What Is a Soft Starter Module?

A soft starter module (SRQ series) is a compact, SCR-based unit designed solely to provide motor soft-start functionality. The SRQ series covers a power range of 1 KW to 15 KW and is recommended for motors up to 10 KW. It does not include soft-stop, built-in protection circuits, or a display panel.

The module supports two control modes: automatic startup on power-on, or startup via an external start/stop button. Its compact dimensions — 43 × 74 × 104 mm for the SRQ-H3P-38 — make it straightforward to integrate into custom control panels or OEM equipment without occupying significant space.

For motors above 10 KW, heat dissipation under sustained load becomes a concern with the module design. In these cases, a complete online soft starter with active cooling is the more reliable choice.

What Is an Online Soft Starter?

The RQ6000 online soft starter is a full-featured intelligent motor soft starter covering a power range of 5.5 KW to 630 KW, making it suitable for small workshop motors through to large industrial drives.

Unlike the soft starter module, the online soft starter remains in the power circuit throughout operation — the SCRs continue to conduct rather than switching to a bypass contactor after startup. This design supports both soft-start and soft-stop, and includes a large LCD display, six-button keypad, and comprehensive built-in protection functions.

The unit supports multiple starting modes including current-limiting start and voltage ramp start, allowing adjustment to match different load characteristics such as pumps, fans, conveyors, and compressors.

How It Works

Both products use SCR phase-angle control to limit motor inrush current during startup. At startup, the SCRs fire at a controlled angle, gradually increasing the output voltage from an initial value (typically 150–250V on the SRQ module, adjustable) up to full rated voltage (380V). This ramps the motor current smoothly rather than allowing the 6–8× inrush current typical of direct-on-line starting, reducing it to approximately 2–4× rated current.

On the soft starter module, the ramp time is adjustable up to 30 seconds. On the RQ6000 online soft starter, both the starting mode and ramp parameters are fully configurable via the keypad, and the unit can also perform a controlled soft-stop by gradually reducing output voltage when a stop command is received — preventing water hammer in pump systems and reducing mechanical stress on connected equipment.

Soft Starter Module vs Online Soft Starter

When to Choose a Soft Starter Module

A soft starter module is ideal for OEM integration, custom control cabinets, or applications with limited space, where only soft-start is needed. Its simple control allows either automatic startup on power-on or connection to a start/stop button. The compact design and cost-effectiveness make it the preferred choice when size and budget are critical, and additional protections can be implemented externally.

Recommended for motors up to 10 KW. For motors in the 10–15 KW range, the module can still be used, but heat dissipation under sustained or frequent cycling loads should be carefully considered. A properly sized heatsink is required, and external protection relays are necessary since the module has no built-in fault protection.

When to Choose an Online Soft Starter

Choose the RQ6000 online soft starter when independent installation, built-in protection, and a wider power range are required. The complete unit is suitable for motors from 5.5 KW to 630 KW and is the clear choice for any application above 15 KW, where the soft starter module's limitations in heat dissipation and protection become significant.

For motors in the 10–15 KW range, the online soft starter is the recommended option if the application involves frequent starts, continuous duty cycles, or environments where built-in overload, phase-loss, and overtemperature protection add value. The soft-stop function is also critical in pump applications to prevent water hammer.

Complete units simplify setup, reduce the need for external protection wiring and auxiliary components, and ensure safe, reliable operation even for facilities without specialized electrical engineering staff.

Key Specifications

Soft Starter Module (SRQ series):

• Input voltage: 150–250V AC adjustable initial voltage, rated 380V three-phase
• Power range: 1 KW–15 KW (SRQ-H3P-38: 1–4 KW aluminum housing; up to 15 KW copper housing)
• Ramp time: adjustable up to 30 seconds
• Control: power-on auto-start or external start/stop button
• Cooling: external heatsink required
• Dimensions: 43 × 74 × 104 mm (SRQ-H3P-38)

Online Soft Starter (RQ6000 series):

• Input voltage: AC 380V (−10% to +15%), 50Hz/60Hz
• Power range: 5.5 KW–630 KW
• Starting modes: current-limiting start, voltage ramp start, kick-voltage ramp start
• Stop modes: free stop, soft stop (1–60s adjustable)
• Protection: overload, overcurrent, overvoltage, undervoltage, phase loss, phase imbalance, overtemperature, underload
• Communication: optional Modbus RTU (RS485)
• Analog output: 4–20 mA
• Cooling: forced fan cooling
• Display: large LCD, Chinese/English selectable
• Protection rating: IP20 (≤55 KW) / IP00 (≥75 KW)

Get a Customized Solution

Looking for the right soft starter solution for your application? Visit softstarterpro.com to explore the SRQ soft starter module series and the RQ6000 online soft starter, designed for OEM equipment and industrial motor control across a wide power range.

Whether for compact control panels, custom control cabinets, or standalone industrial installations, these solutions provide smooth motor startup, reduced mechanical stress, and enhanced equipment longevity.

Contact our technical team today for a tailored recommendation based on your motor rating, duty cycle, and installation requirements. Reach out via email at ann@softstarterpro.com or WhatsApp at +8618006697926 for a free quote or consultation.