Vfd Sizing Calculator

VFD Sizing Calculator

Determine the appropriate Variable Frequency Drive size based on your motor's specifications.

function calculateVFD() { var motorHP = parseFloat(document.getElementById("motorHP").value); var motorFLA = parseFloat(document.getElementById("motorFLA").value); var serviceFactor = parseFloat(document.getElementById("serviceFactor").value); var resultDiv = document.getElementById("vfdResult"); if (isNaN(motorHP) || isNaN(motorFLA) || isNaN(serviceFactor)) { resultDiv.innerHTML = 'Error: Please enter valid numbers in all fields.'; resultDiv.style.display = 'block'; resultDiv.style.borderColor = '#d9534f'; resultDiv.style.backgroundColor = '#f2dede'; return; } if (motorHP <= 0 || motorFLA <= 0 || serviceFactor <= 0) { resultDiv.innerHTML = 'Error: Input values must be greater than zero.'; resultDiv.style.display = 'block'; resultDiv.style.borderColor = '#d9534f'; resultDiv.style.backgroundColor = '#f2dede'; return; } var requiredAmps = motorFLA * serviceFactor; var resultHTML = '

Recommended VFD Sizing

' + 'Recommended VFD Horsepower (HP): ' + motorHP + ' HP' + 'Minimum Continuous Amp Rating: ' + requiredAmps.toFixed(2) + ' Amps' + 'Select a VFD that meets or exceeds both the motor HP and the calculated minimum amp rating. Always consult the VFD manufacturer\'s specifications to ensure compatibility.'; resultDiv.innerHTML = resultHTML; resultDiv.style.display = 'block'; resultDiv.style.borderColor = '#0073aa'; resultDiv.style.backgroundColor = '#e9f5ff'; }

Understanding VFD Sizing

A Variable Frequency Drive (VFD) is a type of motor controller that drives an electric motor by varying the frequency and voltage of its power supply. VFDs are widely used for energy savings, process control, and reducing mechanical stress on equipment. However, selecting the correct size VFD is critical for reliable and safe operation. An undersized VFD will constantly trip on overcurrent faults and can be damaged, while an oversized VFD is unnecessarily expensive and may not provide accurate motor protection.

Key Factors for Sizing a VFD

The most reliable way to size a VFD is to use the motor's nameplate data. Here are the essential parameters:

• Motor Horsepower (HP) or Kilowatts (kW): This is the primary power rating of the motor and serves as the starting point for VFD selection. The VFD's HP rating should always be equal to or greater than the motor's HP rating.
• Motor Full Load Amps (FLA): This is the most critical factor. The FLA is the amount of current the motor draws when operating at its rated full load and horsepower. The VFD must have a continuous amperage rating that is equal to or greater than the motor's FLA. Our calculator multiplies this by a service factor to ensure a safety margin.
• Voltage and Phase: The VFD must be compatible with the incoming line voltage (e.g., 460V, 3-Phase) and must be able to output the voltage required by the motor. Ensure the VFD's input and output voltage ratings match your system and motor.
• Application Load Type: Loads are generally categorized as Variable Torque or Constant Torque.
  • Variable Torque (VT): Loads like centrifugal fans and pumps, where the torque requirement is low at low speeds and increases with the square of the speed. These are less demanding. A service factor of 1.1 to 1.15 is often sufficient.
  • Constant Torque (CT): Loads like conveyors, mixers, and positive displacement pumps, where the torque requirement is consistent across the entire speed range. These are more demanding and require a VFD with higher overload capacity. A service factor of 1.25 to 1.5 is recommended.

How to Use the VFD Sizing Calculator

1. Enter Motor Horsepower (HP): Find the HP rating on your motor's nameplate.
2. Select Motor Voltage: Choose the voltage that matches your motor's nameplate (e.g., 230V or 460V).
3. Enter Motor Full Load Amps (FLA): This is the most important value. Find the "FLA" or "Amps" rating on the motor nameplate. Do not use generic chart values if the nameplate is available.
4. Set the Service Factor: Use 1.15 for standard applications like fans and pumps (Variable Torque). Increase this to 1.25 or more for demanding applications like conveyors or crushers (Constant Torque).
5. Calculate: Click the button to see the recommended VFD HP and the required minimum continuous amp rating.

Example Calculation

Let's say you need to size a VFD for a conveyor system motor with the following nameplate data:

• Horsepower: 15 HP
• Voltage: 460V
• Full Load Amps (FLA): 21 Amps

Since a conveyor is a heavy-duty, constant torque application, we will use a service factor of 1.25.

Calculation:

Required Amps = Motor FLA × Service Factor
Required Amps = 21 A × 1.25 = 26.25 Amps

Result:

You should select a VFD with a rating of at least 15 HP and a continuous output current rating of at least 26.25 Amps. When looking at manufacturer catalogs, you would choose the next standard VFD that meets both these criteria.

Disclaimer

This calculator provides a general recommendation and should be used as a starting point. Always verify your selection by consulting the official documentation from the VFD manufacturer and consider all application-specific factors like ambient temperature, altitude, braking requirements, and control methods. For critical applications, consulting with a qualified electrical engineer or automation specialist is highly recommended.