Easy Reduced-Voltage Autotransformer Starter kVA Sizing Calculator — Instant Motor Starter Screening

Reduced voltage autotransformer starters optimize motor starting torque while minimizing inrush current and mechanical stress.

This article details KVA sizing calculations, selection criteria, screening methods, and instant starter performance analysis.

Principles of reduced-voltage autotransformer motor starting

Autotransformer reduced-voltage starters connect a tapped autotransformer between the supply and motor to apply a controlled reduced voltage during starting. This method reduces starting current proportionally to the applied voltage while allowing better starting torque than other reduced-voltage methods for the same voltage reduction.

How an autotransformer starter operates

• Autotransformer primary is connected to the full line; secondary taps provide one or more reduced-voltage outputs.
• During start, the motor is supplied from a reduced-voltage tap; after a preset time or speed, a bypass contactor connects the motor directly to full line voltage.
• The autotransformer carries only the difference in power between the line and the reduced output; therefore the autotransformer KVA rating can be significantly less than the full motor kVA.

Advantages and limitations

• Advantages:
  • High starting torque at reduced currents compared to resistive or primary-reactor starters.
  • Lower autotransformer KVA than full-transformer equivalents because of autotransformer action.
  • Adjustable tap settings commonly provide 50%, 65%, and 80% voltage starts for flexibility.
• Limitations:
  • Requires a bypass contactor and interlocking to prevent paralleling taps and line inrush during changeover.
  • Cost and size higher than simple soft starters for equivalent performance.
  • Protection coordination and fault current contribution must be assessed because autotransformers add impedance to the circuit.

KVA sizing fundamentals and governing equations

Correct autotransformer KVA sizing guarantees reliable starting performance and prevents transformer saturation or overheating. The sizing procedure uses motor-rated quantities, chosen reduced-voltage tap ratio, and motor kVA at full load.

Core formulas (expressed in plain HTML)

Autotransformer_KVA ≈ Motor_kVA × (1 − V_start_fraction)

Autotransformer_current_primary ≈ Motor_kVA × (1 − V_start_fraction) × 1000 / (sqrt(3) × V_line)

Explanation of variables and typical values

• Motor_kVA — Apparent power of motor at full load in kilovolt-amperes (kVA). Typical values depend on motor size; see the motor FLA table below.
• V_line — Line-to-line supply voltage (V). Common industrial values: 230 V, 400 V, 415 V, 440 V, 460 V, 690 V.
• I_full_load — Motor full-load current (A) at rated voltage. Typical values appear in the motor FLA table below.
• V_start_fraction — Ratio of starting tap voltage to line voltage (unitless). Typical tap settings: 0.50, 0.65, 0.80.
• Autotransformer_KVA — KVA rating per three-phase autotransformer assembly required to start the motor under the chosen reduced-voltage setting.

Detailed derivation and justification

The derivation assumes that the motor is approximately an inductive load where starting current scales nearly linearly with applied voltage for reduced-voltage starts, and apparent power requirement of the autotransformer is the difference between line apparent power and secondary apparent power supplied to the motor. Therefore:

At reduced voltage V_start = V_line × V_start_fraction, the motor apparent power at starting approximately equals Motor_kVA × V_start_fraction (first-order approximation ignoring slip and transient dynamics). The autotransformer must handle the difference between the full line kVA drawn from the supply and the kVA delivered from the reduced tap, which simplifies to Motor_kVA × (1 − V_start_fraction).

Practical correction factors and safety margin

Manufacturers often recommend applying a service factor multiplier (1.10–1.25) to the computed autotransformer KVA to account for:

• Higher-than-rated locked-rotor currents for some motor designs and heavily loaded machines.
• Frequent starts or extended starting time.
• Ambient temperature and ventilation limitations.

Thus recommended purchasing KVA = Autotransformer_KVA × 1.10 (typical) to 1.25 (aggressive margin).

Common motor full-load current and kVA reference table

Notes: FLA values are typical approximations from standard motor tables at 60 Hz and 50/60 Hz practice. Motor_kVA values are calculated using Motor_kVA = sqrt(3) × V_line × I_FL / 1000.

Typical autotransformer tap settings and expected starting effects

Interpretation

Because starting torque scales roughly with the square of voltage for induction motors, a 65% voltage start yields approximately 42% of rated torque, often enough for loaded starts depending on load inertia and friction.

Instant motor starter screening: methodology and criteria

Instant screening is a rapid procedure to determine whether an autotransformer starter is suitable for a motor/application and to compute preliminary KVA sizing. The screening focuses on motor size, required starting torque, supply characteristics, and duty cycle.

Screening checklist (rapid)

1. Record motor nameplate: voltage, horsepower (HP), rated current (A), service factor, locked rotor current if available.
2. Determine load type: constant torque, variable torque, or high-inertia load; required starting torque percentage of rated torque.
3. Select desired start tap (50%, 65%, 80%) based on torque requirement.
4. Compute Motor_kVA and Autotransformer_KVA using formulas above.
5. Apply service factor (×1.10 recommended) for purchase sizing.
6. Check supply short-circuit capacity (ISC) to confirm protective device and transformer coordination.
7. Verify thermal withstand and continuous rating at maximum expected ambient temperature.

Screening pass/fail criteria (example thresholds)

• Pass: Autotransformer calculated KVA ≤ commercially available standard KVA and starting torque meets application requirement.
• Conditional: Calculated KVA near catalogue limit — consult manufacturer for custom unit or forced cooling.
• Fail: Required starting torque cannot be achieved at the highest tap (80%) or supply short-circuit contribution prohibits safe operation.

Two complete worked examples with step-by-step calculations

Example 1 — 200 HP motor at 460 V starting on a 65% tap

Scenario: A 200 HP (150 kW nominal) induction motor at 460 V, 60 Hz needs reduced-voltage starting to limit inrush current. Required starting torque ~50% of rated torque. Proposed tap: 65% (0.65).

Step 1 — Obtain motor FLA (use reference table):

• I_full_load (from table) = 278.4 A at 460 V

Step 2 — Compute motor apparent power (Motor_kVA):

Step 3 — Compute autotransformer KVA requirement using chosen tap:

Step 4 — Apply service factor margin (typical 1.10):

Step 5 — Verify starting torque adequacy:

Approximate starting torque fraction ≈ V_start_fraction^2 = 0.65^2 = 0.4225 (≈42% of rated torque). Because required torque was ~50%, 65% tap may be marginal. Options:

• Choose a higher tap (80%) to yield ~64% torque; recompute KVA for 80% (Autotransformer_KVA = 222.7×0.20 = 44.54 kVA; purchased ≈ 50 kVA).
• If mechanical load can be unloaded or torque requirement reduced, 65% may be acceptable; conversely select 80% to meet torque.

Conclusion: For torque requirement 50%, select 80% tap and purchase a 50 kVA autotransformer (after margin check). For pure inrush current limitation where 42% torque is acceptable, select 90 kVA unit at 65% tap based on KVA sizing with margin.

Example 2 — 50 HP motor at 400 V starting on 50% tap for heavy inertia load

Scenario: A 50 HP (37 kW) pump motor at 400 V, frequent starts, requires high starting torque to overcome static friction and fluid load. Proposed tap: 50% (0.50).

Step 1 — Obtain motor FLA (from table):

• I_full_load = 80.2 A at 400 V

Step 2 — Compute motor_kVA:

Step 3 — Compute autotransformer KVA requirement:

Step 4 — Apply service factor (×1.10 for frequent starts):

Step 5 — Check starting torque:

Torque fraction = 0.50^2 = 0.25 (25% of rated torque). Because the pump requires high starting torque, 50% tap may be insufficient. Options:

• Use 65% tap: Autotransformer_KVA = 55.46 × 0.35 = 19.41 kVA; Purchased ≈ 22 kVA — torque ≈ 42% rated.
• Alternatively choose a mechanical assist or soft starter with torque boost if 25% torque is inadequate.

Conclusion: Although KVA sizing for 50% tap yields a small autotransformer (~30 kVA purchased after margin), the torque may be insufficient for heavy-inertia pump starts; therefore evaluate 65% tap or alternative starter technology.

Protection, contactor selection, and coordination

Autotransformers modify the distribution of fault currents and introduce additional impedance in the motor supply path. Proper selection of contactors, overload relays, and short-circuit protective devices is mandatory.

Protection checklist

• Bypass contactor rated for full motor starting and making capacity; interlock logic to prevent tap and bypass contactor being closed simultaneously.
• Thermal overload protection set to motor full-load current; consider current during reduced-voltage start to avoid nuisance trips due to extended start time.
• Short-circuit protection: confirm fuse or circuit breaker interrupting rating against maximum available fault current and autotransformer contribution.
• Earth-fault and differential protections where required for sensitive equipment.

Mechanical and thermal considerations

• Autotransformer cooling: ensure ambient temperature and enclosure allow natural convection or provide forced ventilation where required.
• Physical space and weight: larger KVA autotransformers are heavy and require proper mounting and lifting provisions.
• Vibration and harmonic considerations: if the motor-driven process produces harmonics, consult transformer manufacturer for derating guidance.

Operational considerations and best practices

• Limit start time: reduce-duty cycles and avoid prolonged reduced-voltage starting; typical start time 2–10 seconds depending on load inertia.
• Sequence changeover carefully: ramp or timed bypass transition should minimize torque dip and avoid transient current spikes.
• Coordinate soft-start or anti-periodic re-start logic to prevent restarting under locked rotor conditions or process stalls.
• Document start-performance data during commissioning: record starting currents, voltages, and motor speed profile for future troubleshooting and optimization.

Standards, normative references and authoritative guidance

Designers and specifiers should consult applicable standards and manufacturer documentation. Important references include:

• NEMA MG1 — Motors and Generators: guidelines on motor ratings, locked-rotor currents, and thermal limits. (https://www.nema.org)
• IEC 60034 — Rotating electrical machines — general standards for motor testing and characteristics. (https://www.iec.ch)
• IEC 60947 series — Low-voltage switchgear and controlgear, for contactors and starters. (https://www.iec.ch)
• IEEE Std 141 (Green Book) — Power distribution for industrial plants (useful for coordination and voltage-drop considerations). (https://www.ieee.org)
• Manufacturer application notes from major transformer/starter OEMs (ABB, Siemens, Schneider Electric) for detailed autotransformer sizing curves and inrush behavior.

Limitations of the simplified KVA calculation and when to perform detailed studies

The Motor_kVA × (1 − V_start_fraction) approach is an industry-accepted first-order sizing method but has limitations. More detailed studies are required when:

• The motor has atypical locked-rotor characteristics or high slip designs.
• Loads are highly dynamic (frequent cycling, pump dead-head starts, compressors with locked valve conditions).
• Supply system short-circuit capacity is low or protection coordination margins are tight.
• Multiple motors start simultaneously from the same transformer — aggregate loading and voltage drop must be modelled.

Example of composite application: plant start coordination

When several motors are started sequentially from the same supply transformer, autotransformer sizing must consider cumulative effect and upstream voltage drop. Use the following steps for screening:

1. List all motors with starting tap and computed autotransformer KVA contributions.
2. Estimate simultaneous start scenarios and compute aggregate motor_kVA demand during peak transient.
3. Check supply transformer capacity and voltage drop under combined starts; if unacceptable, stagger starts with programmable logic controller (PLC) timing.
4. Verify protective devices can clear faults without nuisance operation during start transients.

Commissioning checks and measuring targets

• Record starting current per phase and compare against predicted values within ±15%.
• Verify autotransformer temperature rise after a sequence of starts matches manufacturer curves.
• Confirm bypass transition does not create unacceptable transient voltage dips in the process network.
• Implement data logging on the first 10 start cycles to assess real-world performance and update settings.

Summary of practical rules-of-thumb for quick sizing

• Use Motor_kVA × (1 − V_start_fraction) to obtain preliminary autotransformer KVA.
• Apply 10% margin for routine designs; apply up to 25% margin for frequent or difficult starts.
• Prefer 65% tap for a balance of current reduction and torque for general-purpose starts.
• Always confirm torque requirement vs. voltage squared rule: Torque ≈ (V_start_fraction)^2 × rated torque.

References and further reading

• NEMA MG 1 — Motors and Generators. National Electrical Manufacturers Association. https://www.nema.org/standards
• IEC 60034 — Rotating electrical machines. International Electrotechnical Commission. https://www.iec.ch
• IEC 60947 series — Low-voltage switchgear and controlgear. International Electrotechnical Commission. https://www.iec.ch
• IEEE Std 141 — IEEE Recommended Practice for Electric Power Distribution for Industrial Plants. https://www.ieee.org
• ABB Application Note — Autotransformer Starters (manufacturer application guidance). https://new.abb.com
• Siemens — Autotransformer starter selection guide and technical data. https://www.siemens.com

For final selection and procurement, consult autotransformer manufacturer catalogs and request short-circuit contribution data, impulse withstand level, and factory test certificates. Where necessary, perform time-domain simulation or manufacturer-backed transient analysis to verify performance under specific process conditions.