VFD Sizing

TPI Designs, Manufactures, Procures and Installs many types of conveyors

Sizing VFDs for the Required Torque Load

Torque is an applied force that tends to produce rotation and is measured in lb-ft or lb-in. All loads have a torque requirement that must be met by the motor, the purpose of which is to develop enough torque to meet the requirements of the load.

An important component of the torque requirement is “Starting Torque”, or “Break-away Torque”, which is the amount torque required to get the load moving; after the load is moving, the torque demand will drop off to that which is required to handle the required capacity. Achieving this level of power may require up the VFD to deliver up to 400% of rated motor current.

NOTES:

  1. It may be necessary to up-size the VFD one or two steps to provide the level of starting torque required for the application.

  2. Other than a higher initial cost, oversizing a VFD is never a bad idea; an upsized VFD will absolutely run cooler, which is always a positive step.

Definitions:

  • Starting torque (Break-away torque): torque required to start a load in motion (potentially 150% to 200% greater than the running torque).

  • Accelerating torque: torque required to bring the load to operating speed within a given time.

  • Running torque: torque required to keep the load moving when uniformly loaded at normal operating speeds.

  • Peak running torque: occasional torque spike required by the load, such as having an additional load dropped on a conveyor.

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