Soft Starter vs VFD

A soft starter controls how a motor starts and stops; a VFD controls how the motor runs. If the motor reaches full speed and stays there, a soft starter is usually the correct and cheaper choice. If the process needs the motor speed to change while it runs, only a VFD can do that.

So the selection rule is one question, not a feature comparison: after the motor reaches normal running condition, do you still need to change its speed or torque? Answer "no" and a soft starter is sufficient — a VFD then adds cost, panel space, continuous heat and continuous harmonics for capability the process never uses. Answer "yes" and no amount of soft-starter tuning will substitute for a drive.

The Question Behind the Specification

A customer has a 75 kW motor.

One supplier recommends a Soft Starter.

Another recommends a VFD.

Both solutions can start the motor smoothly.

Both can reduce the electrical and mechanical stress associated with direct starting.

But their purpose is fundamentally different.

A Soft Starter mainly controls what happens while the motor starts and stops.

A Variable Frequency Drive controls how the motor operates throughout its running cycle.

That single distinction solves a large part of the Soft Starter vs VFD question.

So before asking:

“Which is better?”

ask:

“After the motor reaches normal running condition, do I still need to control its speed or torque?”

If the answer is No, a Soft Starter may be the more appropriate and economical solution.

If the answer is Yes, a VFD usually becomes the stronger candidate.

This guide explains how to make that decision properly for pumps, fans, compressors, conveyors and other industrial motor applications.

Soft Starter vs VFD — Quick Answer

Requirement	Soft Starter	VFD
Reduce starting current	Yes	Yes
Smooth acceleration	Yes	Yes
Smooth stopping	Yes, application dependent	Yes
Continuous speed control	No	Yes
Advanced torque control	Limited/application dependent	Yes
Motor normally runs at full speed	Excellent application	Can work, but may be unnecessary
Variable process demand	Limited	Excellent application
Initial cost	Generally lower	Generally higher
Panel space	Generally lower	Generally higher
Heat during continuous operation	Low when bypassed	Continuous drive losses
Supply-side harmonic consideration	Primarily during thyristor-controlled operation	Continuous consideration
Motor-side PWM effects	No during bypass/full-voltage running	Yes
Long motor cable considerations	Similar to conventional supply after bypass	Requires VFD-specific evaluation
Energy-saving potential from speed reduction	No	Yes, where process permits
Process automation capability	Moderate	High
Full-speed pump application	Often excellent	Depends on process
Variable-flow pump application	Limited	Excellent

The correct selection should come from the application rather than from motor kW alone.

What Is a Soft Starter?

A Soft Starter is an electronic motor-starting device that controls the voltage supplied to an AC motor during acceleration.

Instead of connecting the motor immediately to full line voltage, semiconductor devices—normally thyristors—control how the voltage is applied.

This allows the motor to accelerate progressively.

IEC 60947-4-2 covers semiconductor motor controllers, starters and soft starters intended for applicable AC circuits up to 1,000 V. The current IEC publication incorporates the 2020 edition and its 2024 amendment, with a corrigendum issued in 2026.

A Soft Starter can therefore help control:

• Starting current
• Starting torque
• Acceleration
• Mechanical shock
• Stopping behaviour
• Motor-start related voltage dip

What Happens After the Motor Reaches Full Speed?

This is critical to understanding a Soft Starter.

Once the motor reaches normal operating speed, many Soft Starter systems use an internal or external bypass arrangement.

The motor then effectively operates from the normal line supply rather than having its full operating power continuously processed in the same manner as a VFD.

That makes a Soft Starter particularly attractive when the process requirement is:

Start smoothly → Run continuously at full speed → Stop smoothly

Typical examples can include suitable:

• Pumps
• Fans
• Compressors
• Conveyors
• Crushers
• Mixers
• Material-handling equipment

What Is a VFD?

VFD means:

Variable Frequency Drive

A VFD converts incoming electrical power and generates an output with controlled frequency and voltage for the motor.

Because induction-motor speed is strongly related to supply frequency, controlling frequency allows the drive to control motor speed.

A VFD therefore remains actively involved throughout normal motor operation.

For applicable adjustable-speed power-drive systems, IEC 61800-5-1:2022 addresses electrical, thermal, energy and other safety requirements.

The Most Important Selection Question

Imagine two identical 90 kW pump motors.

Pump A

The pump:

• Starts
• Runs at 100% speed
• Stops
• Has no requirement to regulate flow by motor speed

The engineering requirement is mainly:

Reduce starting current and mechanical stress.

A Soft Starter may be an excellent solution.

Pump B

The pump must maintain pressure according to changing process demand.

Sometimes it needs:

45% speed

sometimes:

70%

sometimes:

100%

Now the requirement is not simply motor starting.

It is:

Continuous process control.

A VFD becomes much more relevant.

Same motor rating.

Completely different correct solution.

Soft Starter Controls Starting — VFD Controls the Process

This is the easiest way to remember the difference.

Soft Starter

Primarily asks:

“How should the motor reach full speed?”

VFD

Also asks:

“At what speed should the motor operate right now?”

That difference affects:

• Energy consumption
• Process control
• Automation
• Panel construction
• Cooling
• Harmonics
• Maintenance
• Cost

Starting Current: Soft Starter vs VFD

Both technologies can reduce the severe starting-current demand associated with direct-on-line starting.

But they achieve it differently.

Soft Starter

The Soft Starter progressively controls motor voltage.

Starting current can be limited according to:

• Motor
• Load
• Starting torque requirement
• Starter settings

But reducing current also affects available motor torque.

This is important for high-torque loads.

VFD

A VFD starts the motor at low frequency and controls voltage/frequency through the acceleration process.

This allows much greater control of motor current and torque.

Therefore, for demanding applications requiring controlled torque from low speed, a VFD can have a major advantage.

Do Not Reduce Starting Current Without Checking Torque

This is one of the biggest motor-starting mistakes.

Suppose the customer says:

“I want the lowest possible starting current.”

That sounds reasonable.

But the motor still has to accelerate the load.

A centrifugal pump starting against normal conditions has a very different torque requirement from:

• Loaded conveyor
• Crusher
• Positive-displacement pump
• High-inertia machine

If Soft Starter current limiting is set too aggressively, the motor may not develop sufficient accelerating torque.

The result can be:

• Long starting time
• Motor heating
• Soft Starter thermal stress
• Failure to accelerate
• Trip

The correct starting current is therefore:

Not necessarily the lowest possible current.

It is the current required to develop sufficient torque while staying within the electrical-system constraints.

In our FAT and commissioning experience, this is the single most common soft-starter complaint we are called about — and it is almost never a faulty starter. A current limit set at 250% of motor FLC on a loaded conveyor or a positive-displacement pump will often stall the acceleration, the ramp timer expires, and the starter trips on "failure to start". Raising the limit to a value that the load actually needs, typically in the 350–450% range for such loads, usually clears it — but the real figure must come from the motor torque-speed curve and the load torque data, not from a default setting. Our recommendation: ask for the load torque requirement at the enquiry stage, and let the electrical system constraint set the ceiling, not the target.

Which Gives Better Starting Torque?

For applications requiring advanced torque control at low speed, a VFD generally provides significantly more capability because it controls both motor frequency and voltage.

A Soft Starter primarily controls motor terminal voltage during starting.

That distinction becomes important for applications such as:

• Loaded conveyors
• Crushers
• High-inertia machines
• Controlled acceleration systems
• Process machinery

For simple full-speed applications, however, that additional VFD capability may provide no useful commercial benefit.

Soft Starter vs VFD for Pumps

Pumps are one of the most common applications where this decision arises.

But “pump” alone is not enough information.

Case 1 — Pump Always Runs at Full Speed

Suppose:

• Motor = 75 kW
• Pump runs continuously at rated speed
• Starting current needs reduction
• Water hammer during stopping is a concern
• Flow does not need continuous speed-based regulation

A Soft Starter can be a very strong solution.

Its purpose directly matches the problem:

Smooth start + controlled stop + full-speed running

The stated purpose of a soft starter is to reduce current surge and mechanical stress through controlled acceleration — nothing more, and nothing about running speed.

Case 2 — Pump Must Maintain Variable Pressure or Flow

Suppose the pump must respond to a pressure transmitter.

Demand changes continuously.

Now running at full speed and throttling a control valve may not be the most effective operating strategy.

A VFD can adjust pump speed according to actual process requirement.

Possible benefits include:

• Pressure regulation
• Flow regulation
• Reduced throttling
• Lower operating speed during low demand
• Process automation

This is where VFD technology offers functionality that a Soft Starter cannot provide during normal full-speed operation.

Does a VFD Always Save Energy?

No.

This deserves a direct answer because it is one of the most common sales claims.

Installing a VFD does not automatically create energy savings.

Energy saving depends on how the application actually operates.

Consider a pump that must run:

24 hours/day at 100% speed

whether controlled by:

• DOL
• Soft Starter
• VFD

If there is no useful opportunity to reduce operating speed, the VFD's speed-control capability may provide little or no process-energy benefit.

The VFD itself also has conversion losses.

When Can a VFD Save Significant Energy?

Energy-saving potential becomes particularly attractive where process demand varies and motor speed can genuinely be reduced.

This commonly applies to suitable centrifugal:

• Pumps
• Fans
• Blowers

Instead of running continuously at full speed and controlling output mechanically, a VFD can reduce motor speed according to actual demand.

But the actual saving should be estimated from:

• Load profile
• Operating hours
• Required flow/pressure
• Existing control method
• Motor/pump/fan characteristics
• Drive efficiency

Do not sell the VFD purely using:

“VFD saves 30% electricity.”

without studying the application.

This is the item we push back on at clarification stage, because the payback figure in the enquiry is often calculated from a speed reduction the process cannot actually accept. On water treatment and cooling-water duties we have seen the pump's minimum acceptable speed turn out to be around 80% because of static head — and at 80% speed the affinity-law saving is a fraction of what a straight cube-law calculation promises. Our recommendation: ask for the operating-hours histogram against required flow before quoting any saving. If the customer cannot produce one, quote the panel and leave the saving out of the offer.

Can a Soft Starter Save Energy?

The main purpose of a Soft Starter is not continuous speed-based energy optimization.

Once a bypassed Soft Starter has brought the motor to full speed, the motor normally continues to operate at line frequency.

Therefore, if a pump is unnecessarily running at full speed when the process needs only half the output, a Soft Starter cannot continuously reduce the motor speed to solve that problem.

Its primary benefits are instead related to:

• Starting
• Stopping
• Reduced stress
• Electrical-system impact
• Equipment life
• Process smoothness

Soft Starter vs VFD for Water Hammer

Water hammer can occur when fluid velocity changes too rapidly.

Depending on the pumping system, abrupt stopping can create pressure transients.

A suitable Soft Starter may provide controlled soft-stop functionality.

A VFD can provide even more detailed control of deceleration and pump speed.

But neither device should be considered a guaranteed cure for every hydraulic issue.

The actual system should consider:

• Pump type
• Check valve
• Pipe length
• Static head
• Flow
• Deceleration
• Control strategy

Electrical control and hydraulic system behaviour must be coordinated.

Soft Starter vs VFD for Fans

Full-Speed Fan

If the fan:

• Starts
• Reaches rated speed
• Operates continuously at rated speed

a Soft Starter may provide the required reduced-stress starting without the additional complexity of a VFD.

Variable-Airflow Fan

If airflow demand changes and the fan speed must respond to:

• Temperature
• Pressure
• Process demand
• Building automation

then a VFD becomes far more useful.

The question is again:

Does speed need to change after startup?

Soft Starter vs VFD for Conveyors

Conveyor applications require more careful consideration.

A Soft Starter can provide:

• Reduced mechanical shock
• Controlled acceleration
• Reduced belt stress
• Full-speed operation

Soft Starters can offer a smaller footprint and lower cost than VFDs in applications where continuous variable speed is not required.

But a VFD may be better when the conveyor requires:

• Variable speed
• Process synchronization
• Controlled low-speed operation
• High starting torque
• Frequent acceleration/deceleration
• Speed matching with another conveyor

Do not write:

“Conveyor = VFD.”

First understand the process.

Soft Starter vs VFD for Compressors

The correct selection depends strongly on compressor type and operating philosophy.

For a compressor that:

• Starts
• Runs continuously at full speed
• Uses another method for unloading/capacity control

a Soft Starter may be sufficient.

Where compressor capacity must be controlled through speed, a VFD may be considered.

Also verify:

• Manufacturer's minimum speed
• Lubrication
• Cooling
• Torque
• Operating envelope

Never apply variable speed to process equipment without confirming that the mechanical machine is designed for it.

Which One Is Better for DG Supply?

Generator-fed motors need additional attention.

Compared with a strong utility-connected bus, a generator source may experience larger:

• Voltage dip
• Frequency disturbance

when a large motor starts.

Both a Soft Starter and VFD can help control starting demand.

But the correct choice still depends on process requirements.

If only starting current must be reduced:

Soft Starter may be adequate.

If the motor also requires variable speed:

VFD may be appropriate.

For a VFD supplied from a DG, also consider:

• Rectifier harmonics
• Generator impedance
• AVR behaviour
• Total nonlinear load
• Other VFDs/UPS loads
• Harmonic mitigation

The generator should therefore be evaluated as part of the complete electrical system.

Harmonics: Major Difference Between Soft Starter and VFD

This is an important technical distinction.

Soft Starter

During controlled starting, thyristor switching creates a non-sinusoidal operating condition.

However, once a bypassed Soft Starter reaches normal running condition, the semiconductor starter is effectively bypassed and the motor operates on the normal AC supply.

Therefore, harmonic behaviour associated with the starter is mainly relevant during the starting/stopping period.

VFD

A conventional VFD continuously converts power while the motor runs.

Therefore, supply-side harmonic current is a continuous design consideration whenever the VFD is operating.

Depending on the system, the design may need to consider:

• DC choke
• Input reactor
• Passive filter
• Active harmonic filter
• Low-harmonic drive

The appropriate solution depends on the network and required harmonic performance.

VFD Also Creates a Motor-Side PWM Issue

Supply-side harmonics are only one side of VFD engineering.

The VFD output is a PWM waveform.

This can create additional motor-side considerations including:

• dV/dt
• Reflected-wave voltage
• Motor insulation stress
• Long cable effects
• Common-mode currents
• Bearing-current concerns
• EMC

Depending on cable length and equipment, an installation may require:

• Output reactor
• dV/dt filter
• Sine-wave filter
• Appropriate VFD cable
• Grounding measures

A Soft Starter running in bypass does not create the same continuous PWM motor-output condition.

This can make Soft Starter installations considerably simpler for full-speed motor duties.

Panel Heat: Soft Starter vs VFD

Thermal design is another significant difference.

Soft Starter

During starting, semiconductor devices generate heat.

After bypass operation, losses generally reduce significantly.

Therefore, continuous enclosure heat load can be comparatively low.

VFD

The VFD power electronics remain active whenever the motor runs.

That means continuous:

• Semiconductor losses
• Heat
• Cooling requirement

Panel design may therefore require:

• Larger enclosure
• Fans
• Filters
• Air conditioning in demanding environments
• Derating
• Greater spacing

Temperature should be calculated from actual equipment losses and ambient conditions—not guessed from motor kW.

The order of magnitude is worth internalising. A VFD at approximately 97% efficiency on a 110 kW load dissipates roughly 3 kW inside the enclosure, continuously, all day. The same motor on a bypassed soft starter dissipates a small fraction of that once the bypass closes. We have opened VFD panels in chemical plants where the drive was correctly selected but the enclosure was sized as though it held a contactor starter — the drives were derating themselves every afternoon and the customer read it as a drive fault. Heat load is a design input, not an afterthought; see our panel heat dissipation and cooling guide for the calculation approach.

Panel Size

In many comparable full-speed motor applications:

Soft Starter panels can be smaller than VFD panels.

Soft Starters are typically less expensive and require less panel space than VFDs where their functionality is sufficient for the application.

A VFD panel may additionally require space for:

• Drive ventilation clearance
• Input reactor
• Output reactor
• dV/dt filter
• Harmonic filter
• Braking resistor
• Bypass system

depending on the project.

Initial Cost: Which Is Cheaper?

For a comparable motor rating, a Soft Starter is generally less expensive than a VFD.

This is especially true as motor ratings increase.

But initial equipment price should not be the only decision criterion.

Compare:

Purchase cost + panel cost + installation + energy + maintenance + downtime + process benefit

A ₹3 lakh additional VFD cost might be easily justified if process speed control saves ₹10 lakh annually.

The same VFD could be wasted money if the motor always operates at full speed and the only requirement was a smooth start.

Lifecycle Cost Is More Important Than Purchase Price

Consider two applications.

Application A

Soft Starter costs less.

Motor runs at 100% speed continuously.

There is no useful speed-control requirement.

Here the Soft Starter may also be economically superior over its lifecycle.

Application B

VFD costs more initially.

The pump spends most of its operating time below full process demand.

Reducing speed significantly reduces energy use.

Here the VFD may provide much lower lifecycle cost.

That is why procurement should not ask only:

“Soft Starter price?”

vs

“VFD price?”

The correct question is:

“Which technology creates the lowest lifecycle cost for the required process?”

Maintenance Comparison

Soft Starter

Advantages can include:

• Fewer continuous power-electronic functions
• Lower thermal duty after bypass
• Relatively straightforward system architecture
• Simpler replacement in many applications

Possible maintenance items include:

• Bypass contactor
• Cooling system where used
• Thyristor module
• Connections
• Control electronics

VFD

Additional considerations may include:

• Cooling fans
• DC-link capacitors
• Power semiconductor modules
• Control boards
• Filters
• Reactors
• Communication modules

Modern VFDs are highly reliable when applied correctly.

But they are inherently more complex devices.

Maintenance capability should therefore be part of the equipment-selection decision.

Does a Soft Starter Need a Bypass Contactor?

It depends on the Soft Starter design.

Some devices contain an integrated bypass.

Others may use an external bypass contactor.

Do not automatically add an external bypass contactor without checking the selected product architecture.

Several soft starter ranges include integrated bypass contacts as standard, which removes the separate bypass contactor from the panel.

The panel designer should understand:

• Starter type
• Internal bypass
• External bypass requirement
• Emergency bypass philosophy

before finalizing the BOM.

We have received approved drawings showing an external bypass contactor across a starter that already had integrated bypass contacts — a duplicated contactor, extra busbar work and extra enclosure width, all for no function. The reverse mistake is worse: sizing the enclosure for an integrated-bypass unit and then discovering at BOM stage that the selected rating in that range needs an external contactor. This is worth confirming from the starter datasheet on the very first GA drawing.

VFD Bypass Is a Different Concept

A VFD bypass arrangement usually allows the motor to operate directly from the line if the VFD is unavailable or if the process philosophy permits bypass operation.

A typical bypass could involve:

VFD Mode

or

DOL/Bypass Mode

But bypass engineering requires attention to:

• Interlocking
• Motor speed before transfer
• Protection
• Process suitability
• Motor starting current
• VFD isolation
• Maintenance
• Control logic

A motor that normally requires a VFD because the electrical network cannot tolerate DOL starting cannot simply be given a manual DOL bypass without studying the consequences.

Do You Need a VFD Bypass?

Not always.

Ask:

Is motor operation critical if VFD fails?

If NO:

Bypass may not be justified.

Can the process safely operate at full motor speed?

If NO:

Bypass may be useless.

Can the electrical network start the motor directly?

If NO:

A simple DOL bypass may not work.

Is redundancy more important than bypass?

Perhaps the better solution is:

Duty VFD + Standby VFD

rather than:

VFD + DOL bypass

The architecture should come from operational philosophy.

Soft Starter vs VFD for Frequent Starts

The number of starts per hour matters greatly.

A motor starting:

twice per day

has a completely different duty from one starting:

20 times per hour.

For Soft Starters, frequent starting affects:

• Thyristor heating
• Thermal model
• Starter sizing
• Bypass duty
• Motor thermal condition

For VFDs, frequent controlled starts can be easier electrically, but the complete mechanical and drive duty still requires evaluation.

Always provide the supplier with:

• Starts/hour
• Start duration
• Loaded/unloaded start
• Motor current
• Ambient temperature

rather than only motor kW.

Heavy-Duty Applications

Suppose the motor drives:

• Crusher
• Loaded conveyor
• Mixer
• High-inertia load

Do not select Soft Starter or VFD from catalogue motor kW alone.

The engineer should review:

• Starting torque
• Breakaway torque
• Acceleration time
• Load inertia
• Current limit
• Motor thermal capacity
• Required overload

In some cases, the same 75 kW motor may require different starter/drive sizing depending on the load.

Soft Starter vs VFD for Speed Control

This answer is simple.

Soft Starter

Not intended for continuous normal variable-speed operation.

VFD

Designed specifically for adjustable-speed motor control.

If your customer says:

“Motor should operate between 25 Hz and 50 Hz depending on process.”

You are no longer simply solving a starting problem.

You need a speed-control solution.

Can a VFD Run Above 50 Hz?

Technically, many VFDs can produce output frequencies above the motor's base frequency.

But that does not automatically mean the motor and driven machine should operate above rated speed.

Before overspeed operation, verify:

• Motor manufacturer limits
• Mechanical equipment limits
• Bearing limits
• Fan/pump limits
• Torque capability
• Voltage/frequency behaviour
• Safety margin

Drive capability is not the same as machine capability.

Process Control Is Where VFD Has a Major Advantage

A VFD can receive process signals such as:

• 4–20 mA pressure
• Flow
• Temperature
• Level
• PLC command
• Fieldbus communication

and continuously change motor speed.

This allows closed-loop process control.

For example:

Pressure low → increase speed

Pressure high → decrease speed

A Soft Starter can communicate status and provide advanced control functions, but after normal full-speed running it does not provide the same continuous speed-control capability.

Soft Starter Can Still Be a Smart Device

Do not confuse:

No continuous speed control

with:

No intelligence.

Modern Soft Starters can provide features such as:

• Motor protection
• Current monitoring
• Torque control
• Communication
• Diagnostics
• Event information
• Pump functions

So the decision is not:

Basic Soft Starter vs intelligent VFD.

The real distinction remains:

Full-speed starting/stopping vs continuous variable-speed control.

Soft Starter vs VFD — Detailed Comparison

Parameter	Soft Starter	VFD
Main function	Controlled motor start/stop	Continuous motor speed/torque control
Starting current control	Yes	Yes
Starting torque control	Moderate/advanced depending on unit	Advanced
Continuous speed variation	No	Yes
Full-speed application	Excellent	Possible
Variable-speed process	Not suitable as normal solution	Excellent
Initial cost	Lower	Higher
Panel footprint	Usually smaller	Usually larger
Continuous heat	Low after bypass	Continuous
Supply harmonics	Mainly during thyristor-controlled period	Continuous design issue
PWM output to motor	No in normal bypass running	Yes
Long motor cable issue	Conventional considerations	Requires VFD-specific evaluation
Process energy optimization	Limited	High potential where variable speed helps
Process control	Moderate	High
Communication	Available	Common
Maintenance complexity	Lower	Higher
Starting on weak network	Good when correctly selected	Excellent control capability
Heavy low-speed torque	Limited vs VFD	Stronger capability
Soft stopping	Yes	Yes
Reversing/speed command	External arrangement	Drive controlled
Dynamic braking	Limited/not main function	Available with suitable drive architecture

How to Choose: A Practical Decision Tree

Question 1

Does the process require continuous speed variation?

YES → Evaluate VFD

NO → Continue

Question 2

Does the motor normally run at 100% speed after starting?

YES → Soft Starter becomes a strong candidate

Question 3

Is smooth acceleration required mainly to reduce mechanical/electrical stress?

YES → Soft Starter may be sufficient

Question 4

Does the application require high controlled torque at low speed?

YES → Evaluate VFD

Question 5

Does process demand vary significantly?

YES → Evaluate VFD energy/process benefit

Question 6

Is cost, simplicity and smaller panel size highly important while speed control is unnecessary?

YES → Soft Starter often has the advantage

Example 1 — 110 kW Cooling-Water Pump

Requirements:

• Full-speed running
• Four starts/day
• Smooth acceleration
• Reduce electrical-system impact
• Reduce water hammer
• No variable flow requirement

A properly engineered Soft Starter may be the logical solution.

Why pay continuously for speed-control capability that the process never uses?

Example 2 — 110 kW Process Pump

Requirements:

• Process pressure varies continuously
• Pressure transmitter provides feedback
• Motor speed must adjust automatically
• Energy optimization important

VFD is the stronger candidate.

The VFD is not being purchased simply as a starter.

It becomes part of the process-control system.

Example 3 — 75 kW Conveyor

Requirements:

• Conveyor always operates at one speed
• Mechanical shock during startup causes belt wear
• No need for low-speed operation

Soft Starter may solve the actual problem.

But change the requirement to:

• Variable line speed
• Synchronization with another conveyor
• Controlled low-speed operation

and the answer may become VFD.

Example 4 — 55 kW Fan

If airflow demand is always 100%:

Soft Starter may be sufficient.

If airflow demand varies significantly and is currently controlled using dampers:

Evaluate VFD.

Same fan.

Different process.

Different correct solution.

Example 5 — Critical Pump With Bypass Requirement

Suppose process philosophy requires the pump to remain operational even after an electronic starter/drive fault.

Possible architectures could include:

Soft Starter + Bypass

or

VFD + suitable bypass

or

Duty/standby motors

or

Duty/standby VFDs

The correct redundancy philosophy should come from:

• Process criticality
• Network capacity
• Required speed control
• Maintenance strategy

not simply from adding more contactors to the panel.

What Information Should Be Given to the Panel Manufacturer?

Before asking:

“Quote Soft Starter and VFD option.”

provide:

Motor

• kW
• Voltage
• Full-load current
• Rated speed
• Efficiency
• Motor insulation information
• Duty

Load

• Pump/fan/conveyor/compressor/etc.
• Starting torque
• Load inertia
• Loaded/unloaded start
• Required acceleration
• Required deceleration

Process

• Fixed or variable speed
• Minimum speed
• Maximum speed
• Pressure/flow control
• PID requirement
• Reversing
• Braking

Electrical System

• Transformer rating
• Transformer impedance
• DG operation
• Available fault level
• Permissible voltage dip
• System harmonics

Installation

• Motor cable length
• Ambient temperature
• Indoor/outdoor
• Panel location
• Ventilation

Control

• Local/remote
• PLC
• SCADA
• Communication protocol
• Emergency stop
• Bypass

Only then can the comparison be technically meaningful.

What Should Be Compared in Two Quotations?

Suppose:

Supplier A

Soft Starter Panel — ₹X

Supplier B

VFD Panel — ₹Y

Do not compare only price.

Compare:

Requirement	Soft Starter Offer	VFD Offer
Motor rating/current	?	?
Starting duty	?	?
Heavy/normal duty	?	?
Speed control	No	?
Bypass	?	?
Harmonic mitigation	N/A/limited need	?
Output filter	N/A	?
Motor cable length	?	?
Communication	?	?
Panel cooling	?	?
Protection	?	?
FAT scope	?	?
Lifecycle energy benefit	?	?

Then decide whether the additional VFD cost provides real project value.

Relevant IEC and Indian Standards

For Soft Starters, the relevant IEC product standard is:

IEC 60947-4-2:2020 + Amendment 1:2024, covering semiconductor motor controllers, starters and soft starters, with the amendment corrected in March 2026.

For Indian projects, BIS currently lists:

IS/IEC 60947 (Part 4/Sec 2):2020

for semiconductor motor controllers, starters and Soft Starters.

For applicable adjustable-speed power-drive systems, IEC publishes:

IEC 61800-5-1:2022

covering electrical, thermal, energy and related safety requirements for adjustable-speed electrical power-drive systems.

Project specifications should also consider:

• Complete panel assembly requirements
• EMC
• Harmonic requirements
• Motor requirements
• Client/project standards

rather than referencing one product standard in isolation.

FAT for Soft Starter Panels

During Soft Starter panel FAT, applicable tests should include:

Component Verification

• Soft Starter make/model
• Current rating
• Duty
• Bypass arrangement
• Breaker/fuse
• Contactor
• Motor protection
• Control supply

Functional Testing

• Start command
• Stop command
• Local/remote
• Ramp settings
• Current-limit settings
• Bypass operation
• Trip simulation
• Reset
• Run feedback
• Fault feedback
• PLC interface
• Communication

Protection

Verify configured functions according to the approved project requirement.

FAT for VFD Panels

For VFD panels, the scope should additionally consider:

• VFD make/model
• Current/kW rating
• Heavy/normal-duty selection
• Frequency limits
• Acceleration
• Deceleration
• Speed reference
• Local/remote
• Run feedback
• Fault feedback
• PLC communication
• Input/output reactor where applicable
• dV/dt filter where applicable
• Harmonic equipment
• Braking arrangement
• Bypass
• Cooling
• Interlocks

The VFD should not simply be powered ON and marked:

PASS.

The required application functions should actually be simulated.

How Wisdom Techno Solutions Approaches Soft Starter and VFD Panels

At Wisdom Techno Solutions, the starting technology should be selected around the motor, load and process, not simply the motor kW.

Wisdom Techno Solutions is a licensed partner for three design-verified switchgear platforms: Rittal Ri4Power (Rittal Solutions Partner since 2021 — the Ri4Power system carries Intertek ASTA design verification to IEC 61439-1 and -2 Edition 3, with copper and aluminium busbar systems up to 6300 A), C&S CX (Licensed Partner since 2023, up to Form 4B Type II) and Siemens SIEPAN Elite/8PU (IEC 61439 Solutions Partner since 2024). We build to each system's verified configuration and routine-verify every assembly in-house at our Vadodara, Gujarat facility, for industrial, EPC and infrastructure projects across India. That places the drive-panel thermal and fault rating inside a verified design envelope.

For motor-control requirements, we engineer customized panels incorporating:

• Soft Starters
• VFDs
• DOL/Star-Delta bypass arrangements where technically required
• PLC interfaces
• Local/remote controls
• Protection
• Metering
• Communication
• Input/output reactors
• dV/dt filtering
• Harmonic-mitigation equipment
• Process interlocks

Before finalizing the panel, the useful engineering questions include:

• Does the motor need continuous speed control?
• What torque is required during starting?
• Is the load high inertia?
• What is the allowable starting current?
• Is DG operation required?
• How long is the motor cable?
• What is the operating environment?
• Is bypass genuinely required?
• Are harmonics relevant?
• What communication does the customer require?

The objective should not be:

“Sell a VFD because it has more features.”

Nor should it be:

“Use a Soft Starter because it is cheaper.”

The objective should be:

Use the simplest technology that fully meets the electrical and process requirement over the equipment lifecycle.

WTS FAT and Inspection Capability

WTS performs applicable panel FAT using in-house calibrated testing instruments and equipment.

Depending on project scope, testing can include:

• HV/dielectric testing
• Insulation resistance testing
• Continuity
• Internal wiring verification
• Protection
• Control logic
• Interlocks
• Soft Starter operation
• VFD operation
• PLC/HMI interface
• Communication
• Bypass logic

We support customer or consultant-witnessed FAT where required.

We also facilitate Third Party Inspection according to project or customer requirements.

Common Soft Starter vs VFD Selection Mistakes

Mistake 1: Selecting VFD Because It Is “Better Technology”

Better for what?

If speed control is unnecessary, its additional functionality may provide no benefit.

Mistake 2: Selecting Soft Starter Only Because It Is Cheaper

If continuous variable-speed operation is required, it cannot replace the VFD.

Mistake 3: Selecting From Motor kW Alone

A 90 kW pump and 90 kW loaded conveyor can require very different solutions.

Mistake 4: Assuming VFD Always Saves Energy

Energy saving requires a process that can actually benefit from reduced speed.

Mistake 5: Ignoring Starting Torque With Soft Starter

Too much current limitation can prevent proper acceleration.

Mistake 6: Ignoring Harmonics With VFD

A continuously operating VFD is a nonlinear load and should be considered in system power-quality design.

Mistake 7: Ignoring Motor Cable Length With VFD

Long cables can introduce dV/dt and reflected-wave concerns.

Mistake 8: Adding Bypass Without Understanding Why

Bypass must match the electrical and process philosophy.

Mistake 9: Ignoring Starts Per Hour

Starting frequency affects thermal sizing.

Mistake 10: Comparing Only Purchase Price

Compare lifecycle value.

Soft Starter vs VFD Selection Checklist

Before making the decision:

Motor

✓ Motor kW/current known ✓ Motor duty known ✓ Starting torque known where critical ✓ Motor suitable for VFD if applicable

Process

✓ Fixed or variable speed? ✓ Minimum speed? ✓ Maximum speed? ✓ Smooth stop required? ✓ Process feedback required? ✓ Energy-saving opportunity?

Electrical

✓ Transformer size known ✓ DG operation considered ✓ Starting-current limit known ✓ Harmonic requirement known ✓ Fault level known

Installation

✓ Cable length known ✓ Ambient temperature known ✓ Panel cooling considered

Commercial

✓ Initial cost compared ✓ Energy profile evaluated ✓ Maintenance considered ✓ Process benefit quantified ✓ Downtime implications considered

Then select the technology.

Conclusion

The choice between a Soft Starter and VFD becomes much easier when the correct question is asked.

If your requirement is:

Start smoothly → Run at full speed → Stop smoothly

evaluate a:

Soft Starter

If your requirement is:

Start smoothly → Continuously change speed/torque according to the process → Stop under controlled conditions

evaluate a:

VFD

A Soft Starter is not an inferior VFD.

A VFD is not simply an expensive Soft Starter.

They are different technologies designed for different operating requirements.

Do not select from motor kW alone.

Study:

• Starting current
• Starting torque
• Load inertia
• Required speed
• Process profile
• Operating hours
• Energy potential
• DG conditions
• Harmonics
• Cable length
• Lifecycle cost

Then choose.

The best solution is not the device with the most features.

It is the device whose features the application actually needs.

Planning a Soft Starter or VFD panel?

Share your motor datasheet, load application, SLD, transformer/DG details, motor cable length and control requirement with Wisdom Techno Solutions for project-specific motor-control panel engineering and selection.

Related Guides

• DOL vs star-delta vs soft starter vs VFD
• VFD harmonics, chokes and dV/dt filters
• MPCB vs MCCB + OLR vs motor protection relay
• VFD bypass panel design
• Product page: VFD and soft starter panel

Frequently Asked Questions

What is the main difference between a Soft Starter and VFD?

A Soft Starter primarily controls motor acceleration and stopping for full-speed applications, while a VFD continuously controls motor frequency and speed during normal operation. The soft starter regulates motor terminal voltage through thyristors during the ramp, then normally hands the motor over to the line supply through a bypass. A VFD converts the supply continuously and produces its own controlled frequency and voltage for as long as the motor runs, which is what makes adjustable speed possible. In practice, the difference decides the answer: full-speed duty favours a soft starter, variable-speed duty needs a VFD.

Which is cheaper: Soft Starter or VFD?

For comparable motor ratings, a Soft Starter is generally less expensive than a VFD, and the gap usually widens as motor kW increases. The saving is not only in the device — a soft starter panel typically needs less enclosure width, less cooling and none of the input reactors, output filters or harmonic mitigation a VFD installation may require. However, purchase price is the wrong basis for the decision on a variable-demand process, where the VFD's energy and process-control benefit can outweigh a higher initial cost within a single year.

Can a Soft Starter control motor speed?

Not as a normal continuous variable-speed controller. It ramps the motor toward normal operating speed. For continuous adjustable-speed operation, a VFD is the appropriate technology.

Can a VFD perform soft starting?

Yes. A VFD accelerates the motor from low frequency upwards, so it can limit starting current and control torque more tightly than a soft starter can, and it retains full speed and torque control after startup. That is why a VFD is often specified on weak networks or DG-fed buses where the permissible voltage dip is tight. The point to note is commercial rather than technical: if soft starting is the only requirement, you are paying for continuous drive capability, continuous losses and continuous harmonic considerations that the process never uses.

Which is better for a pump: Soft Starter or VFD?

For a fixed-speed pump requiring smooth starting/stopping, a Soft Starter can be highly suitable. If flow or pressure must be continuously controlled through pump speed, a VFD is generally the stronger solution.

Does a VFD always save electricity?

No. A VFD only saves energy where the process allows the motor speed to actually be reduced for a meaningful part of the operating hours. A pump or fan that runs 24 hours a day at 100% speed gains no process-energy benefit from a drive, and the drive's own conversion losses — typically a few percent of rated power, dissipated continuously — make the total slightly worse than a bypassed soft starter. Estimate the saving from a real load profile: operating hours against required flow or pressure, the existing control method, and the pump or fan curve.

Does a Soft Starter save electricity?

Its primary purpose is controlled starting and stopping rather than continuous speed-based energy optimization. In bypass operation, the motor normally runs at line frequency.

Which is better for a conveyor?

If the conveyor operates at one speed and needs smooth acceleration, a Soft Starter may be sufficient. If speed adjustment, synchronization or controlled low-speed operation is required, a VFD may be more appropriate.

Which is better for a motor running on DG?

Either can be appropriate. Selection depends on starting current, torque, generator capability and whether continuous speed control is required. A VFD also introduces continuous harmonic considerations on the generator-fed network.

Does a Soft Starter produce harmonics?

Semiconductor phase control creates distortion primarily during its controlled starting/stopping operation. With a bypassed Soft Starter, the motor subsequently operates directly from the AC supply.

Does a VFD produce harmonics continuously?

Conventional VFD input rectifiers are nonlinear loads, so supply-side harmonic behaviour is a consideration during operation. The actual system impact depends on the drive and electrical network.

Does a VFD require an output reactor?

Not always. Requirement depends on the selected drive, motor cable length, motor insulation, switching characteristics and manufacturer recommendations.

Does a Soft Starter require an output reactor?

Normally no. Once a bypassed Soft Starter has brought the motor to full speed, the motor is fed from the ordinary sinusoidal AC supply, so there is no continuous PWM waveform, no reflected-wave voltage and no dV/dt stress on the motor insulation. Motor cable length therefore follows conventional cable sizing and voltage-drop rules rather than drive-specific limits. This is one of the practical reasons soft-starter installations are simpler for long-cable, full-speed motor duties.

Does every Soft Starter need an external bypass contactor?

No. Some Soft Starters include internal bypass arrangements. Check the selected product architecture before designing the panel.

Does every VFD need bypass?

No. A bypass should be provided only where the process genuinely must keep running after a drive fault, and only where full-speed operation is actually acceptable to the process. A DOL bypass is also useless if the motor was given a VFD in the first place because the network could not tolerate direct starting — the bypass would simply reproduce the problem the drive was bought to avoid. Where availability is the real requirement, a duty/standby drive arrangement is often the better architecture than a drive plus DOL bypass.

What standard applies to Soft Starters?

IEC 60947-4-2:2020 together with Amendment 1:2024 covers applicable semiconductor motor controllers, starters and Soft Starters for AC circuits up to 1,000 V. It is a product standard for the device, not for the assembly that houses it. The complete panel is separately governed by IEC 61439-1:2020 and IEC 61439-2:2020, so a project specification should reference both rather than quoting one product standard in isolation.

What Indian standard applies to Soft Starters?

BIS currently lists IS/IEC 60947 (Part 4/Sec 2):2020 for semiconductor motor controllers, starters and Soft Starters. It is the Indian adoption of the corresponding IEC part, so the technical requirements align with the IEC text. For Indian projects it is usually cleanest to specify the IS/IEC reference for the device and the IEC 61439 series for the assembly, so that both the starter and the panel are covered.

What IEC standard applies to VFD power-drive safety?

IEC 61800-5-1:2022 addresses electrical, thermal, energy and related safety requirements for applicable adjustable-speed electrical power-drive systems. It covers the drive as a power-drive system rather than as a switchgear component, which is why VFD panel specifications often need to cite it alongside the assembly standard. Harmonic performance and EMC requirements are handled separately and should be stated explicitly in the project specification.

What information is required before selecting Soft Starter or VFD?

Provide motor current/kW, load type, starting torque, load inertia, speed range, starts per hour, process requirement, transformer/DG details, motor cable length, ambient conditions, control philosophy and communication requirements. Motor kW alone is not enough — a 90 kW centrifugal pump and a 90 kW loaded conveyor can need different starter duty ratings and sometimes different technology altogether. Starts per hour and loaded-versus-unloaded starting are the two inputs most often omitted from enquiries, and both directly affect thermal sizing.