The Star–Delta starter is one of the most commonly used methods for reducing the starting current of a three-phase induction motor. It is widely used for medium-size motors. In this method, the motor initially starts in a star (Y) connection and after reaching a certain speed, it is switched to a delta (Δ) connection for normal operation.
Table of Contents
Need for Star–Delta Starter
When an induction motor starts with full line voltage, it draws a very large current (about 5 to 7 times the rated current). This high starting current can:
- Cause voltage drop in the power system
- Produce mechanical stress on motor windings
- Affect other equipment connected to the same supply
The Star–Delta starter reduces the starting current by lowering the voltage applied to the motor during starting.
Principle of Star–Delta Starting
The basic principle is based on the fact that:
- In star connection, the phase voltage is reduced.
- In delta connection, the full line voltage is applied.
For a three-phase system,
\[
V_{phase} = \frac{V_{line}}{\sqrt{3}}
\]
Thus during star starting:
- Voltage across each phase is reduced to \( \frac{1}{\sqrt{3}} \) of the line voltage.
- Starting current and torque are reduced.
Starting Current in Star–Delta Starter
If the motor is started in star connection:
- Phase voltage becomes \( \frac{V}{\sqrt{3}} \)
- Starting current reduces significantly
The line current drawn from the supply becomes approximately:
\[
I_{start} = \frac{1}{3} I_{DOL}
\]
Thus the starting current is about one-third of the DOL starting current.
Starting Torque
Since torque is proportional to the square of the applied voltage,
\[
T \propto V^2
\]
Therefore,
\[
T_{start} = \frac{1}{3} T_{DOL}
\]
So the starting torque is also reduced to one-third of the DOL starting torque.
Components of Star–Delta Starter
A typical star–delta starter includes:
Three Contactors
- Main Contactor
- Star Contactor
- Delta Contactor
Timer
Used to automatically switch the motor from star to delta connection after a few seconds.
Overload Relay
Protects the motor from excessive current and overheating.
Push Buttons
Used for starting and stopping the motor.
Working of Star–Delta Starter
- When the start button is pressed, the motor windings are connected in star configuration.
- The motor starts with reduced voltage, resulting in lower starting current.
- As the motor accelerates and reaches about 80–90% of rated speed, the timer operates.
- The connection changes from star to delta.
- The motor then runs normally with full line voltage.
Advantages of Star–Delta Starter
- Reduces starting current
- Prevents voltage dip in the supply system
- Simple and economical
- Suitable for medium-sized motors
Disadvantages
- Starting torque is low
- Not suitable for high-load starting conditions
- Requires motors designed for delta operation
Applications
Star–Delta starters are commonly used for:
- Pumps
- Fans
- Compressors
- Blowers
- Machine tools
- Industrial drives
Typically used for motors rated above 5 HP or 7.5 kW.
Comparison with DOL Starter
| Feature | DOL Starter | Star–Delta Starter |
|---|---|---|
| Starting voltage | Full voltage | Reduced voltage |
| Starting current | Very high | About one-third of DOL |
| Starting torque | High | About one-third of DOL |
| Cost | Low | Moderate |