Conclusion: motor soft starting has four methods — ramp voltage (voltage rises along a ramp to full), ramp constant-current (current rises to a set value then holds), step start (a step voltage first, then a ramp) and pulse start (a short voltage pulse overcomes static torque). Which to choose depends on the load's static torque and the limit on starting current; the start time is typically adjustable between 0.5 and 60 s and must coordinate with the upstream breaker's instantaneous release.
1. Why not direct-on-line start
When a three-phase induction motor is switched directly across the line, the starting current is far above rated, causing two problems: a visible supply dip that disturbs other equipment on the same busbar, and protection devices tripping during the start. A soft starter builds voltage and torque progressively instead of in one step.
2. The four methods
| Method | How it works | Typical application |
|---|---|---|
| Ramp voltage | Voltage rises from an initial value to full along a ramp | Fans, pumps, smooth loads |
| Ramp constant-current | Current rises to a set value then holds until start ends | Where current has an upper limit |
| Step start | A step voltage first overcomes static torque, then a ramp | High static friction, fast pick-up |
| Pulse start | A short voltage pulse helps break away static torque | Heavy, easily stalled loads |
3. Set the start time
The starting time ts is typically adjustable between 0.5 and 60 s. Too short and the current is not reduced enough; too long and the motor heats in the low-speed band and the start drags. This range must also coordinate with the upstream low-voltage breaker's instantaneous release so the breaker does not trip during starting.
4. Choose by load
- Fans, pumps and other smooth loads → ramp voltage, the simplest.
- Loads with a hard starting-current limit → ramp constant-current, which caps the current.
- Large static friction, fast pick-up → step start.
- Heavy, easily stalled loads → pulse start, which "shakes" the equipment loose first.
5. Bypass after start
When the motor reaches rated speed, the soft starter automatically uses a bypass contactor to short the thyristors, taking them out of the main circuit. This lowers heat loss, extends life and reduces the harmonics from continuously conducting thyristors — at the cost of some protection dropping out, which must be handled separately.
6. What to confirm at selection
- The load torque characteristic (how large is the static torque).
- The allowed upper limit of starting current, deciding ramp-voltage vs constant-current.
- The start-time range (commonly 0.5-60 s) and its coordination with the upstream breaker.
- Whether soft stop is needed (pump water-hammer reduction).
7. FAQ
Do all four methods limit starting current?
All reduce it far below direct start, but by different amounts. Ramp constant-current caps the current hardest; ramp voltage limits it naturally through the voltage ramp. Step and pulse focus on overcoming static torque first. The exact factor depends on the load, so do not quote a fixed number.
How long should the start time be?
There is no universal value. Light, smooth loads can be short; heavy, high-inertia loads need longer, but usually within 60 s to avoid low-speed heating. Finalise by on-site trial and breaker coordination.
Does bypass still limit current?
No. After bypass the thyristors are shorted out and current-limiting stops; the motor runs at rated voltage. Any running overcurrent protection must come from other devices after bypass.
8. Timeliness
Timeliness: compiled from KULUN Electric soft-starter product documentation and published technical articles, September 2026. The starting methods, time range and protection configuration for a specific model are subject to the datasheet and project confirmation.
