The star-delta starter is one of the most commonly used starters for three-phase induction motors. It is popular because it is an efficient and relatively simple method for reducing the starting current of a motor, thus protecting electrical systems from sudden surges during motor startup. The star-delta starter is specifically designed for squirrel-cage induction motors, which are typically designed to run with a delta-connected stator winding under normal operation.
Basic Working Principle
The star-delta starter initially connects the stator windings of the motor in a star (wye) configuration during startup. Once the motor reaches around 80% of its rated speed, the connection switches to a delta configuration. This reduces the inrush current and starting torque during the startup phase. The inrush current is a significant concern, especially for large motors, as it can be 5-7 times the full load current. By using a star-delta starter, this current can be reduced to approximately one-third of what it would be if the motor were started directly with a delta connection.
Star-Delta Connection
When the motor is connected in the star configuration during startup, each phase of the motor receives a reduced voltage, specifically VL / √3, where:
VLis the line voltage.√3is the factor by which the phase voltage is reduced in a star connection compared to the line voltage.
This reduction in voltage decreases the starting current and torque proportionally. Once the motor reaches a predetermined speed (around 80% of its rated speed), the starter transitions the motor to the delta configuration, applying the full line voltage to each phase.
Diagram of Star-Delta Starter Connections
Star Connection during Startup:
In this setup, all three windings are connected together at one point (neutral), forming the star connection.
Delta Connection after Startup:
Star-Delta Starter Circuit
A typical star-delta starter circuit consists of three main components:
- Main Contactor (KM1): Supplies the three-phase power to the motor.
- Star Contactor (KM2): Connects the motor windings in the star configuration during startup.
- Delta Contactor (KM3): Connects the motor windings in the delta configuration during normal operation.
Additionally, a timer is used to control the transition from the star to the delta configuration.
How the Star-Delta Starter Works
Initial Star Configuration
When the switch S is set to the "START" position, the main contactor (KM1) and star contactor (KM2) are engaged. This connects the motor windings in the star configuration, where the voltage applied to each phase is VL / √3. The reduced voltage leads to a lower starting current and torque, as the starting current Istart in star configuration is reduced to:
Istart,star = Istart,delta / √3
Where Istart,delta is the current when the motor is directly started in delta configuration.
Since torque (T) is proportional to the square of the applied voltage (V2), the starting torque in the star configuration is also reduced to one-third of what it would be in the delta configuration:
Tstart,star = Tstart,delta / 3
For example, if the motor is designed to operate at 400 V line voltage in delta configuration, in the star configuration, each winding will receive:
Vphase,star = 400V / √3 = 230V
Transition to Delta Configuration
After the motor reaches approximately 80% of its rated speed, the timer deactivates the star contactor (KM2) and activates the delta contactor (KM3). This reconnects the motor windings in delta configuration, allowing the motor to receive the full line voltage and run at full torque and speed. The current and torque increase as the voltage applied to each winding rises from VL / √3 to VL.
Delta Configuration for Normal Operation
In the delta configuration, each winding of the motor receives the full line voltage (VL). The motor now operates at full capacity, handling the mechanical load with the rated torque.
Calculations of Current and Torque
For a motor rated at 400 V, with a full load current of 50 A and a torque of 100 Nm, the following can be calculated:
In Star Configuration:
- Phase voltage =
400V / √3 = 230V - Starting current =
50A / √3 = 28.87A - Starting torque =
100 Nm / 3 = 33.33 Nm
In Delta Configuration:
- Phase voltage = 400 V (line voltage)
- Full load current = 50 A
- Full torque = 100 Nm
Advantages of Star-Delta Starter
- Reduced Inrush Current: The star-delta starter limits the inrush current during motor startup to one-third of the DOL current, making it suitable for large motors.
- Mechanical Stress Reduction: The lower starting torque in star configuration reduces the mechanical stress on the motor and the connected equipment, enhancing the motor's lifespan.
- Cost-Effective: Compared to other soft starters or variable frequency drives (VFDs), the star-delta starter is relatively inexpensive and easy to implement.
Limitations of Star-Delta Starter
- Torque Reduction: The starting torque is reduced to one-third, which may not be suitable for applications requiring high starting torque.
- Sudden Transition: The transition from star to delta can cause a sudden surge in current and torque, which may lead to electrical and mechanical stress.
- Not Suitable for Low Power Motors: The star-delta starter is typically not used for motors with low power ratings, as the cost-to-benefit ratio decreases for smaller motors.