Electrical

Comparison between Star and delta connections and How Star-Delta starter works?

What is the Function of motor starters?

1. Start and stop the motor

2. Limit inrush current where necessary

3. Permit automatic control when required

4. Protect motor and other connected equipment from sustained overload, no voltage, under voltage, single phasing, etc.

What is meant by a star?

In each 3-phase alternator, six terminals come out from 3 coils placed at 120˚ apart from each other. If any three ends starting or finishing are connected at one point and we get supply from the other three-point the connection is called a star. The junction point is called the Starpoint or neutral point.

Star connection is used where we require a Neutral terminal to obtain Phase voltage

In a star-connected system VL=√3Vph,

In a star Connected system IL=Iphase.

Star-connected systems require less insulation level and are used where a low starting current is required.

star connection

Advantages of star connections

  1. Used for high voltage
  2. Common neutral point
  3. Good for unbalanced loading
  4. Each phase is a separate circuit
  5. Dual voltage applications 
  6. Star connection can distribute the load evenly
  7. Star connection- alternator requires low insulation
  8. Star connection -alternator requires a low number of turns than delta for the same voltage
  9. Ability to spread the load between the phases
  10. Availability of single phase at a lower voltage
  11. The neutral point can be earthed

Disadvantages of star connections

  1. Less torque
  2. Construction involves combining 3 single phases into 1
  3. Secondary distribution, light-duty applications
  4. Construction cost is more expensive

What is meant by Delta?

In each 3-phase alternator, six terminals are so connected that the end of the first coil (assumed) is connected to the start of the second coil and the end of the second coil is connected to the start of the third coil and the end of the third coil is connected with the start of the first coil, the connections are called delta connected. The supply is taken from those connection points.

Delta connection
Star Vs delta

Power in both cases is the same√3V *I* COS ɸ.

line voltage = Phase Voltage so, the Insulation level is high.

For high starting torque, a delta connection is required.

Advantages of Delta connections

  1. More torque
  2. Efficient
  3. Simple motor design
  4. Heavy-duty application
  5. Protection is simple and less costly
  6. Used in rotatory conveyors
  7. While using delta connection, less current per winding for the same power output
  8. Major applications in power generation, transmission, and distribution
  9. The transformer secondary provides all the 3 phases
  10. Construction cost is low

Disadvantages of Delta connections

  1. No common neutral point
  2. Detecting earth ground faults is difficult
  3. Low voltage connection

DELTA CONNECTION IN A 3PHASE INDUCTION MOTOR TERMINAL

A delta connection in 3 phase induction motor is, the opposite end of three windings are connected (or) the end of each winding is connected with the starting point of another winding and three wires are taken out from the winding joints.

Delta connection

STAR CONNECTION IN A 3PHASE INDUCTION MOTOR TERMINAL

A star connection in 3 phase of induction motor in which the three winding each one end is connected in common junction and another end is connected to line voltage or supply voltage separately.

Star connection

Difference between star and delta?

 STARDELTA  
Basic DefinitionIn a three-phase Induction motor, one terminal from each winding is connected to a common point, and the remaining three terminals are connected to the external circuit.In a three-phase Induction motor, terminals of each winding are connected to each other.
Also known asWye connectionMesh connection
Denoted byYΔ
Point connectionCommon point occurs in star connectionThe Common point doesn’t occur in delta connection
Connection of terminals
Neutral pointthe star point or neutral exists in the star connection.The Neutral point doesn’t exist in the delta connection.
Relation between line and phase currentLine current =Phase current.IL = IP.Line current is equal to the√3of Phase Current. IL =√3 IP.
Relation between line and phase voltageLine voltage is equal to √3 the Phase VoltageLine voltage = Phase voltage.
SpeedIn star-connection motor speed is slow due to 1/√3 of the voltage.In delta-connection motor speed is high because each phase gets the total of the line voltage.
Phase voltagePhase voltage is low as 1/√3 times the line voltage.Phase voltage is equal to the line voltage.
Number of turnsRequires a low number of turnsRequires a large number of turns.
Insulation level Low InsulationHigh insulation
Network Typeused in the Power Transmission networks.Used in the Power Distribution networks.
Type of systemBoth three-phase four-wire and three-phase three-wire systems can be derived in star connection.A Three-phase four-wire system can be derived from the Delta connection.
The Required voltage for star and delta wiring230V received by each winding in star connection.415V received by each winding in delta connection.
DistanceFor the short distanceFor the long-distance
Difference between star and delta

Why star delta starter is used?

Star delta starter is used to reduce the current flow at the starting of the three-phase induction motor to avoid sparking, overheating, coil burning, etc. In the star connection, the supply voltage is lower than the delta connection. 

So basically, the star delta starter has both the star and delta connection arrangement and also it has a timer. So, at the starting time, the motor is connected to the Star connection. After a few seconds which is already set in the timer that will connect the motor from the star to the delta connection.

So, at the initial starting stage-star connection helps to reduce the starting current and in the running condition, the delta connection helps to gather enough torque and speed for the motor.

Star delta starter

Application of Star-Delta starter

Star delta starter is preferred for low to medium voltage and low torque squirrel cage induction motor, centrifugal compressors, large and high-power induction motors.

Suitable for applications where the required starting current is low and also the line current drawn must be minimum.

Commonly implemented in industries like- Textile, Food and Beverage, Sugar plants, Paper and Printing, Cold storage, etc.

Advantages of Star-Delta Starter

  1. Star delta starter provides more electrical safety and a greater life span to the induction motors.
  2. It is comparatively cheaper than other reduced voltage methods of starting of induction motor.
  3. Reduce the flow of current at starting which avoids the chances of sparking, overheating, burning of the coil, etc.
  4. It helps to gather more speed and torque to the induction motor at running conditions.
  5. Torque and current characteristics of star delta starter are better than other methods of starting.
  6. Starting current is 1/3 times of direct On-Line (DOL) starting current.
  7. No tap changing device is required to use
  8. It produces high torque per ampere of line current
  9. The design and operation of the star delta starter are simple and rugged.

Disadvantages of Star-Delta starter

Star delta starter can only be useful for 6 terminal delta connected induction motors where all 6 terminals can be accessed.

As the initial starting current compared to nominal current is 1/3 times high, starting torque is also reduced to 1/3 times. Hence it provides only 33% starting torque and if the load connected to the motor requires higher starting torque, then very high transient and stress are produced while changing from star to delta connection. It causes many electrical and mechanical break-down to occur.

It requires more cables from the star delta starter to the motor compared to the DOL starter.

Star delta starter will fail to start the motor if load connected with the motor has load torque higher than 50% of motor rated torque.

In this method of starting during the transition from star to delta, if the motor does not reach at least 80% of its rated speed then the current peak will be high same as in the DOL starter. It causes harmful effects on the contacts of contractors. Hence it would not be reliable.

If the motor too heavily functions then there will not be enough torque to accelerate the motor up to the desired speed before switching over to the delta position.

What happens if the Motor is connected to Delta connection at starting?

The secondary or rotor coil of the induction motor is always short-circuited. So, if the motor is connected to the delta connection, it will get a very high voltage to the winding, the motor will draw a very high starting current which may cause the burning to the coil.

What happens if the motor is always connected to the Star Connection?

The speed and torque of the induction motor are directly proportional to the square of the power supply voltage. So, if we increase the supply voltage, speed and torque also be increased. For this reason, once the motor is started with a normal speed it is changed to a delta connection to gather more speed and torque, that is why the motor will always be in star connection during starting time and change to delta for gathering speed and torque for motor.

What is the Difference between Star Delta Starter and DOL Starter?

DOL starter means Direct On Line Starter, With a DOL starter, the motor is only connected with the Star connection or Delta Connection (where high starting torque is required). It doesn’t change the connection. DOL starter doesn’t reduce the starting current. DOL starter provides a similar voltage to the motor at both starting and running conditions. 

On the other hand, the Star delta starter can reduce the flow of current at starting. It starts the motor with a star connection and runs the motor with a delta connection. Star delta starter is more expensive and complex designed than the DOL starter. Star delta starters are used above 6HP (horsepower).

Related posts

What is an autotransformer?

Ashlin

How to use a multimeter for various measurements?

Ashlin

Introduction to Electrical substation

Ashlin
Follow Us on LinkdIn