Soft Starters


Principles, Selection Tables, Starting Current Reduction

This is a key element in the industry — between direct on line starting (DOL) and frequency inverters.

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Soft Starters – Overview

Soft starters reduce the mechanical and electrical stress on motors during startup by gradually increasing voltage. They are commonly used for pumps, fans, compressors, conveyors and applications where smooth acceleration is required.

1. Typical Starting Current Reduction

Starting Current Reduction – Comparison of Methods
Starting Method Starting Current Mechanical Stress
DOL (Direct On Line) 6–8 × In High
Star‑Delta 2–3 × In Medium
Soft Starter 1.5–3 × In Low
VFD (Inverter) 1 × In Very Low

2. Soft Starter Selection by Motor Power

Soft Starter Selection by Motor Power
Motor Power (kW) Motor Current (A) Typical Soft Starter Size
4 8 12 A
5.5 11 16 A
7.5 15 22 A
11 21 30 A
15 29 37 A
18.5 37 45 A
22 42 52 A
30 58 75 A
37 72 90 A
45 86 105 A
55 105 140 A

3. Typical Applications

Typical Applications for Soft Starters
Application Reason for Using Soft Starter
Pumps Prevents water hammer and pressure spikes
Fans Smooth acceleration, reduced belt wear
Compressors Limits torque shock during startup
Conveyors Prevents jerks and load shifting
Mixers Reduces mechanical stress on the drive

4. Practical Notes

    • Soft starters reduce starting current, but do not control speed.
    • For heavy‑duty loads, choose a model with bypass contactor.
    • Thermal protection must match the motor’s nominal current.
    • For pumps, use pump‑control algorithms to avoid pressure spikes.
    • For applications requiring speed control, use a VFD instead.