Selecting a VFD based only on motor power in kilowatts can lead to incorrect sizing.
The correct selection starts with the motor nameplate current, followed by the load type, required overload capacity, acceleration time, and site conditions.
Normal Duty and Heavy Duty
Some VFD models have two ratings for the same drive.
Normal Duty is generally suitable for loads that require moderate starting torque and limited overload capacity, such as centrifugal pumps and fans.
Heavy Duty is generally suitable for applications that require higher starting torque or greater short-term overload capacity, such as conveyors, mixers, crushers, and positive-displacement pumps.
The available continuous output current may be lower when the same VFD is used in Heavy Duty mode.
Overload ratings are not identical across all manufacturers. For example, one drive may provide 110% or 120% overload in Normal Duty and 150% overload in Heavy Duty for a specified time.
Always check the current and overload ratings in the data sheet of the exact VFD model.
The Simplest Selection Method
1. Read the Motor Nameplate Current
Use the motor rated current at the actual supply voltage.
Do not start the selection from the motor kilowatt rating alone.
2. Identify the Load Type
Determine whether the application is a pump, fan, conveyor, mixer, crusher, compressor, or another type of machine.
Also check whether the motor starts with the load already applied.
3. Determine the Required Overload
Identify the current required during acceleration, how long it is required, and how often the overload may occur.
The VFD must provide enough torque to accelerate the load without reaching the current limit or tripping.
4. Apply Derating Factors
Check the manufacturer’s requirements for:
Ambient temperature
Installation altitude
Switching frequency
Panel ventilation
Mounting clearances
Motor cable length
The VFD output current after derating must remain equal to or greater than the required motor current.
Practical Example 1: Centrifugal Pump
Motor power: 45 kW
Motor nameplate current: 82 A
Application: Clean-water centrifugal pump
Starting condition: Normal acceleration without repeated shocks
Site conditions: No derating required
A Normal Duty VFD may be evaluated first.
The selected VFD must have a continuous Normal Duty output current equal to or greater than 82 A.
Its overload capacity must also be checked to ensure reliable acceleration.
Practical Example 2: Loaded Conveyor
Motor nameplate current: 82 A
Application: Conveyor starting while loaded
Required overload: 150% for 60 seconds
Required temporary current:
82 A × 150% = 123 A
The selected VFD must have a continuous Heavy Duty output current equal to or greater than 82 A after derating.
It must also be capable of supplying 123 A for 60 seconds according to the manufacturer’s data sheet.
The 123 A value is a temporary overload current, not a continuous operating current.
When Is a Larger VFD Required?
A larger VFD may be required when:
The ambient temperature is above the manufacturer’s rated value.
The installation altitude reduces cooling performance.
A high switching frequency is selected.
The application requires a very short acceleration time.
The load has high inertia.
The motor operates for long periods at low speed.
The motor cable is long and additional output equipment is required.
The application includes repeated shocks, jamming, or regenerative operation.
Common Selection Mistakes
Selecting the VFD from kilowatts only.
Assuming that every pump is a Normal Duty application.
Treating 150% overload current as a continuous current rating.
Ignoring ambient temperature, altitude, and switching-frequency derating.
Selecting Heavy Duty from a parameter setting without checking the actual Heavy Duty current rating.
Ignoring braking requirements for high-inertia or regenerative loads.
Conclusion
Start with the motor nameplate current.
Then identify the load type, starting conditions, required overload, acceleration time, and derating factors.
Only after these checks should you select the VFD from the manufacturer’s Normal Duty or Heavy Duty current-rating table.
Two VFDs may have the same kilowatt rating, but they may not provide the same output current or overload capability.
For a suitable VFD recommendation, send Power Wadi:
Motor nameplate photo
Application and load type
Required acceleration time
Minimum and maximum speed
Supply voltage
Ambient temperature
Installation altitude
Motor cable length
Required control method