Rated current
178.7 A
Calculadora de Arranque de Motor e Queda de Tensão
Verifique a queda de tensão no barramento quando um motor arranca — corrente de arranque, queda %, comparação de métodos de arranque e a potência mínima da alimentação ou transformador para permanecer dentro dos limites.
Resultado
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Saída de exemploBus voltage dip at start
6.8 %
90 kW motor · 380 V · Direct-on-line (DOL) · limit 15% · Within limit ✓
Starting current
1162 A
Starting kVA
765 kVA
Bus short-circuit
10.5 MVA
Starting calculation
| Quantity | Value |
|---|---|
| Motor rated current In | P ÷ (√3·U·cosφ·η) = 90 kW ÷ (√3 × 380 V × 0.85 × 0.9) = 178.7 A |
| Starting current multiple | 6.5× (Direct-on-line (DOL)) |
| Starting current Ist | 178.7 × 6.5 = 1162 A |
| Starting apparent power | 90 ÷ (0.9 × 0.85) × 6.5 = 765 kVA |
| Bus short-circuit capacity | 630 kVA ÷ 6% = 10.5 MVA |
| Voltage dip | Sstart ÷ (Sstart + Ssc) = 6.8% vs limit 15% |
Starting-method comparison
| Method | kst | Voltage dip | Verdict |
|---|---|---|---|
| Direct-on-line (DOL) | 6.5× | 6.8% | OK |
| Star-delta (Y-Δ) | 2.2× | 2.4% | OK |
| Soft starter | 3.0× | 3.3% | OK |
| VFD (variable speed) | 1.3× | 1.4% | OK |
Recommendation
Direct-on-line (DOL)
Current method keeps the bus voltage dip within the limit.
Bill of materials
| Equipment | Spec | Qty | Subtotal |
|---|---|---|---|
| DOL starter cabinet | 90 kW · 380 V · Direct-on-line (DOL) (6.5× In) | 1 | POA |
| Total equipment | POA |
Para engenheiros eletrotécnicos e empreiteiros que dimensionam arrancadores de motor e verificam a queda de tensão durante o arranque de motores em sistemas BT/MT.
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Exemplo prático
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Exemplo prático — motor de 90 kW, arranque DOL, transformador de 630 kVA
Bus voltage dip at start
6.8 %
90 kW motor · 380 V · Direct-on-line (DOL) · limit 15% · Within limit ✓
Rated current
178.7 A
Starting current
1162 A
Starting kVA
765 kVA
Bus short-circuit
10.5 MVA
Starting calculation
| Quantity | Value |
|---|---|
| Motor rated current In | P ÷ (√3·U·cosφ·η) = 90 kW ÷ (√3 × 380 V × 0.85 × 0.9) = 178.7 A |
| Starting current multiple | 6.5× (Direct-on-line (DOL)) |
| Starting current Ist | 178.7 × 6.5 = 1162 A |
| Starting apparent power | 90 ÷ (0.9 × 0.85) × 6.5 = 765 kVA |
| Bus short-circuit capacity | 630 kVA ÷ 6% = 10.5 MVA |
| Voltage dip | Sstart ÷ (Sstart + Ssc) = 6.8% vs limit 15% |
Starting-method comparison
| Method | kst | Voltage dip | Verdict |
|---|---|---|---|
| Direct-on-line (DOL) | 6.5× | 6.8% | OK |
| Star-delta (Y-Δ) | 2.2× | 2.4% | OK |
| Soft starter | 3.0× | 3.3% | OK |
| VFD (variable speed) | 1.3× | 1.4% | OK |
Recommendation
Direct-on-line (DOL)
Current method keeps the bus voltage dip within the limit.
Bill of materials
| Equipment | Spec | Qty | Subtotal |
|---|---|---|---|
| DOL starter cabinet | 90 kW · 380 V · Direct-on-line (DOL) (6.5× In) | 1 | POA |
| Total equipment | POA |
Exemplo prático — motor de 250 kW, arrancador suave, rede de 20 MVA
Bus voltage dip at start
4.7 %
250 kW motor · 380 V · Soft starter · limit 15% · Within limit ✓
Rated current
496.5 A
Starting current
1490 A
Starting kVA
980 kVA
Bus short-circuit
20.0 MVA
Starting calculation
| Quantity | Value |
|---|---|
| Motor rated current In | P ÷ (√3·U·cosφ·η) = 250 kW ÷ (√3 × 380 V × 0.85 × 0.9) = 496.5 A |
| Starting current multiple | 3.0× (Soft starter) |
| Starting current Ist | 496.5 × 3.0 = 1490 A |
| Starting apparent power | 250 ÷ (0.9 × 0.85) × 3.0 = 980 kVA |
| Bus short-circuit capacity | 20 MVA (grid) = 20.0 MVA |
| Voltage dip | Sstart ÷ (Sstart + Ssc) = 4.7% vs limit 15% |
Starting-method comparison
| Method | kst | Voltage dip | Verdict |
|---|---|---|---|
| Direct-on-line (DOL) | 6.5× | 9.6% | OK |
| Star-delta (Y-Δ) | 2.2× | 3.5% | OK |
| Soft starter | 3.0× | 4.7% | OK |
| VFD (variable speed) | 1.3× | 2.1% | OK |
Recommendation
Soft starter
Current method keeps the bus voltage dip within the limit.
Bill of materials
| Equipment | Spec | Qty | Subtotal |
|---|---|---|---|
| Soft starter cabinet | 250 kW · 380 V · Soft starter (3.0× In) | 1 | POA |
| Total equipment | POA |
Como foi calculado
- · Corrente nominal: In = P ÷ (√3 · U · cosφ · η).
- · Corrente de arranque: Ist = In × kst, em que kst é o múltiplo da corrente de arranque (≈6,5× DOL, ≈2,2× estrela-triângulo, ≈3× arrancador suave, ≈1,3× VFD).
- · Potência aparente de arranque: Sstart = P ÷ (cosφ · η) × kst.
- · Queda de tensão no barramento: ΔU% = Sstart ÷ (Sstart + Ssc) × 100, em que Ssc é a potência de curto-circuito no barramento.
- · Alimentação/transformador mínimo para cumprir um limite L: S ≥ Sstart × (1 − L) ÷ L × uk.
Normas referenciadas
| Standard | Scope |
|---|---|
| IEC 60034 | Máquinas elétricas rotativas — classificação e desempenho |
| GB/T 755 | Classificação e desempenho de máquinas elétricas rotativas |
| IEEE 3002.7-2018 | Análise de arranque de motores para sistemas de potência |
Perguntas frequentes
What is an acceptable voltage dip during motor starting?
Most standards (IEC 60034, GB/T 755) allow a 15-20% voltage dip during motor starting, with the bus voltage staying above 80-85%. A dip below 80% can stall the motor or trip contactors. The calculator computes the starting dip and flags if it exceeds the limit.
How much current does a motor draw on direct-on-line starting?
Direct-on-line (DOL) starting draws 5-7 times rated current for a few seconds. A 100 kW motor at 0.4 kV (~180 A rated) draws 900-1260 A on start. This inrush causes the voltage dip; the calculator computes the dip and compares DOL, star-delta, soft-start and VFD.
How much does a soft-starter reduce starting current?
A soft-starter ramps voltage and typically limits starting current to 2.5-3.5 times rated (versus 5-7x DOL), reducing the voltage dip proportionally. The starting torque also drops roughly with the square of current, so soft-start suits centrifugal loads. The calculator compares starting methods.
What supply transformer size do I need for a big motor?
The Transformer must supply the motor starting kVA without excessive dip. For a 160 kW motor starting at 3x on soft-start, starting demand is ~480 kVA; a 630 kVA transformer keeps the dip near 10-12%. The calculator sizes the supply from the starting demand and dip limit.
What is star-delta starting and when is it used?
Star-delta starting connects the motor in star for start (reducing voltage to 58% and current/torque to one-third) then switches to delta for run. It cuts starting current to ~2-2.5x rated but the torque drop limits it to light-starting loads like fans and pumps. The calculator compares it against DOL and soft-start.
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