An autotransformer is a type of electrical transformer that has a single winding and operates by using a portion of that winding as both the primary and secondary winding. Autotransformers have the capability to supply a varying secondary voltage to an externally connected load. Customize 1kVA to 100kVA 3 phase auto transformers are mostly used to provide tapped lower starting voltages to motor loads, so they can prevent the motor from the high starting current. We manufactures 3 phase autotransformers in our China facilities which ensure superior quality, trouble free performance, and long working life.


Phase 3-Phase
Capacity 1 kVA to 100 kVA (customized)
Input Voltage 0V to 1140V (customized)
Input Current 2A to 151A
Output Voltage 0V to 1140V (customized)
Frequency 50Hz/60Hz
Winding Material Aluminum wire, copper wire
Cooling Method Self-cooling or wind-cooling
Working Efficiency ≥98%
Protection Grade IP21
Moisture-Proof Type Open type, sealed type

Transformer Materials Design

Autotransformer detail

Copper Coil

Temperature resistance can reach 150 ℃, good insulation and durability, further enhancing the life of autotransformer. high electrical conductivity, excellent heat transfer properties, resistance to corrosion and easy recyclability.

Autotransformer detail

Steel Sheet

Manufactured with excellent quality silicon steel, boasting good magnetic flux performance and high heat dissipation properties, it further improves the overall efficiency of the autotransformer.

Autotransformer detail

Tube Terminal

Stable performance and anti-loosening of tinned terminals with good airtightness and corrosion resistance. Rail-type wiring port, convenient wiring, to ensure that the energized overload capacity.

Autotransformer detail

Cooling Fan

All-round heat dissipation, not only reduce the working temperature of the autotransformer, but also to extend the service life. Cooling fan reducing energy losses and increasing efficiency by facilitating better heat transfer.

Autotransformer customization

Autotransformer Customization

Our chinese factory supplies autotransformers rated up to 100 kVA. If our existing products don't meet your application requirements, our team of engineers can create a transformer solution to meet your specific requirements. Provide us with the specifications you need in detai for your custom autotransformer requirements.

Here are some considerations before customize a autotransformer:

  1. Determine the basic model of the autotransformer, the number of phases (single-phase or three-phase), capacity and quantity.
  2. The input voltage and output voltage of the transformer, the output voltage accuracy can reach ±1%.
  3. When providing secondary voltage, it is best to provide capacity allocation for each output voltage.
  4. What load is the autotransformer used for?
  5. What material is needed, copper wire or aluminum wire.


Power Equipment

Provides efficient voltage conversion by utilizing the principle of electromagnetic induction. Improving power factor, and facilitate efficient power transmission over long distances.

Urban Infrastructure

In urban areas, where space is often limited. It ensures reliable power supply, facilitate load balancing, and enhance the overall efficiency and stability of urban electrical networks.

High Speed Rail

It ensures stable and reliable power supply to the train. The compact design of autotransformer makes them suitable for the demanding requirement of high-speed rail system.

CNC Machine Tool

Autotransformer is a vital part in CNC machine tool, ensuring stable and precise power delivery for optimal performance and accuracy, and protect sensitive electrical components.

Factory Circuit

Voltage regulation, impedance matching, and step-up or step-down applications. Autotransformer plays a crucial role in efficiently distributing power within the factory.

Medical Equipment

Used in power supply to provide the necessary voltage levels for medical devices, such as X-ray machines and MRI scanners to regulate and control the voltage supplied to critical components.

Industrial Equipment

Industrial equipment often requires different voltage levels for various components and machines, and auto transformer provides a reliable and efficient solution for voltage adjustment.

Export Equipment

By tapping into different points along the winding, the autotransformer can vary the voltage ratio and adapt to different electrical systems worldwide, facilitating global compatibility.
Autotransformer function

What can an Autotransformer do?

  1. Voltage regulation. An autotransformer can be used to regulate the voltage of an AC power supply by adjusting the number of turns on the winding.
  2. Impedance matching. Autotransformer can be used to match the impedance of a load to that of a power source, which helps to maximize power transfer efficiency.
  3. Starting and control of motors. This China step up & step down transformer can be used to control the starting and speed of electric motors by varying the voltage supplied to the motor.
  4. Testing and measurement. Auto transformer can be used in testing and measurement applications, such as measuring the voltage and current of a circuit.
  5. Power conversion. 3 phase autotransformer can be used to convert power from one frequency to another, such as in power supplies for electronic devices.

How to Use Autotransformer Correctly?

1. Understand the autotransformer: Familiarize yourself with the specifications of auto transformer, ratings, and its purpose in the electrical system. Ensure you have the necessary knowledge and experience to work with it.
2. Review safety precautions: Before starting work, review the safety guidelines for working with appropriate electrical equipment. Ensure you have protective equipment and follow all safety procedures to prevent accidents.
3. Verify the suitability: Ensure that the autotransformer is the appropriate choice for your application. Confirm that it can handle the voltage and current requirements of the system without exceeding its ratings.
4. Inspect the autotransformer: Perform a visual inspection of the autotransformer to check for any visible damage, loose connections, or signs of wear. If you notice any issues, do not proceed with the installation or operation and contact a qualified technician.
5. Connect the autotransformer: Make the necessary electrical connections as per the manufacturer's instructions. This typically involves connecting the input and output terminals of the autotransformer to the appropriate power sources and loads.
6. Set the taps: Autotransformers often come with multiple tap settings to adjust the output voltage. Select the appropriate tap based on your system requirements. This may adjust a physical switch or use a control panel if the autotransformer has an automatic tap changer.
8. Test the autotransformer: After making the connections, perform tests to verify the autotransformer's functionality. Check for proper voltage regulation, phase balance, and any other parameters relevant to your specific application. Please follow the manufacturer's instructions.
8. Monitor the autotransformer: During operation, keep an eye on the autotransformer for any signs of abnormal behavior, such as excessive heat, unusual noises, or voltage fluctuations. Continuous monitoring helps detect potential issues early and prevent major failures.
9. Maintenance and periodic inspections: Follow the manufacturer's recommended maintenance schedule to keep the autotransformer in optimal condition. Perform regular inspections, cleaning, and any necessary adjustments or repairs as per the guidelines.


Differences between autotransformer and conventinal transformer

  • An autotransformer has only one winding which acts both as a primary and the secondary whereas the conventional transformer has a two separate windings, i.e., the primary and the secondary winding.
  • The auto-transformer works on the principle of self-induction i.e. induce the electromagnetic force in the circuit due to variation in current. The conventional transformer works on the principle of mutual induction in which the emf induces in the coil by changing the current in the adjacent coil.
  • The auto-transformer is smaller in size, whereas the conventional transformer is larger in size.
  • The autotransformer is more economical as compared to a conventional transformer.
  • In an autotransformer, electrical power is transferred from primary to secondary partly by the process of transformation and partly by the direct current. The conventional transformer transfers the electrical power through the electric transformation due to which power loss occurs.
  • The voltage regulation of an auto-transformer is much better than the conventional transformer.
  • The voltage regulation is the change in the secondary terminal voltage from no load to full load.
  • The autotransformer has only one winding. Thus, less conductor is required for winding as compared to the conventional transformer.
  • The primary and secondary windings of the autotransformer are not electrically insulated whereas the windings of the conventional transformer are electrically insulated from each other.
  • The starting current of the auto-transformer is less than the actual current, whereas the starting current of the conventional transformer is one-third of the main current.
  • The auto-transformer is more efficient as compared to the conventional transformer.
  • The leakage flux and resistance of an auto-transformer are low because it has only one winding whereas it is high in the conventional transformer.
  • The autotransformer has less impedance as compared to conventional current. The smaller impedance results in the large short circuit current.
  • The cost of the autotransformer is very less whereas the conventional current is very costly.
  • The losses in the auto-transformer are less as compared to the conventional transformer.
  • The output voltage of the secondary transformer varies when the sliding contacts are used in the secondary winding whereas the output voltage of the conventional transformer always remains constant.
  • The autotransformer is used as a voltage regulator, in the laboratory, in the railway stations, as a stator in an induction motor, etc., whereas the conventional transformer is used to step-up and step-down the voltage in the power grid.

How efficient is an autotransformer?

An autotransformer is a type of electrical power transformer that consists of a single winding and has a common portion shared by both the input and output voltage. It is designed to transfer electrical energy between circuits with different voltage levels. The efficiency of an autotransformer can vary depending on several factors, including the design, construction, and operating conditions.

In general, autotransformers tend to have higher efficiency compared to traditional two-winding transformers. This is because autotransformers, when manufactured, utilize a common winding for both the input and output, resulting in reduced copper losses and improved overall efficiency. Additionally, autotransformers typically have a simpler design and fewer components, which can contribute to higher efficiency.

The efficiency of an autotransformer is primarily affected by two factors: resistive losses and core losses. Resistive losses occur due to the resistance of the winding wire, resulting in power dissipation in the form of heat. Core losses include hysteresis and eddy current losses, which occur in the transformer's magnetic core.

To maximize efficiency, autotransformers are typically designed with low-resistance windings and high-quality magnetic cores to minimize losses. However, it's important to note that the efficiency of an autotransformer can also depend on the specific load conditions, as well as factors such as temperature and frequency.

In practical applications, autotransformers can achieve efficiency levels ranging from around 90% to 98% or higher, depending on the design and operating conditions. It's important to consult the manufacturer's specifications and efficiency curves for a specific autotransformer to get accurate efficiency values for that particular model.

What are the advantages of autotransformer?

  • Wholesale price, shipped directly from reliable China factory.
  • Saving in size and weight.
  • Size is very smaller.
  • Voltage regulation is much better.
  • Excitation current requirement is low.
  • For manufacturing autotransformer copper is used with less requirement.
  • In conventional transformer step up and step down voltage are fixed while in auto transformer output varies as per the requirement.

Does an autotransformer have a neutral?

No, an autotransformer does not have a neutral wire. In an auto transformer, the primary and secondary windings are connected to each other, and there is a single shared winding that acts as both the primary and secondary winding. Due to this configuration, there is no separate neutral connection in an auto-transformer. The purpose of a neutral wire is to provide a return path for current in a single-phase or three-phase AC system. It is typically used in transformers with separate windings for primary and secondary, such as isolation transformers or distribution transformers. In those transformers, the neutral wire is connected to the center tap of the secondary winding, which allows for a balanced distribution of current in three-phase systems or provides a reference point for single-phase systems. However, in an auto-transformer, since there is a direct connection between the primary and secondary windings, the neutral wire is not required. The common winding in an auto-transformer carries a portion of the load current, and the voltage between the primary and secondary is determined by the winding ratio.

How power is transferred in autotransformer?

In an auto transformer, power is transferred through the process of electromagnetic induction. An autotransformer is a type of transformer that consists of a single winding with two or more taps, rather than separate primary and secondary windings. It is used for voltage transformation and to provide a variable voltage output.

The power transfer in an auto transformer occurs as follows:

  • Input Power: The input power is connected to the primary winding of the auto transformer.
  • Magnetic Field: When an alternating current (AC) passes through the primary winding, it creates a changing magnetic field around the winding.
  • Induced Voltage: The changing magnetic field induces a voltage in the same winding, which is the output voltage. The induced voltage is proportional to the turns ratio between the input and output taps.
  • Voltage Transformation: The auto transformer taps are connected to different points on the winding, allowing for voltage transformation. The voltage difference between the input and output taps determines the voltage transformation ratio.
  • Load Connection: The load is connected to the output tap of the auto transformer, and the induced voltage is transferred to the load.
  • Power Transfer: As the load draws current, power is transferred from the input to the output of the auto transformer. The power transfer is determined by the voltage and current levels at the input and output.

Why auto transformer has higher efficiency?

An autotransformer has higher efficiency compared to a conventional transformer due to several reasons:

  • Reduced Copper Losses: In an autotransformer, the primary and secondary windings are electrically connected, and they share a common section called the "common winding." This shared winding reduces the amount of copper used in the construction of the transformer, resulting in lower resistive losses. As a result, there are fewer copper losses in an autotransformer, leading to increased efficiency.
  • Lower Magnetizing Current: The autotransformer design typically results in a lower magnetizing current compared to a conventional transformer. The magnetizing current is the current required to establish the magnetic field in the transformer core. By reducing the magnetizing current, the autotransformer reduces the core losses, thereby improving efficiency.
  • Reduced Size and Weight: Autotransformers are physically smaller and lighter than conventional transformers for a given power rating. The reduced size and weight result from the elimination of one complete winding and the reduced amount of copper and iron used. Smaller transformers tend to have better efficiency due to reduced losses in the windings and the core.
  • Voltage Regulation: Autotransformers can provide voltage regulation by tapping into different points along the winding. By adjusting the tapping point, the voltage can be controlled more precisely, which helps optimize efficiency. Conventional transformers may require separate regulation mechanisms or multiple windings to achieve the same level of voltage control, leading to additional losses and reduced efficiency.

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