Autotransformer starting is a reduced-voltage starting method that feeds a motor through a tapped autotransformer during startup, applying a chosen fraction of full voltage, then reconnecting the motor directly to the line once it is near speed. Taps are typically provided at around 50, 65, and 80 percent of line voltage, letting the engineer trade starting torque against inrush by picking the tap that suits the load. Its defining advantage over other reduced-voltage methods is efficiency of torque: because a transformer reduces line current more than it reduces motor current, an autotransformer delivers more starting torque per amp drawn from the line than a comparable resistor or wye-delta start. This page explains the tap-selection trade-off, the closed-transition Korndorfer arrangement, and the transition signature the controller records.
Autotransformer Starting in one line: Autotransformer starting supplies a motor through a tapped autotransformer during startup, applying a reduced voltage of about 50, 65, or 80 percent, then switching the motor to the full line once it approaches running speed. The tap sets the trade-off between reduced inrush and available starting torque. Its key strength is that the transformer reduces the line-side inrush more than it reduces motor torque, giving better torque per line amp than wye-delta or resistor starting, and the closed-transition Korndorfer connection avoids a break in supply during the changeover.
An autotransformer is a single tapped winding that can deliver a chosen fraction of its input voltage from an intermediate tap. In an autotransformer starter, the motor is connected to a tap during startup so it sees a reduced voltage, which reduces both the current it draws and the torque it develops. Because motor current is roughly proportional to the applied voltage, starting at, say, 65 percent voltage substantially lowers the current the motor pulls, taming the inrush the supply has to provide compared with a full-voltage start.
The feature that sets autotransformer starting apart from other reduced-voltage methods is what happens on the line side. A transformer that reduces voltage steps up current, so the current drawn from the line is less than the current flowing into the motor. The net effect is that the line-side inrush is reduced by roughly the square of the voltage ratio, while the motor torque, which follows the square of the motor voltage, is reduced by the same factor. This means the autotransformer gives more starting torque for each amp taken from the supply than a wye-delta or primary-resistance start at an equivalent motor voltage, which is why it is favored where the supply is limited but the load still needs meaningful breakaway torque.
The tap is the primary tuning knob. A lower tap, such as 50 percent, cuts the inrush hardest but leaves the least starting torque, suited to loads that start very easily. A higher tap, such as 80 percent, preserves more torque for a harder-starting load at the cost of a larger inrush. The engineer picks the lowest tap that still develops enough torque to accelerate the specific load through its start, balancing gentleness on the supply against the need to actually get the machine turning and up to speed.
Like other two-step reduced-voltage methods, an autotransformer start has to hand the motor over from the reduced-voltage tap to the full line, and how that changeover is done matters. In an open-transition scheme the motor is briefly disconnected during the switch, and when it reconnects to full voltage its coasting, out-of-phase voltage can cause a current and torque surge at reconnection, much like the jolt seen in an open-transition wye-delta start. That surge partly undoes the point of starting gently.
The Korndorfer connection is the classic closed-transition solution, and it is the arrangement most autotransformer starters use. It sequences the contactors so that during the changeover part of the autotransformer winding is left in series with the motor as an inductor, keeping the motor energized through the transition with no open gap. The motor is never disconnected, so there is no out-of-phase reconnection and no surge; it is smoothly handed from the reduced-voltage tap to the full line. The cost is a third contactor and a more intricate switching sequence than a simple open-transition starter.
The choice again trades cost against smoothness, but in practice autotransformer starters are usually built closed-transition because the Korndorfer sequence is well established and the surge-free changeover is a large part of the method's appeal. Knowing whether a given starter is Korndorfer closed-transition or a simpler open-transition design matters for troubleshooting, because an unexpected current or torque surge at the transition instant points to an open-transition changeover, a mis-sequenced contactor, or a Korndorfer contactor failing to pull in on schedule.
An autotransformer start is governed by timing. The starter holds the motor on the reduced-voltage tap long enough to accelerate it close to running speed, then triggers the transition to full voltage. That dwell time is set to match how long the specific motor and load take to spin up on the reduced voltage: too short and the motor is still drawing heavy current when it hits full line, undermining the gentle start; too long and the motor labors at reduced torque for longer than necessary and the autotransformer, which is only rated for intermittent starting duty, is stressed. The transformer's short-time rating is a real constraint, because it is sized to carry starting current briefly, not continuously.
The controller or SCADA system records this as a distinctive current signature: a reduced but still elevated current plateau while the motor is on the tap, then a step change at the transition as the motor reconnects to full voltage and briefly draws more before settling to running current. In a closed-transition Korndorfer start that step is smooth; in an open-transition design it can show as a spike at the changeover. The timing between energization and transition, and the shape of the current around it, are exactly what a monitoring system captures to confirm the start ran as designed.
For remote and unmanned sites, that logged signature is where cloud SCADA adds value. Because the transition is time-based and the autotransformer has a limited starting-duty rating, a start where the transition fires late, where the current plateau runs longer than usual, or where a surge appears at the changeover is worth catching before it damages the transformer or nuisance-trips the start. Surfacing the start current profile and transition timing lets an operator confirm the tap and timing are still right for the load, and comparing successive starts flags a motor whose acceleration is drifting longer, an early hint that the driven machine is loading it down.
A transformer reduces line-side current more than motor-side current, because stepping voltage down steps current up. So for a given motor voltage during start, an autotransformer draws less current from the line than a wye-delta or resistor start would for the same motor torque. The result is better starting torque for each amp taken from the supply, which is why autotransformer starting is preferred where the supply is limited but the load still needs meaningful breakaway torque.
They are the fractions of full line voltage the autotransformer applies to the motor during startup. A lower tap like 50 percent cuts inrush the most but leaves the least starting torque, suiting easy-starting loads, while a higher tap like 80 percent preserves more torque for harder-starting loads at the cost of more inrush. The engineer selects the lowest tap that still develops enough torque to accelerate the specific load through its start.
The Korndorfer connection is the standard closed-transition arrangement for an autotransformer starter. It sequences the contactors so that during the switch from the reduced-voltage tap to full line, part of the autotransformer winding stays in series with the motor as an inductor, keeping the motor energized with no open gap. This avoids the out-of-phase reconnection surge that an open-transition changeover would cause, at the cost of an extra contactor and a more complex switching sequence.
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