Choosing the right soft starter comes down to five factors: motor current, connection method, bypass type, start duty and operating environment. Work through them in order to select the model best suited to your installation.
Step 1: Confirm motor full load current and voltage
Size the soft starter to the motor's nameplate full load current (FLC), not its horsepower. Two motors with the same horsepower rating can draw different current. Also confirm the supply voltage, which determines whether you need a low voltage or medium voltage soft starter.
Step 2: Choose the motor connection method
In-line connection places the soft starter between the supply and the motor and is the simplest to install. Inside-delta connection places the soft starter inside the motor's delta windings, where it carries about 58% of motor current. This lets a smaller soft starter control a larger motor, but it requires six-lead motor wiring.
Step 3: Choose the bypass type
A bypassed soft starter routes current around the SCRs once the motor reaches full speed, which reduces heat and enclosure size. Internally bypassed models save panel space. Externally bypassed or non-bypassed models suit applications that need the soft starter to keep controlling the motor while it runs.
Step 4: Define the start duty
Determine the start current, start time and number of starts per hour your application needs. High-inertia loads such as crushers, fans and centrifuges need longer, heavier starts and may require a larger soft starter than the motor FLC alone suggests.
Step 5: Check operating conditions
Ambient temperature, altitude and enclosure rating all affect a soft starter's capacity. Above the rated ambient temperature (commonly 40°C) or altitude (commonly 1,000 m), the soft starter must be derated.
Get the full selection guide
For worked examples and selection tables, download Benshaw's free eBook “Where Do I Start? What You Need to Know to Select the Ideal Soft Start Solution.”
Frequently asked questions
Do I need a bypass soft starter?
Usually yes, for standard pumps, fans and conveyors, because it reduces heat and panel size. Non-bypassed models suit applications needing continuous SCR control.
Can a soft starter replace a VFD?
Yes, if you only need to control starting and stopping. If you need to vary speed while running, you need a VFD.
What happens if I undersize a soft starter?
It may trip on overload during starting or overheat in heavy-duty applications.
Electrical Solutions to Mechanical Problems
Benshaw is widely recognized as a leading provider of applied motor controls and technologies. We thrive on complex applications and work diligently to bring cost-effective, advanced motor control and protection solutions to our customers. With operations spanning the globe, Benshaw now offers the broadest family of globally supported, globally certified, low and medium voltage soft starters in the industry.