Soft Starter vs Variable Frequency Drive (VFD)
Choosing the Right Motor Starting Method
When designing or upgrading a motor control system, one of the most common decisions is whether to use a soft starter or a variable frequency drive (VFD).
Both technologies improve motor starting compared with traditional direct-on-line starting, but they serve different purposes. Understanding the differences helps ensure the correct solution is selected for the application.
What is a Soft Starter?
A soft starter gradually increases the voltage applied to a motor during startup. This allows the motor to accelerate smoothly while reducing the high inrush current associated with direct-on-line starting.
Once the motor reaches full speed, the soft starter typically bypasses its power electronics and the motor runs directly from the supply.
Advantages of Soft Starters
- Reduced motor starting current
- Reduced mechanical stress on equipment
- Lower cost than VFD systems
- Simpler installation and configuration
- Suitable for fixed-speed applications
Typical Applications
Soft starters are commonly used in:
- Pumps
- Fans
- Compressors
- Conveyors
- Crushers
- Mixers
These systems normally run at full speed during operation, so only controlled starting is required.
What is a Variable Frequency Drive (VFD)?
A variable frequency drive (VFD) controls both the voltage and frequency supplied to a motor. This allows the motor speed to be continuously adjusted during operation.
Because the motor speed can be varied, VFDs provide much greater flexibility and control over the process.
Advantages of VFDs
- Full variable speed control
- Significant energy savings in many applications
- Improved process control
- Reduced mechanical wear
- Advanced monitoring and protection features
Typical Applications
VFDs are commonly used for:
- Pump flow control
- HVAC systems
- Fans and ventilation
- Conveyor systems
- Industrial machinery
- Process control systems
Key Differences
| Feature | Soft Starter | Variable Frequency Drive |
| Motor speed control | No | Yes |
| Start current reduction | Yes | Yes |
| Energy savings during operation | Limited | Significant |
| Cost | Lower | Higher |
| Complexity | Low | Medium–High |
| Typical use | Fixed speed motors | Variable speed applications |
Which Should You Choose?
A soft starter is typically the best choice when:
- The motor runs at fixed speed
- Only controlled starting is required
- Budget is a key factor
- The process does not require speed control
A VFD is typically the best choice when:
- Variable speed control is required
- Energy efficiency is important
- The process needs precise control
- Equipment load varies during operation
Need Help Choosing?
Motor Control Scotland designs and supplies both soft starter panels and inverter drive systems tailored to the requirements of each installation.
If you're unsure which solution is best for your application, our engineers can help you select the most suitable motor control system.