Using Automatic Star-Delta-Star Starter

An AC induction motor can be single phase, poly-phase, brushed or brushless. Since industries are consuming major part of the power, we have to concentrate on energy saving from this area...

Ac Induction Motor

Power is the basic necessity for the economic development of a country. The power should be handled in an efficient manner to meet the power requirement. This  article presents the effective usage of star – delta starter in a power saving way in particular applications. Delta star Converter Module is the system to be interfaced with the existing Star Delta starter. When the load on the motor is less than 50% of full load, it switches the motor to operate in star to save energy. When the load increases beyond 50%, it automatically switches the motor to operate in Delta without disturbing the working of motor. This module is recommended for applications where load changes are not too fast. This is suitable for any capacity motor by using appropriate Current sensing IC and calibrating the same.

By implementing this, we can achieve:

  • Energy saving.
  • Contribute to reduction in maximum demand.
  • Improvement in power factor.

Efficient use of energy and its conservation assumes even greater importance in view of the fact that one unit of energy saved at the consumption level reduces the need for fresh capacity creation by 2 times to 2.5 times. Moreover, such saving through proper usage of energy can be achieved by at less than 1/5th the cost of fresh energy.

Therefore, conservation and proper management of energy saving are very important things. In industries, more than 80% of the motors are AC induction motors. An AC induction motor can be of single phase, poly-phase, brushed, or brushless. Since industries are consuming major part of the power, we have to concentrate on energy saving from this area. Here in this article, a proposal for energy saving in induction motors is being introduced apart from reducing fossil fuel consumption.

Literature Review

There are number of articles published on the performance  of induction motor under variable non sinusoidal voltages connection with phase controlled converters. The idea of applying variable voltage to optimize the performance under fractional load is also utilized by many authors in energy saving. Currently available products are costly and do not fully provide entire protection.

Low voltage application is provided for starting of induction motor. This is achieved by using star to delta conversion. Star delta is used in attempt to reduce the start current applied to the motor. This article aims to provide low voltage at starting i.e., at starting the motor is switched in star mode. Therefore, in this article, at starting of motor protection is provided by applying reduced voltage. There is no energy saving provision provided in these article.

Cyclic load converter equipment is made to save power on variable load machine, especially for those  which are running in no load or partial load for long time. Using delta to star change over during no load or partial load, the power is saved upto 30 to 40% of the no load power. The simulation of proposed system is done in the PLC simulation software. The logic which has been developed in the PLC is somewhat complicated.

Protection at the starting of induction motor by providing low voltage application by using simple conventional star-delta-starter provides provision for energy saving by using PLC simulation software but the logic which is developed is much complicated.

Therefore in comparison with above mentioned articles, we have developed energy saving provision by using conventional star-delta-star starter but in effective manner, by using arduinouno software/hardware device and also we have developed alternative method of energy saving by using PLC simulation software.

Conventional Star -Delta-Star Starter

The star delta starting is a very common type of starter. Star delta starter is used more than any other type of starters. This method uses the reduced supply voltage in starting. This is achieved by low starting current by first connecting the stator winding in star configuration, and then after the motor reaches a certain speed, throw-switch changes the winding arrangements from star to delta configuration. While starting the motor in star, the starting current can be reduced one-third of the current with compared to delta connection. Since the windings are connected in star in the starting, the windings will get the line voltage across it. Since the torque developed by an induction motor is proportional to the square of the applied voltage, star- delta starting reduces the starting torque by one – third that obtainable by direct delta starting.

Star Delta Starter
Fig 1.Star Delta Starter

Proposed Energy Saving by using Automatic Star -Delta-Star Starter

Now 3 phases supply is given to the induction motor, current sensing element connected in series with any one phase. At start motor will start in star connection, as soon as motor reaches rated speed the stator winding connected in delta. As the load on motor increases current sensing element sense the current flowing through the circuit and the output of this current sensing element (i.e., voltage) fed to the arduinouno device i.e., it is connected to the input pin of arduino. For comparing the output of ACS712 current sensor with a fixed reference current value, we are connecting a resistor pot to one of the input pin of arduino to set a constant reference current value. Arduino compares the current sensing element output (i.e., voltage) with a set reference current limit. As the Current sensing element input is greater than set limit, it send signal to control circuit to make change of motor connection from star to delta. Similarly, when load on motor decreases then current sensing element output (i.e., voltage) deceases as compared to set limit, so control circuit switches the motor connection from delta to star.

Block Diagram Of Automatic Star Delta Star Starter
Fig. 2 Block Diagram of Automatic Star-Delta-Star Starter

Relationship between PStar and PDelta

VL is same for Delta & Star load

Zph is same for Delta & Star load

For Delta connected load:

Delta Connection
Fig.3 Delta Connection

PDelta= 3VPhIphCosθph

As,  Iph= Vph / Zph

PDelta= 3VPh( Vph / Zph) Cosθph

PDelta= 3 (VPh) 2Cosθph / Zph

As,  Vph= VL( For Delta Connection )

PDelta= 3 (VL) 2 Cosθph / ZphWatts…….. (I)

For Star connected load:

Star Connection
Fig.4. Star Connection

PStar= 3VPhIphCosθph

As, Iph= Vph / Zph

PStar= 3VPh( Vph / Zph) Cosθph

PStar= 3 (VPh) 2 Cosθph / Zph

As,  Vph= VL/ √3 ( For Star Connection )

PStar= 3 (VL / √3 )2 Cosθph/ Zph

PStar= VL2 Cosθph/ ZphWatts…….. (II)

So from (I) & (II) we can see that the power in the Delta connection is 3 times greater than that of the Star connection.

Performance of Energy Saving by using Star Delta Star Starter:


Control Circuit 1
Fig.5. Control Circuit 1

In this article, 5HP DELTA-DELTA connected 3 phase induction motor has been used.ACS712 current sensing IC is connected in series with one of the phase of 3 phase induction motor. However, ACS712 current sensing IC comes in three variants providing current range for ±5A [ACS712-05B], ±20A[ACS712-20B] & ±30A[ACS712-30B].

According to full load current carrying capacity of 3 phase induction motor ACS712 current sensing IC has been selected. Full load current carrying capacity of 5HP 3 phase induction motor is 8.4A, therefore ±20A [ACS712-20B] has been used. Further this ACS712 IC is fed to ARDUINO UNO device (software and hardware) device. As shown in above figure Vcc pin of ACS712 is connected to 5V supply pin of arduino and the Ground(GND) and Vout pin of ACS712 IC is fed to Ground(GND) and analog pin A3 of arduino. Relay is connected to the digital pin D8 and ground pin of arduino device.

The current flowing in the motor is measured through ACS712 IC which is nothing but measuring current value. Arduino programming has been developed for measurement of current and comparison of measured value with reference value for relay operation. A constant value is set as a reference value.

Relay operates in following condition,

Condition 1: If the current flowing during star mode is greater than reference 1, it transforms contactor from star to delta.

Condition 2: If the current flowing during delta mode is less than reference 2, it transforms contactor from delta to star.


Reference1=set for conversion from star to delta.

Reference2=set for conversion from delta to star.

Therefore, according to relay condition i.e., when the relay is high then the motor get will connect in DELTA mode. Similarly, when relay is low then the motor will get connected in STAR mode. In this way Energy Conservation takes place.

Control Circuit 2:

Control Circuit 2
Fig.6. Control Circuit 2

When START push button is pressed STAR contactor get energized through R1 and D1. Due to energization of STAR contactor its auxiliary contacts S1 and S2 becomes NC and NO respectively. Because of S1 MAIN contactor gets energized.

When MAIN gets energized its auxiliary contacts M1 and M2 becomes NC. To keep the MAIN contact energized auxiliary contact M1 is used and M2 to hold the STAR contactor, as START push button is not latch button.

The signal from control circuit 1 actuates the relay contacts in control circuit 2.R1 becomes NC and R2 becomes NO. As a result STAR contactor get de-energized and DELTA get energized. The NC contacts of STAR and DELTA in circuit is used to provide interlocking.

Analytical Results of Energy Saving for 5Hp Motor:

Following are the various results taken on the motor:

Current Input Power







Torque Output Power







Graphical Representation of Various Parameters of Induction Motor:

Output Power Vs Current:

Graph Of Output Power Vs Current
Fig. 7 Graph of Output Power Vs Current

The graph of output power Vs current is plotted by taking power output on x-axis and line current on y-axis. From graph, we see that at no load the current in delta mode is large as compare to current in delta mode.

Output Power Vs Efficiency:

Graph Of Output Power Vs Efficiency
Fig.8 Graph of Output Power Vs Efficiency

The graph of output power Vs efficiency is plotted by taking power output on x-axis and efficiency on y-axis. From the graph, we see that efficiency for star mode increases in linearly and after 40% of loading it becomes constant. The efficiency for delta gradually get increases and after cross over point it increases. So, we can say that after cross over point delta mode is more efficient than star.

Output Power Vs Losses:

Graph Of Output Power Vs Losses
Fig. 9 Graph of Output Power Vs Losses

The graph of output power Vs losses is plotted by taking power output on x-axis and losses on y-axis. At no load losses in star mode are less as compare to losses in delta mode. So, after the cross over point, losses in star mode get increases.

Energy Saving Results:

From above graph of output power Vs losses

Energy saving in star:

At, output power = 1000 watts

Losses in star = 336 watts

Losses in delta = 495 watts

Therefore energy saving in induction motor running in star mode

at output power = 1000 watts is,

Power saving = Losses in delta – Losses in star

= 495 – 336

= 159 watts

For a time period of 1 hour running in star at these load the energy saving will be,

Energy saving = Power saving (watts) × Operating time period (hr) = 159 × 1

= 159 watt-hr.

Energy saving in Delta :

At, output power = 2000 watts

Losses in star = 886 watts

Losses in delta = 746 watts

Therefore, energy saving in induction motor running in Delta mode at output power = 1000 watts is,

Power saving =Losses in Star – Losses in Delta = 886 – 746 = 140 watts.

For a time period of 1 hour running in star at this load the energy saving will be,

Energy saving = Power saving (watts) × Operating time period (hr) = 140 × 1 = 140 watt-hr.

Advantages, Disadvantages & Applications:


  • Energy saving is possible.
  • Contributing to reduction in maximum demand.
  • Improvement in power factor.
  • After removing the load motor reconnected to star.
  • Economic as compare to auto transformer.
  • Energy Saving is more as compare to DOL starter.
  • It has better efficiency.


  • Contactor switching losses take place.
  • Six terminals must be required on terminal box.
  • At the time of starting motor must be in star mode.


  • Use for starting large capacity motor.
  • Use in lathe machine at mechanical workshops.
  • Use in wood processing machine.
  • Use in Compressor.
  • Use in press machine.


This article intended to meet the requirements of modern age starter of induction motor with additional provision of energy saving in general starting & accelerating period, and discussing a method to improve the total efficiency of the induction motors used in press Machines, Injection Molding Machines, Agitators Conveyors and Textile Mills etc. The life span of the machine is also increased by implementation of this starter.


I would like to thank Dr. J. B. Dafedar, Principal, NKOCET, Solapur, Prof. V. S. Shirwal, HOD, Electrical Department, NKOCET, Solapur and my colleagues for their most support and encouragement.

Adarsh J Mehta




Prof. Adarsh J. Mehta
Assistant Professor, Electrical Engineering Department, DBATU University,
N K Orchid College of Engineering & Technology, Solapur


  1. The error in the power diagram of Star Delta is that the yellow phase is connected to the yellow phase, ie the other end of the yellow phase is also supplied with the yellow phase, which is incorrect. There will be an explosion. Fix your circuit.

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