A soft starter, a variable frequency drive and a direct on line contactor all get a large fan turning. They differ enormously in what they cost and what they do afterwards.
The decision usually comes down to two questions: does the supply tolerate the inrush, and does the fan need speed control in service?
Direct on line is cheapest and draws six to eight times full load current at start. A soft starter cuts that to around three times and reduces mechanical shock, though it gives no running benefit. A VFD does both, adds full speed control and typically saves 25 to 45 percent of fan energy, at the highest cost.
What each method does
| Direct on line | Soft starter | VFD | |
|---|---|---|---|
| Starting current | 6–8 × FLC | 2–4 × FLC | ≈ 1 × FLC |
| Mechanical shock | High | Low | None |
| Running speed control | None | None | Full |
| Energy saving | None | None | 25–45% |
| Relative cost | Lowest | Medium | Highest |
| Panel heat | Minimal | Low, bypassed after start | Continuous |
| Harmonics | None | Brief during start | Continuous, needs assessment |
Notice the last two rows. A drive brings continuous heat and harmonics that a soft starter does not.
Why fan inertia matters
A large fan wheel is a flywheel, and accelerating it takes time.
During that time the motor draws high current and heats up, so a long start-up is thermally demanding. Meanwhile the belt, coupling and bearings take a mechanical shock at the moment of connection.
Consequently direct on line starting becomes harder to justify as fan size grows, particularly where the fan cycles frequently.
When direct on line is fine
- Motors below roughly 7.5 kW on a robust supply.
- Fans that start infrequently, such as once or twice a day.
- Sites where supply capacity easily absorbs the inrush.
- Simple installations where panel cost dominates.
- Standby and emergency fans that rarely run.
- Finally, applications where speed control brings no benefit.
Direct on line remains the most reliable method available, precisely because it contains almost nothing to fail.
Where a soft starter earns its place
Soft starting sits in a narrower band than it used to, since drives have fallen in price.
It still makes sense where the supply cannot handle inrush, where mechanical shock damages belts or ductwork, and where the fan genuinely runs at fixed speed all the time. Meanwhile it avoids the harmonics and continuous losses that a drive brings.
Ask whether the fan will ever need to run slower. If the answer is yes, a drive replaces the soft starter and pays for itself in energy. If the answer is genuinely no, a soft starter is the cheaper and simpler choice.
When a VFD wins outright
- The load varies, so part-load running saves real energy.
- The supply is weak and cannot take inrush current.
- Process control needs adjustable airflow rather than damper throttling.
- The fan starts frequently, since drives eliminate start stress entirely.
- Commissioning flexibility matters, because speed can be trimmed later.
- Finally, the site has an energy target to meet.
Our part-load energy guide covers the savings, and the VFD compatibility guide covers the checks before fitting one.
Practical points on all three
Whichever method you choose, three details deserve attention.
Motor thermal protection must suit the method, since a long start heats the winding. Ramp times need setting to the actual load rather than left at default. Finally, check that the fan does not sit in a resonance band, which matters most on drives that sweep the speed range.
For drive system requirements, IEC 61800 sets out the framework, including harmonics and electromagnetic compatibility.
Fan starting FAQ
Does a soft starter save energy?
No. It reduces starting current and mechanical stress, though once the motor is running it behaves exactly like direct on line. Only a drive saves running energy.
Can I use star-delta starting instead?
It reduces inrush to around a third, and it is still used on larger motors. However, the transition produces a current spike, and a soft starter handles the same job more smoothly.
How long should a soft start ramp be?
Long enough to accelerate the wheel without excessive current, often 10 to 30 seconds on a large fan. Set it to the actual load rather than leaving the factory default.
Do drives cause harmonic problems?
They can, particularly with several on one supply. A harmonic assessment is worth doing on larger installations, and filters or higher pulse drives address it.
Direct on line for small and simple, a soft starter for fixed-speed fans on a weak supply, and a drive whenever the fan will ever run slower than full duty.
