Auto-transformer starter cabinets
- Autotransformer: 65% / 80% taps, dry-type (standard) or oil-immersed (high power, outdoor)
- Starting torque: about 64% of full torque on the 80% tap, about 42% on the 65% tap — select the tap against the load resisting torque
- Starting current: about 0.64 × direct-on-line current on the 80% tap, measured at the transformer primary
- Settings: starting time adjustable 10–60 s; the overcurrent relay also provides transformer overload protection
- Components: Schneider / ABB / Siemens / Chint; brand can be specified, or customer-supplied components shipped to our plant
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; starter and transformer can share one cabinet or be split (split recommended at ≥160 kW) |
|---|---|
| Standard sizes (W×D×H) | 800×600×1800 / 1000×800×2000 mm, sized by transformer capacity, custom build available |
| Enclosure construction | 2.0 mm cold-rolled carbon steel, separate transformer and electrical compartments, ventilation louvers |
| IP rating | IP54 standard / IP65 or outdoor type optional |
| Transformer mounting | Bottom channel steel load-bearing with vibration pads; oil-immersed transformer per site conditions |
| Cable entry | Bottom cable entry, busbar outgoing to the motor |
| Rated operating voltage | AC 380V / 400V, 50 Hz; control circuit AC 220V |
|---|---|
| Applicable Power | 15–315 kW; choose this cabinet type first for heavy-duty starting |
| autotransformer | QZB dry-type (standard) / oil-immersed (outdoor, high power), 65% / 80% taps |
| Transition method | Time relay 10–60 s automatic transfer; step-down power is cut before full voltage is applied (closed transition optional) |
| Protection configuration | Overcurrent relay (transformer side) + thermal overload relay (running side); phase loss and overload |
| Metering and indication | Ammeter / voltmeter; step-down starting / full-voltage running / fault indication |
| Component brands | Schneider / ABB / Siemens / Chint; customer-supplied components accepted for factory installation |
| Open transition, standard | Momentary power loss at transfer, then full voltage is applied; simple design, most widely used |
|---|---|
| Closed transition | No power interruption at transfer, lower surge; suited to interlocked duty where even a momentary loss of supply is not acceptable |
| Compared with star-delta | Choose autotransformer starting for heavy loads (higher torque); choose star-delta for light loads (lower cost) |
| Compared with soft start | On a limited budget with single-speed operation, choose autotransformer starting; if you need an adjustable current ramp, choose a soft starter cabinet |
| Multiple motors from one unit | One autotransformer cabinet starts several motors in sequence (interlocked timing control), reducing total investment |
| Assembly standard | GB/T 7251.1 / IEC 61439-1 |
|---|---|
| Transformer testing | Winding DC resistance, turns ratio and insulation resistance reports supplied with the unit |
| Routine tests (every unit) | Insulation resistance, power-frequency withstand voltage, and a full powered start-transfer sequence run twice with currents recorded |
| Temperature rise | Transformer temperature rise at full load is verified to its insulation class; terminal connections ≤70 K |
| Documents with shipment | Schematic diagram, terminal list, BOM, transformer and complete-cabinet test reports |
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.
Torque Tap Calculated for Your Application
Before startup, we calculate the load resistance torque curve and specify the 80% or 65% tap in the proposal — no trial and error on site.
Dedicated Transformer Compartment
The transformer is compartmentalized away from contactors and relays, each with its own air path, so hot air does not bake components and accelerate aging.
Unit-by-Unit Energized Start Test
Each unit undergoes 2 complete reduced-voltage start transitions at the factory, with current waveforms recorded to verify transition timing and protection settings.
Transformer Base Channel Steel
Transformer weight is transferred through bottom channel steel with vibration damping pads — neither transport nor operation relies on the side panels for structural support.
Permanent Terminal Labels
The 65%/80% tap terminals carry permanent labels for instant identification during maintenance, eliminating wrong-tap connections.
Wire Number = Circuit Number
All conductors are fitted with wire number sleeves matching the circuit numbers on the schematic. Delivery includes the complete schematic and terminal list.
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.
Ball Mills and Grinding Equipment
Typical loaded-start scenario; the 80% tap torque covers static resistance torque.
Crushers
Jaw or impact crushers starting with a loaded cavity still require high initial torque — autotransformer starting is preferred.
Reciprocating Compressors
For start-under-pressure conditions, the tap position is calculated based on unloading capability.
High-Power Centrifugal Fans
Started with the damper closed; where grid capacity is insufficient, the 65% tap reduces inrush.
Deep Well Pumps and Line Shaft Pumps
High static head means high resistance — we size the tap after calculating starting torque.
Replacement for legacy XJ01 cabinets
We can also keep the existing transformer and replace only the control section — send site photos and we will issue a retrofit plan.
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 actually separates autotransformer starting from star-delta?
Torque: star-delta delivers only 1/3 of full-voltage starting torque; the 80% tap on an autotransformer gives about 64%. For heavy-load starting, autotransformer is the only option. Motor compatibility: star-delta works only with delta-connected motors; autotransformer works with any connection. Cost: autotransformer adds a transformer and runs 30–50% more. Light load — choose star-delta. Heavy load — choose autotransformer.
65% tap or 80% tap?
80% is the default: 64% torque, enough to start most heavy loads, with current at about 64% of direct-on-line. Choose 65% where the grid is weak, transformer capacity is small, or voltage dip is a concern (42% current, but torque also drops to 42% — the load resistance torque must be checked). If you are unsure, tell us the load type and we will run the starting calculation free of charge.
Dry-type or oil-immersed transformer?
For typical indoor installations, dry-type (QZB) — simple maintenance, no oil contamination. For outdoor or high-power duty (≥200 kW), oil-immersed is an option: better heat dissipation, but site fire protection requirements apply. We supply dry-type by default; oil-immersed is quoted per project.
How many consecutive starts are allowed?
The autotransformer is rated for short-time duty and will overheat under consecutive starts. As a rule of thumb, no more than 2 consecutive starts from cold; further starts require a cooling interval (about 30 minutes or more). For frequent starting duty (multiple starts per hour), use a soft starter cabinet or VFD panel instead — thyristors do not have this limitation.
What about 315 kW and above?
For high-power heavy loads, we recommend evaluating a soft starter cabinet (thyristor current-limit ramp) or a medium voltage switchgear solution directly; above 315 kW the transformer size and cost of an autotransformer scheme no longer make sense. Send us the motor data and duty conditions and we will provide a comparison.
Does the transfer cause a power interruption?
Open transition interrupts power briefly (tens of milliseconds) before applying full voltage — imperceptible to most equipment. Closed transition bridges with a third contactor, so there is no interruption and lower current surge; choose it for interlock-sensitive duty.
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.
Heavy-load starting calculation, free of charge
Send us the motor nameplate and load type (ball mill / crusher / fan) and we will calculate starting torque and specify the tap free of charge · 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.