Variable Frequency Drives (VFDs) have achieved significant advances in industrial motor control technology delivering energy efficiency, exact speed regulation and more operational flexibility. 

However, by selecting the correct VFD size is essential to make sure of the optimal performance, longevity and safety. 

Oversizing leads to the unnecessary costs while under sizing can result in thermal stress, reduced motor lifespan and also potential system failures. 

Our VFD Sizing Calculator is a professional tool designed to simplify by providing accurate recommendations based on the engineering best practices & industry standards.

What is a Variable Frequency Drive?

A Variable Frequency Drive is an electronic device which controls the speed and torque of the three phase (3 phase) AC motors by varying the frequency and voltage of the electric power supplied to the motor. 

VFDs are the essential components in modern industrial automation, HVAC systems, pump control, conveyor systems and countless other applications where accurate motor control is needed.

Advantages of VFD

• Reduces the power consumption by up to 50% in the variable load applications,

• Provides a smooth acceleration and deceleration,

• Eliminates the mechanical stress from direct-on-line (DOL) starting,

• Enables efficient process control and monitoring,

• Reduces the heat & mechanical stress on motors and

• Significantly lower operational & maintenance costs.

VFD Sizing Fundamentals

Motor Power Ratings

Motor power is commonly measured in kilowatts (kW) or horsepower (HP). The relationship between these units is fundamental.

1 HP = 0.7457 kW

1 kW = 1.341 HP

The motor nameplate provides the rated power output which serves as the baseline for the VFD selection. 

This power rating represents the maximum continuous power that the motor can safely deliver under normal operating conditions.

Service Factor (SF)

The Service Factor (SF) is a multiplier applied to the motor rated power to consider for the operating conditions and also mechanical stress. 

Most industrial motors possess service factors (SF) ranging from 1.0 to 1.15.

• SF = 1.0: Standard duty with optimal efficiency.

• SF = 1.15: Higher tolerance for the overload & harsh conditions.

• SF > 1.15: Special duty applications that require enhanced durability.

The minimum VFD rating should always satisfy

VFD Rating ≥ Motor Power x Service Factor

Supply Voltage Considerations

Supply voltage availability differs by region and facility:

• 230V: Single phase (or) three phase (common in residential & light industrial)

• 400V: Three-phase (standard in Europe and many regions)

• 480V: Three-phase (standard in North America)

• 575V: Three-phase (high power industrial applications)

• 690V: Three-phase (very large industrial installations)

Voltage selection impacts VFD availability, cost, efficiency and cable requirements. 

Always verify your industry required voltage before selecting a VFD.

Application-Based Duty Cycles 

Different applications require varying demands on VFDs. 

The VFD sizing calculator ensures application specific margins to ensure adequate head room for reliable operation.

Variable Torque Applications (10% Margin)

Examples

Centrifugal fans, cooling tower fans, irrigation pumps & water circulation pumps.

Variable torque loads require less torque at the lower speeds reducing power requirements proportionally. 

Constant Torque Applications (15% Margin)

Examples

Positive displacement devices include pumps, screw compressors, conveyor systems and crushers.

Constant torque loads used to maintain the consistent force requirements regardless of the speed demanding maximum power throughout the operating range. 

High Inertia and Frequent Starts (20-25% Margin)

Examples

Industrial hoists, jaw crushers, rolling mills, large flywheel systems, punch presses & printing presses.

The 20-25% buffer ensures critical heat protection for the entire VFD and the motor.

Servo-Like Cycles (Custom Margin)

Examples

Robotic systems, process control applications, accurate manufacturing equipment and quality-critical tasks.

Custom margin calculations or specific VFD classes (such as servo drives) may be required for certain control characteristics, response times and stability requirements.

How do you calculate VFD size?

The basic formula utilized is

VFD Size (kW) = Motor Power (kW) x Service Factor (SF)

The service factor which typically ranges from 1.1 to 1.5 other than for any unexpected load increases and also ensures reliability.

VFD Sizing Calculation

Step-1: Find Motor Power Input

Convert the motor power (kW) to kilowatts (kw) if specified in horsepower (HP).

Motor Power (kW) = Motor Power (HP) x 0.7457

Step-2: Compute Minimum VFD Rating

Enter the service factor (SF) to determine the minimum (small) acceptable VFD rating.

Minimum VFD Rating = Motor Power (kW) x Service Factor (SF)

Step-3: Application Margin

Add the application specific margin to make sure that the thermal headroom.

VFD with Margin = Minimum VFD Rating x (1+Margin %)

Step-4: Standard VFD Size

Choose from the standard commercial Variable Frequency Drives (VFD) sizes that may exceed the calculated rating.

Common Sizes

0.75, 1.5, 2.2, 3.7, 5.5, 7.5, 11, 15, 18.5, 22, 30, 37, 45, 55, 75, 90, 110, 132, 160, 200, 250, 315, 400, 500 kW.

Step-5: Calculate Estimated kVA

Estimate the VFDs apparent power requirement using conservative assumptions.

kVA = kW/(Power Factor X Efficiency)

Standard assumptions: PF = 0.9 & Efficiency = 0.95

Industry Standards 

International (IEC Standards)

• Based on the IEC 60034 (motor standards).

• IEC 61800 series (power electronic equipment).

• Conservative approach that is suitable for the most applications.

European Standards (CE Marking)

• Comply with machinery directive 2006/42/EC.

• EMC Directive 2014/30/EU.

• Low Voltage Directive 2014/35/EU.

• Requires the detailed risk assessment & documentation.

NEMA Standards (North America)

• Based on the NEMA MG-1 (motor & generator standards).

• NEMA ICS series (industrial control systems).

• Slightly different margin recommendations than IEC.

Australian Standards (AS/NZS)

• Based on the IEC standards with regional modifications.

• Consider the tropical climate conditions.

• Requires compliance with the workplace for all safety regulations.

Proper VFD sizing is a fundamental aspect to industrial reliability, energy efficiency and cost effectiveness. 

The VFD Sizing Calculator provides engineers, technicians and procurement professionals with a reliable and also standards-based tool for accurate VFD selection.