Hello! Welcome to the next step in our journey. In the previous lesson, you successfully performed a critical engineering task: calculating the full-load current for the 3 kW exhaust fan motor and selecting the appropriate MPCB and contactor from a manufacturer's catalog. You now have the specific components for the job.
Today, we'll bridge the gap between that parts list and a functional design by creating the power circuit diagram for the Direct-On-Line (DOL) starter. This diagram is the master plan for your motor's power wiring. It shows how the high-voltage, high-current power flows from the supply, through the protective and switching devices, and finally to the motor itself. Mastering this fundamental diagram is essential before we dive into the EPLAN software.
Understanding the Power Circuit Diagram
An electrical schematic is typically divided into two parts: the power circuit and the control circuit.
- The Power Circuit (our focus today) handles the heavy lifting. It's drawn with thick lines and shows the main current path for the three-phase supply (L1, L2, L3) to the motor.
- The Control Circuit (which we'll cover later) is the "brain." It operates at a lower voltage and uses signals from pushbuttons, timers, and sensors to tell the power circuit's components (like contactors) when to switch on or off.
For now, we will focus exclusively on getting power to the motor safely.
The Key Components and Their Symbols
Every component in an electrical schematic is represented by a standard graphical symbol. Since your background is in electrical engineering, these might be familiar, but a quick review is always helpful. The components we selected for the 3 kW motor are:
- MPCB (Motor Protection Circuit Breaker): Our Schneider Electric
GV2ME14. This single device provides both short-circuit protection (magnetic trip) and overload protection (thermal trip). - Contactor: Our Schneider Electric
LC1D12. This is an electrically operated switch. When its coil is energized by the control circuit, it closes its main power contacts to let current flow to the motor. - Motor: The 3 kW, 400V three-phase induction motor for the exhaust fan.
The following resource provides an excellent overview of the standard symbols used in motor control circuits.
[PDF] Motor Control Handbook - NHP
This handbook from NHP is a great reference for standard motor control practices. We'll use it to familiarize ourselves with the common graphical symbols.
Please review pages 16 and 17. On page 16, locate the symbols for a Circuit breaker and Contactor. On page 17, locate the symbol for a Thermal over-load contact. These are the fundamental building blocks of our diagram.
Assembling the DOL Power Circuit
Now, let's connect these symbols to create the DOL power circuit. The logic is simple and follows the flow of power from the source to the load.
- Source: The circuit begins with the three-phase power supply, labeled L1, L2, and L3.
- Protection: The first component after the supply is always the main protective device. In our case, this is the MPCB (
-Q1). Its purpose is to disconnect the entire circuit in case of a major fault like a short circuit. - Switching: Next comes the contactor (
-K1). We only show its three main power contacts in the power diagram. These are normally open, meaning they only close and allow current to pass when the contactor's coil is energized. - Overload Protection: After the contactor, we place the overload protection (
-F1). Although our MPCB has this function built-in, in many diagrams, it is shown as a separate symbol for clarity. This device specifically protects the motor from drawing too much current over a period of time, which would cause it to overheat. - Load: Finally, the power is delivered to the three-phase motor (
-M1).
The image below shows this exact sequence in a standard IEC diagram, along with the physical components for reference. The power circuit is on the left, and the control circuit is on the right.
Now, let's look at a professionally drawn schematic for a DOL starter. This is the standard you should aim for.
[PDF] Motor Control Handbook - NHP
Let's return to the NHP Motor Control Handbook to see the complete, standard DOL power circuit diagram.
On page 45, examine the diagram titled "Main circuit". Notice the clear top-to-bottom flow of power: L1, L2, L3 -> Q1 (Circuit Breaker/MPCB) -> K1 (Contactor) -> F1 (Overload Relay) -> M1 (Motor). Also, note the standard IEC terminal numbering (e.g., 1/3/5 for inputs, 2/4/6 for outputs).
Visualizing the Circuit in Operation
To solidify your understanding of how these components interact, the following video provides an excellent animated walkthrough of a DOL starter in action.
DOL Starter | Direct On-Line Starter | Explain with Circuit Diagram
This video from ISCA Engineering provides a clear, animated explanation of how a DOL starter works.
Please watch the following segments: First, watch the animation from this timestamp. It will show you the flow of power as the start button is pressed and what happens during an overload trip. Then, watch the component breakdown from this segment for a refresher on the role of the circuit breaker, contactor, and overload relay.
From Paper to EPLAN
The diagram we have just constructed is the blueprint we will use in EPLAN. In the software, you won't be "drawing lines" so much as "placing symbols and connecting them logically." The software then helps manage the connections, numbering, and cross-references.
Here is a screenshot of what a completed DOL power circuit looks like inside EPLAN. This is exactly what you will soon be creating.
The process for creating this in EPLAN involves inserting symbols from a library and connecting their pins. The following tutorial provides a glimpse into this process. You don't need to perform these steps now, but it's useful to see where we are headed.
Getting Started With EPLAN - How to Layout a Basic Electrical Panel ...
This tutorial demonstrates the practical steps of creating a power circuit in EPLAN. While it shows a more complex Star-Delta circuit, the fundamental actions are identical for our DOL starter.
Briefly scroll through the section titled "EPLAN – Power Circuit Diagram" to get a feel for the workflow. Pay attention to how the user inserts symbols for circuit breakers and contacts from the symbol library and places them onto the schematic page. This is the core activity of schematic design in EPLAN.
Conclusion
In this lesson, you have learned how to translate a list of selected components into a standard electrical power circuit diagram for a DOL motor starter.
Key Takeaways:
- A power circuit illustrates the high-current path from the three-phase supply (L1, L2, L3) to the motor.
- The standard component sequence for a DOL starter is: Protection (MPCB) -> Switching (Contactor) -> Motor. An overload relay is also included in this path, either as part of the MPCB or as a separate unit.
- Each component has a standard graphical symbol and follows IEC terminal numbering conventions (e.g., inputs 1, 3, 5; outputs 2, 4, 6).
- This manually-drawn schematic is the essential blueprint you will implement in EPLAN.
You now have a solid understanding of both the components required and the schematic layout for your 3 kW exhaust fan. In the next lesson, we will apply these same principles to a more advanced starter type: the Star-Delta power circuit for the 15 kW conveyor motor.