How to Choose a Motor Soft Starter?

Published: July 22, 2025

Choosing the right soft starter for your motor comes down to one key principle:
The soft starter’s ratings must match your motor’s requirements.

However, many users feel overwhelmed by complex motor parameters. Understanding the basic motor formula can make this task much clearer:
P = 3 · U · I · PF · η

Where:

• P = Rated Power

• U = Rated Voltage

• PF = Power Factor

• η = Efficiency

• I = Rated Current (also called Full Load Amps, FLA)

After understanding these motor parameters, you only need two steps to select the right soft starter.

Step 1 — Match the Rated Voltage

The rated voltage of your motor and soft starter should be the same. This ensures that the motor will operate safely and the soft starter will work within its designed range.

Step 2 — Check the Rated Current

Once you know the motor’s rated current (from either the nameplate or by calculation),
choose a soft starter with a current rating 10%–30% higher than the motor’s FLA. This margin helps extend the soft starter’s service life, especially in demanding applications.

A Simple Guide for Beginners

What if you don’t have all the motor’s exact parameters? No problem — we’ve grouped common motor applications into four categories based on their typical power factor, efficiency, and starting current profile:

Class 10 – Light Loads

• Default

• Agitators

• Band Saws

• Tumblers

• Low Inertia Fans

• Unloaded Positive Displacement Pumps

(High current utilization; Smooth start-up; Soft starter selection is straightforward)

Class 10B – Standard Industrial Loads

• Chillers

• Lathes

• Escalators

• Transformers

• Unloaded Mixers

• Centrifugal Pumps

• Unloaded Conveyors

• Plastic & Textile Machines

• Equalized Air Compressors

(Common industrial applications)

Class 20 – Heavy or High Inertia Loads

• Ball Mills

• Rolling Mills

• Pelletisers

• Grinders

• Flour Mills

• Hammer Mills

• Circular Saws

• Loaded Mixers

• Loaded Conveyors

• Vibrating Screens

• Flywheel Presses

• Fully Loaded Air Compressors

(High inertia or load demand; Expect higher starting currents; Allow extra margin when selecting a soft starter)

Class 30 – Severe Duty Loads

• Crushers

• High Inertia Fans

• Wood Chippers

(Very high inertia or impact loads; Recommend selecting a soft starter with a higher current rating or considering a variable frequency drive)

Recommended Motor - Soft Starter Matching Table

We’ve created a motor-soft starter matching table based on the above four categories. The motor power listed in columns 2 to 5 shows the maximum power that the soft starter (in the first column) can handle. And this table applies to various voltage ratings.

Soft Starter (kW)

Class 10 Motor (kW)

Class 10B Motor (kW)

Class 20 Motor (kW)

Class 30 Motor (kW)

5.5

4.3

4.2

3.7

3.0

7.5

5.9

5.7

5.1

4.1

11

8.6

8.4

7.5

6.1

15

11.7

11.4

10.2

8.3

18.5

14.4

14.1

12.6

10.2

22

17.2

16.7

15.0

12.1

30

23.4

22.8

20.4

16.5

37

28.9

28.1

25.2

20.4

45

35.1

34.2

30.6

24.8

55

42.9

41.8

37.4

30.3

75

58.5

57.0

51.0

41.3

90

70.2

68.4

61.2

49.5

115

89.7

87.4

78.2

63.3

132

103.0

100.3

89.8

72.6

160

124.8

121.6

108.8

88.0

185

144.3

140.6

125.8

101.8

200

156.0

152.0

136.0

110.8

220

171.6

167.2

149.6

121.0

250

195.0

190.0

170.0

137.5

280

218.4

212.8

190.4

154.0

320

249.6

243.2

217.6

176.0

350

273.0

266.0

238.0

192.5

400

312.0

304.0

272.0

220.0

450

351.0

342.0

306.0

247.5

Choosing the Soft Starter Type

The most widely used soft starters today are solid-state voltage reduction soft starters.
Depending on whether the soft starter bypasses itself after the motor reaches full load current,
they can be classified as:

• Continuous Running Soft Starters

• External Bypass Soft Starters

• Internal Bypass Soft Starters

You can click here for a detailed comparison.

Default Wiring Configurations

In most three-phase systems, the default wiring is for the soft starter to be wired in series with the motor connected in delta. This is also the basis of our selection table.

If the soft starter and motor are both connected in delta, the soft starter measures phase current, which is about 58% of the line current. This allows the soft starter to control a larger motor, but the wiring becomes more complex and the voltage requirement doubles compared to the default. If you have wiring needs that differ from this setup, feel free to contact our technical support team.

3-Phase RVSS Soft Starters

Designed to ensure smooth motor startups, reduce mechanical stress, and provide reliable protection throughout operation.

Boost your motor's performance today!