VSDs are often described by kW, but kW alone is not enough for final drive selection. The motor's rated current, voltage and the actual load duty are critical.
Start With Motor kW
Motor kW is a useful first filter because it places the motor into the approximate drive size range. It helps narrow the options, but it should be followed by a current check.
Then Check Full-Load Current
Compare the motor nameplate full-load current with the VSD's continuous output-current rating for the intended duty. The selected drive needs sufficient current capacity after any applicable derating.
Two motors with the same kW rating can have different nameplate currents. That is why quoting from kW alone can produce the wrong drive.
Normal Duty vs Heavy Duty
Many VSD families publish different ratings for lighter variable-torque duty and higher-overload duty. Pumps and fans are often candidates for normal or pump/fan duty, while loaded conveyors, crushers, mixers and some process machinery can require higher starting torque and overload capacity.
The application decides the duty. Do not select the lighter rating simply because it produces a higher kW number on the datasheet.
Voltage and Phase Still Matter
The drive input must suit the supply, and its output must suit the motor winding voltage. A current rating from the wrong voltage class is not a valid substitute.
Ambient Temperature and Altitude
High ambient temperature, altitude and restricted panel ventilation can reduce the usable rating of a VSD. Manufacturer derating rules should be checked when the drive is installed outside its standard conditions.
Starting Torque, Braking and Deceleration
Loads with high breakaway torque, frequent starts, rapid deceleration or regenerative energy may need a different drive size or braking arrangement. These requirements should be identified before purchasing the VSD.
Long Motor Cables
Long cables between the VSD and motor can affect the installation and may require changes to cable selection, switching-frequency settings or output filtering. Follow the drive and motor manufacturers' recommendations.
One VSD Running Multiple Motors
Multi-motor applications require specific engineering. The total current, individual motor protection, operating sequence and whether motors can be switched on the VSD output all need to be considered. Do not simply add kW values and select the next drive size.
A Practical Planning Checklist
- Motor nameplate photo
- Motor kW
- Motor full-load current
- Motor voltage and phase
- Available supply
- Application and load type
- Starting torque or loaded-start requirement
- Acceleration and deceleration time
- Braking requirement
- Motor cable length
- Ambient temperature, altitude and enclosure conditions
- Communications and control requirements
Technical Note
This guide provides general guidance. Final drive sizing, protection, braking, filtering, cable selection, programming and installation must be verified against the exact manufacturer documentation and site requirements.
Related catalogue examples: Variable Speed Drives. For an equipment enquiry, contact Softcore Group Industrial Solutions at info@softcoregroup.co.za.