What is motor starting current?

Motor starting current (inrush current) is the initial surge of current when a motor is first energized. For DOL (direct-on-line) starting, it is typically 6–8 times the rated full-load current and lasts until the motor reaches operating speed.

Why is starting current so high?

At standstill, the rotor does not generate back-EMF, so the motor winding impedance is very low, resulting in high current. As the motor accelerates, back-EMF builds up and current drops to the normal running value.

What is the difference between DOL and star-delta starting?

DOL (direct-on-line) applies full voltage immediately, giving 6-8× rated current. Star-delta starts in star connection (1/3 voltage per winding), reducing starting current to about 1/3 of DOL values (2-3× rated), then switches to delta for normal running.

How does a VFD reduce starting current?

A Variable Frequency Drive (VFD) controls both voltage and frequency, ramping them up gradually. This allows the motor to start with only 1–1.5× rated current while still providing adequate starting torque.

What is the NEMA code letter for locked rotor current?

NEMA code letters (A through V) indicate the locked rotor kVA per HP. For example, code letter F means 5.0–5.6 kVA/HP. The starting current can be calculated as I_start = (kVA/HP × HP × 1000) / (√3 × V).

Ferramenta de Engenharia Gratuita

Calculadora de Corrente de Partida do Motor

Calcule a corrente de partida (inrush) do motor para os métodos DOL, estrela-triângulo, soft starter e VFD. I_partida = k_partida × I_nominal.

DOL 6–8× Estrela-Triângulo 2–3× VFD 1–1,5×

Potência Nominal do Motor (kW)

Tensão de Linha (V)

Fator de Potência cos φ (0–1)

Eficiência η (0–1)

Método de Partida

Multiplicador de Partida k (×I_rated)

Predefinições rápidas

Resultados

Corrente de Partida I_start

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Corrente Nominal a Plena Carga I_rated

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Multiplicador de Partida k

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Potência Aparente de Partida

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Método de Partida

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Corrente Nominal

Corrente de Partida

Comparação de Métodos de Partida

Método	Multiplicador de Corrente	Torque	Uso Típico
DOL	6–8 × I_nominal	Completo	Motores pequenos < 7,5 kW
Estrela-Triângulo	2–3 × I_nominal	~33%	Bombas, ventiladores, 7,5–75 kW
Partida Suave	2–4 × I_nominal	Variável	Esteiras transportadoras, compressores
VFD	1–1,5 × I_nominal	Completo	Qualquer, melhor controle

Exemplo — Motor de 15 kW, Partida DOL

Dados: P = 15 kW, V = 400 V, cos φ = 0,85, η = 0,91, DOL k = 7

I_nominal = 15000 / (√3 × 400 × 0,85 × 0,91) = 28,0 A

I_partida = 7 × 28,0 = 196,0 A

Calculadora de Corrente de Partida de Motor | Ferramenta Online Gratuita

Resultados

Corrente Nominal

Corrente de Partida

Comparação de Métodos de Partida

Loja

Apoio

Contato