Thyristor soft starter for a three-phase induction motor

An induction motor started direct on line draws six to seven times its full load current, and that inrush dips the supply voltage, shocks the coupled machinery and shortens the motor's life. A soft starter reduces the applied voltage during acceleration by phase-controlling back-to-back thyristors in each line, ramping the firing angle down until the motor reaches full voltage. This project builds one for a small three-phase motor and measures the starting current envelope against a direct-on-line start and a star-delta start on the same machine, which is the comparison that gives the report its results.

Electrical & Electronics (EEE)Machines & drivesPower electronics & convertersProtection & switchgearHardware build

Rs 12,928 to Rs 31,877

All-in cost

5.2 weeks

With 3 of you

B.E. mini, B.E. final year

Usually taken as

Parts on the list

What it will really cost

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.

Split betweenstudents

Line	Cost
Components and materials From the bill of materials	Rs 9,746 to Rs 23,840
Spares and replacements 15 % — you will destroy something	Rs 1,462 to Rs 3,576
Consumables and hardware Wire, connectors, headers, screws, heatshrink	Rs 1,170 to Rs 2,861
Perfboard and assembly	Rs 150 to Rs 400
Report, printing and binding Two or three bound copies, plots and the plagiarism check	Rs 400 to Rs 1,200
Total	Rs 12,928 to Rs 31,877
Each, split 3 ways	Rs 4,309 to Rs 10,626

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 5.2 week figure assumes 12 person-weeks of work shared between 3; students past 4 add coordination rather than output, so the estimate stops improving there.

Block diagram

Supply / source
Sensing
Control
Power stage
Load / output
Feedback path

The same chain in words

1Three-phase 415 V supply through a 4 pole MCB
2Back-to-back thyristor pairs in each of the three lines
3Zero-crossing detection on each phase to give the firing reference
4Pulse transformers or opto-triacs isolating the gate drive from the controller
5Microcontroller running the firing angle ramp and the timing
6Current transformers on each line, logged during the start
7Bypass contactor closing across the thyristors once the motor is up to speed
8Three-phase induction motor with a loading arrangement

How it works

Each line has two thyristors connected back to back so both half cycles can be controlled. Delaying the firing point within each half cycle reduces the RMS voltage the motor sees.

The controller detects each phase's zero crossing and fires its thyristors at a delay measured from that instant. Starting from a large delay applies a small voltage; reducing the delay toward zero applies full voltage.

The firing angle ramps down over the set start time. The motor accelerates on reduced voltage, so the current drawn is far lower than a direct start, and the mechanical shock to the driven machine is much gentler.

Once the motor is at speed the bypass contactor closes across the thyristor stack. The thyristors are then out of circuit, so their conduction losses and the harmonics they cause disappear for the whole of the running period.

Current transformers log the current envelope through the start, and the same is repeated with the thyristors bypassed for a direct-on-line reference start.

Design calculations

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.

Motor full load current

1.63 A for a 0.75 kW motor at 415 V

I = P / (sqrt(3) x V x pf x efficiency)

750 / (1.732 x 415 x 0.8 x 0.8) = 1.63 A. Always use the nameplate figure if there is one; this calculation is for sizing when there is not.

Direct-on-line inrush

About 11 A for the same motor

Starting current is typically 6 to 7 times full load current

This is the number the soft starter exists to reduce, and it is what the current transformer logs will show.

Torque against voltage

At 50 percent voltage the motor develops 25 percent of its torque

Torque is proportional to the square of the applied voltage

This is the central constraint of the whole method. Current falls in proportion to voltage but torque falls as its square, so reducing the voltage too far leaves a loaded motor unable to break away — and a motor that cannot accelerate sits at locked rotor current, which is worse than the direct start you were avoiding.

Initial voltage and ramp

Start at 40 percent voltage, ramp to 100 percent over 5 s

Start above the voltage that develops breakaway torque

Forty percent gives 16 percent torque, which is enough for an unloaded or fan-type load but not for a loaded conveyor. The starting voltage must be chosen from the load's breakaway torque, and the report should say which load it was set for.

Firing angle range

About 150 degrees at start, ramping to near 0

Delay measured from the zero crossing, in electrical degrees

One millisecond of delay is 18 electrical degrees at 50 Hz. The relationship between firing angle and RMS voltage is not linear, so the ramp in angle is not the same shape as the ramp in voltage — computing one from the other is a design step, not an approximation.

Thyristor rating

16 A modules for a 1.63 A motor

Rate for the surge, not for the running current

The devices carry the full starting current for several seconds with no thermal help from a bypass, so they are chosen for I squared t capability and for the peak inverse voltage of a 415 V system, which needs at least 1200 V devices.

Bill of materials

Item	Qty	Unit cost
Thyristor modules Back-to-back SCR pairs, 16 A, 1200 V, one per line	3	Rs 450 to Rs 1,400
Gate drive MOC3021 opto-triac drivers or pulse transformers	6	Rs 30 to Rs 110
Microcontroller board Arduino Mega or STM32 with three capture inputs	1	Rs 350 to Rs 900
Zero-crossing detectors PC817 with series resistors, one per phase	3	Rs 12 to Rs 40
Current transformers Split core CT, 20 A, one per line, with burden resistors	3	Rs 250 to Rs 700
Bypass contactor 9 A, 3 pole, 230 V AC coil	1	Rs 500 to Rs 1,300
Snubber components 100 Ohm 2 W with 0.1 uF 630 V film, per device	3	Rs 40 to Rs 120
Heatsinks Extruded heatsinks with thermal pads for the modules	3	Rs 120 to Rs 400
Three-phase motor 0.75 kW (1 HP), 415 V, 4 pole Almost always borrowed from the machines lab — check before budgeting for one.	1	Rs 4,500 to Rs 9,000
MCB and enclosure 16 A four pole MCB, metal panel with DIN rail	1	Rs 1,200 to Rs 3,000
Wiring and terminals 2.5 sq mm wire, ferrules, terminal blocks, lugs	1	Rs 400 to Rs 1,000

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 8,428 at the low end without them.

What you need to be able to do

Skills

Embedded C / Arduino
Mains & 3-phase wiring

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.

Equipment and access

Soldering stationrequired
CRO / DSO
Power electronics labrequired
Machines lab (motors, loading)required

Anything marked required is a blocker, not a difficulty. Arrange the access before you commit to the project, not after.

What goes wrong

This project switches 415 V and needs a real three-phase motor. It cannot be done safely outside a lab with a supervised supply, an isolator and an RCD. Confirm you have machines lab access before choosing it, because the alternative is an unfinished project.
Thyristors fail short circuit, not open. A failed device puts full line voltage through with no control at all, so an upstream MCB sized for the motor is not optional.
The dv/dt across a thyristor can turn it on without a gate pulse, which produces a half-cycle of uncontrolled current. Fit the RC snubbers from the first build, not after the first unexplained bang.
Reducing the voltage too far on a loaded motor is worse than a direct start, because the motor cannot accelerate and sits at locked rotor current for the whole ramp. State the load you designed for.

How to make it a stronger project

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.

Add current feedback so the ramp adjusts to hold a current limit rather than following a fixed time ramp — this is what commercial soft starters actually do.
Add a soft stop for pump loads and demonstrate the reduction in water hammer, measured with a pressure sensor.
Measure the harmonic content of the line current during the ramp and discuss why the bypass contactor matters for supply quality as well as for losses.

Writing the report

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 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. A final year report is normally 60 to 90 pages with a substantial literature survey, and many departments expect a paper submission alongside it.

The full report guide — front matter, conventions and page counts

Abstract

One 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.

Introduction

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.

Literature survey

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.

Methodology and block diagram

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.

Design and calculations

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.

Hardware and software implementation

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.

Results and discussion

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.

Conclusion and future scope

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.

References and appendices

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.

Viva questions with answers

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.

Why does an induction motor draw such a large current at start?

At standstill the slip is one, so the rotor conductors are cut by the rotating field at full supply frequency and the rotor behaves like a short-circuited secondary of a transformer. The impedance seen by the supply is the leakage impedance alone, with no back EMF from rotation to oppose it, so the current is limited only by that small impedance. As the rotor speeds up the slip falls and the effective rotor resistance rises, so the current comes down.

How does a soft starter differ from a variable frequency drive?

A soft starter only varies voltage, at fixed supply frequency, and it is bypassed once the motor is running. A VFD varies voltage and frequency together, so it controls speed continuously throughout operation and it can develop full torque at any speed. A soft starter is much cheaper and is used where only the start needs controlling.

Why is torque proportional to the square of voltage?

The airgap flux is proportional to the applied voltage, and the rotor current induced is also proportional to that flux. Torque is the product of flux and rotor current, so it goes as the square of the voltage. That is why halving the voltage leaves only a quarter of the torque available.

What is the advantage of a soft starter over star-delta starting?

Star-delta gives one fixed step: about a third of the direct-on-line current and a third of the torque, followed by a transition that itself produces a current and torque transient as the motor is briefly disconnected and reconnected. A soft starter ramps continuously, so there is no transition transient, the starting voltage can be tuned to the load, and the ramp time is adjustable.

Why bypass the thyristors once the motor is running?

Because each conducting thyristor drops around a volt and carries the full motor current continuously, which is a real power loss and a lot of heat for the life of the run. Bypassing also removes the harmonic distortion that phase control injects into the supply. The thyristors are only needed during the few seconds of the start.

Why must thyristors be rated far above the motor's full load current?

Because during the ramp they carry the starting current, several times full load, for seconds at a time with no bypass to help them. The limiting parameter is the surge and I squared t rating rather than the continuous rating, and the voltage rating has to cover the peak of the line-to-line voltage plus switching transients.

Tools for the calculations above

Motor full load current calculator 3 phase power calculator Overload relay setting calculator Induction motor load test

Normally taken as a B.E. mini project (5th/6th sem) or B.E. final year project (7th/8th 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.