Starting a large motor across the line — applying full voltage instantly — draws enormous inrush current. A motor that runs at 100 amps at full load may draw 600 to 800 amps during the first few seconds of starting. That inrush can sag voltage across the facility, trip overcurrent protection, disturb other equipment, and stress the motor windings and driven equipment.
Autotransformer reduced voltage starting is one of the oldest and most reliable methods for managing this problem. By starting the motor at a reduced voltage through an autotransformer, inrush current is reduced while still delivering enough torque to accelerate the load. Once the motor approaches operating speed, the autotransformer is bypassed and the motor runs on full line voltage.
Safety note: This article is educational content for engineers, specifiers, and maintenance professionals. Motor starter selection and installation must be performed by qualified personnel in accordance with the NEC, motor manufacturer requirements, and applicable local codes. This article does not contain wiring instructions.
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Why reduced voltage starting matters
Search intent includes: motor inrush current reduction, across the line motor starting, large motor starting methods
When a squirrel-cage induction motor starts, it behaves electrically like a short circuit for a brief period. The rotor is stationary, there is no back-EMF, and the motor draws locked-rotor current — typically 5 to 8 times the full-load amperage.
This matters for three reasons:
Voltage sag. The high inrush current flowing through the supply system impedance causes voltage to drop. Other equipment on the same system — lights, controls, computers, other motors — sees this voltage dip. Depending on the system capacity and the motor size, the sag can be severe enough to cause nuisance tripping of contactors, malfunction of sensitive electronics, or visible lighting flicker.
Utility requirements. Many utilities impose limits on motor starting current, particularly for large motors (above 50 or 100 horsepower). Exceeding these limits can result in penalties or a requirement to install reduced-voltage starting equipment.
Mechanical stress. Full-voltage starting delivers full locked-rotor torque instantly. For equipment with high-inertia loads or loads that are sensitive to shock (conveyors, compressors, crushers, pumps), the mechanical jolt of across-the-line starting can damage couplings, gears, or the driven equipment.
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How autotransformer starting works
Search intent includes: autotransformer motor starter, reduced voltage starter, RVSS autotransformer
An autotransformer starter uses a three-phase autotransformer with multiple voltage taps to provide reduced voltage to the motor during the starting period. The process follows a sequence:
1. Start: The motor is connected to the autotransformer, which supplies a fraction of the line voltage (determined by the selected tap). The motor begins to accelerate at reduced voltage. 2. Transition: Once the motor has accelerated to near operating speed (determined by a timer or current-sensing relay), the autotransformer is disconnected. 3. Run: The motor is connected directly to the full line voltage for normal operation.
The autotransformer is only in the circuit during starting — typically 5 to 30 seconds depending on the load. Once the motor is running, the autotransformer is de-energized and contributes no losses to the running circuit.
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Tap selection: 50%, 65%, and 80%
Search intent includes: motor starting autotransformer taps, 50 65 80 percent tap motor starting, motor starting torque current
Most autotransformer starters provide three voltage taps, expressed as a percentage of line voltage:
50% tap: The motor receives half the line voltage during starting.
- Starting current is reduced to approximately 25% of locked-rotor current (current reduction is proportional to the square of the voltage ratio).
- Starting torque is reduced to approximately 25% of locked-rotor torque.
- This tap provides the maximum inrush reduction but the lowest starting torque. Use it only when the load is light during starting (such as an unloaded compressor or a centrifugal pump).
65% tap: The motor receives 65% of line voltage.
- Starting current is reduced to approximately 42% of locked-rotor current.
- Starting torque is reduced to approximately 42% of locked-rotor torque.
- This is the most commonly used tap, balancing inrush reduction with adequate starting torque for moderate loads.
80% tap: The motor receives 80% of line voltage.
- Starting current is reduced to approximately 64% of locked-rotor current.
- Starting torque is reduced to approximately 64% of locked-rotor torque.
- This tap provides the least inrush reduction but the most starting torque. Use it when the load requires significant torque to begin moving.
The tap selection is application-specific. The goal is to choose the lowest tap (most current reduction) that still provides enough torque to accelerate the load within an acceptable time. A motor that takes too long to accelerate on a low tap will overheat during starting.
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Korndorfer (closed transition) starting
Search intent includes: Korndorfer starting
The basic autotransformer starting sequence described above is an open transition — there is a brief moment during the transition from autotransformer to full voltage when the motor is disconnected from all power. This momentary interruption can cause a transient current spike as the motor reconnects at full voltage.
Korndorfer starting (also called closed transition starting) avoids this interruption. The sequence is:
1. The motor starts on the autotransformer tap (reduced voltage). 2. The autotransformer neutral is opened, converting the autotransformer winding into a series reactor. 3. The motor is connected to full line voltage through this reactor, which limits the transition transient. 4. The reactor is then bypassed, and the motor runs on full line voltage.
The advantage is a smoother transition with no power interruption and reduced transient current. Korndorfer starting is standard on most modern autotransformer starters for motors above 50 horsepower.
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Autotransformer starter vs. soft starter vs. VFD
Search intent includes: autotransformer starter vs soft starter, autotransformer starter vs VFD
Autotransformer starters are one of several reduced-voltage starting methods. The alternatives include:
Soft starter (solid-state reduced voltage starter):
- Uses thyristors (SCRs) to ramp voltage gradually from zero to full over a programmable time.
- Provides smooth, stepless acceleration with adjustable ramp rates.
- Smaller footprint than an autotransformer starter.
- Lower starting efficiency than autotransformer starting because the SCRs create harmonic currents and the motor receives distorted voltage during the ramp.
- Generates heat in the SCRs during starting.
Variable Frequency Drive (VFD):
- Controls both voltage and frequency, providing precise speed and torque control from zero to full speed.
- The most sophisticated starting method, with programmable acceleration profiles.
- Remains in the circuit during running, providing continuous speed control, which can deliver energy savings for variable-torque loads (fans, pumps).
- Most expensive option. Generates harmonics that may require filtering. Adds a potential failure point in the running circuit.
Where the autotransformer starter still wins:
- Simplicity and reliability. The autotransformer starter has no semiconductors in the power path during running. Once the motor transitions to full voltage, the starter is out of the circuit. There is nothing to fail, generate harmonics, or add losses during normal operation.
- Starting torque per starting amp. Autotransformer starters deliver more torque per amp of line current than other reduced-voltage methods at the same voltage reduction. This is because the autotransformer reduces the voltage to the motor while also reducing the line current by the same ratio (current transformation), whereas methods like primary resistance or soft starters reduce motor voltage without this transformer action.
- Durability in harsh environments. With no sensitive electronics, autotransformer starters tolerate heat, dust, vibration, and power quality variations better than solid-state alternatives.
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Frequently Asked Questions
How do I know which tap to select? The correct tap depends on the motor’s locked-rotor torque, the load’s breakaway torque requirement, and the maximum allowable starting current. Start with the 65% tap as a default. If the motor fails to accelerate the load within the timer setting, move to the 80% tap. If the utility or facility requires further current reduction and the load allows it, try the 50% tap. The motor manufacturer’s starting data and the load’s torque-speed curve inform this decision.
Can an autotransformer starter be used with any motor? Autotransformer starters are designed for squirrel-cage induction motors. They are not used with wound-rotor motors (which have their own starting methods), synchronous motors (which require different starting procedures), or single-phase motors.
How many starts per hour can an autotransformer starter handle? The autotransformer is energized only during starting, but each start heats the winding. Most starters are rated for 2 to 4 starts per hour, with a minimum rest period between starts to allow cooling. Frequent-start applications may require a larger autotransformer or a different starting method.
Is an autotransformer starter still relevant with VFDs available? Yes. For applications where the motor runs at fixed speed and the only requirement is reduced starting current, an autotransformer starter is simpler, less expensive, and more reliable than a VFD. The VFD’s continuous speed control is unnecessary overhead if the motor runs at one speed.
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What to send XFMRDirect
To get a quote on an autotransformer starter or starting autotransformer, gather the following:
1. Motor horsepower, voltage, and phase. 2. Motor full-load amps and locked-rotor amps (from the nameplate or motor data sheet). 3. Type of load being driven (pump, compressor, fan, conveyor, crusher, etc.). 4. Whether the load is started loaded or unloaded. 5. Maximum allowable starting current (if specified by the utility or facility standard). 6. Whether open transition or closed transition (Korndorfer) is required. 7. Desired starting tap (50%, 65%, 80%) if known, or describe the application and we can recommend. 8. Enclosure type and installation environment.
XFMRDirect can recommend the right autotransformer starter for your motor and application and confirm pricing, availability, and lead time.