Hello readers, welcome to the new post. In this post, we will have a look at What is AutoTransformer Starter? An autotransformer is a certain type of transformer that comes with one isolated winding for step-up or step-down voltage. Auto here is not any type of automatic process, but it is the action of a single winding that works alone. In this post, we will learn the working principle, advantages, and other parameters of auto transformer starters. Let’s get started with What is AutoTransformer Starter?
Introduction: Auto Transformer Starter
An automotive transformer starter is an electrical transformer that comes with a single winding. The AUTO designation refers to a single winding that operates independently. In this transformer part of the same winding work as both primary and secondary windings.
In conventional transformers, separate primary and secondary windings are connected through the electrically conductive path.
The autotransformer winding comes with three electrical connections with winding. As part of winding, double-duty autotransformer winding comes with the benefit of small size, less weight, and less cost as compared to the dual-winding transformer.
Its disadvantage is that electrical isolation between primary and secondary circuits exits. But it comes with less leakage reactance, fewer losses, low excitation current, and a high VA rating of size and mass.
The common application of an autotransformer is a style of traveler voltage converter that helps 230-volt devices be used on 120-volt supply circuits.
The autotransformer comes with many taps for adjusting the voltage at the end of the long distribution for accurate excess voltage loss that is used on the voltage regulator.
Working Principle of Auto Transformer Starter
The autotransformer comes with a single winding with two end terminals and one or more terminals at middle tap points. This transformer has primary and secondary windings that come with common points in turns.
The part of winding shared with both primary and secondary is a common component. The part of winding not shared by both primary and secondary is a series part.
The primary voltage is given across the two terminals. The secondary voltage is obtained from two terminals; one terminal is common with primary voltage terminals.
As voltage per turn is the same in both windings, every produces voltage in proportion to the number of turns. In this transformer, part of the output current moves directly from input to output, and some parts are shifted inductively, helping a small, less-weight, and low-cost core to be used for single winding.
However, the voltage and current ratio of autotransformers can be defined in the same formula as two winding transformers.
V1/V2= N1/N2
Different Types of Induction Motor and Features
Auto Transformer Symbol
Autotransformer comes with singular winding, in which primary and secondary winding are connected both magnetically and electrically. Compared to conventional transformers comes with two separate windings is insulated from each other.
Autotransformer Diagram
- The autotransformer diagram can be seen here
Advantages and Disadvantages of Auto Transformer Starter
Advantages
- For the transformation ratio of two, the size of the autotransformer is about fifty percent of a 2-winding transformer. According to transformation ratio 2, the size will be 95 percent.
- The autotransformer comes with high efficiency as compared to two-winding transformers. Since less ohmic loss and core loss are due to the reduction of transformer material.
- The autotransformer is less costly and small in size.
- It has good voltage regulation since voltage loss in resistance and reactance of a single winding is less.
Disadvantages of Auto Transformer
- Due to the electrical conductivity of primary and secondary windings, a lower voltage circuit is a chance to be affected due to high voltage. To avoid lower voltage circuits, it becomes necessary to make low voltage circuits for handling high voltage.
- The leakage flux between primary and secondary windings is small and has less impedance. It causes short-circuit currents during fault conditions.
- Connections on the primary and secondary sides needed to be the same except for using interconnection-starring connections. It causes complications due to varying primary and secondary phase angles, especially in delta connections.
- Due to common neutrality in star or star-connected autotransformers, there is no chance of Earth neutral on a single side. Both sides must be neutral, either isolated or earth.
- It is not easy to maintain the electromagnetic balance of winding when voltage adjustment tapping is connected. It must be known that the provision of tapping on the autotransformer increases transformer frame size.
Applications of Auto Transformer Starter
- It adjusts in resistance type heater.
- It is part of cutting styrofoam.
- It is used for speed control of the fan.
- It adjusts AC power supply.
- Helps the line voltage compensation
- It regulates DC power supply of high-current.
Soft Starter vs. Auto Transformer
Soft Starter
- It can easily start by pushing the button.
- It stares at the motor without any jerks, smoothly and gradually getting synchronous speed. So helps to increase the working life of the motor.
- The valve on the delivery side needed to work during switching the motor ON or OFF.
- There is no electrical moving contact and no oil, so not starting it causes.
- There are no limits to the number of starts and not of stops per hour.
- Sot starting and soft stopping prevent back thrust on NRV or pipeline that is protected from back thrust during switching off.
- Inrush (current is restricted to 200% of rated full load current)
Auto Transformer Starter
- Its starting method is the same as a soft starter.
- The motor faces heavy jerks during staring and due to a sudden inrush of current. It causes high damage to motor winding that can burn.
- It needed a valve for both operations ON and OFF.
- It does not have soft starting and soft stopping, so accurate operation of valves is important.
- It has movable parts, so sparking is unavoidable.
- Since the motor winding gets heated, there is a limited number of starts and stops per hour.
- Inrush current is a maximum of 400% of the FLC.
Theory of Autotransformer Starter
Autotransformer work is based on electromagnetic induction and voltage transformation through single windings.
- The single winding is wounded on a magnetic core created with silicon steel or ferromagnetic material.
- AC voltage is given at input points.
- It causes a fluctuating magnetic field in the core according to Faraday’s law of induction.
- The fluctuating magnetic field induces voltage over the length of the winding.
- Voltage regulation is done through tapping winding at different points.
- Taps close the input, producing low voltage, and taps produce high voltage.
- There is no isolation between the input and output circuit since the single-winding design
Limitations of Auto Transformer Starter
- Its limitations are low power factor, high cost, and limitation of space due to larger size.
- There is no isolation between primary and secondary windings.
- It is non-economical for a low voltage ratio between output and input; for this case, its size and cost increase.
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FAQs
What is the autotransformer starter?
An autotransformer starter needs difficult cabling since it comes with 3 terminals used for connection with the motor. The main objective of the autotransformer motor starter is to reduce the starting current of the motor to the voltage ratio of the transformer’s square.
What is an autotransformer and how does it work?
An autotransformer comes with a single winding with a tap point between the primary and secondary. The tap point can be set for getting the required output voltage.
What is the difference between a VFD and an autotransformer starter?
The basic difference is that a VFD can change the speed of the motor, and a soft starter only controls the starting and stopping of that motor.
What is a starting transformer?
The starting transformer is used as a current limiter for starting three-phase induction motors, reducing power supply dips, such as with weak networks and larger motors.
Why use an autotransformer starter?
The autotransformer started being used for reducing applied voltage with a motor from starting. If a larger motor is used for longer time in the circuit, then voltage loss occurs, so it can cause damage to electrical instruments that operate on the same supply line.
Where is the autotransformer starter used?
It is used for limiting the starting current of the induction motor. This technique is used for the high rating of the squirrel cage induction motor. This method is best for motors having ratings of more than 15 HP.
What are the types of autotransformers?
Which starter is mostly used?
Direct Online (DOL) Starter since it provides full voltage to the motor during startup. Best for small motors that do not need precise speed control or acceleration
What are the common uses for an autotransformer?
It is commonly used in power applications for interconnection systems operating at different voltage types, such as 132 kV to 66 kV for transmission.
It is also used in industries for machines built for 480 V supplies to operate on a 600 V supply.