Star-Delta Motor Starting Wiring and Principle

Overview

Star-delta reduced-voltage starting is a common method for induction motors. Because induction motors draw a large starting current, large-capacity motors often use a star-to-delta changeover at startup. This simple reduced-voltage method connects the stator windings in a star configuration during startup and then switches to a delta configuration after acceleration, which reduces the starting current and lessens the impact on the power supply.

Main Components of the Wiring Diagram

• QS: circuit breaker
• KM1: main contactor
• KM2: star contactor
• KM3: delta contactor
• FR: thermal overload relay
• KT: timer relay (energize-delay type)
• SB1: stop button
• SB2: start button

Starting Procedure

1. Close the circuit breaker QS and press the start button SB2. KM1 (main contactor) and KM2 (star contactor) are energized and close, connecting the motor in a star configuration for startup.
2. After the preset delay of the timer relay KT, for example 10 seconds, the normally closed contact of KT opens. KM2 de-energizes and releases, and KM3 (delta contactor) is energized and closes.
3. The motor is then reconfigured to delta connection and runs in delta mode, completing the star-delta start sequence.

Applications

This method effectively reduces starting current and mitigates disturbances to the power grid. It is suitable for large motors where limiting inrush current is important.

Advantages

• Reduced starting current: With the stator connected in star during startup, the starting current is approximately one third of the current when starting directly in delta, reducing stress on the supply network.
• Simple and economical: Compared with other reduced-voltage starting methods, the star-delta method has the simplest structure and lowest cost.
• Improved efficiency under light load: In some cases with light load, operating in star can better match rated torque to load, which may improve efficiency and reduce energy consumption.

Disadvantages

• Low starting torque: Starting torque is proportional to the square of the per-phase voltage. In star connection the per-phase voltage is reduced by 1/√3, so starting torque drops to about one third of the torque in delta connection. This sacrifices torque to reduce starting current.
• Limited applicability: Because of the reduced starting torque, star-delta starting is mainly suitable for no-load or light-load start conditions.

In summary, star-delta reduced-voltage starting is an effective method to limit starting current and reduce supply disturbances, but its limited starting torque restricts its use to no-load or light-load starts.