Electrical
Motor Starters Explained: DOL, Star-Delta and VFD
Short answer
Direct-on-line (DOL) starters connect a motor straight to the supply and suit small motors. Star-delta starting reduces start-up current for larger motors. Soft starters ramp voltage smoothly, and variable frequency drives (VFDs) control speed as well as starting, saving energy on pumps and fans. The right choice depends on motor size, load and supply capacity.
Comparison
| Method | Starting current | Speed control | Typical use |
|---|---|---|---|
| DOL | High (about 6 to 8 times full-load current) | No | Small motors, strong supply |
| Star-delta | Reduced to about a third | No | Medium motors starting with light load |
| Soft starter | Controlled ramp | Limited | Pumps, conveyors, fans |
| VFD | Low, controlled | Yes | Pumps, fans, process lines needing speed control |
Indicative values; check the motor and starter datasheets.
Choosing
- Check what the supply or generator can handle at start-up.
- Match the starter to the load: heavy loads need high starting torque.
- Use VFDs where flow or speed varies; they can cut energy use.
- Add overload, short-circuit and phase failure protection.
Common problems
- Motor trips on start: current limit, wrong overload setting or supply issue.
- Overheating: overload, poor ventilation or phase imbalance.
- Harmonics from VFDs: may need filters. See power factor correction.
Industrial support
We build and service motor control panels. See industrial electrical services.
Common questions
Can I run a three-phase motor on single-phase?
With a VFD designed for single-phase input, small motors often can. Check ratings before connecting.
What does a VFD save?
On pumps and fans, running slower can cut energy use substantially, because power falls rapidly with speed.
Why does my motor trip the breaker at start?
Start-up current may exceed the breaker setting or the supply capacity. A technician can check and correct it.