Online Tool Store
Motor Starting Method Selector
Compare direct-on-line, star-delta, soft start, and VFD starting for a motor, on inrush current, starting torque, and cost.
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Math & Science
Motor Starting Method Selector
Frontend preview — no upload or external service.
Comparison
22 kW on a limited supply: DOL draws roughly 6-8 times full load current. Star-delta cuts inrush to about a third but also cuts starting torque to a third — adequate for a fan, marginal for a loaded conveyor.
How the Motor Starting Method Selector works
- Enter the motor rating and the type of load, which determines how much starting torque you need.
- Note any supply constraint, since inrush current is usually the reason not to start direct-on-line.
- Match torque to the load: star-delta reduces current and torque together, which is fine for a fan and often not for a loaded start.
The method
Reduced-voltage starting cuts current and torque, and torque falls with the square of the voltage reduction.
star-delta applies about 58% of line voltage, giving roughly 33% of DOL current and 33% of DOL torque
That proportional torque loss is why a method chosen purely to protect the supply can leave a loaded conveyor unable to start at all.
FAQ
When is direct-on-line acceptable?
On a supply strong enough to absorb the inrush and a load that tolerates the mechanical shock. It is the cheapest and simplest method, and for smaller motors on a robust supply it remains the default.
Star-delta or soft starter?
Soft starters ramp voltage smoothly and avoid the transition transient that star-delta produces when switching. Star-delta is cheaper and still widely used where that transient does not matter.
When is a VFD the right answer?
When you need speed control anyway, or need full torque at low current, or want the smoothest possible start. It is the most capable and the most expensive, and it introduces harmonics that may need consideration.
How we compare
| Feature | Online Tool Store | A graphing calculator | A stats package |
|---|---|---|---|
| Torque and current together | ✓ | One at a time | Yes |
| Load type considered | ✓ | ✗ | ✓ |
| No supplier account | ✓ | ✓ | ✗ |
| Full system design | ✗ | ✗ | ✓ |
Motor Starting Method Selector shows torque falling with current, because a method chosen only to protect the supply can leave the load unable to start.