How to Size a Frequency Converter?

Choosing a frequency converter requires more than matching its kVA rating to the equipment wattage. The converter must match the required voltage, frequency and phase while providing enough capacity for running current, power factor, startup demand and continuous operation. This guide explains the main sizing steps for imported equipment, industrial machinery, motors, test systems and 400Hz loads.

What Does a Frequency Converter Do?

A frequency converter supplies controlled AC power at a different frequency, such as 50Hz to 60Hz, 60Hz to 50Hz, or 50/60Hz to and from 400Hz. Depending on the configuration, it may also change or regulate the output voltage.

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

Step 1: Check the Equipment Nameplate

Record the equipment specifications before calculating capacity.

The converter input must match the available site supply, while its output phase, voltage and frequency must match the equipment. For example, equipment marked “3PH, 400V, 50Hz” requires three-phase 400V 50Hz output.

Step 2: Calculate the Running Load in kVA

Frequency converters are normally rated in kVA. Use the equipment voltage and current whenever available.

Single-Phase Load

For a 230V load drawing 8A:

Three-Phase Load

For a 400V three-phase load drawing 18A:

When Only kW Is Listed

An 8kW load with a power factor of 0.8 requires 10kVA. Do not assume that kW and kVA are equal unless the power factor is 1.0.

Step 3: Allow for Continuous Operation

For stable continuous operation, a practical target is to keep the load at about 70% to 80% of the converter rating.

For the 12.47kVA example:

Select the next suitable catalog size. In this case, a 20kVA converter provides more appropriate continuous-duty capacity than a 15kVA unit.

Step 4: Account for Starting and Inrush Current

Motors, pumps, compressors, transformers and other inductive loads may draw much more current during startup. The required capacity depends on the starting method, mechanical load, acceleration time, startup frequency and actual inrush current.

These multipliers are preliminary only. Measured or manufacturer-specified starting current is more reliable.

Motor Starting Example

A 400V motor drawing 7A while running and 32A during startup requires:

The converter must support both the continuous load and the short startup demand. Overload capability should not replace proper sizing.

Step 5: Size for Multiple Loads

Calculate the maximum demand that will occur at the same time rather than adding every possible startup current.

For loads of 1.2kVA, 2kVA and 1kVA, plus 2kVA of additional motor starting demand, the peak requirement is 6.2kVA. After allowing operating margin, a 10kVA converter may be more suitable than a 5kVA model.

Step 6: Check Output-Frequency Derating

Some converters provide full rated capacity only within a specified frequency range. For applicable PowerHome models, full capacity is available up to 120Hz, while output capacity is reduced above 120Hz. At 400Hz, usable capacity may be approximately 50% of the nameplate rating.

For a continuous 4kVA, 400Hz load:

The next suitable size would be 10kVA. Always verify the derating specified for the selected model.

Step 7: Match Voltage, Phase and Wiring

Capacity alone does not confirm compatibility. Single-phase converters are commonly used for appliances, laboratory instruments and small machines, while three-phase models are used for industrial machinery, CNC equipment, pumps, compressors and production systems.

Before ordering, confirm the input voltage and phase, Wye or Delta wiring, neutral availability, required output voltage and frequency, and maximum load current.

Quick Frequency Converter Selection Table

The table assumes a continuous load of approximately 80% of the converter rating. Motors, pumps, compressors, transformers and other high-inrush loads may require additional capacity.

Common Sizing Mistakes

• Selecting by watts alone: Watts do not account for power factor, apparent power or startup current.
• Matching converter kVA directly to running kVA: Continuous loads require operating margin.
• Ignoring startup current: A converter may run a motor normally but shut down during starting.
• Matching frequency but not voltage or phase: All three output parameters must match the equipment.
• Confusing a frequency converter with a VFD: A VFD primarily controls motor speed, while a static frequency converter supplies controlled AC power to motors or mixed loads.
• Ignoring 400Hz derating: Apply the high-frequency capacity reduction specified for the model.

Final Selection Checklist

• Available input voltage, phase and wiring configuration
• Required output voltage, phase and frequency
• Total simultaneous running current or kVA
• Power factor, starting current and startup duration
• Continuous or intermittent duty
• Ambient conditions and required operating margin
• 400Hz derating, when applicable
• Future expansion requirements