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"Driving
Belt Conveyors"
With
trilingual summary
Belt
conveyors handling bulk material impose several restrictions on the driving
motor. Locked-rotor torque must be sufficient to overcome static friction in the
belt system, but not so high as to damage the belt by overstretching it.
Likewise, motor breakdown torque must not be so high as to cause slippage
between the belt and its driving pulley. Between those points, motor torque must
remain high enough to overcome the constant accelerating torque demanded by the
belt.
Because
pulley speed in the range of 50 to 125 rpm requires multi-stage gearing in the
drive, a 4-pole motor is the most economical choice. A 6- or 8-pole rating would
still require more than one gear reduction stage, and would be more expensive.
For
ratings of 150 kw and below, the NEMA C motor can often be used. Double-cage
rotors are common for higher output as well, although NEMA does not define
Design C above that level. However, in larger ratings particularly, such a
design is often unsatisfactory for several reasons. First, both locked-rotor and
breakdown torques can be unacceptably high. Secondly, the minimum or pullup
torque at an intermediate speed may be too low.
Although
a single-cage using deep, narrow rotor bars more closely approaches conveyor
torque requirements, it has not proved popular. Adjustable-speed drives and
solid-state starters, allowing close control of motor torque during starting,
tend to be favored because they do not require special motor design. Wound-rotor
or d-c motor drives, also offering flexible torque control, are seldom used
because of cost and maintenance requirements.
Large
conveyor systems may require two, three, or even four large motors. For the
simple two-motor geared drive, motors of identical design may not share the load
equally because of slight, unavoidable differences in their speed-torque
characteristics. Forced to run at the same speed, the two units may divide the
load in a ratio closer to 40-60 percent than 50-50.
From "Driving
Belt Conveyors," published in the
Electrical Apparatus May 2008 issue
. Visit our online webstore to
order
copies. © 2008
Barks Publications, Inc. All Rights Reserved.
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