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).

免費工程工具

馬達啟動電流計算器

計算 DOL、星三角、軟啟動器及 VFD 方式的馬達啟動（湧入）電流。I_start = k_start × I_rated.

DOL 6–8× 星三角 2–3× VFD 1–1.5×

馬達額定功率 (kW)

線電壓 (V)

功率因數 cos φ (0–1)

效率 η (0–1)

啟動方式

啟動倍數 k （×I_rated）

快速預設

結果

啟動電流 I_start

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額定滿載電流 I_rated

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啟動倍數 k

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啟動視在功率

—

啟動方式

—

額定電流

啟動電流

啟動方式比較

方法	電流倍數	扭矩	典型用途
DOL	6–8 × I_rated	全壓	小型馬達 < 7.5 kW
星三角啟動	2–3 × I_rated	~33%	泵浦、風機，7.5–75 kW
軟啟動器	2–4 × I_rated	變頻	輸送機、壓縮機
VFD	1–1.5 × I_rated	全壓	任何類型，最佳控制

範例 — 15 kW 馬達，DOL 啟動

已知： P = 15 kW, V = 400 V, cos φ = 0.85, η = 0.91, DOL k = 7

I_rated = 15000 / (√3 × 400 × 0.85 × 0.91) = 28.0 A

I_start = 7 × 28.0 = 196.0 A