, and not vice versa. Further, this concept works for AC applications only.
If we just short the output with a wire instead of connecting the inductor, then also you get a variable inductor (for AC application). Refer circuit in Figure 2 for making such an arrangement.
However, the range of this variable inductor in this application of autotransformer may not be in our control.
You can watch a related video on this topic, output shorting of the transformer as an inductor as an experiment.
Extra knowledge as per experience
In one application of the autotransformer, we used a variable-type autotransformer in one of the testing products named Variable voltage surge generator. The company I worked for, got a patent for this product with me as one of the inventors.
Whenever we design some device for high voltage application, then testing at high voltage is also done. But we always apply voltage slowly to avoid any unwanted flashover. So autotransformer is always required for such kind of testing.
This testing equipment was used to generate a high-voltage surge to test the over-voltage protection of the thyristor. As different thyristor has different voltage level, we need a variable voltage surge generator. The variac type autotransformer was used to change the input voltage continuously.
Keep learning by reading about the advantages and disadvantages of the autotransformer.
Further reading, How to use open circuit CT transformer.
Then read a transformer-related post on the Difference between CT and PT transformers.
Keep learning Why power factor improvement
Further, read the differences between the MCB breaker and fuse.
I hope that you enjoyed reading the article on the Application of the autotransformer.
If so, then subscribe to my YouTube channel.
About the author – G K Agrawal B.Sc and B.Tech (from HBTU Kanpur), Retd. Sr DGM Design (BHEL), the inventor of patents, has lifelong industry experience in the electrical and electronics design field of R&D. He worked for BHEL. He shares his experience and knowledge on blogs and YouTube. Read the profile here.
