Soft Starters vs. VFDs: Which One Should You Use?

Imagine you’re tasked with getting a massive industrial motor up and running. Flip the switch and you’re hit with a surge of current, lights flicker, and the machinery jolts from a dead stop to full speed. Not ideal, right? This is why soft starters and VFDs (variable frequency drives) are so crucial in motor control – they both tame the inrush current and mechanical stress of motor startup, but in very different ways. Choosing the right solution — soft start vs variable frequency drive — can impact everything from your equipment’s lifespan to energy efficiency and operational costs. In this post, we’ll compare soft starters and VFDs, explain how each works, outline their pros and cons, and help you determine which one you actually need for your industrial motors.

What is a Soft Starter?

A soft starter is an electronic device that smoothly ramps up the voltage to an AC motor, thereby limiting the sudden inrush of current during startup. In other words, it provides a “soft” start instead of a jarring full-voltage kick. Soft starters use solid-state components (typically pairs of SCR thyristors) to temporarily reduce the supply voltage and limit the torque when the motor begins to turn. By gradually increasing the voltage over a set ramp-up time, the motor accelerates gently to full speed. Once the motor reaches full speed, many soft starters employ a bypass contactor to connect the motor directly to the line, avoiding additional losses in the electronics.

Key features of soft starters:

• Reduced Inrush Current: By lowering the initial voltage, soft starters dramatically cut the startup current spike, protecting the motor’s windings and preventing voltage sags in the electrical system. This helps avoid tripping breakers and minimizes stress on the power circuit (circuit protection benefit).

• Lower Startup Torque: Because torque is roughly proportional to the square of voltage, reducing voltage also curtails the torque surge. This prevents the mechanical shock to gears, belts, and couplings that a direct-on-line start can cause. For example, conveyors and belt-driven systems use soft starters to relieve torque spikes and tension on startup. Pumps benefit from soft stop features that reduce water hammer in pipes, enhancing system safety.

• Simple Control: Soft starters are only active during startup (and optionally during shutdown). They do not adjust the motor’s normal running speed – once at full speed, the motor runs directly off line power (often via an internal bypass). This makes soft starters ideal for constant-speed applications where you just need to limit the start kick, and not vary the operational speed.

• Compact & Cost-Effective: Soft starters are usually smaller and simpler than VFDs, especially in higher horsepower ranges. They often take up less space in a control panel and come at a lower cost than an equivalent VFD for the same motor. Fewer components and a straightforward function make them a more economical choice when full speed control isn’t required.

In summary, a soft starter is a “gentle handshake” for your motor – easing it into motion to protect both the motor and the mechanical drivetrain. However, once the motor is up to speed, a soft starter steps aside and plays no further role in controlling the motor.

What is a Variable Frequency Drive (VFD)?

A VFD (Variable Frequency Drive) is a more sophisticated motor control device that adjusts the speed of an AC motor by controlling the frequency and voltage of the power supplied to the motor. Unlike a soft starter, which only modulates voltage during startup, a VFD fully converts incoming AC power into DC and then back into a variable frequency AC output. This allows it to ramp the motor smoothly through startup and continue to control motor speed and torque at any point during operation.

How a VFD works, in a nutshell, is by using a three-stage power electronics system:

1. Rectifier: Converts the incoming AC (e.g. 480 V, 60 Hz) to DC.

2. DC Bus: Filters and smooths the DC (using capacitors/inductors) to provide a stable link.

3. Inverter: Chops the DC back into AC at the desired frequency and voltage using high-speed switching (typically IGBTs with PWM – pulse width modulation). This synthesized AC can be of any frequency (and corresponding voltage) to control motor speed.

Key features of VFDs:

• Full Speed Control: VFDs can continuously adjust motor speed from zero to above rated speed (if desired), allowing for complete control throughout the run cycle. You can increase, decrease, or hold a precise speed as process needs demand. This makes VFDs essential for variable-speed applications where you might, for example, ramp a pump up and down to match flow demand or control a fan’s speed to regulate air flow.

• Soft Start and Stop: A VFD inherently provides a soft start (by starting at a low frequency/voltage and ramping up) and can also perform controlled deceleration (soft stopping) by ramping down the frequency. This yields a consistent acceleration and deceleration profile for the motor, which can be tuned for the application.

• Torque Control & Performance: Because it controls frequency, a VFD can deliver 100% torque at even 0 speed if needed (holding a load steady), and can provide high breakaway torque for heavy loads from standstill – something soft starters cannot do. Modern drives often include features like dynamic torque control, letting you limit or boost torque as required by the process.

• Energy Savings: Perhaps the biggest advantage, VFDs allow significant energy efficiency gains in many applications. By reducing motor speed when full speed is not needed, a VFD cuts down power use dramatically (thanks to the affinity laws for pumps and fans). For example, running a fan at 80% speed can use roughly half the energy compared to full speed. Eaton notes that a VFD “can provide energy savings of up to 50 percent”, translating to major cost savings over the equipment’s life. We’ll dive deeper into energy comparisons later.

• Advanced Functionality: VFDs are essentially smart devices. They often come with digital displays, self-diagnostics, and communication protocols to integrate with control systems. They provide protections(overload, undervoltage, etc.), programmable I/O, and sometimes even built-in PLC-like logic. Many VFDs include features like automatic fault detection, remote control, and even safety interlocks (e.g. “safe torque off” function) to meet modern industrial safety standards. This robust feature set can reduce the need for additional control hardware in a system.

• Versatility: VFDs can control motor speed on the fly, enable quick reversing without extra contactors, and even perform braking (by dissipating energy or regenerating it) for rapid stops. They work not only with standard induction motors but also with certain synchronous and permanent magnet motors (in appropriate configurations), broadening their use in advanced applications.

In short, a VFD is the “maestro” of motor control – it not only starts the motor smoothly but can conduct the motor’s speed and torque like an orchestra throughout operation. The trade-off for all this capability is complexity, size, and cost, which brings us to the crucial differences between these two solutions.

Soft Start vs. Variable Frequency Drive: Key Differences

Both soft starters and VFDs reduce the stress of motor startup, but beyond that shared goal they diverge significantly. Let’s break down the key differences between soft starters and VFDs in terms of functionality, capabilities, and impact on your system:

• Speed Control: The most fundamental difference is speed control. A soft starter only controls the motor’s acceleration during startup (and deceleration if a soft stop is used) – once the motor is at full speed, the soft starter is out of the circuit (bypassed) and cannot slow down or speed up the motor. In contrast, a VFD can control the motor’s speed continuously throughout its operation, from zero to full speed and any point in between. This means soft starters are suited for fixed-speed motors, whereas VFDs excel in variable-speed applications where process control or frequent speed adjustments are needed.

• Method of Control: A soft starter only reduces voltage to limit current and torque during start. It does not change the supply frequency, so it cannot alter the motor’s synchronous speed. A VFD, on the other hand, modulates both frequency and voltage to control motor speed and torque. Essentially, the soft starter is a voltage controller, while the VFD is a frequency (and voltage) controller that completely manages the power sent to the motor.

• Torque and Performance: Because of their control methods, VFDs can deliver full torque even at low speeds or standstill, and can provide extra torque for heavy-load startup if needed. Soft starters inherently reduce torque during startup (since they reduce voltage), so they cannot provide full torque at low speeds – the motor may struggle with heavy loads until it reaches near full voltage. If an application requires high starting torque or holding torque at zero/low speed, a VFD is the only choice. Soft starters are adequate for normal starting torque needs but not for high-torque demands like certain conveyors or positive displacement pumps under load (those often need VFDs or other solutions).

• Energy Efficiency in Operation: Soft starters have no effect on energy consumption once the motor is at full speed, because they effectively step out of the circuit (via bypass contactor) after the start. The motor then runs at full voltage directly from the line. In a constant-speed scenario, this is actually more efficient: the soft starter itself doesn’t consume significant power after startup, so the system efficiency remains very high (usually ~99% efficient). VFDs, by contrast, are active in the circuit at all times and do incur a continuous efficiency penalty (typically about 2-4% losses in the drive electronics). A standard VFD might be ~97–98% efficient at full load. However, the ability of VFDs to throttle speed can save far more energy in variable load situations than they lose in inherent efficiency. We will compare this trade-off shortly in the energy section.

• Size and Complexity: A soft starter is generally smaller, lighter, and simpler than a VFD for the same motor rating. It consists mainly of a few thyristors and a basic control module. A VFD is a more complex piece of equipment, packed with power electronics (rectifier diodes or thyristors, DC bus capacitors, IGBTs, filters) and requires more robust cooling and often EMI/RFI shielding. The complexity of VFDs means they typically have larger physical footprints than soft starters, especially in higher HP (kilowatt) ranges. If panel space is at a premium and speed control isn’t needed, a soft starter’s compactness can be a big advantage.

• Upfront Cost: Cost differences can be significant. Soft starters are usually much cheaper than VFDs of the same power rating, particularly as motor size grows. VFDs can cost 2–3 times more than a soft starter initially due to their complexity and components. For large motors, this often makes soft starters the go-to choice if they meet the requirements, because a large VFD can be prohibitively expensive (and requires more infrastructure to support). Keep in mind, however, that operational cost savings with a VFD (via energy efficiency) might offset the higher purchase price over time. It’s a classic CAPEX vs OPEX consideration.

• Harmonics and Power Quality: Because a soft starter only briefly controls the voltage during startup (and uses SCRs that conduct for part of each AC cycle), it introduces minimal harmonic distortion into the network, and typically does not require special harmonic filters. Once the motor is running and the soft starter is bypassed, it’s as if the motor is directly on line – no ongoing distortion. In contrast, a VFD’s rectifier and inverter inherently draw and create non-sinusoidal waveforms, injecting harmonic currents back into the supply. VFDs often require filters or line reactors to mitigate harmonics and meet power quality standards. This adds to system cost and complexity. Power factor is also impacted: soft starters have little effect on power factor after start, while VFDs typically have a front-end power factor of ~0.95 (good, but not unity, unless using active front ends).

• Motor Heating and Wear: A soft starter’s reduced-voltage start means less heating in the motor during startup compared to across-the-line starting (and it only affects the motor during that brief period). Once running, the motor sees normal sinusoidal voltage. A VFD, however, outputs a PWM-modulated waveform which can cause additional heating in the motor (due to waveform harmonics, eddy currents, etc.). Most modern motors are VFD-compatible, but older motors might run hotter on VFD power. Additionally, bearing wear can be an issue with VFDs due to induced shaft currents (often mitigated by shaft grounding and filters). Soft starters don’t create these high-frequency issues because they don’t alter frequency.

• Additional Features: Soft starters are relatively limited in features – they typically offer adjustable start ramp time, maybe adjustable starting voltage or current limit, and sometimes a soft stop feature. High-end soft starters might include motor protection functions (overload, phase loss, etc.) and diagnostics, but still nowhere near the flexibility of a VFD. VFDs come loaded with features: programmable acceleration profiles, multi-speed presets, PID controllers for process control, communications (Ethernet/IP, Modbus, etc.), diagnostic codes, and more. If your application demands a lot of control finesse or integration into automation systems, a VFD provides those tools out-of-the-box, whereas a soft starter would need external controls to achieve similar capabilities.

• Maintenance: Soft starters, having fewer components, often require less maintenance. Once installed, there’s not much to tend to except maybe tightening connections and verifying that the bypass contactor (if external) is functioning. VFDs require a bit more care: the cooling fans and capacitors have limited lifespans and may need periodic replacement, and keeping the drive free of dust and within temperature is important. Also, troubleshooting a VFD is more complex (given all the internal electronics and software) compared to a soft starter, which either works or not for the most part. However, VFDs do provide fault codes and diagnostics which can aid in maintenance, whereas a soft starter might simply trip without detailed insight.

In summary, a soft starter is a simpler, cheaper device that gently starts (or stops) a motor, ideal for applications that run at full speed. A VFD is a more advanced system that can not only soft-start but also vary the motor’s speed and torque on demand, at a higher cost and complexity. Your choice will depend on what your application needs – and next, we’ll look at typical use cases to illustrate that decision.

Pros and Cons of Soft Starters and VFDs

Now that we’ve covered the general differences, let’s distill the advantages and disadvantages of soft starters vs VFDs. This section provides a quick overview of the pros and cons of each solution:

Soft Starter Advantages

• Lower Cost (Especially Upfront): Soft starters are more budget-friendly than VFDs for the same motor size, often by a factor of two or more. This makes them attractive for large motors where VFD prices soar. You get the essential startup protection without breaking the bank.

• Smaller Size & Simplicity: With fewer components, soft starters usually have a smaller footprint and are easier to integrate into control panels. Installation and setup are straightforward (usually just setting ramp time and maybe current limit). Fewer things can go wrong, and no complex programming is needed.

• Reduced Mechanical & Electrical Stress: By limiting start current and torque, soft starters protect your motor and machinery from the strain of abrupt starts. This can extend equipment life – belts won’t slip as much, gear teeth won’t shear, and motors run cooler during startup. It’s a gentle approach that also avoids big voltage dips in your facility when a large motor kicks in.

• High Efficiency at Full Load: Once the motor is at speed and the internal bypass is closed, a soft starter adds virtually no additional losses. The motor runs as if directly connected to the mains, achieving ~99% efficiency. For continuous full-speed operation, this means no ongoing energy penalty from the control device, which is better than a VFD in such scenarios.

• Minimal Electrical Noise: Soft starters generate very little electromagnetic interference or harmonic distortion on the supply line (since they only briefly chop the waveform during starting). There’s typically no need for extra filters or line reactors to maintain power quality.

Soft Starter Disadvantages

• No Speed Control After Start: The biggest limitation is that soft starters cannot adjust the motor’s running speed. If your process later demands the motor slow down or speed up, a soft starter has no ability to do that – the motor is tied to the fixed supply frequency once started. This inflexibility makes it unsuitable for variable-speed needs.

• Limited Torque Boost: Soft starters inherently reduce voltage and torque during acceleration. If your load requires a heavy torque push to get moving (e.g. a loaded conveyor or a pump that starts against high head pressure), the soft starter may not get you there quickly or at all. It cannot provide torque beyond what the reduced voltage allows, so high-starting-torque applications might see sluggish starts or require an oversizing strategy.

• Fewer Features: Soft starters are single-purpose devices. They lack the rich feature set of VFDs – no constant speed regulation, no advanced braking, no built-in process control logic, etc. You may need additional equipment for things like motor overload protection or sequencing if the soft starter doesn’t include those.

• Still Draws High Running Current: Once at full speed, the motor draws full line current as normal (though startup was reduced). Unlike a VFD, a soft starter can’t offer running energy savings by lowering speed. So in applications with varying load, a soft starter misses out on potential energy reduction, and the motor will always run at full power consumption even if the load is light.

• Not Ideal for Frequent Start-Stop Cycles: If an application requires very frequent starting and stopping of the motor, a soft starter will still cause some heat each time during startup and can only ramp so fast. VFDs might handle rapid cycling better by controlling deceleration and re-acceleration more smartly. Soft starters also might need a cool-down between starts if done back-to-back, to avoid overheating the SCRs.

VFD Advantages

• Complete Speed Control: Dial in any speed you need. This is the number one reason to use a VFD. You can match motor speed to process requirements precisely – whether it’s slowing down a pump to 50% or fine-tuning a conveyor speed. This improves process control, efficiency, and flexibility in a way a soft starter cannot.

• Significant Energy Savings: In variable torque applications (fans, pumps, compressors), VFDs can dramatically cut energy usage by reducing speed during low demand periods. Instead of running a motor full bore and wasting energy (or using mechanical throttling), a VFD can trim speed to only use the power needed. This often yields energy (and cost) savings up to 30-50%, quickly paying back the VFD’s higher initial cost. With rising energy prices and efficiency goals, this is a major advantage.

• Smooth Acceleration & Deceleration: VFDs allow you to program acceleration and decel ramps exactly as needed – preventing not just start-up jolts but also sudden stops. This is great for avoiding water hammer in pipelines (slowly ramping down a pump) or preventing product damage (gradually stopping a conveyor rather than an abrupt halt). The consistent acceleration profile also reduces mechanical wear and enhances safety for connected equipment.

• High Starting Torque & Performance: Unlike soft starters, VFDs deliver robust torque across the speed range. Need to start a heavy load from zero speed? A VFD can apply a high torque at low frequency to get it moving. Need to hold a position or inch the motor slowly? A VFD can do that too, even provide full torque at 0 RPM if the motor is capable. This makes VFDs suitable for heavy-duty and high-precision applications (e.g. cranes, elevators, mixers, crushers) where starting torque or speed control under varying loads is crucial.

• Built-in Motor and System Protection: VFDs often include a suite of protective functions – overcurrent protection, overvoltage, undervoltage, phase loss detection, motor thermal modeling, etc. They can automatically trip to prevent damage to the motor or themselves if something goes wrong. Many have diagnostic displays or fault codes that aid in troubleshooting. This can enhance safety and reduce the need for separate protective relays.

• Multifunctionality: A single VFD can sometimes replace multiple components. For instance, a VFD can eliminate the need for a separate reversing contactor (it can reverse rotation electronically), or a flow control valve (by controlling pump speed), or even a soft starter (since it inherently soft starts). It can interface with PLCs directly, using analog/digital inputs or communication networks to control speed. All this can simplify the control system and provide a high degree of automation.

• Improved Power Factor at Light Loads: When running a motor at less than full load/speed, a VFD will generally draw less reactive current from the mains compared to an unloaded motor on the line. While a motor on a soft starter (or direct line) at no load still draws magnetizing current (lower power factor), a VFD’s DC-link isolates the motor’s reactive component from the supply. In practical terms, this means the system power factor seen by the utility can be better with lots of VFD-controlled motors, especially if lightly loaded. (Note: At full load, a standard drive’s input PF is ~0.95, so not unity, but some drives include active front ends or filters to correct PF).

VFD Disadvantages

• Higher Initial Cost: There’s no way around it – VFDs cost more upfront. You’re paying for advanced capability and components. They also often require auxiliary components like input/output filters, EMI shielding, or extra cooling, which add cost. In large motors, the cost difference is substantial, making the investment harder to justify if energy savings or process needs are minimal.

• Larger and Heavier: All those electronics take up space. VFDs tend to be bulky, and for high HP drives you might need significant panel or floor space, plus clearances for heat dissipation. If you’re retrofitting into a tight cabinet, a VFD might not fit where a simple starter could. Additionally, heat output from a VFD can be several percent of the motor’s power – that heat must be vented or cooled, whereas a bypassed soft starter runs cool.

• Complexity and Tuning: Installing and configuring a VFD is more involved. You must set the motor parameters, choose appropriate ramp profiles, and possibly tune the drive for stable operation (especially with high dynamic loads or multiple motors). There’s also more that can go wrong – firmware bugs, parameter misadjustment, noise issues, etc. Maintenance personnel need training to service and troubleshoot VFDs, whereas a soft starter is comparatively plug-and-play.

• Maintenance & Reliability Factors: VFDs have active electronic components that age – DC bus capacitors dry out over years, cooling fans wear out, and power semiconductors can fail under surges. They are also more sensitive to environmental factors: dust, heat, and moisture are enemies of drives. A soft starter, once bypassed, has very little continuously stressed electronics, so it can be very reliable long-term. VFDs typically have a MTBF (mean time between failures) that might be shorter than a simple starter, so you need to manage their upkeep (e.g. regular fan/capacitor replacements as preventive maintenance).

• Harmonic Distortion & Electrical Noise: As noted, VFDs inject harmonics into the supply which can affect other equipment (transformer heating, interference with sensitive devices, etc.) if not mitigated. They also emit electromagnetic noise due to rapid switching – this can require shielded cables and proper grounding to avoid interference with nearby instrumentation. Soft starters, in contrast, cause far less of these issues. Dealing with harmonics may involve installing filters or using multi-pulse or active-front-end drives, all adding to complexity.

• Potential Motor Stress if Not Managed: While VFDs bring many benefits, improper use can cause issues: for example, running a standard motor at very low speed for extended time can overheat it (since its fan isn’t moving enough air). Or driving a motor above nameplate speed can cause mechanical stress. Also, the high dv/dt pulses from VFD output can stress motor insulation, especially on long cable runs. Mitigation like dv/dt filters, or using inverter-duty motors, is sometimes needed. So, a VFD isn’t simply “set it and forget it” – you must ensure the motor and system are compatible with the VFD’s mode of operation.

Having laid out the pros and cons, you can see that soft starters shine in their simplicity and cost-effectiveness for basic start/stop duty, whereas VFDs provide unparalleled control and efficiency for variable-speed needs, at the expense of higher cost and complexity. Next, let’s consider some typical applications and scenarios to solidify when you would choose one over the other.

Applications and Use Cases

The choice between a soft starter and a VFD often “depends on your application”. Here are some common scenarios and guidelines on when to use a soft starter vs. a VFD:

When to Use a Soft Starter

Use a soft starter for applications that run at constant speed and mainly need reduced stress on startup:

• Pumps with Fixed Output: If you have a pump that runs at full speed to move fluid and you only need to avoid pressure surges and pipe stress, a soft starter is ideal. For example, in irrigation or water distribution where pumps kick on and off, a soft starter will prevent water hammer by gradually starting and stopping the pump. Once running, the pump sees full line power (max flow) which is what the process needs anyway.

• Conveyors and Material Handling: Many conveyor systems run at a fixed speed but can benefit from a gentler start to avoid jerking the belt and spilling or shifting the load. A soft starter can smoothly accelerate the conveyor, then a mechanical clutch or full-speed operation takes over. No fancy speed changes required. Similarly, belt-driven machinery, crushers, grinders, or fans that always operate at one speed can use soft starters just to alleviate startup strain.

• Large Motors with Infrequent Start/Stop: For very large induction motors (think hundreds to thousands of horsepower) driving things like compressors, blowers, or mills that do not require speed variation, soft starters are often the practical choice. They limit the massive inrush current such big motors draw, avoiding voltage sags in the plant and oversizing of generators or transformers. And compared to a VFD, the soft starter for a 1000 HP motor is much smaller and cheaper. As long as the process doesn’t need speed control, it’s a straightforward solution.

• Weak Power Systems or Backup Generators: In facilities where the power supply is limited (or running off a generator), starting large motors across-the-line can be prohibitive. Soft starters come to the rescue by limiting the current draw to a manageable level, allowing a motor to start without tripping the generator or dimming the lights. This is common in remote sites and marine applications.

• Basic Industrial Machines: Simple machines like large saws, presses, or pumps in older industrial setups might not have sophisticated control systems. Adding a soft starter can be a quick retrofit to reduce electrical and mechanical shocks during startup without altering the machine’s operation otherwise.

• Budget-Sensitive Projects: When funds are tight and the process requirements are basic, a soft starter often delivers the most bang for the buck. For example, if you have a ventilation fan that just needs to start smoothly and then run, a soft starter will be far cheaper than a VFD and will accomplish the goal of reducing startup current (preventing breaker trips or motor stress).

In these cases, soft starters provide a cost-effective, low-complexity solution to start motors gently. You’ll get the benefit of reduced startup wear and tear, without the need for the motor speed to fluctuate once it’s running.

When to Use a VFD

Choose a VFD whenever speed control, energy savings, or advanced control features are required by the application:

• Variable Flow Fans and Pumps (HVAC and Process): Arguably the most widespread use of VFDs is in centrifugal fans and pumps. HVAC systems, for instance, use VFDs on fans to regulate building airflow based on demand. Instead of dampers or on/off cycling, the fan speed modulates, saving huge amounts of energy. Likewise, in process industries, pumps with VFDs can adjust flow or pressure setpoints continuously. Whenever you see the need for variable flow or pressure, a VFD is usually the answer for the combination of control and efficiency.

• Energy Efficiency Requirements: If an application runs long hours with varying loads, and energy constitutes a big part of operating costs, VFDs become attractive. For example, a pump that runs 24/7 but at half capacity most of the time – a VFD will drastically cut energy use (vs running full speed and throttling flow). Companies with green initiatives or seeking to reduce energy costs will often justfy VFDs on this basis alone, using the projected energy savings (often 20-50%) to calculate a short payback period.

• Process Control & Automation: In manufacturing lines, the ability to fine-tune motor speeds can improve product quality and throughput. For example, conveyor lines might need different speeds at different stages, or mixers/blenders might ramp up slowly to avoid splashing then speed up for a thorough mix. VFDs allow programming these profiles. With their communication abilities, they can be controlled by a central PLC or even IIoT systems for recipes, feedback loops, etc. Precision applications like extruders, winches, elevators, or CNC machines rely on VFDs (often in tandem with feedback sensors) to achieve the required speed/torque profiles and positional control.

• Frequent Start/Stop or Reversing: If your operation involves starting and stopping a motor frequently, a VFD handles this more gracefully than a soft starter. Since it can decelerate the motor under control, it avoids wear from plugging or mechanical braking. Additionally, if you need to reverse the motor direction often, a VFD can do so electronically with no additional hardware – perfect for something like an overhead crane or hoist that moves in both directions. Soft starters cannot reverse on their own (you’d need contactors) and are not ideal for very frequent cycling.

• Multi-Speed or Multi-Motor Systems: In some systems, one VFD can control multiple motors (if they are always supposed to run in unison, like multiple fans on a header, although careful sizing and controls are required). Or a process might require a motor to run at say, two or three distinct speeds (e.g., slow speed for setup, fast speed for production). While a soft starter typically just ramps to full, a VFD can have preset speeds or be commanded to any speed. For example, a centrifugal separator might start slow to load, then speed up to operational rpm.

• Mechanical Stress and Special Ramp Profiles: Some loads are extremely sensitive – for instance, a large centrifuge or water pump might need a very specific acceleration curve to avoid resonance or surges. VFDs let you shape the ramp (S-curves, etc.), which can be a requirement in those cases. Also, if you need to apply dynamic braking or hold a shaft in position, VFDs with braking units or regenerative capability are essential (soft starters simply let the motor coast on power off).

• Non-Standard Motor Types: If you have synchronous motors or permanent magnet AC motors for high-efficiency applications, these generally must have a VFD for startup and speed control. Soft starters only work with standard induction motors (and even then, mainly squirrel-cage type). VFDs, especially in vector control modes, can manage these motor types and even do sensorless control of speed/torque.

In summary, use a VFD when you need more than just a smooth start – if you require speed variation, optimized energy use, or intelligent control, VFD is the tool for the job. The investment in a VFD pays off in process optimization, energy savings, and sometimes even improved product quality or system longevity.

Energy Efficiency Considerations

Energy efficiency is a major factor in modern motor control decisions. Both soft starters and VFDs can save energy in different ways, but their impact varies greatly during startup vs running:

• During Motor Startup: In terms of energy during the actual starting event, both soft starters and VFDs reduce the stress and wasted energy compared to an across-the-line start. A soft starter’s gentle voltage ramp minimizes sudden current surge and excessive heat generation in the motor windings during start. A VFD typically uses an even more efficient start – by controlling frequency and voltage, it can bring the motor up to speed with minimal excess current, essentially eliminating the big inrush completely. The difference in energy consumption just during the few seconds of startup is usually not significant in the big picture (starts are infrequent and brief), but the stress reduction is where the “savings” matter (less thermal and mechanical damage means less maintenance/longer life).

• Running at Full Speed: If your motor will always run at its rated speed and load, a soft starter has the edge in efficiency during operation because it gets out of the way. As noted earlier, with the bypass contactor closed, the soft starter itself draws negligible power – the motor sees normal line power at near unity power factor. In contrast, a VFD driving a motor at full speed will typically be about 97-98% efficient. That means a small percentage of the energy is lost as heat in the VFD. For example, if you have a 100 kW motor at full load, the VFD might waste 2-3 kW as heat continuously. Soft starter: 0 kW loss after start. So, in a steady-state full-speed scenario, the system with a soft starter will consume slightly less electricity than with a VFD (on the order of a couple percent). As one expert put it, “if it is going to run at a fixed speed all the time, the soft starter is the clear answer” because after the motor is up to speed the soft starter “shuts down and not use any power… there’s no efficiency loss” in running.

• Running at Partial Speed/Load: The game completely changes here. If your motor does not need to run at full capacity all the time, a VFD allows you to capitalize on huge energy savings by reducing speed. Thanks to the affinity laws, the power required by a centrifugal load (like a fan or pump) drops roughly with the cube of the speed. So even a modest reduction in speed can yield a big drop in energy use. For instance, running a fan at 80% speed might use only ~50% of the energy compared to full speed! A soft starter cannot provide any running speed reduction – if you tried to throttle flow mechanically (valves, dampers) while still running the motor at full speed, you’d waste a lot of energy. This is where VFDs shine: they tailor the motor’s output to exactly what’s needed, avoiding wasted energy. Over time and long duty cycles, these savings are enormous. That’s why VFDs are strongly promoted for energy efficiency in HVAC, pumping, and other variable load systems. The upfront cost of the VFD is often recovered through lower electricity bills.

• Lightly-Loaded Motors: Even if a motor runs at full speed, sometimes it is lightly loaded (e.g., an oversized motor). Some modern soft starters have an “energy saver” mode that can reduce voltage a bit for lightly loaded motors to improve power factor and slightly reduce core losses. However, the efficiency gain is relatively small and this only works for steady light loads. A VFD in contrast could slow the motor (if the process permits) to actually reduce the real power use significantly. If the motor must run at full speed despite light load, a VFD doesn’t help much with efficiency (you might actually lose a tiny bit due to the VFD losses). In such a case, neither device has a strong advantage – it might be better to right-size the motor or add capacitor banks for power factor.

• Duty Cycle and Idle Periods: If a process involves a lot of idle time or varying cycles, VFDs can also save energy by slowing down or idling a motor instead of stopping it completely or running it unloaded. For example, some conveyor systems use VFDs to run motors at a crawl when no product is present, then ramp up when needed, instead of repeated start-stop. This can save energy (and also reduce wear from starting). Soft starters, in contrast, would just allow an on/off cycling – during off there’s zero energy use, which is good, but if the cycling is frequent you might prefer to keep things moving slowly with a VFD to avoid repeated start losses and wear.

To summarize the efficiency aspect: If your motor application is strictly full speed, full load whenever it’s on, a soft starter setup will eke out a bit more efficiency by virtue of no continuous losses. But if there’s any opportunity to run at reduced speed or load, a VFD will enable very large energy savings that dwarf the VFD’s own losses. That’s why VFDs are a cornerstone of energy-efficient industrial design and often come with utility incentives. The decision often hinges on this question: Is there energy to be saved by running this motor slower? If yes, lean toward a VFD. If no, a soft starter is efficient and simple.

Cost Considerations

Cost is often the deciding factor in the soft starter vs VFD debate, so let’s break down the considerations:

• Initial Purchase Cost: As noted earlier, soft starters have a clear advantage in upfront cost. They are simpler devices and generally priced significantly lower than VFDs for the same motor rating. For small motors (say under 10 HP), the difference might be modest – VFD prices have come down and you might find a basic VFD not much more expensive than a soft starter. But as motor size increases, VFD costs climb faster than soft starter costs. For a 250 HP motor, a soft starter could be a fraction of the cost of a full-fledged VFD. This is why in many heavy industries (mining, oil & gas, etc.), you’ll see few large VFDs unless absolutely needed, whereas soft starters (or traditional reduced voltage starters) are common for starting big motors economically.

• Installation and Ancillary Costs: There can be extra costs around each solution. Soft starters usually require an external bypass contactor (if not built-in) and appropriate overload protection. They are relatively simple to wire and don’t require special cables. VFDs may need output filters (dV/dt filters or sine wave filters) for long motor leads, harmonic filters or line reactors on the input to satisfy power quality, and often shielded cables to prevent EMI issues. They also might need a cooling arrangement (like venting cabinet heat or providing air conditioning in a drive room). All these add cost and complexity to a VFD installation. Plus, the labor to program and commission a VFD might be more than a soft starter which typically just has a couple settings.

• Operating Cost – Energy: This ties back to the Energy Efficiency section. A VFD can reduce operating (energy) costs, sometimes dramatically, if it allows running at lower speeds. Thus, while you pay more upfront, you save money every hour the motor runs at less than full throttle. It’s wise to do a ROI (return on investment) calculation: estimate how many hours per year the motor will run at reduced load and how much power (kWh) that saves, then see how many years of savings equal the price difference of the VFD. Many VFDs pay for themselves in a couple of years or even months in high-use cases. On the other hand, if a motor runs only occasionally or always needs full power, a VFD won’t save energy and thus has no payback – the soft starter would be the more cost-effective choice over the equipment life.

• Maintenance and Downtime Costs: Consider the cost of potential downtime or repairs. A soft starter is pretty robust; if it fails (maybe an SCR burns out), you can often bypass it or swap it relatively easily, and it’s cheaper to replace. A VFD failure can be more involved – it might require a specialist to diagnose, and the part cost is higher. If a critical motor (like a production-critical fan or pump) is on a VFD, you might need a spare drive on hand due to lead times, which is another investment. Some companies mitigate this by using a “soft starter + bypass + VFD” combo on critical applications: the VFD runs the motor most of the time for efficiency, but a bypass contactor and either a soft starter or DOL path is available to keep the motor running (at full speed) if the VFD fails. This ensures uptime but obviously at higher initial cost. For non-critical applications, this isn’t necessary, but it’s a thought for critical processes – and a soft starter can be part of that backup plan.

• Utility or Power Factor Penalties: In some regions, utilities charge extra for poor power factor or high harmonic distortion. Using large numbers of VFDs could potentially incur such costs if mitigation isn’t done (harmonic filters, etc.). Soft starters, after starting, don’t contribute to harmonics and the motor runs at normal power factor (which might be ~0.85-0.9 lagging). VFDs have good displacement power factor but do introduce harmonics that lower the true power factor. The cost of harmonic mitigation should be considered if you plan on many drives. Sometimes this is not a big deal, but for very large installations it can be.

• Futureproofing and Flexibility: A cost-related angle: using a VFD gives you flexibility for future changes (since you can adjust speed easily). If you install a soft starter and later find you need to change the process speed, you’d have to buy a VFD at that point (effectively paying twice). Some companies opt for VFDs upfront even if initially they plan to run at full speed, just to have the option to tweak speed later or to accommodate process expansions. While this is not a direct cost “saving”, it can avoid future costs or modifications. Essentially, think long-term: are there likely benefits in the future that would make the VFD worth it beyond the current situation?

In essence, soft starters win on immediate cost and simplicity, whereas VFDs often win in the long run if they unlock energy savings or process improvements. Evaluate both the capital expenditure (CapEx) and the operational expenditure (OpEx). For a simple application that just needs gentle starting, the cost calculus will favor a soft starter every time. For a complex or energy-intensive application, a VFD often justifies its higher price by delivering savings and performance.

Safety and Motor Protection Aspects

Both soft starters and VFDs contribute to safer operation of motors and connected machinery, but there are nuances in how they affect safety and protection:

• Mechanical Safety: A soft starter’s gentle ramp greatly reduces mechanical shock. This is a safety factor for equipment – less stress on couplings, shafts, and gearboxes means lower risk of sudden failures or accidents (for example, a gearbox tooth breaking due to a torque spike could cause a machine jam or projectile hazard). In pumping systems, as mentioned, soft starters prevent the “hammer” effect that can burst pipes or cause valves to slam. By relieving torque spikes and tension in the system, soft starters protect not just the motor but the entire driven mechanism, contributing to a safer and smoother operation.

• Electrical Safety (Inrush & Voltage Dips): Reducing inrush current isn’t just about the motor – it also protects the electrical network. A massive inrush can cause voltage drop that might reset other equipment or dim lights, potentially unsafe in some environments (imagine hospital or emergency systems experiencing a sag). Soft starters and VFDs both mitigate this, making the electrical system more stable during motor starts. In facilities with multiple large motors, using soft starters/VFDs can prevent cumulative stresses on switchgear and transformers, thereby preventing nuisance trips or overheating – which is a safety improvement (reducing the chance of an electrical fire or arc flash incident from overstressed gear).

• Built-in Motor Protection: Many soft starters come with basic motor protection features – like adjustable current limit, overload (class 10/20) settings, phase loss detection, etc. They will trip to protect the motor if it draws excessive current for too long (for example, if the motor is jammed or a heavy start exceeds set limits). This saves the motor from burning out and prevents dangerous overheating. VFDs typically have even more advanced protection: they constantly monitor motor current, voltage, and sometimes temperature. They can detect conditions like overcurrent, overvoltage, stalled rotor, even ground faults in some cases, and shut down before catastrophic failure. They also control the voltage/frequency in a way that inherently limits the current to safe levels (unless told to do otherwise). In short, both devices tend to protect the motor from abuse, but VFDs provide a finer level of control and broader protection functions (including things like stall prevention, phase imbalance protection, etc. as part of their programming).

• Stopping and Emergency Situations: In terms of stopping a motor, soft starters typically let the motor coast to stop when you cut power (unless a soft-stop is configured, which just ramps voltage down to let it coast more gently). For emergency stopping, a soft starter doesn’t offer much beyond maybe faster removal of power (which is basically just turning it off). VFDs can perform controlled braking – for instance, they can inject DC or use dynamic braking resistors to slow a motor quickly in an emergency without mechanical brakes. Moreover, many VFDs have a feature called Safe Torque Off (STO), which, when wired into an emergency stop circuit, will quickly disable the drive’s output in a certifiably safe manner (used in safety-critical applications to meet standards like ISO 13849). Soft starters don’t have an equivalent SIL-rated electronic safe stop; you’d rely on a contactor to disconnect power. So, for machinery where fast or controlled stopping is a safety issue (e.g. saws, centrifuges, lifts), VFDs can enhance safety by bringing things to a halt in a managed way or interfacing with safety systems more directly.

• Overspeed and Motor Damage Prevention: By fixing the motor to line frequency, a soft starter inherently prevents overspeed (motor can’t go faster than 100% sync speed from the supply). A VFD, however, can overspeed a motor if commanded (some drives allow 120% or more frequency). While typically you’d configure the max frequency to a safe limit, there’s a theoretical risk if misconfigured or a control fault that a motor could be driven beyond its rated speed, which could be dangerous mechanically. Therefore, with VFDs, one should always set appropriate limits and understand the failure modes. Most VFDs will fault to off if they lose control or if something goes awry, but it’s worth noting from a safety perspective: a soft starter is simple and fails “safe” (it just won’t start or it will connect full voltage if SCRs short, which is like DOL start), whereas a VFD has more complex failure modes (most result in no output, some could result in uncontrolled output – though rare).

• Harmonics and Thermal Effects on System: From a broader system safety angle, VFD-caused harmonics can overheat transformers or neutrals if not managed, potentially a fire risk in extreme cases. Using proper mitigation is a safety must for large drive installations. Soft starters, causing negligible harmonics after start, don’t have this issue. Additionally, VFDs in certain cases can induce bearing currents in motors that cause bearing pitting – if those bearings fail, it could lead to motor seize-up. The fix is using grounded shafts or filters, which is usually done in critical systems.

• Environmental and Operational Safety: VFDs often allow smooth speed changes that can make the work environment safer. For example, a conveyor VFD can have a “jog” mode for maintenance, moving the belt slowly for inspection – a soft starter can’t do that (it would just jolt it to near full speed). Also, the ability to run a motor at a slow speed can sometimes eliminate the need for workers to physically intervene (like using a slow rotation to position something, rather than manual effort). This can reduce risk of injury. On the other hand, one must ensure cooling for motors at low speeds if people are around – a motor that overheats because its fan isn’t cooling at low RPM could pose a hazard.

In conclusion, both devices promote safety by reducing strain on equipment, which indirectly protects personnel and infrastructure. Soft starters provide a simple protective start that avoids many immediate hazards of inrush and mechanical shock. VFDs go further, offering precise control that can be leveraged for safety (controlled stops, speed limits, interlocks) and generally have more comprehensive protective controls built-in. The best practice is to integrate whichever device with appropriate safety circuits and protective devices: for example, use proper fusing/circuit breakers, motor overload relays (if not built into the starter/drive), and emergency stop circuits that remove power or trigger safe torque off as needed. When applied correctly, both soft starters and VFDs will make your motor-driven system safer and more reliable.

Conclusion: Choosing the Right Solution

So, soft starter or VFD – which one do you need? The decision comes down to your application’s requirements and priorities:

• If your motor runs at full speed all the time, and you only need to reduce the stress of startup, a soft starter is likely the best choice. It’s economical, easy to use, and it will protect your system during those few critical seconds of motor start (and stop). You’ll avoid huge current draws and mechanical shocks without a lot of complexity. In many cases, especially with large horsepower motors that don’t need speed control, a soft starter gets the job done in a simple and cost-effective way.

• If you need to vary the motor’s speed or torque during operation, or you’re aiming for energy savings by running at partial loads, then you should opt for a VFD. The VFD will give you complete control over the motor and can greatly improve efficiency and process performance. Yes, you pay more upfront, but you gain flexibility and potential long-term savings (both in energy and reduced wear). For any application where adjustability, efficiency, or advanced control is desired – from HVAC fans to conveyor systems to complex machinery – a VFD is the appropriate solution.

Sometimes the answer is straightforward, but other times you might have a borderline case. Consider these guidelines as a quick recap:

• Choose Soft Starter if: budget is tight, only startup current/torque needs to be managed, motor runs at fixed speed, and no significant energy saving would come from speed control. Typical for fixed-speed pumps, fans, compressors, conveyors, or very large motors where VFD costs are prohibitive.

• Choose VFD if: speed adjustments are required (now or potentially in the future), the load has varying demand, energy efficiency is a goal, or you need features like soft braking, reversing, or precise acceleration. Typical for variable-flow systems, heavy-duty start/stop cycles, and integrated automated processes.

In some cases, you might even use both in different roles: for example, using a VFD on a primary process pump for fine control, but soft starters on auxiliary pumps that just start and run. The key is to match the tool to the task. Don’t pay for a Ferrari (VFD) when all you need is a reliable sedan (soft starter) to get from A to B – but also, don’t try to cut corners with a soft starter if the application truly demands the muscle and brains of a VFD.

Finally, always ensure the device (soft starter or VFD) is properly sized and configured for your motor and application. Consult with the manufacturer’s guidelines or an application engineer if in doubt. Both soft starters and VFDs have specific selection criteria (motor FLA, start duty, environment, etc.) to ensure safe and effective operation.

In summary, both soft starters and VFDs are valuable motor control solutions in the world of industrial motor control and circuit protection. Understanding their differences in capability, cost, and complexity will help you choose the right one for your needs. With this knowledge, you can improve your system’s reliability, efficiency, and longevity. If you’re still unsure which to choose for a particular application, consider reaching out to a motor control specialist or the manufacturers (e.g., Eaton, ABB, Schneider Electric) – they often provide selection guides and support to ensure you get the optimal solution.

Need help selecting or sourcing a soft starter or VFD? Contact our team of experts for personalized assistance. We can help analyze your motor control needs, offer the latest solutions from top brands, and guide you towards the most cost-effective and efficient choice. Empower your operations with the right motor control – and keep your motors running smoothly and efficiently!

Frequently Asked Questions (FAQ)

How does a soft starter work?

A soft starter works by gradually increasing the voltage supplied to an AC motor during startup, which limits the inrush current and reduces initial torque. It uses solid-state devices (SCR thyristors) connected in series with the motor; by controlling the firing angle of these SCRs, the soft starter starts the motor at a reduced voltage and then slowly ramps the voltage up to full over a preset time. This gentle voltage ramp results in a smooth acceleration of the motor instead of a sudden jolt. Once the motor reaches near full speed, the soft starter often activates a bypass contactor that connects the motor directly to line power, allowing it to run at full voltage with no additional losses.

Can you use a VFD instead of a soft starter?

Yes, you can use a VFD (Variable Frequency Drive) in place of a soft starter, and in fact a VFD inherently provides soft-start capability and more. A VFD can absolutely perform the function of a soft starter – it will smoothly ramp up the motor’s speed by increasing frequency and voltage, eliminating high startup current. In scenarios where you only need a soft start, a VFD would accomplish that but it’s likely overkill if you don’t need the extra features. The main considerations are cost and complexity: a VFD typically costs more than a soft starter and requires a bit more setup. If you anticipate needing speed control or torque control at any point, a VFD is a better long-term choice and can stand in for a soft starter. But if speed control isn’t needed at all, using a VFD “just” as a soft starter might not be cost-effective. In summary: a VFD can do everything a soft starter does (and then some), so it can substitute for one – just weigh the added benefits against the higher investment.

Do soft starters save energy?

Soft starters themselves do not save energy during the normal running of the motor. Their main benefit is reducing the electrical and mechanical stress at startup. The energy used to start the motor with a soft starter is slightly less than an across-the-line start (because current is limited), but this difference is minimal in the context of overall energy consumption. Once the motor is at full speed, a soft starter typically bypasses and the motor draws the same power as it would without the soft starter. So, in continuous operation, there’s no ongoing energy reduction – the motor runs at full voltage and full speed, consuming full power. The indirect energy benefit is that by avoiding voltage dips and mechanical shocks, you might prevent inefficiencies or wear in the system. However, if you’re looking for energy savings during operation, a VFD is the solution to reduce energy by controlling speed. The exception could be some soft starters that have an “energy optimization” mode for lightly loaded motors (reducing voltage a bit to cut core losses), but the savings from that are marginal. In summary, soft starters are about protecting equipment, not about running efficiency, whereas VFDs are the go-to for actual energy efficiency improvements in motor systems.

Is a VFD the same as a soft starter?

No, a VFD is not the same as a soft starter, though they share some similarities in purpose. Both devices are used to smoothly start motors and reduce inrush current, but a VFD is a far more capable device. A soft starter simply ramps up voltage to the motor to limit start current and then is usually bypassed; it cannot control the motor’s speed once running. A VFD (Variable Frequency Drive), on the other hand, varies the supply frequency and voltage to the motor, allowing full control of motor speed and torque throughout operation. In essence, a VFD includes the function of a soft start (since starting at low frequency/voltage inherently gives a soft start) but also offers continuous speed control and many other features. You can think of a VFD as a superset of a soft starter. Because of this, VFDs are typically larger, more complex, and more expensive than soft starters. In summary: a VFD can do what a soft starter does (soft start/stop) plus regulate speed, whereas a soft starter only manages the start/stop transition and then sits idle. They are different tools – one for basic start protection, the other for full-fledged motor control.