Variable Frequency Drive (Vfd)

CEMA
Engineering Standard 351-2007
Variable Frequency Drive (VFD)
Selection
For Screw Feeders
Approved: September 19, 2007

Published by the CEMA Screw Conveyor Section
as a service to the conveying industry.

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DISCLAIMER
The information provided in this document is advisory only. Theserecommendations are provided
by CEMA in the interest of promoting better engineering approaches to certain equipment
applications.
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only be made with the agreement and understanding that the user and the user’s company assume
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component, or mechanical or electrical device designed or manufactured using this information.
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CEMA Engineering Standard 351-2007

Variable Frequency Drive (VFD)Selection For Screw Feeders
Screwfeeders are designed to operate under flooded conditions with a head load of product on
the screw or multiple screws. The unit must be designed to overcome the static condition of the
screws and the force exerted by the product head load. The product head load exerts a downward
force on the screws and creates frictional resistance. As a result, additional torque is required to
start and operate ascrew feeder when compared to a screw conveyor.
Special consideration is required when designing screw feeders. The starting torque requirements
of screw feeders can be as much as 2-1/2 times the demand running torque. Torque is a function
of horsepower and speed. Most screw feeders for industrial applications operate at speeds below
30-rpm. The lower screw feeder speed provides higher torqueat the drive shaft.
Screw feeders are volumetric metering devices. A fixed volume of product is discharged with
each revolution of the screw. The volumetric capacity of the screw feeder is based on the volume
available in the last pitch before the shroud and the speed of the unit. All calculations are based
on the capacity given in cubic feet per hour.
Procedure for Calculating Demand...