Principle of Star-Delta Starter
The star-delta starter is a common reduced-voltage starting device for three-phase asynchronous motors.
Its core principle involves altering the motor winding configuration to reduce startup voltage and current,
minimizing grid impact. Below is a detailed analysis:
I. Basic Principle
The starting current of a three-phase asynchronous motor is typically 4–7 times its rated current. Direct
starting causes:
Grid voltage fluctuations
Overheating of motor windings
Excessive mechanical stress
Principle of Y-△ Starter
**Principle of Y-△ Starter**
I. Basic Principle
**I. Basic Principle**
The starting current of a three-phase induction motor is typically 4–7 times its rated current. Direct-on-line (DOL)
starting may cause:
- Grid voltage fluctuation
- Overheating of motor windings
- Excessive mechanical stress
**Y-△ starting** addresses these issues through:
1. **Startup (Y Connection):**
1. **Startup (Y Connection):**
Windings are connected in **wye (Y)** configuration, with each phase winding subjected to **phase voltage**
(\(U_{\text{ph}}=\frac{U_{\text{ln}}}{\sqrt{3}}\); e.g., 220V phase voltage for 380V line-to-line voltage), reducing
starting current to **1/3** of DOL starting current.
2. **Normal Operation (△ Connection):**
2. **Normal Operation (△ Connection):**
After the motor reaches stable speed, windings switch to **delta (△)** configuration, with each phase winding under
**line-to-line voltage** (380V) for full-voltage operation.
II. Operational Sequence
**II. Operational Sequence**
1. **Startup Phase (Y Connection):**
1. **Startup Phase (Y Connection):**
- **KM1 (main contactor)** and **KM2 (wye contactor)** close; **KM3 (delta contactor)** remains open.
- Y-connection reduces voltage and starting current.
2. **Transition Phase:**
2. **Transition Phase:**
- Time relay (KT) delays, then **KM2 opens**. After a brief interval (to prevent short circuit), **KM3 closes**.
- Windings switch to △ configuration for full-voltage operation.
III. Control Circuit Schematic
**III. Control Circuit Schematic**
Power supply → KM1 → Motor windings ↘ KM2 (Y-contactor) ↘ KT (Time relay) ↘ KM3 (△-contactor)
IV. Advantages & Disadvantages
| Advantages | Disadvantages |
| Simple structure, low cost | Starting torque = 1/3 of △ starting torque |
| Easy maintenance | Suitable only for light-load/no-load starting |
| No additional components (resistors/reactors) | Momentary current surge during transition |
V. Applications
**V. Applications**
- Three-phase induction motors for **light-load/no-load starting** (e.g., pumps, fans).
- Motors rated for △ configuration (nameplate: "△/380V").
- Power grids with limited capacity requiring starting current limitation.
VI. Conclusion
**VI. Conclusion**
The Y-△ starter reduces startup voltage via **Y-△ winding transition**, balancing starting current and torque
requirements—a cost-effective solution.
Always select the appropriate starting method based on motor load characteristics.
Translation Notes
1. **Term Standardization**:
- Uniformly use the standard terms of the International Electrotechnical Commission (IEC):
- "Y-△" instead of "star-delta" for
- "contactor" instead of "contact"
- Use engineering standard symbols for formulas: \(U_{\text{ln}}\) (line voltage), \(U_{\text{ph}}\) (phase voltage).
2. **Sentence Optimization**:
- Use the passive voice to emphasize objectivity (such as "are connected", "is subjected").
- Supplement engineering examples after technical parameters (such as "e.g., 220V phase voltage for 380V line-to-line voltage").
3. **Format Alignment**:
- Retain the original hierarchical structure (I. II. → 1. 2.) to adapt to the reading habits of technical documents.
