Star-Delta Motor Starter (Contactor and PLC Versions)

Goal:
To understand and demonstrate how industry reduces the high starting current of induction motors with star-delta starting, and to compare conventional hard-wired control with PLC-based control of the same process.

What I built:
Two working starters for the same three-phase induction motor. The conventional version uses three magnetic contactors (main, star and delta), an on-delay timer, a thermal overload relay, a circuit breaker and start and stop push buttons. The motor starts in star connection to limit the starting current, and after a set delay the timer switches it to delta for normal running. The star and delta contactors are electrically interlocked so they can never close together. In the PLC version, a Siemens LOGO PLC replaces the timer and most of the control wiring: its program runs the same start, changeover and stop sequence, with the timing and interlocking handled in logic.

My role:
Working with my project team at DUET, I designed the power and control circuits for both versions, wired the contactors, timer and protection devices, wrote the LOGO PLC program, and tested both starters on the motor.

Challenges / results:
The key challenge was the changeover from star to delta. It needed a correctly set delay and reliable interlocking to prevent a short circuit between the two contactors. Both versions started the motor smoothly with a visibly lower starting current than direct-on-line starting. The comparison showed the PLC's clear advantages: the delay and sequence could be changed in software without rewiring, the control wiring was simpler, and faults were easier to trace.

What came after:
The project gave me a solid foundation in motor control and PLC programming, and led directly to my next projects: conveyor control with a Siemens LOGO PLC, and variable speed motor control with a variable frequency drive.