Comparison

VFD vs Soft Starter: Which One for an AC Induction Motor?

A soft starter eases the start and then runs at full speed. A VFD controls speed the whole time. Compare them by load, energy use, harmonics and cost.

Electrical & Power · Last reviewed October 15, 2026

Short answer: use a soft starter if the motor only needs a gentler start and then runs at full speed. Use a VFD if you need speed control, or if the load is a fan or pump whose flow varies.

	Soft starter	VFD
What it does	Ramps voltage during the start, then is bypassed at full speed	Varies frequency and voltage and controls speed continuously
Speed control	None. Full speed only	Full range
Starting current	Reduced, typically about 2 to 4 times full-load current	Can be held near or below full-load current
Starting torque	Reduced. Torque falls with the square of the voltage	Full torque available at low speed
Energy saving while running	None	Large on variable-torque loads (fan and pump affinity laws)
Harmonics	Very low once bypassed	Significant. May need line reactors or filters
Cost and size	Lower	Higher
Typical uses	Fixed-speed conveyors, crushers, compressors, pumps	HVAC fans, variable-flow pumps, process lines

How to choose

Does the process need speed control? If yes, use a VFD.
Is the load a centrifugal fan or pump with varying flow? Usually a VFD, because power falls roughly with the cube of speed.
Is the only problem inrush current or mechanical shock at start? A soft starter is enough.
Is the supply weak, such as a generator or a long feeder? Compare both. A VFD limits inrush the most.
Does the load start with high inertia or under load? Check the torque. Reduced-voltage starting may not accelerate it.

The fan example

For a centrifugal fan where the affinity laws apply, shaft power varies about with the cube of speed. At 80% of full speed:

power ratio ≈ 0.80³ = 0.512, so 10 kW at full speed becomes about 5.12 kW

This is a shaft-power estimate, not a prediction of electricity savings. Static pressure, efficiency changes, minimum-speed limits and drive losses all change the real result. Do not apply the cube law to every load. A constant-torque load such as a conveyor does not follow it.

Common mistakes

Buying a VFD only to start softly when a soft starter would do.
Ignoring harmonics, cable length and motor insulation when a VFD feeds the motor.
Assuming a soft starter saves energy while running. It does not.
Running a self-cooled motor at low speed for long periods without checking its cooling.

Two scenarios

A loaded conveyor at fixed speed on a weak supply. The motor must start under load and the utility limits starting current, but the belt never needs a different speed. A soft starter with a current limit is the simpler and cheaper choice. Check that the reduced voltage still gives enough torque to move the loaded belt.

A supply fan whose airflow changes through the day. The load follows the fan laws, so running at 80% speed uses about half the power. A VFD pays for itself in energy savings and also gives a soft start as a side effect.

Cost and life cycle

Purchase price: a soft starter is usually cheaper than a VFD of the same rating.
Energy: a VFD saves energy only on loads that can run slower. On a fixed-speed load it saves nothing.
Installation: VFDs may need line reactors, filters, shielded cable and attention to cable length. Soft starters need little beyond a bypass contactor.
Maintenance: both have electronics and fans that age. VFDs have more parts that need setup and spares.

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Questions

Can a soft starter control speed?

No. It only shapes the start and stop. Once the motor is up to speed it runs at the supply frequency.

Does a soft starter need a bypass contactor?

Many do, to remove the heat and the losses of the power electronics at full speed. Some are rated for continuous operation without one.

Can one VFD run several motors?

It can, with separate overload protection for each motor and attention to the cable lengths. Check the drive manufacturer's rules.

Does a VFD protect the motor?

It includes thermal and current protection, but the settings must be configured to the motor nameplate.

Do I need a line reactor with a VFD?

Often yes. A reactor reduces harmonic current and protects the drive from supply disturbances. The drive manual and the supply conditions decide.

Can a soft starter slow a motor down?

It ramps voltage for the start and the stop. It cannot hold a motor at a lower speed. Some models have a slow-speed jog function, but it is not speed control.

Educational material. Examples use simplified, hypothetical inputs and are not a design, a construction document or professional engineering advice. Confirm requirements with the codes and standards adopted where the work is built and with a licensed engineer.

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