Reduced-voltage starting for large motors — without the torque penalty of star-delta.
An auto-transformer starter reduces the voltage applied to a large squirrel-cage induction motor at start-up, limiting the inrush current that would otherwise stress the supply network, trip upstream protection, and cause voltage dips that affect other equipment. Standard taps at 50%, 65%, and 80% of supply voltage reduce starting current to 25%, 42%, and 64% of the DOL value respectively. Unlike a star-delta starter, the motor receives voltage from the transformer secondary — not from a re-wired winding — so it can develop full starting torque at any tap position. This makes auto-transformer starting the preferred choice for loaded conveyors, compressors, and high-inertia loads where star-delta starting torque is insufficient.
Sakthi manufactures auto-transformer starters from 5 kVA to 500 kVA with open frame or enclosed construction, wound on CRGO laminated cores with Class F or H insulation. The starter is timed by a contactor-and-timer circuit — after a settable acceleration time, the circuit transitions to direct supply. Custom tap positions and panel-mounted automatic starters with ammeters and timers are available on request.
Typical applications
Steps down supply voltage to 50–80% to limit starting current of large squirrel-cage induction motors where DOL is not permitted.
Used to start high-inertia loads like centrifugal pumps, blowers, and air compressors in industrial plants and waterworks.
Reduces starting torque and current surge for large HVAC fans, chillers, and cooling tower motors.
Integrated into automatic reduced-voltage starter panels with timers and contactors for fully automatic starting cycles.
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