Free Engineering Tool
VFD Selection Calculator
Size a Variable Frequency Drive for your motor. Calculate rated current, required VFD current rating with overload margin, and recommended drive size.
Results
Motor Full-Load Current
VFD Current Rating
Where the overload margin accounts for transient conditions and safety factor.
Standard VFD Sizes
| kW (400 V) | Typical FLA (A) | VFD Frame |
|---|---|---|
| 0.75 | 2.1 | Size A |
| 1.5 | 3.8 | Size A |
| 4.0 | 9.5 | Size B |
| 7.5 | 17 | Size B |
| 15 | 32 | Size C |
| 22 | 46 | Size C |
| 37 | 73 | Size D |
| 55 | 106 | Size D |
| 75 | 145 | Size E |
| 110 | 210 | Size E |
⚠️ Note: Always verify VFD selection against manufacturer datasheets. Derate for ambient temperature >40°C, altitude >1000m, or switching frequency >4 kHz.
Calculate the motor FLA, then select a VFD with continuous current rating ≥ FLA plus overload margin. Typical margin is 10% for variable torque (fans/pumps) and up to 50% for constant torque applications.
Normal duty (ND): 110% overload for 60s, for variable torque loads. Heavy duty (HD): 150% overload for 60s, for constant torque loads. Same VFD is often rated higher kW in ND than HD mode.
Yes, consider one frame size up for: high starting torque, frequent starts/stops, ambient >40°C, altitude >1000m, or high switching frequency. Oversizing also extends VFD lifetime.
Match VFD input to your supply: 230V (small), 400V (Europe), 480V (N. America), 690V (large industrial). VFD output voltage matches the motor rating.
VFDs work best with inverter-duty motors. Standard motors may need derating and bearing insulation above 30 kW to prevent EDM bearing damage from PWM-induced shaft voltages.
The procedures described on this page were developed and field-tested by the Vibromera team using the Balanset-1A, a portable dual-channel balancer and vibration analyzer for on-site rotor balancing.
Professional field balancing instruments and software. Used in 50+ countries.
Nikolai Shelkovenko
Nikolai Shelkovenko is a vibration analysis engineer and the founder and CEO of Vibromera. For more than 15 years he has balanced rotating equipment in the field rather than on a test bench: mulchers, industrial fans, crushers, centrifuges, shafts and spindles. That work is what the Balanset instruments grew out of — they were designed as a tool a specialist can carry to the machine and use alone, on site, not as laboratory equipment. Vibromera was founded in 2017 and has been based in Porto, Portugal, since 2023. Development, assembly and support of the Balanset line all happen here. The flagship instrument is the Balanset-1A, a portable analyser for single- and two-plane balancing and for vibration diagnostics. Nikolai is personally involved in customer support, in working through difficult balancing cases and in the development of the software. He works with customers worldwide, in any language.
