Star-delta starting is the standard reduced-voltage start for a motor whose load is light at start-up, and implementing it in a PLC rather than with a mechanical timer turns it from a wiring exercise into a control project. This build runs the full sequence in ladder logic: main contactor and star contactor close together, the motor accelerates in star at a third of the direct-on-line current, the star contactor drops out, a deliberate dead time lets the arc clear, and the delta contactor closes. Around that sit the interlocks and the fault handling — because the one thing this circuit must never do is close the star and delta contactors together.
Not just the parts. Teams budget from a component list, then run out in week six on wire, connectors and the driver board they destroyed. These are the lines that catch them.
| Line | Cost |
|---|---|
Components and materials From the bill of materials | Rs 10,340 to Rs 41,020 |
Spares and replacements 15 % — you will destroy something | Rs 1,551 to Rs 6,153 |
Consumables and hardware Wire, connectors, headers, screws, heatshrink | Rs 1,241 to Rs 4,922 |
Report, printing and binding Two or three bound copies, plots and the plagiarism check | Rs 400 to Rs 1,200 |
| Total | Rs 13,532 to Rs 53,295 |
| Each, split 3 ways | Rs 4,511 to Rs 17,765 |
Indicative bands last checked in August 2026 — a guide, not a quotation. Prices move and vary by seller, and anything you can borrow from the lab comes straight off the total. The 3 week figure assumes 7 person-weeks of work shared between 3; students past 4 add coordination rather than output, so the estimate stops improving there.
The same chain in words
On the start command the PLC energises the main and star contactors together. The motor windings are connected in star, so each winding sees the line voltage divided by root three.
A timer runs for the star period while the motor accelerates. The period is set from how long the machine takes to approach its running speed on the actual load, not from a default value.
At the end of the star period the PLC de-energises the star contactor and starts a short dead-time timer. Nothing is energised into the delta path during this interval.
After the dead time the delta contactor is energised, the windings are reconnected in delta, and the motor runs at full voltage.
The overload relay trip contact, the emergency stop and both interlock auxiliaries are inputs to the logic, and any of them drops the whole sequence. The interlocks are also wired physically in the coil circuits, so the safe behaviour does not depend on the program being correct.
This is the chapter that separates an engineering project from an assembly job. Every value below carries the formula that produced it and the assumptions it rests on — check them against your own ratings before you use them, because your motor is not this motor.
Star line current is one third of the delta line current
In star each winding sees V line divided by root three, so its current is down by that factor; and the line current in star equals the winding current, whereas in delta the line current is root three times the winding current. The two factors multiply to one third.
Torque is proportional to the square of winding voltage
This is why star-delta only suits loads that are light at start — pumps with a closed discharge valve, fans, unloaded compressors. A conveyor already carrying material will not accelerate in star, and the motor will sit drawing star-connected locked rotor current until the timer expires.
I = P / (sqrt(3) x V x pf x efficiency)
3700 / (1.732 x 415 x 0.83 x 0.85) = 7.3 A. Use the nameplate value where one exists.
Main and delta contactors carry winding current, which is line current over root three
7.3 / 1.732 = 4.2 A, and the star contactor carries a third of the line current. In practice all three are bought as the same 9 A frame size, but knowing why they need not be identical is exactly what gets asked in the viva.
Longer than the star contactor's release and arc-clearing time
Closing delta before star has fully opened puts the star point and the delta connection on the line at once, which is a direct short across the supply. This delay is the entire reason open transition starters exist.
Set from the machine's actual run-up time on its load
Too short and the transition happens at low speed, producing a current surge almost as large as a direct start. Too long and the motor sits at reduced torque, heating up without accelerating. Measure the run-up with a tachometer and set the timer from that.
| Item | Qty | Unit cost |
|---|---|---|
PLC Micro PLC, at least 8 digital inputs and 6 outputs Almost always the department's trainer kit — confirm availability before budgeting. | 1 | Rs 4,500 to Rs 14,000 |
Contactors 9 A, 3 pole, 230 V AC coil, with auxiliary contact blocks | 3 | Rs 500 to Rs 1,300 |
Mechanical interlock kit Matched to the contactor series | 1 | Rs 250 to Rs 700 |
Overload relay Thermal overload, adjustable, set to the winding current | 1 | Rs 600 to Rs 1,600 |
MCB and isolator 16 A four pole MCB, rotary isolator | 1 | Rs 900 to Rs 2,200 |
Pushbuttons and E-stop 22 mm start, stop and latching emergency stop | 1 | Rs 350 to Rs 900 |
Indicator lamps 22 mm LED indicators for star, delta, run and trip | 4 | Rs 60 to Rs 180 |
Three-phase motor 3.7 kW (5 HP), 415 V, with all six terminals available Machines lab motor. A motor with only three terminals brought out cannot be star-delta started at all. | 1 | Rs 0 to Rs 12,000 |
Panel and DIN rail Metal panel 400 x 500 mm, DIN rail, trunking | 1 | Rs 1,500 to Rs 3,800 |
Wiring and terminals 2.5 sq mm wire, ferrules, terminal blocks, lugs | 1 | Rs 500 to Rs 1,200 |
Specifications are written as ratings rather than brands, so any equivalent part works. Where a line says to borrow from the lab, do — it is usually the largest number on the page, and the total drops to Rs 9,032 at the low end without them.
None of these blocks you — they are learnable inside a semester. Each one you do not have costs a couple of weeks, so count them into your plan.
Anything marked required is a blocker, not a difficulty. Arrange the access before you commit to the project, not after.
Use these when your guide says the scope is too small, or when you want something in the results chapter that nobody else in your batch will have.
The structure below is what a project report is marked against, with the mistake that costs marks in each chapter. There is no template to download and there will not be one: the writing is your work, and a site that did it for you would be helping you cheat rather than helping you pass.
A diploma report is typically 40 to 60 pages. The design and results chapters carry the marks; the literature survey can be short. A mini project report runs to about 40 to 50 pages, and the literature survey is usually brief. Guides expect one clear objective, met and measured.
The full report guide — front matter, conventions and page countsOne paragraph covering what the problem is, what you built, and what the result was. Written last, even though it appears first.
Loses marks: Describing the problem and the method but never stating a result. If there is no number in your abstract, it is not finished.
Why the problem matters, the context it sits in, and a clear statement of the objectives your project set out to meet.
Loses marks: Objectives written so vaguely that nobody can tell at the end whether they were met. Write objectives you can tick off in the conclusion.
What has already been done, what each approach achieved, and the specific gap your work addresses.
Loses marks: A list of paper summaries with no comparison and no gap. The survey exists to justify your design choice, so end it by saying what you chose and why.
The system as a whole: the block diagram, what each block does, and the signal or power path between them.
Loses marks: A block diagram copied from a datasheet or another report. Draw yours to match what you actually built, including the parts that are inconvenient.
Every component value and rating, with the formula, the assumptions and the arithmetic that produced it. This is the chapter that distinguishes an engineering project from an assembly job.
Loses marks: Component values with no justification. If a resistor is 10 kOhm, the report must say why it is not 1 kOhm.
The circuit as built, the PCB or wiring, the flowchart, and the parts of the code that carry the actual logic.
Loses marks: Twenty pages of pasted source listing. Include the flowchart and the few functions that matter, and put the full listing in an appendix.
What you measured, in tables and graphs, compared against what you predicted in the design chapter — including where they disagreed.
Loses marks: Photographs of the working model presented as results. A photograph is evidence that it exists, not evidence that it works. Results are measurements.
Whether each objective was met, what the measured performance was, and what the honest next step would be.
Loses marks: Future scope written as a wish list of unrelated features. It should follow from a limitation you actually hit.
Every source in a consistent style, plus datasheets, full code listings and the complete bill of materials.
Loses marks: Citing a blog or a video where a datasheet or a standard exists. Cite the primary source for anything you relied on.
The demonstration is half the assessment. These are the questions this project invites, and the answers are written the way you would give them out loud.
Two effects multiply. In star each winding sees the line voltage divided by root three, so the winding current falls by that factor. And in star the line current equals the winding current, whereas in delta the line current is root three times the winding current. Together those give one third of the line current that a delta connection would draw.
Torque is proportional to the square of the winding voltage. In star that voltage is down by root three, and the square of one over root three is one third. So current and torque both fall to a third — the ratio of torque to current is unchanged, which is a fundamental limit of any reduced-voltage starting method.
Open transition disconnects the motor completely during the changeover, so the motor is momentarily unpowered and its residual flux can be out of phase with the supply when delta closes, producing a current surge. Closed transition keeps the motor connected through resistors during the changeover, so there is no disconnection and no surge, at the cost of extra contactors and a resistor bank.
Because a contactor takes tens of milliseconds to physically open and its arc takes longer still to extinguish. If delta closes while the star contacts are still conducting, the star point and the delta connections are both on the supply at once, which is a phase-to-phase short. The dead time guarantees that cannot happen.
Because in a star-delta starter the overload relay is placed in the winding circuit, not the line, so the current flowing through it during normal running is the line current divided by root three. Setting it to the nameplate line current would mean it never trips until the motor is drawing about 1.7 times its rated current, which is not protection.
Because a program can fail in ways wiring cannot — a corrupted output image, a scan that hangs with an output latched, or an output card whose transistor has failed short. The hardware interlock makes the dangerous state physically impossible regardless of the controller's state, and that is the principle behind every safety interlock in industrial practice.
Normally taken as a Diploma project (C-20, 6th sem) or B.E. mini project (5th/6th sem) project. If yours is a different course, the extensions above scope it up and the simpler half of the design scopes it down — the arithmetic does not change. Back to all projects.