Soft starting an electric motor — how it works
A soft starter limits the inrush current of an electric motor by gradually raising the voltage on the windings from a reduced level to full. Instead of a sudden current spike of 5-7 times the rated value, there is a gradual ramp over 5-30 seconds. The motor accelerates without shock loads on the mains, mechanical drive train and the equipment itself.
The principle is simple: inside the soft starter, thyristors (or triacs) control the firing angle of the sinewave. At start-up they pass only part of each half-cycle, so voltage is low. The angle increases gradually — voltage rises — the motor spins up. When full voltage is reached, the thyristors open completely or a bypass relay shorts them out — the motor runs directly from the mains.
Why you need a soft start
Direct-on-line (DOL) starting of an induction motor draws 500-700% of rated current. For a 22 kW motor that is around 250A instead of 45A running current:
- Voltage sag — lights flicker, other equipment restarts.
- Mechanical shocks — sudden shaft acceleration stresses couplings, belts, gearboxes.
- Water hammer in piping — when starting a pump.
- Generator overload — on standalone sites.
A soft starter solves all of these by limiting inrush current to 200-350%.
How thyristor control works
A soft starter uses phase-angle voltage control. Each phase has two thyristors (anti-parallel) or one triac.
- Start (large angle) — thyristor fires late, voltage ~30-50%. Motor begins turning slowly.
- Ramp (angle decreases) — voltage ramps up over the set ramp time.
- Running (angle = 0) — thyristors fully on or bypass relay closes the power circuit.
Ramp time is adjustable from 1 to 60 seconds. For pumps typically 10-20 seconds, conveyors 5-10.
Soft starter vs VFD
Full comparison in a separate article. In brief:
| Parameter | Soft starter | VFD |
|---|---|---|
| Control | Start and stop only | Continuous speed control |
| Running mode | Motor at full voltage | Adjustable frequency |
| Energy saving | During start only | Continuous |
| Price (22 kW) | ~8,000-15,000 UAH | ~20,000-35,000 UAH |
If you just need a gentle start — soft starter. If you need speed control — VFD.
Where soft starting is used
- Pumps — the most common application. Soft starters for pumps.
- Fans — reducing startup jolts.
- Compressors — limiting inrush current.
- Conveyors — preventing belt slip on sudden start.
- Crushers, mills — starting under high inertia.
Key soft starter parameters
- Rated current — at least equal to motor current. 10-20% margin.
- Ramp time — 1 to 60 seconds.
- Starting voltage — 30% to 70% of rated.
- Current limit — 200-350% of rated.
- Bypass relay — shorts the thyristors after full voltage is reached.
Frequently Asked Questions
Does a soft starter save electricity?
During running — no, the motor is on mains directly. For continuous savings you need a VFD.
Can a soft starter regulate speed?
No. A soft starter controls voltage, not frequency. For speed control you need a VFD.
Which soft starter for a 15 kW motor?
Rated current is about 32A. Choose a soft starter for 32A or more, ideally 38-40A.
Does a soft starter work with a single-phase motor?
Standard three-phase soft starters — no. Special single-phase models exist but are less common.
How much does a soft starter cost?
For 7.5 kW — from 5,000 UAH. For 22 kW — from 8,000 UAH. For 55 kW — from 20,000 UAH.
Summary
A soft starter is a simple and affordable way to protect a motor and mains from shock starting loads.
Our catalogue has soft starters for motors from Danfoss, Schneider, Veichi. We will match one to your motor.
