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Lesson 6

Star-Delta Starter Control Circuit with Interlocking

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Welcome back. In the previous lesson, we designed the fundamental latching start/stop circuit, which is the "brain" for controlling a single contactor. This circuit provides the essential operator controls and safety features needed for the 3kW exhaust fan motor in our project.

Now, we will tackle a more complex challenge: the control circuit for the 15kW conveyor motor's star-delta starter. This requires coordinating three separate contactors—Main, Star, and Delta—in a precise sequence. This lesson focuses on drawing the basic control circuit that incorporates the start/stop logic we've already learned, and introduces the critical concept of electrical interlocking to ensure the starter operates safely.

The Star-Delta Control Sequence

Unlike a simple DOL starter with one contactor, a star-delta starter requires a specific sequence of operations:

  1. Start: When the operator presses the "Start" button, the Main and Star contactors must energize simultaneously. This connects the motor windings in a star configuration, reducing the starting current.
  2. Transition: After a few seconds, the Star contactor must de-energize.
  3. Run: Immediately after the Star contactor switches off, the Delta contactor must energize. This reconnects the motor windings in a delta configuration for normal full-power operation. The Main contactor remains energized throughout this entire process.

The most dangerous moment is the transition between the star and delta connections. If the Star and Delta contactors were ever energized at the same time, it would create a direct, high-current short circuit between the power phases, leading to catastrophic failure. Our control circuit's primary safety function is to make this scenario impossible.

The Solution: Electrical Interlocking

To prevent this dangerous condition, we implement an electrical interlock. The logic is simple and robust:

  • The circuit for the Star contactor will only be complete if the Delta contactor is OFF.
  • The circuit for the Delta contactor will only be complete if the Star contactor is OFF.

We achieve this by wiring a normally closed (NC) auxiliary contact from each contactor into the coil circuit of the other. The following animation clearly explains the risk and the interlocking solution.

Star Delta Control Circuit Wiring Explained | Animated | Easy

This animated video from the Baz Controls channel provides an excellent visualization of why the interlock is necessary and how it's implemented.

First, watch the segment from the beginning to understand the short-circuit risk. Then, continue watching from that point to see how normally closed auxiliary contacts are used to create the electrical interlock.

As you saw, adding an NC contact from the delta contactor in series with the star coil (and vice versa) creates a fail-safe mechanism. If the delta contactor is on, its NC contact is open, cutting power to the star coil. It's a physical guarantee that both cannot be energized simultaneously.

Building the Star-Delta Control Circuit

Now, let's construct the complete control circuit diagram. We'll build upon the latching circuit from the previous lesson and integrate the sequencing and interlocking logic. The circuit below is a standard representation of an automatic star-delta starter control.

A typical control circuit for an automatic star-delta starter. K1 is the Main contactor, K2 is the Delta contactor, K3 is the Star contactor, and T1 is the On-Delay Timer.

Let's trace the operation of the circuit shown in the "Control Circuit Diagram of Automatic Star / Delta Starter" image:

  1. Start & Latch: When the ON pushbutton is pressed, current flows to the coil of the Main contactor (K1) and the Timer (T1). The normally open (NO) auxiliary contact of K1 (terminals 13 & 14) closes, creating a latching path around the ON button, just as we saw in the previous lesson. The system is now energized and latched.

  2. Star Connection: With the main circuit latched, current also flows into the sequencing part of the circuit. Initially, the timer's internal contact T (terminals 55 & 56) is in its normally closed state. Power flows through this NC timer contact, then through the NC interlock contact of the Delta contactor (K2), and finally to the coil of the Star contactor (K3). Both K1 (Main) and K3 (Star) are now on, and the motor starts in the star configuration.

  3. Timer Action & Transition: The timer T1 has been counting since it was energized in step 1. When the preset time (e.g., 10 seconds) elapses, its contacts switch state. The NC contact (55-56) opens, and the NO contact (67-68) closes.

  4. Delta Connection:

    • When the timer's NC contact (55-56) opens, it cuts power to the Star contactor (K3), which de-energizes.
    • Simultaneously, the timer's NO contact (67-68) closes. Power now flows through it, then through the NC interlock contact of the Star contactor (K3), and finally to the coil of the Delta contactor (K2). The Delta contactor energizes, and the motor now runs in the delta configuration.

Notice how the interlocks work:

  • The path to the star coil K3 is only complete if K2 is off (its NC contact is closed).
  • The path to the delta coil K2 is only complete if K3 is off (its NC contact is closed).

This simple, elegant use of auxiliary contacts is the foundation of safe sequential control in motor starters. For a detailed textual breakdown of this sequence, the following resource is very helpful.

Automatic Star Delta Starter - Power, Control & Wiring Diagram

This article from Electrical Technology provides a step-by-step written description of the circuit's operation.

Please read the section titled Operation & Working. This text describes the same sequence we just traced, which will help reinforce your understanding of the logic flow. Note that the component labels (K1, K2, K3) may differ from our diagram, but the principle is identical.

To see the entire process in a fluid animation, let's return to the video from Baz Controls.

Star Delta Control Circuit Wiring Explained | Animated | Easy

This animation puts all the pieces together, showing the complete sequence from start-up to the final delta running state.

Watch the animated demonstration from this point to see the circuit in action. Then, watch the detailed step-by-step walkthrough from the final segment, which breaks down the entire sequence slowly, highlighting the latching and interlocking actions.

This circuit, including the start/stop logic, the timer, and the crucial electrical interlock, is exactly what you will need to implement in EPLAN for the 15kW conveyor motor.

Conclusion

In this lesson, you have advanced from a simple single-contactor control circuit to the more complex sequential logic required for a star-delta starter. You have learned how to build upon a basic latching circuit to control multiple contactors in a specific order.

Key Takeaways:

  • A star-delta control circuit sequences three contactors: Main, Star, and Delta.
  • The Main and Star contactors energize first, followed by a transition where the Star de-energizes and the Delta energizes.
  • An electrical interlock is a critical safety feature that prevents the Star and Delta contactors from being energized simultaneously.
  • The interlock is created by wiring a Normally Closed (NC) auxiliary contact from each contactor in series with the coil of the other.

We've treated the timer as a key component for sequencing, but we haven't explored it in depth. In our next lesson, we will focus specifically on the on-delay timer, examining how its properties and contact arrangements make it the perfect device for managing the automatic switching sequence in this circuit.