How do I size a VFD for a motor?

Calculate the motor's full-load current (FLA), then select a VFD with a continuous current rating equal to or greater than the motor FLA. Add margin for overload capability — typically 110% for normal duty (variable torque) or 150% for heavy duty (constant torque).

What is the difference between normal duty and heavy duty VFD rating?

Normal duty (ND) is for variable-torque loads like fans and pumps — typically 110% overload for 60 seconds. Heavy duty (HD) is for constant-torque loads like conveyors and mixers — typically 150% overload for 60 seconds. The same VFD is often rated for a higher kW in ND vs HD.

Should I oversize the VFD?

It is common practice to select one frame size above the motor rating for: high starting torque requirements, frequent starts/stops, high ambient temperature, or altitude above 1000m. Oversizing by one frame also extends VFD lifetime.

What voltage should I select for the VFD?

The VFD input voltage must match your supply. Common ratings: 230V single-phase (small motors), 400V three-phase (Europe/Asia), 480V three-phase (North America), 690V (large industrial). The VFD output voltage matches the motor rating.

Can I use a VFD with any motor?

VFDs work best with inverter-duty motors (designed for PWM waveforms). Standard motors can be used but may need derating above base speed, and bearing insulation may be required for motors above 30 kW to prevent EDM bearing damage.

免費工程工具

VFD 選型計算器

為您的馬達選定變頻器尺寸。計算額定電流、含過載裕度的所需 VFD 電流額定值，以及建議的驅動器尺寸。

標準 / 重載過載裕度標準尺寸

馬達額定功率 (kW)

供電電壓 (V)

馬達功率因數 (cos φ)

馬達效率 η (0–1)

負載類型

過載裕度 (%)

快速預設

結果

所需最小 VFD 電流額定值

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馬達滿載電流

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馬達視在功率

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負載 / 過載

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建議 VFD 尺寸

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馬達滿載電流

VFD 電流額定值

其中過載裕度已考量瞬態條件與安全係數。

標準 VFD 尺寸

kW (400 V)	典型 FLA (A)	VFD 機架
0.75	2.1	尺寸 A
1.5	3.8	尺寸 A
4.0	9.5	尺寸 B
7.5	17	尺寸 B
15	32	尺寸 C
22	46	尺寸 C
37	73	尺寸 D
55	106	尺寸 D
75	145	尺寸 E
110	210	尺寸 E

⚠️ 注意： 務必依據製造商數據表驗證 VFD 選型。環境溫度 >40°C、海拔 >1000m 或切換頻率 >4 kHz 時需降額使用。