Star-Delta Starter Symbol
Definition: The Star-Delta Starter symbol represents a reduced-voltage motor-starting device drawn as a labeled rectangle with a Y-Δ (wye-delta) designation and six terminals — three supply inputs (L1, L2, L3) and three motor outputs (U, V, W) — denoting a starter that initially connects a three-phase motor in star (wye) configuration to limit starting current, then switches to delta configuration for full-voltage running, as defined in IEC 60947-4-1 (contactors and motor starters) and referenced in NEMA ICS 2.
Also known as: wye-delta starter symbol, Y-delta starter symbol, star-delta motor starter, reduced-voltage starter YD, Stern-Dreieck Starter symbol.
What the Star-Delta Starter symbol means
The Star-Delta Starter symbol identifies a motor-starting method that reduces the voltage applied to each motor winding during starting by connecting the windings in a star (wye) configuration rather than the normal delta configuration. In star configuration, each winding receives 1/√3 (approximately 57.7%) of the line voltage, reducing starting current and torque to one-third of the delta values. After a preset time delay (typically 5–15 seconds), the starter switches the motor connections from star to delta, restoring full line voltage to each winding for normal running operation.
In electrical schematics, the star-delta starter symbol appears as a labeled block with the Y-Δ annotation and a triangular polygon inside the rectangle representing the delta winding topology. The six-terminal arrangement (L1–L3 in, U–V–W out) signals that the device contains the three contactors (main contactor, star contactor, and delta contactor) and the timing relay required to implement the switching sequence. Star-delta is the most widely used reduced-voltage starting method for three-phase motors rated from approximately 5 kW to several hundred kilowatts.
How to identify the Star-Delta Starter symbol
The Star-Delta Starter symbol is a square or rectangular box labeled 'Y-Δ' (or 'Y-' as shown in the SVG) with a triangle (delta symbol) inside the lower portion of the box, representing the delta configuration. Three terminal stubs at the top connect to the supply lines (L1, L2, L3) and three stubs at the bottom connect to the motor terminals (U, V, W). The Y-Δ label and triangle inside clearly distinguish this starter from a plain DOL contactor (rectangle only), a soft starter (ramp waveform inside), and an autotransformer starter (transformer symbol inside).
Function in a circuit
A star-delta starter manages the two-stage starting of a three-phase induction motor that is wound for delta operation. Stage 1 (star): the main contactor closes, the star contactor closes (shorting the motor's winding end-terminals together to form a neutral point), and the motor starts with each winding at 1/√3 of line voltage. Starting current is approximately 1/3 of DOL starting current; starting torque is also reduced to 1/3 of DOL torque. Stage 2 (delta): after the timing relay triggers (typically 5–15 s), the star contactor opens, a brief interlocking pause occurs, then the delta contactor closes, connecting the winding end-terminals directly in delta for full-voltage running. The transition between star and delta creates a current transient that must be managed by the timing relay interlock.
Standards: IEC vs ANSI
| IEC 60617 | IEC 60947-4-1 defines low-voltage contactors and motor starters including star-delta starters. IEC 60617 does not assign a unique graphical symbol for a star-delta starter block; the labeled Y-Δ rectangle with L1–L3 / U–V–W terminal notation is the accepted IEC convention in motor control diagrams. |
|---|---|
| ANSI/IEEE 315 | NEMA ICS 2 covers industrial control and systems motor starter requirements including reduced-voltage starters. ANSI Y32.2/IEEE 315 does not define a specific star-delta block symbol; North American practice uses a labeled rectangle with the same Y-Δ annotation and L/T terminal layout as IEC. |
| Key difference | No glyph difference exists between IEC and ANSI star-delta starter symbols. Both use a labeled rectangle. Terminal designations may vary slightly: IEC uses L1/L2/L3 (supply) and U/V/W (motor); North American practice uses L1/L2/L3 (supply) and T1/T2/T3 (motor) in some conventions. |
Terminals / pins
| Pin | Name |
|---|---|
| l1 | L1 |
| l2 | L2 |
| l3 | L3 |
| u | U |
| v | V |
| w | W |
Typical values
Typical motor size range: 5 kW to over 400 kW three-phase. Supply voltage: 380–690 V AC, 50/60 Hz. Starting current: approximately 1.5–2.5× FLA (vs 5–8× FLA for DOL). Starting torque: approximately 33% of DOL starting torque. Transition time (star to delta): typically 5–15 seconds (set by timer relay). Requires motor rated for delta connection at line voltage (i.e., motor Δ voltage = supply voltage).
Where the Star-Delta Starter symbol is used
- Large centrifugal pump and compressor motor starters where current limiting is needed on constrained supply networks
- Conveyor drive motors requiring reduced current at starting to prevent upstream protection device tripping
- Milling and lathe motor drives in industrial machinery where the supply authority limits starting current
- HVAC centrifugal fan and compressor starters in commercial buildings to minimise voltage dip on the distribution bus
- Aggregate crushers and belt conveyors in mining and quarrying where large motors must start under light load
- Water pump stations where utility tariff agreements require motors above a specified kW to use reduced-voltage starting
Example
In a 75 kW cooling water pump motor control schematic, the star-delta starter symbol shows L1, L2, and L3 feeding from an upstream MCCB and U, V, W connecting to the motor terminals. The internal logic (shown in a separate control circuit) has the main contactor KM1 closing first, the star contactor KM3 closing simultaneously, and a 10-second timer relay setting the transition time. After 10 seconds, KM3 opens (star disconnects) and KM2 (delta) closes, completing the star-to-delta transition with the motor running on full 400 V across each winding.
Key facts
- The Star-Delta Starter symbol represents a reduced-voltage motor-starting device with six terminals: three supply inputs (L1, L2, L3) and three motor outputs (U, V, W), labeled 'Y-Δ' and governed by IEC 60947-4-1.
- In star (Y) connection, each motor winding receives 1/√3 (≈57.7%) of line voltage, reducing starting current and torque to 1/3 of direct-on-line (DOL) values.
- After a programmable time delay (typically 5–15 seconds), the starter switches from star to delta, applying full line voltage across each motor winding for normal running operation.
- Star-delta starting requires the motor to be wound for delta connection at line voltage — a motor rated 230 V delta can be star-delta started from a 230 V supply, not from a 400 V supply.
- The star-delta starter comprises three contactors (main, star, and delta) and a timing relay; the block symbol represents all three contactors combined in a single starting device block.
- The transition from star to delta produces a current transient; 'open transition' starters (most common) briefly disconnect the motor during switching, while 'closed transition' starters use a resistor to provide a current path during the switch, reducing the transient.
- Star-delta starting reduces starting torque to 33% of DOL torque, making it unsuitable for high-inertia loads or loads requiring high starting torque (conveyors with loaded belts).
- The IEC schematic terminal convention uses L1, L2, L3 for supply and U, V, W for motor; North American NEMA convention uses L1, L2, L3 for supply and T1, T2, T3 for motor output.
Frequently asked questions
What does the star-delta starter symbol look like in a circuit diagram?
The Star-Delta Starter symbol is a square rectangle labeled 'Y-Δ' (Y-delta) with a triangle (delta symbol) drawn inside the lower part of the box. Three terminal stubs emerge from the top (L1, L2, L3 — supply inputs) and three from the bottom (U, V, W — motor outputs). The Y-Δ label and triangle clearly distinguish it from a DOL contactor, soft starter, or VFD symbol.
What does a star-delta starter do in an electrical circuit?
A star-delta starter reduces motor starting current by first connecting the motor windings in star (wye) configuration, which reduces voltage across each winding to 1/√3 of line voltage and limits starting current to 1/3 of DOL values. After a time delay of 5–15 seconds, the starter switches to delta configuration, applying full line voltage to the motor for normal running.
What is the difference between a star-delta starter and a soft starter?
A star-delta starter uses mechanical contactors to switch between two fixed winding configurations (star and delta), providing a discrete two-step starting process with a current transient at the Y-Δ transition. A soft starter uses electronic SCRs to continuously ramp voltage from a low initial value to full voltage, providing a smooth, stepless acceleration with no transition transient. The soft starter provides more precise control but at higher cost.
What standard governs star-delta starters?
IEC 60947-4-1 defines performance and testing requirements for low-voltage contactors and motor starters, including star-delta (wye-delta) starters. In North America, NEMA ICS 2 governs motor starter performance requirements. Neither IEC 60617 nor ANSI Y32.2 defines a unique graphical symbol; a labeled Y-Δ rectangle with L1–L3 and U–V–W terminals is the accepted convention.
What motors can be started with a star-delta starter?
Star-delta starters can only be used with three-phase induction motors that are wound for delta connection at line voltage and have all six winding end-terminals accessible in the terminal box (three phase winding start terminals and three end terminals). A motor labeled '400 V delta / 690 V star' can be star-delta started from a 400 V supply. Motors with internally delta-connected windings (only three terminals accessible) cannot use star-delta starting.
How many terminals does the star-delta starter symbol have?
The star-delta starter symbol has six main power terminals: L1, L2, and L3 on the line (supply) side at the top, and U, V, and W on the motor side at the bottom. Additional control circuit terminals for the main contactor coil, star contactor coil, delta contactor coil, and timer relay are shown in a separate control diagram cross-referenced to the power circuit.
Why does star-delta starting reduce torque as well as current?
Torque in an induction motor is proportional to the square of the stator voltage. In star configuration, each winding voltage is reduced to 1/√3 of the delta value, so torque is reduced to (1/√3)² = 1/3 of the delta (DOL) starting torque. This makes star-delta starting unsuitable for loads requiring high breakaway torque, such as heavily loaded belt conveyors or positive-displacement pumps.
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