Soft starter cabinets
- Starting method: thyristor voltage-controlled soft start; voltage ramp / current limit / torque control selectable; starting current limited to 2–4 times rated
- Power range: 15–500 kW (low voltage AC 380V / 400V); higher power available as medium voltage soft start (quoted separately)
- Bypass operation: bypass contactor engages after starting is complete, thyristors drop out — no extra losses or harmonics in running condition
- Protection: overload, phase loss, short circuit, overheat, phase imbalance, locked rotor; multiple-start limit supported
- Testing: on-load start/stop test on every unit; starting time, initial voltage and current-limit multiple set to the customer's motor nameplate
Specifications
The table below covers 80% of factory distribution room cases. Anything outside it we can still build — those are engineered items, quoted separately, with an indicative price the same day.
| Type | Floor-standing soft starter cabinet / wall-mounted soft starter box; one cabinet per motor |
|---|---|
| Standard sizes (W×D×H) | 600×600×1800 / 800×600×2000 mm; size selected by power and heat dissipation requirements |
| Enclosure construction | 2.0 mm cold-rolled carbon steel, bent and welded with reinforcing ribs, electrostatic powder coated |
| Surface treatment | Degrease → derust → phosphating → electrostatic powder coating 60-80 μm, baked at 200 °C for 30 minutes |
| IP rating | IP54 standard / IP65 optional (GB 4208 / IEC 60529) |
| Thermal design | Thyristor heat sink sized to power rating; exhaust at cabinet top; forced-air fan added for higher power |
| Installation | Floor mounting, leave ≥800 mm service access in front of the cabinet |
| Cable entry | Cable entry at bottom with gland plate; main circuit outgoing lines can be brought out via copper busbar |
| Suitable power | Low voltage 15–500 kW (AC 380V / 400V, three-phase); medium voltage options quoted separately for higher power |
|---|---|
| Starting method | Voltage ramp start, current limit start, torque control start — selectable to match load characteristics |
| Starting current | In current limit mode, settable to 2–4 times rated current (3 times by default) |
| Starting time | Adjustable 0–60 s; for heavy-load starting, initial voltage and ramp time can be set in stages |
| Bypass method | After startup completes, the bypass contactor closes and the thyristors drop out, so there is no additional loss in run mode |
| Startup frequency | Designed around the soft starter's permitted number of starts per hour, including interval time and thermal capacity protection |
| Protection | Overload, phase loss, short circuit, overheating, phase imbalance, locked rotor, and startup timeout protection |
| Control interface | Local pushbuttons / remote start-stop; optional PLC interface, communication port, and supervisory monitoring |
| Standalone local control | One soft starter cabinet per motor, with door-mounted start-stop pushbuttons, ammeter, and status indication |
|---|---|
| duty/standby | Two motors, each with its own soft starter cabinet, or one cabinet with dual circuits and automatic fault transfer |
| Remote / PLC control | Accepts start-stop and reset signals from a PLC or supervisory system; returns run, fault, and current signals |
| Heavy-duty starting solutions | For high-inertia loads (fans, crushers), use torque control or staged starting; upsize the soft starter where necessary |
| Soft starter + bypass cabinet | Soft starter and bypass contactor mounted in separate cabinets to spread heat dissipation at high power |
| Mixed use with VFDs | VFD speed control for main process loads, soft starting for large auxiliary motors, sharing the incoming line and branch circuits |
| Reference standards | GB/T 7251.1 / IEC 61439-1 (assemblies); GB/T 14048.6 (soft starters) |
|---|---|
| Clearance & creepage | Meets pollution degree 3 and overvoltage category III requirements |
| Insulation resistance | ≥1 MΩ (500V megohmmeter, measured with thyristors disconnected) |
| Power frequency withstand voltage | Main circuit 2500V / 1 min, control circuit 1500V / 1 min |
| Startup performance verification | Measured startup current curve and startup time against setpoints; records delivered with the cabinet |
| Routine tests (every unit) | Circuit checks, insulation and dielectric withstand, start-stop operation, protection trip tests, on-load start-stop testing |
| Documents with shipment | Electrical schematic, terminal list, soft starter parameter setting sheet, BOM, test report, factory photos |
Five steps to specify your run
A configuration summary is generated as you go. It travels with your inquiry, saving a round of confirmation.
Build details
Systems have been in service for decades, and the failure points are always the same few: poor busbar contact, water ingress at the door seal, high temperature rise in the correction cabinet, and mis-phased outgoing feeders. The six hard rules below are written for exactly these points — every cabinet can be traced to them.
Starting current held in check
The starting current is set between 2× and 4× rated current according to the motor and load characteristics. For other equipment on the same busbar, that difference is what decides whether the lights flicker when the breaker closes.
Drops out once the start is complete
After the start is complete, the bypass contactor closes and the thyristors drop out of the circuit. In running mode there is no thyristor voltage-drop loss and no harmonic injection into the grid — a major advantage of soft starting over a VFD.
Staged settings for heavy-load starts
For high-inertia loads such as fans and crushers, initial voltage and ramp time are set in stages to prevent a mid-start stall from tripping the breaker on overcurrent.
Start counts worked out in advance
Start intervals and permitted starts per hour are checked against the soft starter's thermal capacity curve. For frequent start/stop duty we recommend going up one frame size or switching to a VFD.
Bypass transfer timing verified by test
The timing sequence from bypass contactor pickup to thyristor dropout is measured at the factory to prevent a current surge at the moment of transfer.
Load test with start/stop cycles
Start/stop cycles are run under load before shipment, and starting current and starting time are recorded. The parameter sheet ships with the cabinet so the same values can be reproduced on site.
Photos & enclosure construction
Below are photos taken in our own shop — from the complete unit down to the internal busbars and secondary wiring. Every build detail can be checked item by item.
Image key: 01–05 are shop photos (the numbers match the shot list). The line drawing shows the front elevation of the enclosure; dimensions match the specification table above.
Where these cabinets go
These are the routine applications — we can talk scheme and configuration directly. For the industry view, see the industry pages.
Pumps and fans
Reduces water hammer and damper surge on startup, lowering impact loads on the piping.
Belt conveyors and conveying lines
Soft start and soft stop reduce material spillage and belt slippage; multiple units can be started in sequence.
Crushers and ball mills
High-inertia heavy-load starting; torque control reduces the impact on both the grid and the machinery.
Air compressors and compressors
Reduces the impact of starting current on other equipment on the same busbar and extends contactor life.
Centrifuges and mixers
Prevents sudden changes in starting torque from damaging the shaft train and gearbox.
Limited expansion capacity in older distribution rooms
When transformer capacity is insufficient, current-limited starting gets large motors running and avoids the cost of a capacity upgrade.
Commercial terms
Specification dated September 2026; quotations follow this.
| Drawings & design | Free |
| MOQ | 1 unit (5 or more recommended for production) |
| Production lead time | 15-25 days after drawing approval |
| on-time delivery | 98% |
| Warranty | 12 months |
| Loading port | Ningbo / Shanghai, China |
| Component brands | Schneider / ABB / Siemens / Chint |
| Factory visit & dispatch confirmation | Factory visits welcome; finished-cabinet photos and video before dispatch |
| Capacity | 5,800+ units / year |
Documents for your engineering team
Selection manual, outline dimensions and RFQ checklist — for the customer's electrical engineer. All three documents are being organized by product line; download links go live once complete.
Six questions procurement and electrical engineers ask most
What is the difference between a soft starter cabinet and a VFD panel?
The core difference is whether speed control is needed. A soft starter only acts during the starting phase and is bypassed once the motor is up to speed — in running mode it behaves exactly like a direct-on-line motor. A VFD panel provides full-range speed control and can change speed to match the process. If you only need a smooth start, a soft starter is cheaper and produces fewer harmonics; if you need speed control for energy savings, that is when a VFD is required.
Does a soft starter cabinet always need a bypass contactor?
We recommend it. Without a bypass, the thyristors stay energized continuously, which causes voltage drop losses and heat — costing you both electricity and maintenance. We supply a bypass contactor as standard, and it is automatically switched in once starting is complete.
Can it start frequently?
This is limited by the thermal capacity of the soft starter. Generally, 3–6 starts per hour is a reliable range; above that, we recommend upsizing or switching to a VFD. For frequent start/stop duty, tell us the number of starts per day and we will size the unit based on thermal capacity.
How low can the starting current be limited?
Typically to 3 times rated current; on request, down to 2–2.5 times. The lower the limit, the longer the starting time — for heavy-load applications you need to confirm the motor can actually start. We will recommend a value based on your load characteristics.
How many motors can one cabinet handle?
One soft starter generally controls one motor. Multiple motors can each have their own soft starter mounted in the same cabinet, or a one-controls-many scheme can start them sequentially. The specifics depend on motor power and the required starting interval.
For a retrofit of older equipment, can the existing control cabinet still be used?
In most cases, yes. Keep the existing distribution and feeder circuits and simply insert the soft starter cabinet into the motor circuit. The original control buttons can be wired into the new cabinet for remote signaling. Before the retrofit, we review the original system drawing and provide a wiring plan.
Other cabinet types in the same main circuit
Related products use color-block placeholders for now (product photos not yet complete; they will be replaced together). Internal links will be filled in once live URLs are confirmed.
Want to eliminate the inrush from large motor starting?
Send us the motor power, load type, and daily start count, and we will provide a free soft starter cabinet configuration and capacity calculation · quotation within 24 hours. Direct from the source factory, MOQ 1 unit, 15–25 day delivery, 1-year warranty.
ISO 9001:2015 · CE(LVD+EMC) · CQC · IP65
When you write, include: single-line diagram or primary scheme, switch room layout, load list, and component brand requirements. You do not need complete drawings — capacity and cabinet type are enough to start.