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.

Free Engineering Tool

VFD Selection Calculator

Size a Variable Frequency Drive for your motor. Calculate rated current, required VFD current rating with overload margin, and recommended drive size.

Normal / Heavy Duty Overload Margin Standard Sizes

Motor Rated Power (kW)

Supply Voltage (V)

Motor Power Factor (cos φ)

Motor Efficiency η (0–1)

Duty Type

Overload Margin (%)

Quick presets

Results

Min VFD Current Rating Needed

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Motor Full-Load Current

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Motor Apparent Power

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Duty / Overload

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Recommended VFD Size

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Motor Full-Load Current

VFD Current Rating

Where the overload margin accounts for transient conditions and safety factor.

Standard VFD Sizes

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

⚠️ Note: Always verify VFD selection against manufacturer datasheets. Derate for ambient temperature >40°C, altitude >1000m, or switching frequency >4 kHz.