Global EV Market Growth and Its Impact on Power Electronics Demand

A practical guide for electrical engineers, power-industry professionals, and decision makers.

Introduction: EV Market Growth and Power Electronics

Electric vehicles (EVs) are reshaping the automotive and power sectors. In 2025, global EV sales are projected to exceed 20 million units — a dramatic jump from only a few years earlier. For electrical engineers and decision makers, this rapid EV market growth translates directly into rising power electronics demand.

Why EV Adoption Drives Power Electronics Demand

EVs depend on power electronics at every level: traction inverters, onboard chargers, DC–DC converters, and battery-management circuits. As adoption accelerates, the market size for these components is expected to reach tens of billions of dollars. Each EV sold means additional units of high-value power electronics shipped, tested, and serviced.

Key Power Electronics Components in EVs

• Traction Inverters: Convert DC from the battery to AC for the motor, central to EV performance.
• Onboard Chargers (OBCs): Manage AC charging, now expanding to bidirectional vehicle-to-grid functions.
• DC–DC Converters: Step down high battery voltage to power auxiliaries and infotainment systems.
• High-Voltage Contactors and Power Modules: Ensure safety and reliability under demanding thermal and vibration stress.

Wide-Bandgap Devices: SiC and GaN

The EV boom has accelerated the shift from traditional silicon devices to wide-bandgap semiconductors such as silicon carbide (SiC) and gallium nitride (GaN). These materials enable:

• Higher efficiency and lower energy loss
• Smaller, lighter, and more compact designs
• Higher operating temperatures and simpler cooling

For engineers, mastering SiC and GaN design (gate drivers, EMI control, thermal management) is now a competitive advantage.

Impact on Power Grids and Infrastructure

The rise of EVs affects more than vehicles — it transforms the power grid. Key challenges include:

• Charging load: Fast-charging stations place heavy demand on local distribution networks.
• Vehicle-to-grid (V2G): Bidirectional chargers create opportunities for grid support and renewable integration.
• Power quality: Large-scale EV charging can increase harmonics, requiring advanced converters and filters.

Supply Chain and Manufacturing Considerations

With global EV market growth, supply chains are under pressure. SiC wafers, capacitors, and packaging capacity are critical bottlenecks. OEMs and suppliers need to:

1. Diversify sourcing for key semiconductors
2. Invest in high-voltage testing and assembly
3. Focus on reliability engineering to avoid costly field failures

Strategic Actions for Engineers and Decision Makers

• Upskill teams in SiC/GaN design and simulation
• Perform total cost of ownership analysis for device choices
• Coordinate early with utilities for fleet or depot charging
• Design modular and serviceable power units
• Monitor supply-chain trends and secure long-term agreements

Risks and Forecast Uncertainty

While EV adoption is growing fast, forecasts differ by region and technology readiness. Policy changes, raw material costs, and manufacturing scale for SiC and GaN will shape outcomes. Flexible strategies and scenario planning are essential.

Conclusion: A Growing Market, A Rising Challenge

The global EV market is creating one of the largest growth opportunities for power electronics. Engineers, manufacturers, and decision makers who act now — mastering wide-bandgap devices, planning for grid integration, and building resilient supply chains — will be positioned to lead.

Takeaway: Power electronics is no longer just a subsystem — it is the heart of EV performance, safety, and energy efficiency.

Understanding IEC 60909 for Short-Circuit Calculations

Short-circuit calculations are a daily requirement for electrical engineers who design, operate, or protect power systems. Knowing the prospective short-circuit currents in a network is essential for selecting breakers, relays, busbars, cables, and ensuring overall safety. The IEC 60909 standard gives engineers a common framework for calculating these short-circuit currents.

This article explains IEC 60909 in simple language, focusing on why it matters, what it covers, how to use it, and what engineers should remember in practice.

Why short-circuit calculations matter

Whenever a fault occurs — line-to-line, line-to-earth, or three-phase — the system experiences a huge surge of current. This short-circuit current:

Creates thermal stress (heating of conductors, cables, and equipment).

Produces mechanical stress (electrodynamic forces on busbars, contacts, windings).

Must be cleared safely by circuit breakers and protection relays.

Determines the required withstand ratings of all equipment.

Without reliable calculation methods, engineers risk underestimating these currents, leading to unsafe equipment operation, fires, or blackouts.

IEC 60909 provides formulas, assumptions, and a structured method to compute these currents under realistic system conditions.

What is IEC 60909?

IEC 60909 is an international standard titled: Short-circuit currents in three-phase a.c. systems.

It provides:

Rules for calculating short-circuit currents under different fault conditions.

Simplified methods for both balanced and unbalanced faults.

Guidance on modeling equipment (generators, transformers, lines, motors).

Correction factors for real-world effects (voltage tolerances, transformer impedance, etc.).

It is widely used by utilities, consultants, and industrial plant engineers as the reference method for fault current calculations.

Types of short-circuit currents in IEC 60909

The standard distinguishes between several key values:

Initial symmetrical short-circuit current (Ik" or I"k)

The RMS value at the moment the short circuit occurs (immediately after fault initiation).

Important for calculating breaker making capacity and dynamic forces.

Peak short-circuit current (Ip)

The maximum instantaneous current peak during the first half-cycle.

Critical for mechanical design (busbar bracing, switchgear withstand strength).

Steady-state short-circuit current (Ik)

The current that remains after the transient effects decay.

Used to check equipment thermal withstand and long-duration effects.

Breaking current (Ib)

The current that circuit breakers must interrupt at the moment of arc extinction.

Fault types covered

IEC 60909 covers both balanced and unbalanced faults:

Three-phase short circuit (3Φ) → usually the highest current, used for equipment dimensioning.

Two-phase short circuit (2Φ).

Two-phase to earth fault (2Φ–E).

Single-phase to earth fault (1Φ–E) → very common in distribution networks.

The standard provides symmetrical component methods to calculate the currents for these faults.

Key assumptions in IEC 60909

To keep calculations consistent, the standard makes some simplifications:

Voltage factor (c):

Faults are calculated using a factor to account for system voltage variations.

Example:

1.05 × nominal voltage for maximum short-circuit current.

0.95 × nominal voltage for minimum short-circuit current.

Equipment modeling:

Generators, transformers, lines, and motors are modeled using equivalent impedances.

Synchronous machines contribute strongly at the start but decay over time.

Induction motors contribute transient current for a short duration after fault inception.

Neglecting arc resistance:

For worst-case calculations, arc resistance is neglected (since it would reduce fault current).

How to apply IEC 60909 in practice

A practical workflow for engineers:

Gather system data

Voltage levels, transformer ratings and impedances, line lengths and impedances, generator/motor data.

Create equivalent impedances

Convert all equipment to per-unit or Ohm impedances at a common base.

Select fault location

Choose busbars or nodes where faults will be studied.

Apply IEC 60909 formulas

Compute initial symmetrical current, peak current, and steady-state current.

Check equipment ratings

Compare calculated values with breaker making/breaking capacity, busbar withstand strength, cable thermal limits, and relay settings.

Document results

Maintain clear calculation sheets or simulation reports for compliance and safety records.

IEC 60909 vs. simulation tools

Modern engineers often use software (ETAP, DIgSILENT PowerFactory, SKM, CYME) to do these calculations automatically. These tools usually implement IEC 60909 in the background.

However, understanding the theory and equations is important:

To validate software results.

To spot unrealistic inputs or outputs.

To explain results to auditors, regulators, or clients.

Example (simplified)

Let’s say you want to calculate the three-phase fault current at a 11 kV bus fed by a transformer.

Transformer: 20 MVA, 11/66 kV, 10% impedance.

Short-circuit power base = (MVA × 100) / Z% = (20 × 100) / 10 = 200 MVA.

Equivalent short-circuit current at 11 kV = (200 MVA) / (√3 × 11 kV) ≈ 10.5 kA.

From this base value:

Initial symmetrical short-circuit current Ik" ≈ 10.5 kA.

Peak current Ip ≈ k × √2 × Ik", where k ≈ 1.8 (from IEC factors). → about 26.7 kA.

This simple calculation shows how IEC 60909 guides you to realistic numbers used for breaker sizing.

Why IEC 60909 remains important

Even with advanced simulation tools, IEC 60909 provides:

Consistency → all engineers use the same base rules, ensuring comparable results.

Safety → ensures no underestimation of fault currents.

Compliance → many regulators and utilities require IEC 60909-based studies.

Reliability → helps prevent equipment failure and system blackouts.

IEC 60909 is not just a theoretical document. It is a practical tool that every electrical engineer should understand. It helps you calculate fault currents in a structured way, check equipment ratings, and design safe power systems.

If you are an engineer working in utilities, industrial plants, data centers, or renewable energy projects, mastering IEC 60909 is essential for ensuring your designs are safe, reliable, and compliant.

Electric Trucks: Driving the Future of Freight

 Electric trucks (ETs) have rapidly emerged as a transformative force in the transportation and logistics sector. As governments, businesses, and consumers seek cleaner, more efficient, and sustainable alternatives to traditional diesel trucks, the adoption of electric trucks has accelerated. 

Benefits of Electric Trucks

Environmental Advantages

One of the primary benefits of electric trucks is their ability to reduce greenhouse gas emissions and improve air quality. Diesel-powered trucks are significant contributors to carbon dioxide (CO₂), nitrogen oxides (NOₓ), and particulate matter emissions, which contribute to climate change and urban pollution. Electric trucks, powered by batteries, eliminate tailpipe emissions entirely and can significantly lower the carbon footprint, especially when charged using renewable energy sources.

Reduced Operating Costs

Electric trucks often have lower operating costs compared to their diesel counterparts. Electricity is generally cheaper per kilometer than diesel fuel, and electric drivetrains have fewer moving parts, reducing maintenance expenses. Components such as oil filters, exhaust systems, and gearboxes, which require frequent servicing in conventional trucks, are absent or simplified in electric models.

Quieter Operations

Electric trucks produce significantly less noise than diesel trucks. This makes them ideal for urban deliveries and night-time operations, where noise pollution is a concern. Reduced noise also improves the working environment for drivers and reduces disturbances in residential areas.

Performance and Efficiency

Electric trucks provide instant torque, delivering better acceleration and smoother operation compared to diesel trucks. Regenerative braking captures energy during deceleration, improving overall efficiency and reducing brake wear.

Downsides of Electric Trucks

High Upfront Costs

One of the major challenges is the high initial purchase price. Although operating costs are lower over time, the upfront investment for electric trucks is significantly higher due to expensive battery technology.

Limited Range

Current battery technology limits the driving range of electric trucks, particularly for heavy-duty long-haul transport. While light and medium-duty trucks can efficiently handle city and regional routes, long-haul operations often require frequent recharging or large battery packs, which add weight and reduce payload capacity.

Charging Infrastructure

The lack of widespread charging infrastructure is a critical bottleneck. While passenger EV charging networks have expanded, charging stations capable of supporting high-capacity trucks (including megawatt charging) are still in early stages of deployment.

Longer Downtime for Charging

Recharging an electric truck takes longer than refueling a diesel truck, particularly when using standard chargers. This can impact fleet productivity unless fast-charging or battery swapping solutions are available.

Battery Degradation and Recycling

Over time, batteries lose capacity, reducing range. Additionally, battery disposal and recycling remain environmental and logistical challenges that need more mature solutions.

Growth of Electric Trucks in the Past Few Years

Electric trucks have moved from experimental pilots to commercial adoption in recent years, driven by advancements in battery technology, government incentives, and stricter emissions regulations.

• Global Sales Growth: In 2020, electric truck sales were in the tens of thousands, dominated by China. By 2024, global sales of medium- and heavy-duty electric trucks surpassed 90,000 units, with growth of nearly 80% year-on-year, largely due to urban logistics, regional haulage, and last-mile delivery demands.

• China Leads the Market: China has been the dominant player, accounting for more than 80% of global sales, thanks to strong policy support, subsidies, and domestic manufacturing capacity.

• Europe and North America Catching Up: Europe has set aggressive carbon reduction targets for heavy-duty vehicles, pushing manufacturers and fleet operators toward zero-emission options. In the U.S., companies like Tesla, Freightliner, Volvo, and Nikola have launched or expanded electric truck models for freight and logistics.

• Diverse Segments: Light- and medium-duty trucks (delivery vans, refuse trucks, regional haulers) are leading the transition, while long-haul electric trucks are beginning to emerge with models like the Tesla Semi and Mercedes-Benz eActros 600.

Scope for Wider Adoption

Technological Advancements

The pace of innovation in battery technology (e.g., solid-state batteries, higher energy densities) is expected to improve range and reduce costs significantly over the next five years. Megawatt charging systems (MCS) are also being deployed, enabling faster recharging for long-haul trucks.

Government Policies and Incentives

Government initiatives worldwide, such as subsidies, tax incentives, zero-emission mandates, and carbon penalties, will accelerate adoption. The European Union has mandated a 90% CO₂ reduction for heavy-duty vehicles by 2040, and several U.S. states are adopting Advanced Clean Truck (ACT) rules.

Total Cost of Ownership (TCO) Parity

Analysts project that by 2027–2030, electric trucks will reach TCO parity with diesel trucks in most urban and regional duty cycles. High fuel prices, improved battery economics, and declining maintenance costs will strengthen the business case.

Integration with Renewable Energy and Smart Grids

Electric trucks can play a key role in grid balancing and vehicle-to-grid (V2G) services, especially for fleet depots with predictable schedules. Pairing with renewable energy sources enhances their environmental benefits.

Corporate Sustainability Commitments

Major logistics companies like DHL, UPS, FedEx, and Amazon are committing to electrify large portions of their fleets to meet corporate sustainability and net-zero targets.

The Road Ahead

Electric trucks are poised to transform freight transport, starting with urban and regional logistics and gradually expanding into long-haul segments as technology matures. Key enablers will include:

• Expansion of high-capacity charging networks (e.g., megawatt chargers).

• Standardization of charging protocols across regions and manufacturers.

• Continued investment in battery recycling and second-life use.

• Government policies that balance incentives with infrastructure funding.

• Integration of autonomous driving technologies, which could further enhance efficiency.

While challenges remain, the trajectory is clear: electric trucks are not a niche experiment anymore—they are becoming a mainstream solution for sustainable freight.

Understanding Harmonics

Harmonics are undesirable components in the sinusoidal waveform of the AC Power supply. Harmonics occur as integral multiples of the fundamental frequency. That is, the third order harmonic will have a frequency of 3 times the fundamental frequency; 150 Hz which is 3 times the fundamental 50 Hz frequency. Harmonics affect power quality and equipment life and efficiency.

It is therefore necessary that Harmonics in any power system be monitored. Should Harmonics be present, they can be rectified by using suitable methods such as filters.

Causes of Harmonics

Harmonics are caused by Non-Linear Loads. The majority of electrical loads are linear meaning that the current varies sinusoidally with the voltage, though it may have a phase displacement.

However, of late, the proliferation of electronic devices such as Variable frequency drives, chopper circuits, inverters, etc cause non-linear loading of the power system. The current does not vary sinusoidally with the voltage. This leads to harmonics in the power system. The fundamental frequency will have many other frequencies superimposed on itself. This causes distortion of the waveform.

Using a mathematical technique known as Fast Fourier Transforms, the distorted AC waveform can be resolved into its component waveforms. Of the measured harmonics, the even harmonics(harmonics whose frequency are the fundamental frequency multiplied by even numbers such as 100Hz(2 *50) or 200Hz(4*50) get cancelled out and have no effect. For the study and management of Harmonics, only the odd harmonics are considered.


Effects of Harmonics

Harmonics have a wide range of effects such as heating of conductors, motors etc which can affect equipment efficiency. Besides, they can cause transient over/under voltages and can cause equipment failure.

Harmonic Analysis

If the problem of Harmonics is suspected, a harmonic analysis needs to be conducted. Harmonic analyzers are dedicated equipment to study the harmonics in a power supply. Typical Analyzers can resolve harmonics upto the 25th order.

Harmonics can be neutralized by means of Harmonic filters. Harmonics filters are usually LC circuits tuned to the frequency of the particular order of harmonics to be neutralized.

Voltage Classification - LV, MV and EHV

AC voltages have been classified in various manners.  In earlier times, there were just two categories LV and HV.  As the level of voltages increases, there was a need for more levels.  However, there was ambiguity as to where each band ended and the other began.  For instance, 11kV can be MV in some systems and HV in another. 

The International Electrotechnical Commission has classified the voltages into the following levels(IEC 60038).  This classification system is fast gaining acceptance. 

Low Voltage           - upto 1000V

Medium Voltage     - 1000V to 35kV

High Voltage           - 35kV to 230 kV

Extra High Voltage  - above 230 kV.


In some situations, the term Ultra High Voltage is used to denote voltages above 800 kV.

In addition, the IEC defines a voltage band known as the Extra Low Voltage with a AC voltage less than 70 V.  See article here.

Braking methods in Induction motors

Braking in induction motors refers to quickly bringing the speed of the motor to zero.  Braking can be categorized into two broad categories viz. mechanical braking and electrical braking.
Mechanical braking involves stopping the shaft by means of a braking shoe.  When the braking is to be done, the supply to the motor is cut off and the brake is applied to bring the motor to a halt.
Mechanical braking used in cranes and hoists.  It is also used in elevators when the elevator has to stop at a specific floor of the building.

Electrical braking involves stopping the motor using electrical means.  Most electrical braking systems have a mechanical brake to hold the shaft in position once the machine has been stopped.
There are two main types of Electrical braking.

1. Plugging
2. Dynamic braking
3. Regenerative braking

Plugging
Plugging involves reversing the supply in two of the phases.  For instance, R and Y can be interchanged.  This leads to a torque being developed in the opposite direction to the rotation of the motor.  This causes the motor to stop at once.  Once the motor stops, the reverse supply is cut off (to prevent the motor from running in the opposite direction).  The rotor is secured by a mechanical brake.
Dynamic Braking can be classified into DC injection braking, AC dynamic Braking and Capacitor Braking.
AC dynamic Braking
In AC dynamic braking, the supply to one of the phases is cut off.  Thus the motor runs as a single phase motor.  This induces negative phase sequence components in the supply and the motor stops.  Another method is to give the remove one phase and give the same phase to two terminals.  For instance, two terminals will have 'Y' phase and one will have 'B' phase.
DC injection braking
In DC injection braking, a separate rectifier circuit produces a dc supply.  When the brake is to be applied, the ac supply to the stator is disconnected and a dc supply is given to two of the phases.  The dc voltage in the stator sets up its own magnetic field.  The conductors of the rotor which is rotating will cut the magnetic field.  As the conductors are short circuited, a high current is produced.  This causes a braking torque to be produced in the rotor.  The current produced in the rotor is dissipated as heat.  This system can be used only when the rotor can withstand the heat which will be produced when the brake is applied.
Capacitive Braking
Here the AC supply to the stator terminals is cut off and the terminals are connected to a three phase capacitor bank.  The capacitors will excite the induction generator.  This sets up a magnetic field which will cut the rotor bars.  The rotor energy is thus converted into heat and the motor is stopped.
Regenerative Braking
In Regenerative braking,  the supply frequency to the stator is reduced.  This is possible with VFDs where the frequency can be varied.  When the supply frequency is reduced, the synchronous speed of the motor is reduced. When the synchronous speed falls below the rotor speed, the induction motor works as an induction generator and power is supplied back to the terminals.  The energy in the rotor is thus recovered.  Due to the loss of energy, the rotor slows down and stops.

Electric Motor - Components

Induction motor rotor - construction

The rotor of the induction motor has a core which is made of electrical steel.

The bars which constitute the squirrel cage are typically made of aluminium or copper. The bars are placed in slots on the rotor core. There is no need for insulation between the bars and the core as the voltage developed in the squirrel cage is very low.induction motor rotor

The rotor bars are skewed in order to prevent magnetic locking. Magnetic locking is also known as cogging.

Magnetic Locking can also be prevented by ensuring that the number of rotor slots is not equal to the number of stator slots.

Motor Bearings – Functions and Types

  

The bearings can be one of the two types, a plain bearing (sliding contact) or an anti-friction bearing (rolling bearing), depending on the design parameters of the machine element, each of the two types of bearings, plain and anti-friction, is available for design with linear motion, radial loads and axial loads.

Bearings may be classified into three general classes

Guide or flat bearings, which support linear motion in machine tables and slides.

Thrust bearings

Thrust bearings, which support rotational motion in machine elements that have axial loads i.e., the load is applied along the central axis of the rotating shaft

Radial bearings

Radial bearings, which support rotational motion in shafts with radial loads i.e., the load is applied along the radius of the rotating shaft.

Anti Friction or Roller Element bearings

Anti – friction bearings or roller – element bearings, as they are often called, use a rolling element (ball or roller) between the loaded surfaces.

Anti-friction bearings are divided into two categories,

a) ball bearings

b) Roller bearings.

Ball bearings have five general types:

Guide, Radial, Thrust, Self – aligning and Angular contact.

Roller bearings have four general types: Cylindrical, Thrust, Spherical and Taper.

Roller and Ball bearing types

Guide bearing: The ball guide bearing is used for linear motion where very low co-efficient of friction and extreme smoothness in operation are desired.

Radial bearing: The first radial bearing is the single – row, deep – groove ball bearing, most widely used anti – friction bearing. Second radial bearing is the cylindrical roller bearing is capable of carrying larger radial loads at moderate speeds than those carries by radial ball bearings using the same size bearing.

Thrust bearing: First the ball thrust bearing is designed for axial (thrust) loads only – no radial loads. Second spherical roller thrust bearing is capable of very heavy axial loads as well as moderate radial loads.

Angular contact ball bearing: The shoulders in this provides for thrust (in one direction only) that is larger than the single row, deep radial ball bearing can handle.

Taper roller bearing: A pair of taper roller bearing is capable of handling both very large axial and radial loads.

Shell bearings

In case of very large motors, shell bearings are used. Grease is used as the lubricant in case of roller and ball bearings.

For Shell bearings, lube oil is used as the lubricant.

The bearings are usually designed to withstand radial loads. However, in some applications such as in the use of gears and belts, the motor may also be subjected to axial loads. In such roller bearingscases, bearings such as angular ball bearings which can withstand axial loads can be used.

   

Laminations in Transformer Core and Motor Stator

 Electric machines, especially AC machines such as transformers and alternators are exposed to alternating magnetic fields during operation.

 

This alternating magnetic field causes the induction of eddy currents in the core of transformers and the stator of motors. The eddy current creates a loss of energy in the form of heat loss and hysteresis loss.

In order to avoid this, the core of transformers and the stator of motors and generators are made of a set of laminated steel sheets. Silicon Steel is used. This steel is cold rolled and has special grain orientation. Each steel sheet is around .3 mm thick.

The sheets are insulated on both sides and laid of top of one another. This arrangement ensures that the eddy current is reduced as it cannot flow over a wide area of cross section. The laminated surfaces need to be very clean. Presence of foreign particles can cause laminar faults which lead to core damage.

Eye Bolts in Motors-An Overview

  

An Eye bolt is an important component of the motor. It is used in lifting the machine. The eye bolt consists of a loop at one end and a threaded end at the other. The threaded end is screwed into the motor body. eye bolt

When the motor or alternator is to be lifted, a sling is connected to the eye bolt and the machine is lifted using a crane. It is important to note the capacity of the eye bolt. Every Eye bolt has a WLL ( Working Load Limit). If this is exceeded the eye bolt will fail.

This can result in injury to crew members or even death. Check the capacity of the eye bolt before lifting.

The eye bolt is intended only for vertical lifting. Angular lifting (lifting in an angle) will cause the bolt to fail quickly. shouldered eye boltAngular eye bolts will have the safe Working load limit. As the angle of lifting increases, the WLL decreases. Thus, while the WLL will be maximum at vertical, it decreases as the angle increases. The angle of lift should be calculated and the safe Working Load Limit determined.

There are special types of eyebolts which can withstand angular load (up to a certain degree, usually 45 degrees. Check the angle with the manual).

These are known as shouldered eye bolts. Check that the machine has shouldered eye bolts before lifting the machine.

Grease in Electric Motors

  

Grease is the most widely used lubricant in electric motors. Grease is used in motors with ball and roller bearings.

The function of grease is to minimize friction and wear,to prevent corrosion and to prevent the entry of foreign objects which can contaminate the bearing.

Grease, thus, has a sealing effect. Grease is a semi-solid lubricant. It is composed of a base oil, additives and a thickener. The base oil can be synthetic or natural. Synthetic oils are used in applications with high temperatures and longer regreasing intervals.

The function of the thickener is to prevent the base oil from leaking. Thickeners are usually metallic soaps. Additives include oxidation and corrosion inhibitors, anti-wear agents.

The grease in motors will have to be replaced over time. The regreasing intervals are based on the bearing manufacturers' recommendations.

Deep Bar Rotors in Induction Motors

Deep Bar Rotors are used in induction motors to increase the torque during starting. Deep bars indicate that the bars which comprise the cage in the rotor are deeper than those in normal rotors.

When an induction motor is started, the slip between the rotor and the stator is high. Thus the frequency of the rotor current is high.

This high frequency results in high reactance in the lower layers of the deep bar. Hence, most of the current flows in the surface of the rotor bars. This results in high current density and increased resistance. This resistance produces high torque during starting.

When the motor reaches its rated speed, the slip frequency drops and the reactance reduces. The current now, flows uniformly across the entire cross section of the rotor bar. The resistance in the rotor drops and the motor runs normally.