VFD vs. Soft Starter: Which One Do You Need?

Key Points

• A soft starter controls a motor only during startup and stop. A VFD (variable frequency drive) controls motor speed the entire time it runs.
• If your motor runs at one constant speed and you just need gentler starts, a soft starter is usually the smarter, lower-cost choice.
• If your process needs variable speed, or you run fans and pumps that could slow down part of the time, a VFD can pay for itself in energy savings.
• VFDs cost more up front and can require inverter-duty motors, extra cooling, and harmonic considerations.
• Witmer Motor Service, one of the leading electric motor sales and service companies since 1964, stocks both and can help you choose the right fit.

When a large motor starts across the line, it can pull six to eight times its normal running current. That jolt stresses the motor, the driven equipment, and your electrical system.

Both soft starters and VFDs solve that problem. But they are not interchangeable, and choosing the wrong one can cost you money for years.

Our team at Witmer Motor Service has been helping customers with motors and controls since 1964. Here is how we help customers decide.

What Is a Soft Starter?

A soft starter is a solid-state device that gradually ramps up the voltage to a motor during startup.

Instead of slamming the motor with full voltage, it brings the motor up to speed smoothly. Many models can also ramp the motor down gently when stopping.

Once the motor reaches full speed, most soft starters close an internal bypass contactor. The motor then runs directly on line power at full speed.

What a soft starter does well:

• Reduces inrush current, typically to about two to four times full load amps
• Reduces mechanical shock on belts, couplings, gears, and shafts
• Helps prevent water hammer in pump systems
• Costs less and takes up less panel space than a comparable VFD

What it cannot do:

• Change motor speed once it is running
• Save energy during normal operation

What Is a VFD?

A variable frequency drive controls motor speed by changing the frequency and voltage of the power supplied to the motor.

It converts incoming AC power to DC, then rebuilds it as adjustable AC output. That gives you full control over speed and torque from start to stop.

What a VFD does well:

• Provides the smoothest possible start, often near full load amps
• Adjusts motor speed to match the actual demand of your process
• Saves significant energy on fans, pumps, and blowers
• Offers advanced diagnostics, motor protection, and automation features
• Can run a three-phase motor from single-phase input on smaller horsepower sizes with proper derating

Trade-offs to know about:

• Higher upfront cost, often two to three times a soft starter
• Generates more heat and may need ventilated or cooled enclosures
• Can create electrical harmonics that affect other equipment
• May require an inverter-duty motor, especially at low speeds or with long cable runs

Why VFDs Save So Much Energy on Fans and Pumps

This is the part most comparison guides skip.

Centrifugal fans and pumps follow what engineers call the affinity laws. Power draw changes with the cube of speed.

That means:

• Running a fan at 80 percent speed uses roughly half the power
• Running it at 50 percent speed uses only about one-eighth the power

If your fan or pump runs at full speed and you control flow with a damper or throttling valve, you are wasting energy. The U.S. Department of Energy identifies adjustable speed drives as one of the most effective ways to cut motor system energy use.

A soft starter cannot deliver these savings, because the motor always runs at full speed.

VFD vs. Soft Starter at a Glance

Which One Does Your Application Need? A Quick Checklist

Choose a soft starter if:

1. Your motor runs at one constant speed
2. Your main goal is reducing inrush current and mechanical stress
3. The motor starts only a few times per day
4. Budget and panel space are tight
5. Common examples include conveyors, crushers, rock grinders, constant-flow pumps, and large compressors

Choose a VFD if:

1. Your process needs different speeds at different times
2. You run fans, pumps, or blowers that do not always need full output
3. You need precise torque, acceleration, or positioning control
4. You want to run a smaller three-phase motor from single-phase power
5. Common examples include HVAC fans, irrigation and booster pumps, mixers, machine tools, and variable-flow processes

Still unsure? That is completely normal. Reach out to our team with your motor nameplate and application details, and we will point you in the right direction.

Do Not Forget About the Motor

The control you choose affects the motor it runs.

Standard motors run on a VFD can see extra heat and insulation stress. NEMA sets guidelines for inverter-fed motors in its MG 1 standard, Part 31. For many VFD applications, an inverter-duty three phase motor is the safer long-term choice.

Soft starters are easier on motors in this respect, since the motor runs on normal line power once started.

Either way, watch for the warning signs of motor failure. The Electrical Apparatus Service Association (EASA) is also a great resource for motor care best practices.

Shop VFDs and Soft Starters at Witmer Motor Service

We carry a wide selection of VFDs and soft starters from trusted brands, including:

• WEG
• Fuji Electric
• Teco-Westinghouse
• KB Electronics
• Leeson

As a family-owned company with a 4.96 star customer rating, we are proud to be one of the most trusted names in motor sales and service. Our customers count on us for honest advice, not upselling.

Frequently Asked Questions

Can a VFD replace a soft starter?

Yes. A VFD can do everything a soft starter does, plus speed control. It just costs more, so it only makes sense if you will use the extra features.

Does a soft starter save energy?

Only a little, by reducing startup demand. Once the motor reaches full speed, energy use is the same as a direct-on-line start.

Do I need an inverter-duty motor for a VFD?

Not always, but it is often recommended. Low-speed operation, long cable runs, and higher voltages all increase the need for an inverter-duty motor.

Can a VFD run a three-phase motor on single-phase power?

Yes, for smaller motors, with the drive properly derated. For multiple machines, a phase converter is often the better option.

Which lasts longer, a soft starter or a VFD?

Soft starters are simpler and generally have fewer components that wear. VFDs last well too when kept clean, cool, and properly sized.

Get Expert Help Choosing Your Motor Control

The right choice depends on your motor, your load, and how your equipment actually runs.

Since 1964, Witmer Motor Service has helped farms, shops, and industrial facilities get it right the first time. Contact us today or browse our VFDs and soft starters.