This is probably the question I get asked most often by clients: "Should I use a VFD or a soft starter?" And my honest answer is always the same: "It depends on whether you need speed control." That single sentence eliminates the wrong choice 80% of the time. But the remaining 20% is where the real engineering decisions happen. Let me walk through the technical differences with actual data from projects we've done.
1. How Each Device Works — The Engineering Fundamentals
Soft Starter: Uses anti-parallel SCR (thyristor) pairs to gradually ramp up the voltage applied to the motor during start-up. Typical ramp time: 2-30 seconds. Once at full speed, the SCRs are bypassed by a contactor — the soft starter is essentially "out of circuit" during running. It controls voltage, not frequency. Motor speed remains locked to line frequency (e.g., 2980 RPM on a 4-pole motor at 50 Hz).
VFD (Variable Frequency Drive): Converts AC line power to DC (rectification), then synthesizes variable-frequency AC output using IGBT inverters with PWM modulation. Controls both voltage AND frequency simultaneously (V/f ratio). This means you can run a motor at any speed from 0-120% of rated speed. During running, the VFD remains in circuit — continuously controlling the motor.
Key insight: A soft starter is a starting device. A VFD is a speed controller. If your process never needs to change speed after the motor is running, a soft starter is usually the better economic choice.
2. Do Soft Starters Save Energy? What the Numbers Say
This is where VFDs shine — but only in the right applications. The affinity laws for centrifugal loads (pumps and fans) tell us:
- Flow ∝ Speed (N)
- Pressure ∝ Speed² (N²)
- Power ∝ Speed³ (N³) — this is the money equation
Real example from a water treatment project: A 110 kW raw water pump was running at full speed with a throttling valve controlling flow to 70%. The valve was dropping 1.8 bar of pressure — literally burning energy as heat and noise. We installed an ABB ACS580 VFD and reduced the motor speed to 82% (matching the actual flow requirement). Power consumption dropped from 110 kW to 62 kW — a 44% reduction. Annual energy savings: (110-62) × 8,000 hours × $0.08/kWh = $30,720/year. The VFD cost $18,500 installed. Simple payback: 7.2 months.
⚠️ Important: Soft Starters Don't Save Energy at Running Speed
Once the bypass contactor engages, the soft starter consumes ~15W of control power. The motor runs at full line voltage and full speed. If your process needs flow control, using a soft starter + throttle valve wastes energy that a VFD would save. I've seen plants with 3 soft-started pumps consuming 40% more energy than necessary because they throttle instead of speed-control.
3. How Much More Does a VFD Cost Than a Soft Starter?
| Motor Size | Soft Starter (installed) | VFD (installed) | Cost ratio |
|---|---|---|---|
| 15 kW | $800-1,200 | $1,800-2,500 | ~1:2 |
| 55 kW | $2,500-3,800 | $5,500-8,000 | ~1:2.2 |
| 110 kW | $4,500-6,500 | $12,000-18,500 | ~1:2.8 |
| 250 kW | $9,000-13,000 | $28,000-42,000 | ~1:3.2 |
| 500 kW | $16,000-22,000 | $55,000-80,000 | ~1:3.5 |
Note: At larger motor sizes, the cost gap widens significantly. For a 500 kW motor, the VFD is 3.5× the cost of a soft starter. This is why I always push back when a client asks "can we put VFDs on everything?" — if you don't need speed control, you're spending an extra $40,000 for no return.
4. VFD or Soft Starter: Which Should You Choose for Your Application?
Pumps (Centrifugal)
VFD if: Flow varies by more than 15% in normal operation, or multiple pumps run in parallel with varying demand. I specify VFDs for virtually all water treatment plant pumps — the energy savings pay for themselves within 12-18 months. See our water treatment solutions for examples.
Soft starter if: The pump runs at constant speed 95%+ of the time. Booster pumps for constant-pressure buildings, irrigation pumps, and standby/backup pumps are good candidates.
Fans and Blowers
VFD almost always. The cubic relationship between speed and power means even a 10% speed reduction saves 27% energy. For HVAC air handling units, AHU supply fans with VFDs typically save 35-50% vs. damper control. Payback is almost always under 2 years.
Soft starter exception: Small exhaust fans (<5.5 kW) that run at full speed continuously. The VFD cost can't justify itself at that power level.
Conveyors
VFD if: Speed needs to change based on upstream/downstream process conditions, or you need controlled acceleration to prevent material spillage. Common in mining, aggregate, and food processing.
Soft starter if: Fixed-speed operation with long belt starts. The soft starter limits belt tension spikes and prevents mechanical shock to gearboxes. I use soft starters for conveyor motors where the belt runs at one speed and the only concern is starting stress.
Crushers, Mills, and High-Inertia Loads
Soft starter (high-torque type) is usually correct. These loads need high starting torque (200-350% of FLA). VFDs can provide this but at significantly higher cost for the same motor rating. A 200 kW crusher motor with soft starter: ~$7,000 installed. Same motor with VFD: ~$55,000. Unless you need to vary the crusher speed for product size control, the soft starter wins by a wide margin.
5. When I Recommend a VFD Over a Soft Starter (Despite the Cost)
Beyond energy savings, there are technical reasons to choose a VFD even when cost seems to favor a soft starter:
- Process pressure/flow control needed: If you'd otherwise install a control valve or damper for regulation, the VFD + sensor feedback loop is more efficient AND more precise (±0.5% vs ±5% with valve positioning)
- Multiple operating points: A pump that runs at 100% during the day and 60% at night — the VFD adapts automatically
- Soft starting of weak power supplies: Generators, weak grids, or long cable runs where voltage drop during DOL starting is unacceptable. VFDs draw ~100% FLA during start vs. 600-800% for DOL
- Regenerative loads: Downhill conveyors, centrifuges, or test benches where the motor generates power. VFDs with regenerative front ends feed energy back to the grid; soft starters can't handle this at all
- Positioning or synchronization: If you later need to add encoder feedback for precise speed matching, a VFD is the only option
6. SENTRADO's Motor Control Expertise
We build both VFD control panels and soft starter panels as part of our VFD control panel and MCC product lines. Our typical approach for new projects:
- Audit all motor loads — power rating, duty cycle, speed requirements, annual operating hours
- Calculate energy savings potential for each variable-torque load (pumps, fans)
- Build a cost-benefit spreadsheet showing installed cost vs. annual savings for each motor
- Recommend VFD for any motor with >4,000 annual running hours where flow/pressure control is needed
- Recommend soft starter for constant-speed loads where the only need is reduced starting stress
On a recent cement plant project (18 motors from 15 kW to 450 kW), we specified 6 VFDs for the variable-airflow fans and 12 soft starters for the constant-speed conveyors and crushers. Total installed cost was $185,000 — compared to $340,000 if we'd put VFDs on everything. The client saved $155,000 in upfront cost while still capturing 95% of the potential energy savings.
Need Help Choosing Between VFD and Soft Starter?
Send us your motor list and operating parameters. We'll provide a free cost-benefit analysis showing exactly which motors benefit from VFDs and which are better served by soft starters.