Soft Starter vs VFD in MCC Applications: When to Use Which
Both soft starters and VFDs reduce motor starting current and mechanical stress compared to full-voltage starting. But they work differently, cost differently, and are suited to different applications. Choosing the wrong one wastes money. Choosing the right one saves money and improves process performance.
How They Work
Soft Starters
A soft starter uses silicon-controlled rectifiers (SCRs or thyristors) to gradually increase the voltage applied to the motor during starting. By controlling the voltage ramp, the soft starter limits inrush current to 2-4x full-load current (versus 6-8x for full-voltage starting).
During starting: SCRs phase-control the voltage, gradually increasing from a low initial voltage to full line voltage over a programmable ramp time.
During running: Once the motor reaches full speed, the SCRs are fully conducting (or bypassed by an internal contactor), and the motor runs on full line voltage at full efficiency. The soft starter is essentially transparent during running.
During stopping: The soft starter can also provide a controlled deceleration (soft stop) by gradually reducing voltage, which is useful for pump applications to prevent water hammer.
Variable Frequency Drives (VFDs)
A VFD converts incoming AC power to DC, then synthesizes variable-frequency AC output to control motor speed. The VFD controls both voltage and frequency proportionally (V/Hz ratio) to maintain motor torque across the speed range.
During starting: The VFD ramps frequency from near zero to full speed, providing controlled acceleration with current limited to 100-150% of motor FLA.
During running: The VFD can run the motor at any speed from near zero to full speed (and above, in some applications). The VFD remains in the circuit continuously.
During stopping: The VFD can decelerate the motor in a controlled manner by ramping frequency down.
Cost Comparison
Equipment Cost
| Motor HP | Soft Starter | VFD | Ratio |
|---|---|---|---|
| 10 HP | $500-800 | $1,200-2,000 | 2-3x |
| 25 HP | $800-1,500 | $2,000-3,500 | 2-3x |
| 50 HP | $1,500-2,500 | $3,500-6,000 | 2-3x |
| 100 HP | $2,500-4,000 | $6,000-12,000 | 2-3x |
| 200 HP | $4,000-7,000 | $12,000-20,000 | 2-3x |
VFDs typically cost 2-3x more than soft starters for the same HP rating.
Installation Cost
Soft Starter MCC Bucket: Similar to a standard starter bucket, perhaps one size taller. No special cabling required. Lower installation cost.
VFD MCC Bucket: Requires more space (often 1-2 sizes taller), input line reactor, potentially output reactor, shielded output cable for long runs, and more complex wiring. Higher installation cost.
Operating Cost
Soft Starter: No energy savings during running (motor runs at full speed on full voltage). Same energy consumption as a full-voltage starter.
VFD: Significant energy savings for variable-load applications. Fan and pump applications can save 30-60% of motor energy at reduced speeds due to the affinity laws (power varies with the cube of speed).
Performance Comparison
| Feature | Soft Starter | VFD |
|---|---|---|
| Starting Current | 2-4x FLA | 1-1.5x FLA |
| Speed Control | No (full speed only) | Yes (0-100%+) |
| Energy Savings at Reduced Load | No | Yes (significant) |
| Torque Control During Start | Limited | Full |
| Running Efficiency | 99%+ (bypassed) | 95-98% |
| Harmonic Generation | Low (only during start) | Continuous |
| Motor Cable Length | No limit | Limited (without mitigation) |
| Motor Bearing Concerns | None | Yes (shaft currents) |
| Physical Size | Smaller | Larger |
| Heat Generation | Low (when bypassed) | Continuous |
| Complexity | Lower | Higher |
When to Use a Soft Starter
Ideal Applications
Centrifugal Pumps (Constant Speed) When the pump runs at one speed and you just need to reduce starting current and prevent water hammer:
- Water booster pumps
- Fire pumps (where code requires across-the-line starting, use a soft starter for reduced voltage)
- Irrigation pumps
- Constant-pressure systems
Large Fans (Constant Speed) When the fan runs at full speed and you just need to reduce starting current:
- Exhaust fans
- Supply fans in constant-volume systems
- Cooling tower fans (non-variable)
Compressors Compressors that run at constant speed benefit from soft starting:
- Reduced mechanical stress on compressor components
- Reduced inrush current for utility demand management
- Air compressors, refrigeration compressors
High-Inertia Loads Applications where long acceleration times make full-voltage starting impractical:
- Centrifuges
- Ball mills
- Large conveyors with heavy loads
Why Not a VFD for These?
If the motor always runs at full speed, a VFD provides no energy savings but adds:
- 2-5% continuous efficiency loss (VFD heat generation)
- Continuous harmonic generation
- Motor bearing current risk
- Higher maintenance complexity
- Higher cost
A soft starter provides the starting benefit without these running drawbacks.
When to Use a VFD
Ideal Applications
Variable-Flow Pumping Systems When flow requirements change and you can save energy by running the pump slower:
- Chilled water secondary pumps
- Condenser water pumps with variable load
- Process water pumps with variable demand
- Booster pump systems with pressure setpoint control
Variable Air Volume (VAV) Fan Systems When airflow requirements change based on demand:
- Supply fans in VAV HVAC systems
- Return fans
- Exhaust fans with variable ventilation requirements
Conveyor Speed Control When process requirements demand variable conveyor speeds:
- Product spacing control
- Accumulation conveyors
- Feed rate control for process equipment
Process Control Applications Any application where motor speed is a process control variable:
- Mixer speed control
- Winder tension control
- Extruder speed control
- Dosing pump speed control
Why Not a Soft Starter for These?
A soft starter cannot vary motor speed. If the process requires speed control, a VFD is the only option. Additionally, the energy savings from variable-speed operation often pay for the higher VFD cost within 1-3 years.
The Hybrid Option: VFD with Bypass
Some applications use a VFD with a bypass contactor:
- VFD provides controlled starting and speed control
- Bypass contactor allows the motor to run on utility power if the VFD fails
- Motor runs at full speed on bypass (no speed control or energy savings)
- Provides redundancy for critical motors
This configuration is common in water treatment plants, data centers, and other critical facilities. The VFD bucket must be larger to accommodate both the VFD and the bypass contactor.
MCC Depot Solutions
MCC Depot builds both soft starter and VFD MCC buckets for all major MCC platforms. We can help you determine the right technology for your application.
- Starter buckets with soft starters
- Custom VFD buckets with any VFD brand
- Bypass contactor configurations for critical applications
- Compatible with Square D, Siemens, GE, and Cutler-Hammer MCCs
Call 307-442-0382 or email sales@mccdepot.com to discuss your application requirements.