Page 29 - MetalForming November 2014

mittent overload capacity, the mechan- ical unit would be sized to:
65/2.50 = 26 hp, plus 10 percent to account for friction and gear losses, or 26x1.1=28.6(roundupto30hp).
A VFD and/or DC drive typically can provide only 150-percent overload, so drive size would be:
65/1.50 = 43.33 hp (about 50 hp when accounting for frictional and gear losses.
That’s right, a 20-hp difference, because eddy-current drives can gen- erate 250 percent, or nearly double the peak output capacity, of equivalent AC or DC drives.
Heat and Longevity of AC Motors
At full speed and load, an AC motor will slip by an amount roughly equal to 3 percent of its synchronous speed, depending on the motor design. This slip represents energy loss in the rotor and converts directly to heat inside the motor, which must be dissipated. This makes AC motors relatively limited in
terms of full-torque thermal speed range. A standard induction motor can- not produce full torque over more than a two-to-one speed range without over- heating, if subjected to continuous duty. Even with substantial modifica- tion, the motor cannot run continu- ously at full torque at very low speeds due to the thermal losses in the rotor caused by slip.
Special inverter duty motors are subject to serious longevity concerns when subjected to continuous full- torque demands. Heat is the enemy of the VFD when used in a variable-speed press application.
However, in eddy-current drive applications, the AC motor runs at near synchronous speed, optimizing life expectancy due to the inherent heat- dissipating design capabilities of the motor. Eddy-current drives typically run well in excess of 20 years before requiring major attention, compared to the 7-10 yr. average life expectancy of a VFD.
Common Stamping Press Applications
Many stampers run their presses in progressive and transfer applications, loading the press and not stopping it until production is complete. This con- tinuous mode of operation demands that the press drive run under high load conditions at all times; such demand on the press drive is aug- mented by the energy that must be restored to the flywheel in slower-speed operation when the flywheel-energy demands may already be compromised by the work being done in the press. That’s where eddy-current drives find a home.
Eddy-current drive controls are sim- ple, inexpensive, easy to maintain and readily available in time-proven suc- cessful design configurations. It is com- mon for analog controls to run for 30 to 40 years before they require service. Their availability and cost effective- ness makes them highly desirable when comparedtotypicalVFDcontrols.MF
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