Star-Delta Control with On-Delay Timer

The Role of the On-Delay Timer

The star-delta starting sequence requires a specific pause after the motor starts before switching to the full running configuration. This pause allows the motor to accelerate to about 80-90% of its rated speed while in the star connection, which significantly softens the current surge during the final switch to delta. An on-delay timer is the perfect device for managing this pause.

Its operation is defined by a "delay on energization":

1. When power is applied to the timer's coil, its internal clock begins to count down from a preset time.
2. Crucially, its contacts—both Normally Open (NO) and Normally Closed (NC)—remain in their normal, unpowered state during this countdown.
3. Once the time elapses, the contacts change state: the NO contact closes, and the NC contact opens.
4. The contacts will hold this new state for as long as the coil remains energized.
5. When power is removed from the coil, the contacts instantly snap back to their normal state.

This behavior allows us to create a precise, automated sequence: use the initial state to run the star contactor, and the timed (delayed) state to run the delta contactor.

Integrating the Timer into the Control Circuit

Let's modify our control circuit to include the timer. The diagram below shows a standard implementation. Here, K1 is the main contactor, K2 is the delta contactor, K3 is the star contactor, and K1T is the on-delay timer.

Let's trace the flow of this automated circuit:

1. START & Latch: When the START button (S2) is pressed, current flows to the main contactor coil (K1) and, in parallel, to the on-delay timer coil (K1T). The main contactor's auxiliary contact K1:13-14 closes, latching the circuit. At this moment, both K1 and K1T are energized, and the timer starts its countdown.

2. Star Connection (t=0): Since the timer has just been energized, its timed contacts are still in their normal state. Current flows through the timer's Normally Closed (NC) contact K1T:17-18 (note: the diagram labels it NO, which is a common but confusing convention; it's a timed NO contact that is initially closed, or more simply, a timer's NC contact). This current then flows through the delta contactor's NC interlock (K2:21-22) and energizes the star contactor coil (K3). The motor is now running with the main and star contactors engaged.

3. Delta Connection (t > delay): After the preset time (e.g., 10 seconds), the timer's contacts "time out" and switch state.

   • The NC contact (K1T:17-18) opens, cutting power to the star contactor coil (K3). The star contactor de-energizes.
   • Simultaneously, the timer's Normally Open (NO) contact (not explicitly shown in this simplified diagram, but its action is what drives the next step) closes. In the diagram shown, this is represented by the logic path opening for K3 and closing for K2. A more explicit diagram would show current flowing through the timer's timed NO contact, then through the star contactor's NC interlock (K3:21-22), energizing the delta contactor coil (K2).

The motor is now running in its final delta configuration, with the main (K1) and delta (K2) contactors engaged. The entire sequence happened automatically after a single press of the START button.

To see this wiring performed with physical components, the following video provides a clear, practical demonstration.

Setting the Timer: An Engineering Decision

The timer's delay isn't arbitrary. It's a key engineering parameter.

• Too short: If the switch to delta happens too early, the motor won't have reached sufficient speed. The result is a high current spike, similar to a DOL start, defeating the purpose of the starter.
• Too long: If the motor runs in the star connection for too long, it can overheat, as it's not designed for continuous operation with reduced voltage. It's also inefficient.

The goal is to set the timer just long enough for the motor to reach about 80-90% of its rated speed. For our project's 15kW (~20 HP) motor, a typical setting would be between 7 and 12 seconds, depending on the conveyor's load. The following guide provides excellent practical advice on determining the right setting.

The Complete Control Logic

By adding the on-delay timer, we've finalized the control logic for our 15kW conveyor motor. This logic, which you will soon be implementing in EPLAN, combines the latching circuit, safety interlocks, and automated sequencing.

The video below offers one more clear, animated explanation of the entire control circuit, reinforcing how all the components work together.

Conclusion

In this lesson, you've moved beyond the basic interlocking of a star-delta starter and have now mastered the method for automating its operation. By incorporating an on-delay timer, the circuit now correctly sequences the contactors with a single command from the operator.

Key Takeaways:

• An on-delay timer provides a time delay after its coil is energized before its contacts change state.
• In a star-delta starter, the timer coil is energized simultaneously with the main contactor.
• The timer's NC contact controls the star contactor, allowing it to run during the initial countdown.
• The timer's NO contact controls the delta contactor, energizing it only after the time delay has elapsed.
• The timer setting is a critical parameter based on the motor's acceleration time under load.

Our control circuits for both the DOL and star-delta starters are now functionally complete. The next and final step in our theory module is to add visual feedback for the operator. In the next lesson, we will specify and wire the fault and status indication lamps that report what the circuit is doing.