VFD vs Soft Starter: Differences, Cost, and Which to Choose
VFD vs soft starter compared — speed control, energy savings, starting torque, cost, and size — plus a clear guide to choosing the right one for your motor.
The quick answer: a VFD (variable frequency drive) controls motor speed continuously by varying the frequency and voltage it delivers. A soft starter only ramps voltage during startup — once the motor is at full speed, it steps aside. If you need variable speed, choose a VFD. If you just need smooth starting on a constant-speed load, a soft starter costs less, runs cooler, and takes up less panel space.
Everything that follows explains why that rule holds, and the situations where it breaks down.
Try it: estimate the payback with the free VFD Energy Savings Calculator for a pump or fan.
What Is a VFD?
A variable frequency drive (also called an adjustable frequency drive, inverter drive, or simply "drive") is a power-electronics device that converts incoming AC power to DC and then synthesizes a new AC output at whatever frequency and voltage the motor needs. By changing the output frequency — from near zero up to 60 Hz or beyond — it controls rotor speed continuously.
The internal stages are: rectifier → DC bus → inverter. The inverter stage uses IGBTs switching at several kilohertz to produce a pulse-width-modulated (PWM) waveform that the motor interprets as smooth AC. For a deeper look at why "inverter" and "VFD" are not the same term, see the companion article VFD vs Inverter.
Key capabilities of a VFD:
- Continuous speed control from near-zero to above nameplate RPM
- Soft start and stop built in — no mechanical shock on the drivetrain
- Energy savings on variable-torque loads (fans, pumps) via the affinity laws — reducing speed by 20 % cuts power consumption by nearly 50 %
- PLC integration via analog reference (0–10 V, 4–20 mA), digital I/O, or fieldbus (Modbus RTU, Profibus, EtherNet/IP, PROFINET)
- Built-in protection: overcurrent, overvoltage, ground fault, motor thermistor input
For a full wiring and programming walkthrough, see VFD Programming with PLC Control.
What Is a Soft Starter?
A soft starter uses a set of SCRs (silicon controlled rectifiers) or thyristors wired in antiparallel across each phase to ramp the voltage applied to the motor from a low initial value up to full line voltage over a programmable acceleration time — typically 2–30 seconds.
Once the motor reaches full speed, a bypass contactor closes, shorting the SCRs out of the circuit. At that point the soft starter draws almost no current and generates almost no heat. At stop, the same SCRs can ramp voltage back down for a soft stop.
A soft starter does not change frequency. It can only reduce starting current and torque by temporarily reducing voltage. That means the motor always runs at synchronous speed minus slip — it cannot be commanded to run at half speed.
Key capabilities of a soft starter:
- Reduced inrush current at start (typically 3–4× FLA versus 6–8× FLA for a direct-on-line start)
- Reduced starting torque — protects belts, couplings, and conveyor contents from shock
- Compact size — typically 30–60 % smaller than an equivalent VFD
- Lower cost — usually 40–60 % less than an equivalent VFD
- Low heat dissipation in bypass mode — suitable for enclosed panels without forced ventilation
- PLC integration via simple digital outputs (run/stop) and relay outputs (at-speed, fault)
For context on how soft starters compare to the classic star-delta starter, a soft starter gives smoother control with less voltage dip and no current spike at transition.
VFD vs Soft Starter: Comparison Table
| Feature | VFD | Soft Starter |
|---|---|---|
| Speed control | Continuous, 0–100 %+ | None — full speed only after ramp |
| Starting current | Typically 1–1.5× FLA | 3–4× FLA (vs 6–8× DOL) |
| Starting torque | Adjustable, high torque at low speed possible | Reduced (voltage reduction = torque reduction) |
| Energy savings (running) | Yes — significant on variable-torque loads | No saving once in bypass |
| Energy savings (starting) | Yes | Modest (less inrush only) |
| Harmonic distortion | Generates harmonics; may need line reactor or filter | Minimal in bypass mode |
| Heat generated | Continuous (PWM losses ~2–3 % of rated power) | Negligible in bypass; only during start/stop |
| Physical size | Larger; requires ventilation clearances | 30–60 % smaller |
| Upfront cost | Higher (roughly 2–3× soft starter for same HP) | Lower |
| PLC control depth | Full: speed reference, ramp rates, PID, bus comms | Basic: run/stop digital outputs, fault relay |
| Bypass contactor | Optional, external | Built in (closes at full speed) |
| Motor compatibility | AC induction; also PMSM with sensor or sensorless | AC induction only |
| Typical applications | Pumps, fans, compressors, conveyors, CNC | Pumps (fixed speed), compressors, conveyors, crushers |
The Key Difference: Frequency vs Voltage
This is the distinction that matters most in selecting between the two:
A VFD controls the motor by changing frequency. It synthesizes an entirely new AC waveform. At 30 Hz the motor runs at roughly half synchronous speed; at 60 Hz it runs at nameplate speed; at 70 Hz it runs above nameplate (field-weakening region). Torque is available across the speed range. The drive is always in the circuit.
A soft starter controls the motor by reducing voltage — temporarily. Because induction motor torque is proportional to the square of applied voltage, reducing voltage to 50 % cuts torque to 25 %. That is useful for limiting mechanical shock during starting, but it is not a speed-control mechanism. The SCRs bypass themselves once the motor reaches line speed, and the device does nothing for the rest of the run.
A practical consequence: if you set a soft starter's ramp time too long on a high-load application, the motor may stall because the reduced torque is insufficient to accelerate the load. A VFD does not have this problem — it maintains rated torque all the way from standstill to full speed (within its current limit).
When to Use a Soft Starter
Choose a soft starter when all three of these conditions hold:
- The motor always runs at or near full speed — there is no process benefit from variable speed.
- Reducing mechanical shock at start is the primary goal — protecting couplings, belts, gearboxes, or delicate conveyor loads.
- Reducing inrush current matters — utility demand charges, generator capacity, or weak distribution circuits.
Best applications for soft starters:
- Fixed-speed centrifugal pumps — the pump always runs at nameplate speed; you just want a gentle start to prevent water hammer.
- Air compressors (across-the-line, fixed-speed) — soft start reduces belt wear and motor heating; the compressor's unloader handles the load cycle.
- Crushers and mills — high breakaway inertia; soft start avoids mechanical hammer.
- Conveyor drives (constant speed) — protects product and drivetrain at start; no need to vary belt speed.
- HVAC fans (two-speed or fixed-speed) — reduces inrush; belt life improves significantly.
- Applications in enclosed panels without VFD-grade ventilation — because the soft starter dissipates almost no heat in bypass mode.
Soft starters are not appropriate when:
- Process flow, pressure, or speed must vary during the run cycle.
- The load requires full torque from standstill at low speed.
- Energy savings during the run phase are a project goal.
When to Use a VFD
Choose a VFD when any of these conditions hold:
- Variable speed is required — process control demands it (flow, pressure, tension, position).
- Energy savings on a variable-torque load — the affinity laws make this the highest-ROI motor upgrade in most facilities.
- Precise PLC-controlled speed reference — closed-loop PID, fieldbus speed commands, multi-speed presets.
- Torque control at low speed — starting a loaded conveyor from zero, tensioning control on a winder.
Best applications for VFDs:
- Centrifugal pumps (variable flow) — throttling a valve wastes energy; slowing the pump with a VFD saves it. A pump running at 80 % of full speed uses roughly 50 % of the power.
- HVAC supply and exhaust fans — variable air volume (VAV) systems are the single largest commercial energy-saving application of VFDs.
- Compressors with variable demand — modulating compressor speed to match demand avoids load/unload cycling losses.
- Conveyors with variable throughput — production rate control via PLC speed reference.
- Mixers and agitators — ramp speed for gentle startup in batch processes; adjust speed by recipe.
- CNC and servo-like axis drives — where a VFD replaces a simpler servo on non-precision axes.
For PLC integration, VFDs accept a 4–20 mA or 0–10 V analog speed reference from a PLC analog output module, or communicate directly over Modbus RTU or an industrial Ethernet fieldbus. The motor start/stop ladder logic tutorial shows the fundamental PLC logic for starting a motor; extending that to a VFD speed reference is a matter of adding an analog output rung or a Modbus write instruction.
Cost and Lifecycle Considerations
Upfront Cost
As a rough rule of thumb (prices vary significantly by manufacturer and features):
- A soft starter for a 10–50 HP motor typically costs $200–$800.
- A VFD for the same rating typically costs $400–$1,800.
The gap widens at higher horsepower because VFDs require larger heat sinks, more robust DC bus components, and more sophisticated control electronics.
Installation Cost
Soft starters are simpler to wire: line in, motor out, control wiring for run/stop. VFDs require additional considerations:
- Input line reactor or EMC filter (often required by utility or to meet CE/UL standards) — adds cost and panel space.
- Output dV/dt filter or sine filter if motor cable runs are long (generally >50 m) — protects motor insulation from PWM spikes.
- Shielded motor cable — required in most VFD installations to contain conducted EMI.
- Increased panel ventilation — VFDs continuously dissipate 2–3 % of rated power as heat.
These installation adders can add 20–40 % to the cost of a VFD installation versus a soft starter installation of the same HP.
Operating Cost and ROI
For constant-speed applications, a soft starter has essentially no ongoing operational difference versus a direct-on-line starter. There is no energy saving during the run phase.
For variable-torque applications (pumps, fans), a VFD's energy savings can deliver payback in 12–36 months. The calculation is straightforward: if a 50 HP fan motor runs at 80 % of full speed for most of its operating hours, the VFD reduces power consumption by roughly 50 %, saving thousands of dollars per year at industrial electricity rates.
Maintenance
Soft starters have fewer active components during normal operation (bypass contactor carries the load; SCRs are idle). VFDs run their IGBT bridge continuously and have cooling fans that require periodic replacement. VFD electrolytic capacitors age over time and are typically rated for 10–15 years of service before needing inspection or replacement.
Summary: Cost vs Capability
| Soft Starter | VFD | |
|---|---|---|
| Best when budget is the primary driver | Yes | No |
| Best when energy savings justify cost | No | Yes |
| Maintenance complexity | Lower | Higher |
| Panel space required | Less | More |
| Replacement parts availability | High | High |
Frequently Asked Questions
What is the difference between a VFD and a soft starter?
A VFD controls motor speed continuously by changing the frequency and voltage of its output — the motor can run at any speed from near-zero to above nameplate. A soft starter only reduces voltage during startup to limit inrush current and mechanical shock; once the motor reaches full speed, the soft starter bypasses itself and plays no further role. The fundamental difference is that a VFD is a running-speed controller, while a soft starter is only a starting-method controller.
Is a VFD better than a soft starter?
Not universally. A VFD is more capable — it does everything a soft starter does and adds continuous speed control and run-phase energy savings. But that capability comes at a higher cost, larger size, more heat, and more installation complexity. On a constant-speed application with no variable-speed benefit, a soft starter is often the better engineering choice because it is simpler, cheaper, and more reliable in that specific use case.
Can a VFD replace a soft starter?
Yes. A VFD can perform a controlled soft start and soft stop, making it a functional superset of a soft starter. Many engineers use an entry-level VFD in place of a soft starter when they anticipate future process changes, or when the VFD's energy savings are expected to offset its higher initial cost. However, replacing a soft starter with a VFD on a motor that genuinely only needs soft starting adds unnecessary cost and complexity.
Which is cheaper, a VFD or a soft starter?
A soft starter is typically 40–60 % less expensive than a VFD of the same motor horsepower rating. When you add installation items (line reactors, shielded cable, filters, extra panel ventilation), the total installed cost of a VFD is often 2–3 times that of a soft starter for the same application.
Choosing Between a VFD and a Soft Starter: Decision Summary
- Variable speed needed? — VFD, full stop.
- Fixed speed, only need smooth starting? — soft starter.
- Pumps or fans with variable flow demand? — VFD; the energy savings typically justify the cost.
- Fixed-speed pump or fan, just preventing water hammer or inrush? — soft starter.
- High-inertia, constant-speed load (crusher, mill)? — soft starter.
- PLC must command speed during the run cycle? — VFD.
- Panel space is severely constrained? — soft starter.
- Budget is the primary constraint and constant speed is acceptable? — soft starter.
- ROI on energy savings is the primary driver? — VFD.
For the PLC wiring and programming side of either device, the VFD Programming with PLC Control guide covers analog and Modbus integration in detail, and the motor start/stop ladder logic tutorial covers the fundamental control circuit that underpins both approaches.
