Page 26 - MetalForming November 2014

Press Applications:
When metalformers consider the horsepower required, the installation costs and life-cycle expectancy, eddy-current drives will outperform VFDs in metal-stamping/forming press applications.
BY DAVID STRONER
Eddy-current and variable-frequency drives (VFDs) have been employed to provide vari- able-speed control for presses for many years. Mechanical presses, which use flywheels to store energy, are segregated by several standards; here we’ll focus on speed, rating, flywheel energy and main motor requirements.
Generally, we specify three speed ranges for press- es: slow (0 to 20 strokes/min.); medium (20 to 40 strokes/min.); and fast (40 strokes/min.) and faster.
When discussing ratings, we say “this press is rated at 1⁄2 in. off bottom stroke,” which means that the press is designed to perform work at its full rated tonnage from 1⁄2 in. before bottom through bot- tom dead center (BDC).
The rating of a variable-speed press will allow for full flywheel energy to be developed at mean speed. Consider a machine that runs at 20 to 40 strokes/min.—the mean speed is 30 strokes/min., which is where you will like- ly find the full flywheel energy developed. You will have an overabundance of energy as speed continues to rise from 30 to 40 strokes/min., but the energy available from 30 to 20 strokes/min. will suffer greatly. The energy available is a function of the square of the velocity of the flywheel.
David Stroner is a metalforming and stamping sales engineer with DSI Dynamatic, Sturtevant, WI: 262/554- 7977; www.dynamatic.com
Eddy Current
vs. Variable Frequency Drives
24 MetalForming/November 2014
www.metalformingmagazine.com
In stamping- press applica-
tions, eddy-cur- rent (EC) drives typically run well in excess of 20 years before requiring major attention, com-
pared to the 7-10 yr. average life expectancy of a VFD. Shown is a
combination air- cooled
adjustable-speed EC drive, brake, belt
ring flywheel and shaft- mounted tachometer in a common
housing, ready for blanking, deep
drawing, progressive-die stamping and other metalforming applications.