Soft Starter vs Autotransformer Starter Compared

How does a soft starter compare to an autotransformer starter? A soft starter ramps motor terminal voltage electronically through back-to-back SCRs per phase under IEC 60947-4-2, while an autotransformer (Korndorfer) starter switches taps on a physical transformer — typically 50%, 65%, and 80% — to apply one or two fixed voltage steps before transitioning to full line voltage. The autotransformer's winding action cuts supply-side current by roughly the tap ratio squared while the motor itself only sees a linear voltage drop, a transformer benefit no SCR-based starter provides on its own. This piece works through torque and current at each tap, open-circuit versus closed-circuit (Korndorfer) transition, moving parts and maintenance, installed cost, and which loads and sites favor each method.

What Is an Autotransformer (Korndorfer) Starter?

An autotransformer starter reduces motor voltage by feeding it through a tapped three-phase transformer instead of switching resistance or thyristors into the circuit. The motor connects to a mid-winding tap during start, then a contactor set switches it to full line voltage once the motor is near running speed. Three taps cover most applications — 50% for light, low-inertia loads, 65% as the common middle setting, and 80% where breakaway torque is high. Compare that with a soft starter: no transformer, no fixed taps, just a continuous electronic ramp set by time and current-limit parameters. Our different types of soft starters article covers the electronic side; this one puts the electromechanical alternative next to it.

The name "Korndorfer" specifically refers to the closed-transition variant, covered below. Plain autotransformer starters without that feature are older, simpler, and cheaper — and still common on legacy panels being retrofitted today.

Starting Torque and Current: Where the Two Technologies Really Differ

Torque falls with the square of applied voltage on both technologies — that part is motor physics, not starter design. At 65% tap or 65% ramp voltage, either method delivers roughly 42% of DOL locked-rotor torque, not 65%. The difference shows up on the supply side, not at the motor shaft.

Here is where the autotransformer earns its keep. Because it is a transformer, the current the utility sees on the primary side is lower than the current the motor draws on the secondary side, by roughly the tap ratio again. Run the numbers at the 65% tap: the motor draws something close to 65% of its DOL locked-rotor current, but the line only supplies about 42% of that DOL current — the transformer's turns ratio does the rest. A soft starter has no such luxury. Line current and motor current are the same number, because there is no transformer between the supply and the motor; whatever the current-limit setting holds the motor to (commonly 300-400% FLC) is also what the incoming breaker and cable must carry. On very large motors where the upstream breaker, cable, and transformer capacity are the binding constraint, that difference decides the project, not preference.

Open-Circuit vs Closed-Circuit (Korndorfer) Transition

Not every autotransformer starter transitions the same way. Open-circuit transition drops the motor off the tap winding for a few milliseconds, then reconnects it at full voltage — during that gap the motor is briefly unpowered and decelerating, so reconnection produces a current transient that can approach DOL inrush for a cycle or two. Closed-circuit (Korndorfer) transition avoids the gap: part of the transformer winding stays in the circuit as an inductor while the tap is switched, so the motor is never fully disconnected. It costs more contactors and a more complex winding, but the transient all but disappears.

A soft starter sidesteps this problem in a different way — there is no tap to switch, so there is no transition event at all. Voltage rises continuously from the pedestal setting to full line voltage over the ramp time, and the bypass contactor closes only once the motor is already at full speed and low slip, when the inrush risk has passed. What we see in the field: plants running old open-transition autotransformer starters on stiff loads sometimes report nuisance breaker trips at the tap-change point that get blamed on the motor, when the starter's transition is the actual cause.

Moving Parts, Maintenance and Panel Footprint

An autotransformer starter carries an iron-core transformer plus two or three tap-changing contactors, all of which occupy panel space and add weight — a three-tap Korndorfer starter for a mid-size motor can be noticeably heavier and deeper than a soft starter of the same rating. Those tap contactors switch on every start, so contact wear accumulates with start frequency the same way any electromechanical contactor wears. A soft starter's wear items are simpler: SCRs conduct during the ramp, and a single bypass contactor closes once per start and stays closed through the run — see soft starter bypass contactor for why that closure matters for heat. Fewer moving parts generally means less periodic maintenance, though SCR failure diagnosis calls for different test equipment than contactor inspection does.

Retrofits are where this comparison gets practical rather than academic. Plants replacing a failed or worn autotransformer starter increasingly drop in a soft starter instead of rebuilding the transformer assembly, provided the incoming breaker and cable were already sized for the motor's DOL current — which they usually were, since the transformer's current-transformation benefit was masking the true motor draw rather than reducing what the breaker had to handle at full voltage.

Installed Cost, Control Features and Long-Term Spares

Cost scales differently for each technology. Autotransformer starter cost tracks the transformer's copper and iron content, which climbs fast with motor size and with how many taps and contactors the design uses. Soft starter cost tracks SCR silicon rating and whether a torque-control platform is specified. Neither is categorically cheaper — at moderate motor ratings a soft starter is usually smaller and lighter for the same job; at very large multi-megawatt motors, an autotransformer's inherent current-transformation benefit can make it cost-competitive because it avoids upsizing the incoming breaker and cable for full DOL-equivalent current.

Control flexibility is not close, though. An autotransformer starter's starting profile is fixed by which tap is wired in; changing it means re-terminating to a different tap, not adjusting a parameter. A torque-control soft starter — ABB PSTX, Schneider Altistart ATS480, Siemens SIRIUS 3RW55 — adjusts ramp shape, current limit, and stopping profile from a keypad or fieldbus, with no rewiring. See ABB PSTX vs Schneider ATS480 vs Siemens 3RW55 for how those three flagship platforms compare on that front. This depends on the application, though: a fan running the same duty cycle every day for twenty years has little use for adjustable torque control, and a well-sized autotransformer starter will outlast the debate.

Which Loads and Sites Favor Each Method

Pumps and other loads sensitive to a smooth speed rise favor a torque-control soft starter — a fixed-tap autotransformer either applies enough voltage to start the pump (with the torque bump that comes at tap-change) or it doesn't, with no linear ramp option in between. Our soft starter for pumps article covers that sizing case in detail. Conveyors and crushers with genuinely high breakaway torque sometimes still use an 80% tap autotransformer starter where the load simply needs more starting torque than a soft starter's current-limit ceiling can safely provide without oversizing the SCRs.

Site conditions matter as much as the load. Long cable runs from a remote substation to a large motor put more weight on the autotransformer's supply-side current advantage, since the upstream infrastructure was sized against a lower apparent current. Panels with limited enclosure depth push toward a soft starter simply on footprint. And sites that already standardized on electronic starters across the plant — for spares commonality and a common HMI — will keep specifying soft starters even where an autotransformer starter would technically work.

Conclusion

Both starters reduce voltage, and both obey the same torque-voltage-squared law at the motor shaft. The autotransformer starter's advantage is on the supply side — a real current-transformation benefit that a soft starter cannot replicate — bought at the cost of a transformer's weight, footprint, and fixed, non-adjustable tap settings. A soft starter trades that supply-side benefit for continuous ramp control, fewer moving parts, and, on flagship torque-control models, closed-loop starting the autotransformer simply cannot do. Match the choice to the constraint that actually binds on the project: upstream current capacity favors the autotransformer; load smoothness, footprint, and control flexibility favor the soft starter. For the broader picture across all reduced-voltage methods, see the soft starter selection guide, and for the electromechanical alternative most often cross-shopped against both of these, soft starter vs star-delta starter.