What is Auto transformer Starter: Working & Its Applications, diagram,advantages

Hello readers welcome to the new post. In this post, we will have a look at What is Autotransformer Starter. 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 autotransformer starter is an electrical transformer that comes with a single winding. The AUTO means a single coin working alone. In this transformer part of the same winding work as both primary and secondary windings.

In conventional transformers, separate primary and secondary winding are connected through the electrically conductive path.

The autotransformer winding comes with three electrical connections with winding. As part of winding performs 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 comes with less leakage reactance, fewer losses low excitation current, and a high VA rating of size and mass.

The common application of autotransformer is a style of traveler voltage converter that helps 230-volt devices to 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 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 proportional to the number of turns. In this transformer part of the output current moves directly from input to output and some parts is 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 of 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 is 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 auto transformer comes with high efficiency as compared to two winding transformers. Since less ohmic loss and core loss due to 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 single winding is less.

Disadvantages of Auto Transformer

• Due to the electrical conductivity of primary and secondary winding lower voltage circuit is a chance to be affected due to high voltage. For avoiding 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 casues short circuit currents during fault conditions.
• Connections on the primary and secondary sides needed to be the same except 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 auto transformer increases transformer frame size.

Applications of Auto Transformer Starter

• It adjusts in resistance type heater.
• It is part of cutting styrofoam
• It 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 gets 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 bak thrust on NRV or pipe line that is protected from back thrust during switching off
• In rush (current is restricted to 200% of rated full load current

Auto Transformer Starter

• Its starting method same as a soft starter
• Motor faces heavy jerks during staring and due to sudden inrush of current. it casues 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 motor winding get heated so 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

Auto transformer 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 produce 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 increases

FAQs

What is the auto transformer 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 motor to the voltage ratio of  transformer’s square

What is auto transformer and its working?

Autotransformer comes with single winding with tap point between primary and secondary. The tap point can be set for getting the requried output voltage.

What is the difference between VFD and auto transformer starter?

The basic difference is that VFD can change the speed of the motor and a soft starter only controls   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 used for reducing applied voltage with a motor from starting. If a larger motor is used for linger time in the circuit, then voltae 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  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.