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.
What it costs, in figures
The parameters above are theory. Here is what sits in stock for the most common ratings, priced as of September 2026.
| Motor | Model | Current | Price |
|---|---|---|---|
| 7.5 kW | ABB PSR16-600-70 | 16 A | UAH 9,489 |
| 15 kW | ABB PSR30-600-70 | 30 A | UAH 13,104 |
| 22 kW | Motortronics VMX-PFE-18 | 41 A | UAH 16,525 |
| 37 kW | ABB PSR72-600-70 | 72 A | UAH 27,586 |
| 55 kW | Schneider ATS130N2C11LT | 105 A | UAH 48,822 |
For comparison, a VFD at the same 22 kW starts at UAH 31,763 with us. Size by the motor current on the nameplate, not by kW — at the same rating, current differs from motor to motor. The full range with a current filter: soft starters.
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.
