Purpose

Explain open-transition star-delta power/control sequencing and verify motor suitability, timing and interlocking.

Learning objectives

• Trace six motor terminals
• Explain current/torque reduction
• Sequence contactors
• Prevent star-delta overlap

A star-delta starter initially connects suitable windings in star, then opens star and reconnects them in delta for running. It requires a motor intended to run in delta at the supply line voltage with all six winding ends accessible.

Core theory

During star start, each winding receives VL/√3. Compared with DOL delta at the same line voltage, ideal supply current and torque are approximately one-third.

An open-transition sequence energises main plus star, de-energises star, allows a safe transition interval, then energises delta. Timing must let the motor accelerate sufficiently without excessive coast or transient current.

Mechanical and electrical interlocking must prevent star and delta contactors closing together, which can create a severe short circuit. Correct phase/winding terminal mapping and transition direction are essential.

Assumptions: Balanced source; Correct star connection.

1. Star relation: Vwinding = VL/√3.
2. Substitute: Vwinding = 400/1.732.
3. Calculate: Vwinding ≈ 231 V.

Each winding receives approximately 231 V during star start.

Reasonableness check: This is 57.7% of the 400 V delta-running winding voltage.

Knowledge check

Answer: Their simultaneous connections can short-circuit the supply/windings. Use both mechanical and electrical interlocking as required by the design.

Practical exercise

Create a state table for main, star and delta contactors including start, transition, run, stop and fault states.

Summary

• The motor must be delta-rated at line voltage
• Star reduces current and torque
• Interlocking is safety-critical