Auto-Transformer Starter Diagram
This is a free printable auto transformer starter diagram: download the diagram as SVG or open it and print to paper or PDF.
Understand how an auto-transformer starter reduces motor starting current by applying a tapped percentage of full supply voltage during startup, then transitions to full voltage once the motor is near running speed.
An auto-transformer starter (ATS) is a reduced-voltage starting method for large three-phase induction motors where direct-on-line (DOL) starting current would cause unacceptable voltage dips on the supply, damage to driven machinery, or exceed utility-imposed starting current limits. The auto-transformer provides a tapped voltage output — typically 50%, 65%, or 80% of supply voltage — to the motor during the starting phase. Because motor starting current is proportional to the applied voltage, applying 65% voltage reduces starting current to approximately 42% of the DOL value (current reduces as the square of the voltage ratio).
The starter circuit uses a three-phase auto-transformer with multiple voltage tapping points and a sequence of contactors to implement the start-to-run transition. During the start phase, two contactors are typically closed: one connecting the auto-transformer star point (forming the auto-transformer neutral) and another connecting the motor terminals to the selected tap. The motor starts and accelerates against the reduced-voltage torque. When the motor approaches running speed — determined either by a timer or by current monitoring — the starter sequences to full voltage.
The transition from reduced voltage to full voltage is a critical moment. A poorly designed transition creates a second transient current surge. The 'Korndorfer' connection method (named after its originator) is commonly used to achieve a smoother transition: the auto-transformer star contactor opens first while the motor remains connected to the tap, then the tap contactor opens while a run contactor closes to connect full supply voltage. This sequences the transition to minimise the reconnection transient.
Auto-transformer starters are used for motors typically in the range of 11 kW to several hundred kilowatts. They are more expensive than star-delta starters but are applicable to motors that cannot be reconfigured for star-delta (motors already connected internally in delta and not accessible for reconfiguration). They provide more flexible voltage reduction (tap selection) than a fixed-ratio star-delta.
The selection of tapping percentage is a balance between starting torque requirements and inrush current reduction. Lower voltage taps reduce inrush more but also reduce available starting torque. Insufficient starting torque under load causes the motor to fail to accelerate, stalling at low speed and drawing sustained high current that risks overheating.
How to wire auto transformer starter diagram
- Confirm motor and supply specifications Record motor nameplate data: kW rating, full-load current (FLA), voltage, frequency, power factor, and number of leads. Confirm that the supply voltage matches the motor and that the supply authority permits the expected starting current. Determine whether the installation requires a specific maximum starting current — this informs the tap percentage selection.
- Select the auto-transformer tap percentage Calculate the starting torque available at each tap as a percentage of DOL starting torque (available torque = tap percentage squared × DOL torque). For a 65% tap, available starting torque is 65% × 65% = approximately 42% of DOL. Verify that this exceeds the load's starting torque requirement with a margin. If not, select a higher tap (e.g., 80%).
- Size the auto-transformer, contactors, and overload relay The auto-transformer is rated in kVA based on the motor starting current and the duration of the start cycle. The contactors must be rated for the currents flowing in each part of the circuit — the line contactors see line current, the tap contactor sees the tap current, and the star contactor closes the auto-transformer neutral. The overload relay is set to the motor's full-load current for running protection.
- Wire the main power circuit Connect the three-phase supply (L1, L2, L3) to the line-side of the main line contactor. The output of the line contactor feeds the auto-transformer primary taps and the motor terminals. The auto-transformer star contactor connects the star point of the auto-transformer during the start phase. Wire the selected tap outputs to the motor terminals. The run contactor, when closed, bypasses the auto-transformer and connects supply directly to the motor.
- Wire the control circuit The control circuit (typically 110V or 230V single-phase, derived via a control transformer or directly from the supply) sequences the contactors via a timer relay and interlocking logic. The start sequence closes the star contactor and line contactor simultaneously. After the timer elapses, the transition sequence opens the star contactor, pauses briefly, then opens the tap contactor and closes the run contactor. Ensure mechanical and electrical interlocking prevents any two conflicting contactors from closing simultaneously.
- Commission and verify the starting sequence Before connecting the motor, test the control circuit sequence with a test lamp on each contactor coil to verify the correct sequence of operations. Connect the motor and start under no-load conditions first. Measure line current with a clamp ammeter during starting and verify it falls within expected limits. Adjust the timer to match the motor's actual acceleration time. Test under full load and confirm smooth acceleration to running speed.
Specifications
| Typical application motor range | 11 kW to several hundred kW three-phase induction motors |
|---|---|
| Standard tap voltages | 50%, 65%, and 80% of line voltage (selectable) |
| Supply current at 65% tap (approximate) | 42% of DOL starting current (65% squared) — auto-transformer ratio effect |
| Starting torque at 65% tap (approximate) | 42% of DOL starting torque (65% squared) |
| Auto-transformer duty rating | Short-time rated — typically 15 s start duration; verify with manufacturer |
| Transition method | Korndorfer (star contactor opens first, then tap-to-run transition) or open-circuit transition |
| Applicable standard | IEC 60947-4-1 (low-voltage motor starters); NEC NFPA 70 Article 430 |
| Control circuit voltage (typical) | 110V AC or 24V AC/DC (via control transformer from main supply) |
Safety warnings
- Auto-transformer starter installation and commissioning must be carried out by a licensed electrician and electrical engineer. High-voltage, high-current three-phase systems present a serious electrocution and arc-flash risk. Comply with NEC (NFPA 70), BS 7671, IEC 60364, AS/NZS 3000, and any applicable local industrial electrical codes.
- The auto-transformer is rated for short-time duty — it is designed to carry starting current for a limited period (typically up to 15 seconds per start). Exceeding the rated start duration or number of starts per hour will cause the transformer to overheat and fail, potentially causing fire. Never exceed the manufacturer's rated start duration or start frequency.
- Electrical and mechanical interlocking between the star contactor and the run contactor is mandatory. If both close simultaneously, the auto-transformer is short-circuited — a potentially catastrophic fault causing severe arc flash and equipment damage. Verify interlocking is functioning correctly before commissioning.
- During transition from start to run, there is a brief open-circuit period. The motor produces back-EMF during this period. Do not reduce the transition dead-time below the minimum required — premature reconnection while back-EMF is high in phase opposition to the supply can cause a current surge exceeding DOL starting current.
- An auto-transformer starter does not protect against mechanical overload during starting. The overload relay protects the motor during running but may not respond fast enough to protect a stalled motor during the start sequence. Ensure the load cannot stall under any fault condition during commissioning.
Tools needed
- Calibrated clamp ammeter (to measure starting and running current on all three phases)
- Multimeter with AC voltage and continuity functions
- Insulation resistance tester (megger) — for verifying auto-transformer and motor insulation before energising
- Torque-limiting screwdriver or wrench (for terminal connections to manufacturer specification)
- Non-contact voltage tester
- Oscilloscope or power quality analyser (for detailed transition transient analysis during commissioning)
- Timer with stopwatch function (for verifying start timer setting against actual motor acceleration time)
Common mistakes
- Selecting too low a tap percentage — motor fails to accelerate under load: The load requires more starting torque than the reduced voltage provides. The motor draws sustained high current without accelerating, overheating the motor and the auto-transformer. Always verify that torque at the selected tap exceeds the load's starting torque requirement.
- Setting the start timer too long: Prolonged starting current through the auto-transformer's windings causes overheating beyond the transformer's short-time rating. Set the timer to just exceed the actual motor acceleration time measured during commissioning.
- Missing or incorrect mechanical interlock between star and run contactors: This is a potentially catastrophic omission. The mechanical interlock physically prevents both contactors from closing simultaneously. Check every interlock for correct function as part of pre-commissioning verification.
- Incorrect tap wiring — motor terminals connected to wrong tap output: Wiring the motor to the wrong tap output gives incorrect starting voltage. With a lower-than-intended tap, the motor may fail to start. With a higher-than-intended tap, the current reduction benefit is reduced. Verify tap wiring against the auto-transformer wiring diagram.
- No control circuit protection: The control transformer secondary must be fused to protect the control circuit wiring. An unfused control circuit can sustain a fault indefinitely, potentially causing fire in the control panel or false contactor operations.
Troubleshooting
- Motor fails to accelerate to running speed during the start phase
- Cause: Starting torque at selected tap is insufficient for the load, or the load has more inertia than anticipated Fix: Select a higher voltage tap on the auto-transformer (e.g., move from 65% to 80%). If the 80% tap does not provide sufficient acceleration, the motor may be undersized for the application, or DOL starting may be required — consult an electrical engineer. Do not extend the start timer indefinitely — this overloads the auto-transformer.
- Large current surge occurs at transition from start to run
- Cause: Star contactor and run contactor operate too close in time, causing a hard voltage step; or back-EMF from the motor is not sufficiently decayed before run contactor closes Fix: Verify the Korndorfer or equivalent transition sequence is correctly implemented: star contactor opens first, pause, then tap contactor opens and run contactor closes. Increase the dead-time between contactor operations if the transition current surge is excessive. Check for correct sequencing in the timer relay or PLC logic.
- Auto-transformer overheats after repeated starts
- Cause: Start frequency exceeds the auto-transformer's rated duty cycle, or single start duration exceeds rated time Fix: Review the auto-transformer manufacturer's specification for maximum start duration (seconds) and starts per hour (or starts per cool-down period). Enforce a mandatory cooling interval between starts. If operational demand requires more frequent starting, a soft-starter or VFD (variable frequency drive) may be a more appropriate solution.
Frequently asked questions
How does an auto-transformer reduce motor starting current?
By applying a fraction of the full supply voltage to the motor during starting. The motor's starting current is proportional to the voltage applied. At 65% tap, the motor receives 65% of line voltage, drawing approximately 65% of the current it would draw from the transformer secondary. The supply current is further reduced because the auto-transformer itself steps up current: at 65% tap, supply current is approximately 42% of DOL starting current (65% squared).
What is the Korndorfer transition in an auto-transformer starter?
The Korndorfer sequence is a specific contactor sequencing method for transitioning from reduced-voltage start to full-voltage run with minimal reconnection surge. The auto-transformer star (neutral forming) contactor opens first, converting the auto-transformer into a reactor. The motor remains connected. Then the tap contactor opens and the run contactor closes, applying full voltage. This staged transition reduces the voltage step at changeover.
What tap percentage should I select for an auto-transformer starter?
Tap selection depends on the load's starting torque requirement and the permissible supply current disturbance. A 50% tap gives maximum current reduction but provides only 25% of DOL torque — suitable only for very light starting loads. A 65% tap is a common general-purpose choice. An 80% tap is used where high starting torque is needed. Start with the lowest tap that provides enough torque to accelerate the load to running speed.
What is the difference between an auto-transformer starter and a star-delta starter?
A star-delta starter reconfigures the motor windings from star (low-voltage) to delta (full voltage) — applicable only to motors with both ends of each winding accessible (6 or 12 leads). An auto-transformer starter reduces supply voltage using an external transformer and works with any 3-lead standard motor. Auto-transformer starters are more expensive but more flexible in tap selection and applicable to a wider range of motors.
How long should the auto-transformer starter remain in the start position?
The timer is set to allow sufficient time for the motor to accelerate from standstill to near-synchronous speed — typically 5 to 15 seconds, depending on the motor's inertia and load characteristics. Starting for too long risks auto-transformer overheating. Too short a start time means the motor may not have reached sufficient speed before full voltage is applied, causing a large reconnection transient. Current monitoring is more accurate than a fixed timer for variable loads.
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