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PLC Applications: Primary Resistance Starter for AC Motor

In this article we will discuss the primary resistance starter for induction motor. What is the basic principle and working of primary resistance starting method? The article will also discuss primary resistance motor starter as a basic PLC application by writing a ladder logic program.


Induction motors are widely used in industry. They have lots of advantages however they inherit a problem of high starting current. As compared to full load current, the starting current is 6 to 10 times higher. This high inrush of current can damage the motor windings and other parts. High inrush current can also affect the source. Therefore it is required to limit the high starting current in induction motor. There are lots of methods for smooth starting of induction motors. Each one has its own advantages and disadvantages. Some of them are

In this article we will discuss the primary resistance starter with detail and also learn how to implement it in PLC. Motor starters are among of the basic PLC applications.

Operating principle of primary resistance motor starter

Primary resistance starting method is based on the reduced starting voltage concept. This method introduces the resistance in series with the stator windings of squirrel cage induction motor. Voltage drop occurs in the added resistors during starting which provides reduced voltage across the stator windings. With the reduced voltage, the starting current is lower than it would be if the motor is started on full line voltage. After a preset delay, the resistance unit is shunted out of the circuit and the motor continues to run on line voltage.

Primary resistance starter arrangement

Above figure shows the primary resistor starter arrangement.  It consists of a main contactor and a second contactor labelled as S2. Resistors are connected with each phase of stator winding in series to reduce voltage. The value of these starting resistors can be determined by the method explained in the article

primary resistance starter arrangment

By pass switches are connected in parallel to with the starting resistors which are controlled by S2 contactor.


The motor is started by closing the main contactor. This supplies power to the motor through series starting resistance. The contactor S2 is open thus the current has to pass through the resistors resulting in voltage drop. Due to voltage drop in these resistors, the motor gets reduced voltage.

primary resistance motor starter at starting time

A timer is installed with a preset time delay. When the delay is over, the contactor S2 is closed closing the parallel switches.

primary resistance starting when motor gained speed

As it can be seen in the above figure that closing the S2 bypass all the resistors. As a result full line voltage is applied to the motor windings.

PLC Applications: PLC program for primary resistance starter

Following figure shows the PLC program for Primary resistance motor starter

PLC applications: Program for primary resistor starter

Rung 1: It consist of start button normally open contacts I1, normally close contacts of stop button I2 and normally open contact of over load relay I3. Main contactor Q1 will only be energized if start is pressed, stop is not pressed and overload is not operated. An input normally open contact depending upon output coil Q1 is added in parallel with the start button contacts. It will ensure that power is supplied to rung 1 even if the start button is released. Adding this parallel path enables the input start button to work as a momentary push button.

Rung 2: It only consist of normally open input dependent upon output coil Q1 and a timer T1. When the main contactor coil Q1 is energized, the motor gets the voltage through the resistor and also the timer starts counting the preset delay. When the preset delay is over, the timer is activated.

Rung 3: It contains input depends upon main contactor Q1, an input relying on timer T1 and output coil Q3. The Output Q3 will only be energized when main contactor Q1 is energized and preset time delay is over.

It can be seen that very easily the primary resistance starter can be implemented with PLC and it is one of the basic PLC applications.


Working and program demonstration is further explained in this video

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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