TST Engineering Services | Module 25 3-Phase AC Induction Motors Infographic Expanded

Introduction & Module Route Map

Module layout

This opening page has been simplified. Use the route map below to work through the topic in logical sections: first understand the motor itself, then compare each starting method and its protection philosophy.

Part A — Motor Construction

Identify the stator, rotor, air gap, rotor cage, end rings, shaft, bearings and cooling arrangement, then explain how the rotating magnetic field is produced.

Part B — Starting Methods

Compare DOL, star-delta, soft starter, VFD and autotransformer starting. Discuss inrush current, starting torque, transition method and typical suitability.

Part C — Protection & Oral Points

Cover overloads, short-circuit protection, undervoltage release, phase failure, thermal sensors, interlocking, VFD cable considerations and safe test / isolation practice.

1. Induction Motor Master Overview

Core principles

This overview introduces the complete operating picture: rotating magnetic field, asynchronous operation, slip, starting current, power factor and the reason induction motors are so widely used for marine auxiliaries.

2. Construction & Operating Principle

Motor anatomy

The construction diagram is kept wide and horizontal, while the explanatory notes have been moved into larger HTML boxes below so the text fits cleanly with no crowding.

1. Construction

The stator core carries the distributed 3-phase windings that create the rotating magnetic field. The squirrel-cage rotor consists of rotor bars and end rings mounted on a laminated rotor core fixed to the shaft.

2. Why it turns

A balanced 3-phase supply produces a rotating magnetic field. This field cuts the rotor conductors, induces EMF and rotor current, and the interaction between the stator field and rotor field produces torque.

3. Slip

The rotor must run below synchronous speed. If there were no slip, there would be no relative flux cutting, no induced rotor EMF, no rotor current and therefore no torque. That is why the induction motor is asynchronous.

4. Inductance & power factor

The windings store energy in the magnetic field and oppose changes in current. The magnetising current needed to establish flux makes the induction motor a lagging power-factor load, especially when lightly loaded.

Main construction parts

Frame / housing and terminal box
Laminated stator core with distributed 3-phase windings
Uniform air gap between stator and rotor
Squirrel-cage rotor bars and end rings
Rotor core, shaft, bearings and cooling fan

Protection devices

Fuses or MCCB for short-circuit protection
Thermal overload relay for sustained overload
Phase failure / single-phasing protection
Undervoltage or no-volt release
PTC or RTD winding temperature protection on larger motors

Formulae: Ns = 120f / P • Slip = (Ns − Nr) / Ns

3. Starting Methods Overview

Selection logic

The chosen starting method depends on available generator capacity, acceptable voltage dip, required starting torque, mechanical load, whether speed control is needed, and how complex the system is allowed to be.

Method	Main purpose	Starting current	Starting torque	Best suited to
DOL	Simple full-voltage start	Highest	High	Small/medium motors or strong electrical systems
Star-Delta	Reduced-voltage start using star connection	Reduced	Low / about one-third DOL	Loads that can accelerate with reduced torque
Soft Starter	Smooth acceleration with voltage ramp or current limit	Controlled	Adjustable but reduced	Pumps, fans, conveyors, fixed-speed duties needing smooth start
VFD	Start + full speed control using variable frequency	Lowest / fully controlled	Controlled through V/f or vector strategy	Variable-speed services and energy saving
Autotransformer	Reduced-voltage start with selectable taps	Reduced	Better matched than star-delta	Larger motors needing reduced current but better torque flexibility

4. Direct-On-Line (DOL) Starting

Full-voltage start

DOL applies full line voltage immediately through the main contactor. It is the simplest and cheapest method but produces the highest starting current and the greatest mechanical shock. The diagram below also shows a reversing arrangement by swapping two phases.

How it works

Start pushbutton energises the main contactor coil.
Main contacts close and full line voltage is applied to the motor.
An auxiliary NO seal-in contact holds the circuit after the start button is released.
Stop button or overload trip opens the control circuit and drops the contactor out.

Advantages / disadvantages

Advantages: simple, robust, cheap, high starting torque.
Disadvantages: highest inrush current, voltage dip risk, mechanical shock to couplings and driven equipment.
Protection: fuses or MCCB, overload relay, undervoltage/no-volt release and phase-failure protection if fitted.

5. Star-Delta Starting

Reduced-voltage start

Star-delta starting reduces current by first connecting the windings in star, then changing over to delta for full-voltage running. It must be correctly interlocked so star and delta contactors can never close together.

Star-delta starter power and line diagram

Star-delta starter auxiliary and control circuit

Sequence

Main contactor and star contactor energise at start.
Each winding receives line voltage divided by √3.
After time delay, star opens, open-transition dead time occurs, then delta closes.
Motor continues running at full line voltage in delta.

Main oral points

Line current and starting torque are reduced compared with DOL.
Starting torque is roughly one-third of DOL.
Suitable only if the load can accelerate with reduced torque.
Open transition prevents overlap but poor timing can cause transients.

Protection / interlocking

Mechanical and electrical interlocks between star and delta.
Timer controls changeover.
Overload relay normally remains in the circuit.
Incorrect overlap can create a phase-to-phase short circuit.

6. Soft Starter & Variable Frequency Drive

Electronic control

A soft starter uses SCRs to ramp voltage during acceleration and deceleration. A VFD rectifies AC to DC and then inverts it to variable-frequency AC, giving both smooth starting and full speed control.

Soft starter

Uses anti-parallel SCRs in each phase.
Can ramp voltage or limit current during start.
Often includes a bypass contactor to reduce heat loss during running.
Does not provide continuous speed control.

VFD

Main power stages: rectifier, DC link and inverter.
Controls motor speed by varying output frequency.
Normally maintains a V/f relationship below base speed.
Useful for pumps and fans where flow can be controlled by speed rather than throttling.

Important VFD oral extras: discuss PWM output, dV/dt or output filters, screened cable practice, EMC, motor cooling at low speed and the possibility of shaft/bearing currents requiring insulated bearings or shaft grounding.

7. Autotransformer Starting

Tap-controlled reduced voltage

The autotransformer section has been rebuilt so the diagram remains horizontal, while the operating notes are moved into larger boxes below. This removes the crowded internal note area and gives the text more space.

Operating sequence

Press START to energise K1 and K2. The motor receives reduced voltage through the selected tap. Timer action then opens K2 and closes K3, transferring the motor to full line voltage for normal running duty.

Advantages

It gives reduced starting current and more flexibility than a simple star-delta arrangement because the selected tap can be matched more closely to the duty and the load characteristic.

Limitations

It uses more switching components than DOL or star-delta and still gives reduced starting torque because torque falls with reduced applied voltage.

Protection devices

Short-circuit protection is provided by HRC fuses or an MCCB. Overload protection is provided by the overload relay. Undervoltage or no-volt release is required, and K2 / K3 interlocking must prevent an incorrect transition.

When to choose it

Use an autotransformer starter when DOL current is too high for the electrical system and when star-delta starting does not provide enough flexibility for the duty or the load characteristic.

8. Final Oral Revision Summary

Exam recap

Construction answer structure

Frame, terminal box, laminated stator core, 3-phase windings, uniform air gap, squirrel-cage rotor, shaft, bearings and cooling fan.

Operation answer structure

Balanced 3-phase supply creates a rotating magnetic field, induces current in the rotor cage, and torque is produced by interaction of stator and rotor fields. Slip is necessary.

Starter selection answer structure

Select the starter based on starting current, torque requirement, supply strength, process need for speed control, mechanical shock limits and system complexity.

TST memory line: DOL is the simplest and hardest on the system; star-delta and autotransformer reduce voltage to reduce current; the soft starter smooths fixed-speed starting; the VFD controls both starting and speed by controlling frequency and voltage.