VFD Sizing: 4 Essential Steps for Correct Selection

Hello everyone, today I’d like to share some key points about VFD sizing (variable frequency drive selection). Many electrical professionals are familiar with variable frequency drives, but their understanding of specific VFD selection may not be comprehensive. This article will outline several critical points, from fundamental principles to special cases, using practical examples for your reference.

Flowchart of VFD sizing process with motor FLA, current rating, torque type, and derating considerations

VFD selection by load type

When selecting a variable frequency drive, the first step is to identify the load type.

  • Variable Torque: Typical applications include VFD fans and VFD pumps. These loads exhibit a square relationship, where output torque is proportional to the square of the speed.
  • Constant Torque: Most industrial motor loads fall under this category, where torque remains relatively constant regardless of speed.

If the load type is incorrectly identified during VFD sizing, even the highest power rating will be ineffective. This is the critical first step that demands attention.

VFD sizing by motor FLA

A common mistake when sizing a VFD for a motor is relying solely on power ratings. The correct approach is VFD sizing based on the motor’s Full Load Amps (FLA).

The method is straightforward:

  • Note the motor’s nameplate-rated current (FLA, Full Load Amps).
  • Select a VFD model with a current rating equal to or greater than this value.

Thus, VFD sizing fundamentally involves selecting current, not power. This is especially critical for specialized motors where power and current do not maintain a fixed ratio—power-based selection often leads to errors.

VFD Sizing Adjustments for Special Cases

In VFD application guides, pay special attention to these scenarios:

  • Specialty motors (e.g., synchronous motors, high-speed motors, wound rotor motors, submersible pump motors, Roots blower motors) typically draw higher currents than standard motors. In such cases, size the VFD one size larger than the motor to ensure motor-VFD compatibility.
  • Frequent starts/stops or shock loads: It’s recommended to size the VFD one size larger than the motor to prevent overcurrent.
  • High altitude (>1000m): Thinner air reduces heat dissipation, necessitating VFD sizing for high altitude—typically one size larger than the motor.
  • High ambient temperature (>40°C): Insufficient cooling may cause VFD overheating faults, requiring increased capacity.
  • Demanding acceleration/deceleration requirements: If acceleration time is significantly shorter than standard values (e.g., 3s instead of 10s), greater current surges occur. VFD sizing for fast acceleration is required, typically one to two sizes larger.

Avoiding Excessive VFD Oversizing

Many customers blindly increase capacity after encountering VFD troubleshooting issues, sometimes selecting units two or three sizes larger than the motor. This approach is actually unreasonable.

The correct method is:

  • One size larger is reasonable (to account for cable length, special motors, etc.).
  • Two sizes or more is wasteful: not only does it increase costs, but it also leads to VFD oversizing risks such as reduced efficiency and increased heat generation.

Therefore, VFD sizing is not merely about selecting a larger unit, but scientifically optimizing based on factors like current rating, torque, and derating.

Conclusion

The core logic of VFD sizing can be summarized as:

  • First determine the load type (constant torque or variable torque).
  • Select the VFD based on the motor’s FLA (Full Load Amperage).
  • Apply appropriate derating for special operating conditions (special motors, altitude, temperature, acceleration/deceleration requirements).
  • Avoid excessive oversizing to maintain cost-effectiveness and reliability.

Only by understanding these details of variable frequency drive selection can true motor-VFD compatibility be achieved, preventing repeated pitfalls in troubleshooting guides and fault analysis.