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How to Choose a Motor Starting Method
Heshan Fernando
Co-founder & COO
A 22 kW motor started direct-on-line draws something like six to eight times its full load current for the first few seconds. On a strong supply that’s a brief dip nobody notices. On a limited supply it dims the lights, trips protection, or upsets other equipment on the same feed.
So you reduce the starting current — and in doing so, you reduce the starting torque, which is the part that catches people out.
Torque falls with the square of voltage
This is the relationship that determines whether a reduced-voltage method will work.
An induction motor’s torque is proportional to the square of the applied voltage. Star-delta starting applies roughly 58% of line voltage during the star phase, which gives about 33% of the direct-on-line current — and about 33% of the torque.
Both reductions come together. You cannot have one without the other with a voltage-reduction method.
For a fan or a centrifugal pump, that’s fine: their load torque is low at low speed and rises with speed, so a third of full torque easily gets them moving.
For a loaded conveyor, a compressor starting against pressure, or anything with high static friction, a third of full torque may not start it at all. The motor sits drawing heavy current at zero speed, which is the worst possible condition, until protection intervenes.
| Method | Starting current | Starting torque | Suits |
|---|---|---|---|
| Direct-on-line | 6-8× FLC | 100% | Strong supply, any load |
| Star-delta | ~33% of DOL | ~33% | Fans, pumps, unloaded starts |
| Soft starter | Adjustable ramp | Reduced, smooth | Most loads, no transient |
| VFD | Low | Full torque available | Any load, plus speed control |
Star-delta’s transition problem
Star-delta switches from the star configuration to delta once the motor is near speed. At that instant the motor is briefly disconnected and then reconnected, and the reconnection produces a current and torque transient — sometimes larger than the starting current it was meant to avoid.
For a fan it doesn’t matter. For a drive train with backlash or a fragile mechanical coupling, that jolt is a real consideration, and it’s the main reason soft starters have displaced star-delta in many applications.
Soft starters and VFDs
A soft starter ramps the voltage smoothly from a set initial level to full over a configurable time. No transition transient, adjustable ramp, and a controlled mechanical start. It reduces torque like any voltage-reduction method, but the ramp is smooth and tunable.
A VFD varies frequency as well as voltage, which means it can deliver full torque at low speed. It’s the only method that genuinely doesn’t compromise starting torque, and it gives speed control for the rest of the operating range. It’s also the most expensive, introduces harmonics that may need consideration, and adds a component that can fail.
The rule of thumb: if you need speed control anyway, a VFD pays for itself. If you only need a gentle start, a soft starter is usually the better value.
Common mistakes to avoid
- Choosing star-delta to protect the supply, on a load that can’t start at a third of torque.
- Ignoring the star-delta transition transient on a mechanically sensitive drive.
- Sizing protection for full load current without accounting for the starting profile.
- Assuming a soft starter reduces current as much as star-delta — the ramp trades current against start duration.
- Specifying a VFD purely for starting, where a soft starter would do at lower cost and complexity.
How to do it with Motor Starting Method Selector
The Motor Starting Method Selector shows current and torque together.
- Enter the motor rating and the type of load.
- Note any supply constraint — that’s usually why you’re reducing starting current.
- Check the resulting torque against what the load needs to break away.
- Take the shortlist to a qualified electrical engineer for the installation design.
Other electrical calculators are in the tools directory.
Frequently asked questions
When is direct-on-line acceptable?
On a supply strong enough to absorb the inrush and a load that tolerates the mechanical shock. It’s 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 star-delta produces. Star-delta is cheaper and still widely used where that transient doesn’t 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’s the most capable and most expensive, and it introduces harmonics that may need consideration.
Final thought
Check the load’s breakaway torque before choosing a starting method. A method chosen only to protect the supply can leave a motor stalled at zero speed, which is worse for everything than the inrush would have been.