- Star Delta Starter Definition: A star delta starter is used for starting 3 phase induction motors, minimizing initial electrical overload by starting in a ‘star’ configuration and then switching to ‘delta’ after reaching a certain speed.
- Circuit Diagram: The circuit includes a TPDP switch that helps in changing the motor connections from star to delta, facilitating effective control of current and torque during startup.
- Reduced Starting Current: By using a star delta connection, the starting current is reduced to one-third of what it would be with direct online starting, leading to lower electrical stress.
- Working Principle: The star delta starter operates on a principle where it starts the motor at a lower voltage and current, then transitions to normal operating conditions to manage power efficiently.
- Application Suitability: Ideal for situations where low starting torque is adequate, the star delta starter excels in environments like driving centrifugal compressors, where minimal starting current is beneficial.
What is Star Delta Starter
A star delta starter is a reduced-voltage starter for a 3-phase induction motor. The stator starts in star, then switches to delta once the shaft is up to speed.
A star delta starter is a reduced-voltage starter. It lowers starting current by reconnecting the stator, with no extra resistor or autotransformer. Line inrush current from the supply is about one-third of a DOL starter in the ideal star-versus-delta comparison. Open transition to delta still produces a current spike. Timers often change over near 80% of running speed; that figure is a setting, not a standard.
Textbook diagrams show a TPDP switch (triple-pole double-throw) that changes the stator from star to delta. Factory starters usually use a main contactor, a star contactor, a delta contactor and a timer.
Standstill symbols:
VL = Supply Line Voltage, ILS = Supply Line Current, IPS = Winding Current per Phase, and Z = Impedance per phase winding at standstill conditions.
As the winding is star-connected, the winding current per phase (IPS) equals the supply line current (ILS).
As the winding is star-connected, the voltage across each phase of the winding is
Hence, the winding current per phase is
Since here, the winding current per phase (IPS) equals the supply line current (ILS), we can write,
If the same motor started in delta from the same three-phase supply points,
Here, ILD = Supply Line Current, IPD = Winding Current per Phase, and Z = Impedance per phase winding at standstill condition.
As the winding is delta-connected, the supply line current (ILD) is root three times the winding current per phase (IPD)
As the winding is delta-connected, the voltage across each phase of the winding is
Hence, the winding current per phase is
Now, we can write,
Comparing supply line currents for star and delta-connected windings gives
Starting current from the mains in star-delta is one-third of direct switching in delta. Starting torque of an induction motor is proportional to the square of the voltage applied to the winding per phase.
The equation shows that the star delta starter reduces starting torque to one-third of DOL starting torque. The one-third figure is versus DOL starting torque, not versus full-load torque. The star-delta starter is equivalent to an autotransformer with a 57.7% tapping.
Advantages of Star Delta Starter
The advantages of star delta starters include:
- Inexpensive
- It needs no autotransformer taps. Winding heat is still present, but there is no extra starter resistor.
- Starting current reduced to 1/3 of direct online starting current.
- Produce high torque per ampere of line current.
Disadvantages of Star Delta Starter
The disadvantages of star delta starters include:
- Starting torque is about one-third of DOL starting torque.
- The motor must bring out six terminals and be rated for delta at line voltage.
Application of Star Delta Starter
As discussed in the above advantages and disadvantages, a star-delta starter suits loads that can start on reduced torque and that need a lower line-current draw.
Loads that need high starting torque use a DOL starter instead.
If the motor is too heavily loaded, star torque may stall the shaft before the delta changeover. An example application for a star delta starter is a Centrifugal compressor.
