MIL-HDBK-1003/3
APPENDIX D (Continued)
they enhance process operations, particularly where flow control
is involved. VFD's soft start capabilities decrease electrical
stresses and line voltage sags associated with full voltage motor
start-ups, especially when driving high-enertia loads. For the
motor to produce the required torque for the load, the VFD must
have ample current capability to drive the motor. It is
important to note that machine torque is independent of motor
speed and that load horsepower increases linearly with rpm.
Individual load types are as follows:
a) Constant torque loads. Constant torque loads
represent 90 percent of general industrial machines (other than
pumps and fans). Examples of these load types include general
machinery, hoists, conveyors, printing presses, positive
displacement pumps, some mixers and extruders, reciprocating
compressors, as well as rotary compressors.
b) Constant horsepower loads. Constant horsepower
loads are most often found in the machine tool industry and
center driven winder applications. Examples of constant
horsepower loads include winders, core-driven reels, wheel
grinders, large driller machines, lathes, planers, boring
machines, and core extruders.
Traditionally, these loads were considered DC drive
applications only. With high performance flux vector VFD's now
available, many DC drive applications of this type can be now
handled by VFD's.
c) Variable torque loads. Variable torque loads are
most often found in variable flow applications, such as fans and
pumps. Examples of applications include fans, centrifugal
blowers, centrifugal pumps, propeller pumps, turbine pumps,
agitators, and axial compressors. VFD's offer the greatest
opportunity for energy savings when driving these loads because
horsepower varies as the cube of speed and torque varies as
square of speed for these loads. For example, if the motor speed
is reduced 20 percent, motor horsepower is reduced by a cubic
relationship (.8 x .8 x .8), or 51 percent. As such, utilities
often offer subsidies to customers investing in VFD technology
for their applications. Many VFD manufacturers have free
software programs available for customers to calculate and
document potential energy savings by using VFD's.
D-1.06
Special Applications of VFD's. If any of the following
operations apply, use extra care in selecting a VFD and its setup
parameters.
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