VFD (Variable Frequency Drive) Calculator

A Variable Frequency Drive (VFD) controls a motor's speed and torque by varying the frequency of its power supply — and sizing one correctly requires accounting for motor load, voltage, efficiency, and protection requirements. Enter your motor power, voltage, power factor, and motor efficiency, then set your service factor, safety margin, and OCPD rating under your chosen standard (NEC or IEC). The VFD Calculator returns the recommended VFD power rating, along with minimum VFD output current, motor full load current, and required conductor ampacity.

Standard *

kW
V
PF
η
SF
SM
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Results

VFD Power Rating

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Min VFD Output Current

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Motor Full Load Current

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Required Conductor Ampacity

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Frequently Asked Questions

What is the formula for VFD selection?

The basic formula is I_VFD ≥ I_motor × (Service Factor or Load Factor). The VFD must handle at least the motor's full load current multiplied by the service factor to ensure proper operation and safety margins.

How to choose VFD size?

VFD size is chosen based on motor power rating, operating voltage, power factor, efficiency, and required service factors. The VFD power rating should meet or exceed the motor power with appropriate safety margins.

How to calculate VFD RPM?

Motor RPM with VFD is calculated as: RPM = (120 × frequency) / number of poles. VFDs control speed by varying the output frequency, allowing precise speed control from 0 to maximum motor speed.

What are the criteria for VFD selection?

Key criteria include motor power rating, voltage compatibility, current requirements, environmental conditions, control features needed, and compliance with electrical codes like NEC or IEC standards.

How to calculate the power of VFD?

VFD power is calculated using: P = √3 × V × I × PF × η, where V is voltage, I is current, PF is power factor, and η is efficiency. Add safety margins for proper sizing.

What's the difference between NEC and IEC standards for VFD sizing?

NEC focuses on North American electrical codes with specific conductor sizing rules, while IEC provides international standards. Both ensure safety but may have different safety factors and installation requirements.

Why do I need a service factor for VFD sizing?

Service factor provides overload capacity for temporary high loads or harsh operating conditions. Typical service factors range from 1.0 to 1.25, with 1.15 being common for general applications.

How does VFD efficiency affect sizing calculations?

VFD efficiency determines actual power consumption and heat generation. Higher efficiency VFDs require less input power and generate less heat, affecting conductor sizing and cooling requirements.