How to Size a Frequency Converter?

A Practical Selection Guide

Choosing a frequency converter is not as simple as matching the converter’s kVA rating to the wattage printed on your equipment. A properly sized frequency converter must match the load’s voltage, frequency, phase configuration, running current, power factor, starting current and continuous operating requirements. Ignoring these factors may result in nuisance shutdowns, unstable output voltage, overheating, unsuccessful motor starts or an unnecessarily oversized converter. This guide explains how to calculate the required converter capacity and select a suitable model for imported equipment, industrial machinery, laboratory testing systems, motors, pumps, compressors and 400Hz applications.

What Does a Frequency Converter Do?

A frequency converter changes incoming AC power into a controlled AC output with a different frequency. Common conversions include:

• 50Hz to 60Hz
• 60Hz to 50Hz
• 50Hz or 60Hz to 400Hz
• 400Hz to 50Hz or 60Hz

Depending on the selected configuration, the converter may also change or regulate the output voltage. This makes it useful for operating equipment designed for another country, electrical grid or testing standard.

PowerHome offers single-phase and three-phase frequency converters with adjustable output voltage, pure sine wave output and selectable output frequencies.

Step 1: Check the Equipment Nameplate

Before calculating the converter capacity, record the following information from the equipment nameplate or technical manual.

The converter’s output phase, voltage and frequency must match the equipment, while the converter input must match the available power supply at the installation site.

For example, a machine marked “3PH, 400V, 50Hz” requires three-phase output. A single-phase frequency converter cannot supply it, regardless of the converter’s kVA rating.

Step 2: Calculate the Running Load in kVA

Frequency converters are normally rated in kVA, which represents apparent power. Equipment may instead be labeled in watts, kilowatts or amps, so the load may need to be converted into kVA.

Single-Phase Load Calculation

For example, a 230V single-phase machine draws 8A:

The machine therefore requires approximately 1.84kVA while running.

Three-Phase Load Calculation

For example, a 400V three-phase machine draws 18A:

The machine requires approximately 12.5kVA during normal operation.

When Only kW Is Listed

When the equipment nameplate lists kW and power factor, use the following formula:

For example, an 8kW machine with a power factor of 0.8 requires:

Do not assume that 10kW always equals 10kVA. Resistive heaters may operate close to a power factor of 1.0, while motors and other inductive loads usually have a lower power factor.

Step 3: Add a Continuous-Operation Margin

A frequency converter should not normally operate at its maximum capacity continuously.

For stable long-term operation, a practical target is to keep the continuous load at approximately 70% to 80% of the converter’s rated capacity.

Using the previous 12.47kVA three-phase example:

The next available size should be selected. A 20kVA three-phase frequency converter would provide a better continuous-duty margin than a 15kVA unit.

Step 4: Account for Starting and Inrush Current

Starting current is one of the most important factors in frequency converter sizing.

Motors, pumps, compressors, transformers and other inductive loads may draw substantially more current during startup than during normal operation. The exact starting demand depends on:

• Motor design
• Mechanical load
• Starting method
• Acceleration time
• Compressor pressure
• Pump head
• Connected transformer size
• Whether a soft starter or VFD is used

These multipliers are preliminary guidelines only. When the actual startup current is available, sizing according to the measured or specified current is more reliable.

Motor Starting Example

For a three-phase motor, calculate both the running and starting apparent power.

Suppose a 400V motor has a running current of 7A and a measured starting current of 32A.

Running load:

Starting load:

Although the motor requires only approximately 4.85kVA after reaching full speed, the converter must also tolerate the startup demand.

Short-duration overload capability should not replace proper sizing. Startup duration, starting frequency and mechanical load can all affect whether the motor starts successfully.

Step 5: Add Multiple Loads Correctly

When several devices are connected to one frequency converter, do not simply add every possible startup current unless all devices start at the same time.

For example:

• Test instrument: 1.2kVA
• Heater: 2kVA
• Fan motor running load: 1kVA
• Additional fan starting demand: 2kVA

The total demand during fan startup is:

After adding an operating margin, a 10kVA converter may be more appropriate than a 5kVA converter.

Step 6: Check Output-Frequency Derating

Frequency converter output capacity may be reduced when operating at high frequencies.

For applicable PowerHome models:

• Full rated capacity is available at output frequencies up to 120Hz.
• Output capacity is reduced above 120Hz.
• At 400Hz, usable capacity may be approximately 50% of the converter’s rated kVA.

400Hz Sizing Example

Suppose an aviation test device requires 115V, single-phase, 400Hz power and has a continuous load of 4kVA.

The next suitable catalog size would be a 10kVA single-phase frequency converter.

A 5kVA converter may provide only approximately 2.5kVA of usable capacity at 400Hz and would therefore be undersized for this application.

Step 7: Match Voltage, Phase and Wiring Type

Capacity alone does not determine whether a frequency converter is compatible with the load.

Single-Phase Applications

Single-phase frequency converters are commonly used for:

• Imported appliances
• Laboratory instruments
• Electronic test equipment
• Small production equipment
• Single-phase motors
• Repair and service benches
• Product certification testing

Three-Phase Applications

Three-phase frequency converters are more suitable for:

• Industrial machinery
• Production lines
• CNC equipment
• Pumps and compressors
• Automotive test systems
• Manufacturing equipment
• Centralized electrical testing
• Aviation and aerospace support systems

Before ordering a three-phase converter, confirm:

1. Input line voltage
2. Input wiring type: Wye or Delta
3. Whether a neutral conductor is available
4. Required output line voltage
5. Required output frequency
6. Maximum load current

Recommended PowerHome Frequency Converters

500VA/1000VA Single-Phase Frequency Converter

Recommended for:

• Small electronic devices
• Low-power imported appliances
• Benchtop instruments
• Small laboratory equipment
• Light service and repair applications

This size range is a practical choice when the continuous load remains below approximately 800VA and the equipment has no substantial startup current.

View 500VA/1000VA Single-Phase Frequency Converter

2kVA/3kVA Single-Phase Frequency Converter

Recommended for:

• Medium-size laboratory instruments
• Imported commercial equipment
• Product testing
• Communication equipment
• Small single-phase machines

Choose 2kVA when the calculated continuous load remains comfortably below approximately 1.6kVA. Select 3kVA when the continuous load is closer to 2kVA or additional starting margin is required.

View 2kVA/3kVA Single-Phase Frequency Converter

5kVA Single-Phase Frequency Converter

Recommended for:

• Larger test benches
• Heating equipment
• Light-duty motors
• Packaging or inspection equipment
• Multiple small devices operating together

A 5kVA model is generally suitable for continuous loads of approximately 4kVA or less when startup demand is moderate.

View 5kVA Single-Phase Frequency Converter

10kVA Single-Phase Frequency Converter

Recommended for:

• High-current single-phase equipment
• Industrial testing systems
• Larger imported machines
• Aerospace or 400Hz test loads
• Multiple synchronized laboratory loads

When operating at 400Hz, account for high-frequency derating. A 10kVA model may provide approximately 5kVA of usable load capacity at 400Hz.

View 10kVA Single-Phase Frequency Converter

15kVA/20kVA Single-Phase Frequency Converter

Recommended for:

• High-power single-phase production equipment
• Large testing systems
• Commercial machinery
• High-current 120V or 240V loads
• Multiple single-phase loads on a centralized supply

These models are suitable when the calculated continuous load exceeds the practical capacity of a 10kVA converter.

View 15kVA/20kVA Single-Phase Frequency Converter

30kVA Single-Phase Frequency Converter

Recommended for:

• Centralized testing laboratories
• High-current production systems
• Large single-phase machinery
• Multiple equipment testing stations
• Heavy commercial applications

View 30kVA Single-Phase Frequency Converter

3kVA/10kVA Three-Phase Frequency Converter

Recommended for:

• Small three-phase machines
• Motor testing
• Electrical product testing
• Repair centers
• Small manufacturing systems

The 3kVA option is suitable for small controlled loads. The 10kVA option provides additional capacity for small machinery and moderate startup demand.

View 3kVA/10kVA Three-Phase Frequency Converter

15kVA/20kVA Three-Phase Frequency Converter

Recommended for:

• Medium-size industrial machines
• Pumps with controlled starting
• Industrial test benches
• Manufacturing equipment
• Automotive electrical testing

Select between 15kVA and 20kVA according to running current, power factor, startup demand and continuous-duty margin.

View 15kVA/20kVA Three-Phase Frequency Converter

30kVA/45kVA/60kVA Three-Phase Frequency Converter

Recommended for:

• CNC machinery
• Industrial pumps
• Production lines
• Compressors
• Factory acceptance testing
• Imported manufacturing equipment

Select among 30kVA, 45kVA and 60kVA according to total simultaneous running kVA and the largest expected startup load.

View 30kVA/45kVA/60kVA Three-Phase Frequency Converter

75kVA/100kVA Three-Phase Frequency Converter

Recommended for:

• Large manufacturing equipment
• Industrial motor systems
• Centralized testing platforms
• High-current production machinery
• Plant-level power simulation

At 400Hz, the usable load must be calculated after applying the applicable output-frequency derating.

View 75kVA/100kVA Three-Phase Frequency Converter

150kVA/200kVA Three-Phase Frequency Converter

Recommended for:

• Heavy industrial machinery
• Large production lines
• Centralized factory testing
• Aviation ground-support systems
• High-power custom projects

These high-capacity models are suitable for large industrial systems that require controlled three-phase voltage and frequency.

View 150kVA/200kVA Three-Phase Frequency Converter

Quick Frequency Converter Selection Table

The table assumes a continuous load of approximately 80% of the converter’s rated capacity. Additional capacity may be required for motors, pumps, compressors, transformers and other high-inrush loads.

Common Frequency Converter Sizing Mistakes

Selecting by Watts Alone: Watts do not show apparent power, power factor or starting current. Use kVA and rated current whenever possible.

Choosing the Same kVA as the Running Load: A 5kVA continuous load should not normally be connected to a 5kVA converter. Additional operating capacity should be reserved.

Ignoring Motor Startup: An undersized converter may operate normally after the motor reaches full speed but shut down every time the motor starts.

Matching Frequency but Not Voltage: Changing 50Hz to 60Hz does not automatically ensure that the output voltage is correct. Voltage, frequency and phase must all match the equipment.

Confusing a Frequency Converter With a VFD: A VFD is primarily designed to control the speed of an AC motor. A static frequency converter supplies controlled AC power at the required voltage and frequency. A VFD is generally not a direct replacement for a frequency converter when the load includes heaters, transformers, electronic circuits, control systems or multiple devices.

Ignoring 400Hz Derating: A converter rated at 10kVA may provide only approximately 5kVA at 400Hz. Always apply the specified high-frequency derating.

Final Frequency Converter Selection Checklist

Before ordering, confirm the following:

• Available input voltage
• Available input phase
• Wye or Delta wiring configuration
• Whether a neutral conductor is available
• Required output voltage
• Required output phase
• Required output frequency
• Total simultaneous running current
• Load power factor
• Largest starting current
• Startup duration
• Continuous or intermittent duty
• Ambient operating temperature
• 400Hz derating, when applicable
• Future expansion capacity