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Ladder Logic PLC code for Auto transformer starter

This tutorial will explain the autotransformer starter for induction motors. Explanation and physical arrangement of auto transformer starter is explained with labelled diagram. A PLC code is developed for the control of auto transformer starter arrangement given. Step by step development and demonstration of ladder logic program for autotransformer starter is also presented.


It is a known fact in the world of electrical engineering that motors draws very high current in starting. The starting current is almost 6 to 10 times higher than full load current. This high current can damage the motor windings as well as the power source. Therefore it is recommended that this current should be reduced. Different types of motor starters are introduced which helps in smooth starting of motor. One of the common methods for implementing motor starter in industry is by using PLC. PLC code is written in ladder logic and applied through simplified hardware to accomplish smooth motor starting.

Most common motor starters are

  • Star delta motor starter
  • Auto Transformer motor starter
  • Power electronics based motor starter.

Star delta starter and auto transformer are conventionally based on relay logic or PLC code. Ladder logic is used conventionally to achieve such automation.

PLC code in Ladder logic for start delta starter is explained in the articlen PLC tutorial for star delta starter

Autotransformer starter

Auto transformer starting method is based upon the fact that reduced voltage is applied to the stator winding of motor at start using tappings of auto transformer, which will limit the starting current. To understand this phenomenon, consider a simple auto transformer

Auto Transformer

It is a single winding transformer which either steps up or step down the voltage. Tappings are provided on the single core to acquire appropriate voltage level. A simple auto transformer is shown in the following figure.

auto transformer

75% voltage is taken from the transformer. In this way, a reduced voltage is applied to the motor due to which it draws less current in starting.

Auto transformer starter arrangement

Following figure shows typical autotransformer starter arrangement for 3 phase motor. There are three contactors used for this purpose. Main contactor, S1, S2 and S3 contactor.

autotransformer starter arrnagment

The auto transformer starter is connected between the main supply and the motor windings. Reduced voltage is supplied to the motor via auto transformer  and it is bypassed after motor gains speed.

Main contactor is used to supply power to the motor. While contactors S1, S2 and S3 are used for the smooth starting. The operation of this arrangement is explained in the demonstration video later in the article.

Working of auto transformer starter

Before starting, the motor and autotransformer starter arrangement is shown in the above figure. All the contacts are open and no current passes through the motor.

When the start button is pressed, the main contactor is closes and supply is given to the motor.

auto transformer starter (when motor is started)

Since it is required to apply less voltage to the motor initially. So contactor S2 and S1 are closed while S3 is open and voltage is applied to the motor via auto transformer. The magnitude of voltage supplied to motor depends upon the tapping of auto transformer. According to the rule of thumb, 69% voltage should be applied to motor for smooth starting.

When the motor gains enough speed, the S2 and S1 are opened and while S3 is closed. It is shown in the following figure.

autotransformer starter (after motor gains speed)

Auto transformer is now bypassed and voltage is supplied directly to the motor. The auto transformer is switched out of the system after a known delay.

These switches of auto transformer starter are controlled by Ladder logic PLC code.

Ladder logic PLC code for autotransformer starter

The following figure shows the PLC code for the autotransformer in Ladder logic. The PLC code is divided into 3 part each called rung or step.

ladder logic PLC code for autotransformer starter

RUNG 1: This step of PLC code consists of Normally open (NO) contact I1 of start push button, Normally close (NC) contacts of stop button I2, NC overload relay contact I3 and main contactor coil Q1. Main contactor (Q1) will energize only when start is pressed, stop is not pressed, overload is not picked. A NO contact dependent on output Q1 is added in parallel with start button I1 to convert it into push button. Once output Q1 is energized, it will continue to remain energized even if start button is released because input dependent on Q1 provides parallel path.

RUNG 2: This step of ladder logic program contains Timer T1 and contactor for S2 and S1 coil switch. Addition of NC contact of relay S3 is added to prevent simultaneous closing of S1 and S2 with S3. S1 and S2 coils will energize when Q1 is energized, Timer T1 is not activated, Q3 is de-energized. Also timer starts counting time as soon as Q1 is energized.

RUNG 3: Last step of starter ladder logic code energize S3 contactor when Main contactor Q1 is energized, timer T1 is activated (after preset delay) and Contactor S1 and S2 are opened. Reason for adding NC contacts of Contactor S1 and S2 is to prevent simultaneous closing of them with S3. S3 contacts will only close if S1 and S2 are open and vice versa.

Further explanation of this program and demonstration is shown in the following video.

If you have any questions please ask in comments

About Syed Noman ud din

Syed Noman ud din is an Electrical Engineer and working in Industry from last 3 years. He writes technical articles for electrical and electronic engineers. He has also published several research publications in renowned international journals.

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