Use a soft starter when the main requirement is controlled starting of a motor that normally runs at fixed speed. If the process needs continuous speed regulation, evaluate a VFD instead.
Soft starter or variable frequency drive?
A soft starter controls the motor voltage during starting and, where supported, stopping. It does not provide the same continuous frequency-based speed regulation as a VFD. The choice begins with the process: does the machine need a gentler start, or must its running speed vary?
A fixed-speed pump may use a soft starter to manage starting. A pump that must vary speed to regulate flow or pressure is a different application and may suit a VFD. A high-inertia fan or loaded crusher needs a careful starting-duty review.
What determines the starter rating?
| Information to supply | Selection implication |
|---|---|
| Motor voltage and rated current | Match the actual electrical rating; do not rely on motor kW alone. |
| Load torque and inertia | The motor must develop enough accelerating torque throughout the start. |
| Starting current and duration | Assess semiconductor and motor thermal duty. |
| Starts per hour and rest time | Repeated starts can require a different rating from an occasional start. |
| Ambient temperature and installation | Apply the selected product derating and cooling requirements. |
| Bypass and protection requirements | Confirm the supplied bypass arrangement and protection scope. |
A lower current limit is not always a better start
If the available starting torque is too low, the motor may accelerate slowly or fail to accelerate. A longer start can increase heating rather than solve the problem. Selection should use the actual load and the model’s starting-duty capability, rather than a single current-limit rule for every machine.
For pumps, also review hydraulic conditions and the stop function needed by the system. For compressors, identify whether starting is loaded or unloaded. For crushers and conveyors, tell the supplier whether material is present during starting.
Where INOMAX AST soft starters fit
INOMAX offers AST6800, AST6200 and AST7000 soft starter families for different requirements. AST6800 and AST7000 are presented with built-in bypass configurations; AST6200 is the online soft starter offering. Select the exact model using its current manual and application duty.
AST7000 development includes INOMAX control boards, SCR firing circuitry, firmware and protection algorithms. Discuss communication and temperature-monitoring options for the actual supplied configuration. For medium- or high-voltage motors, request a dedicated MAX-MVSS solution review instead of applying low-voltage selection rules.
What to send with an enquiry
Provide the motor nameplate, load description, start time, starts per hour, current restriction, installation temperature, voltage, desired stop behaviour and required communications. Include any mandatory certification requirements before the quotation is finalized.
Explore INOMAX low-voltage soft starters or the full motor-control portfolio.
Frequently asked questions
Does a soft starter save energy by controlling running speed?
A conventional soft starter does not provide continuous variable-speed control. Evaluate a VFD if speed regulation is the objective.
Can I add an external bypass to any starter?
No. The allowed wiring and bypass arrangement are model-specific. Use the supplied manual and approved drawings; do not assume an external bypass is supported.