机械外文文献
A Comparison of Drive Starting Mechanisms for
Aggregate Belt Conveyors
Abstract
The purpose of this paper is to describe the torque/speed characteristics,during
starting conditions,of the most common drives used on belt conveyors today. Requirements of a Belt Conveyor Drive
A belt conveyor is considered to be a constant torque device. In other words,the
required driving torque is approximately constant at varying speeds (see figure l).other applications,such as a pump drive,have variable torque requirements(see
figure2).
However,to increase the speed of a conveyor additional torque must be added until
the desired speed is obtained. Newton’s Second Law of Motion governs this
relationship.
Fma, ,
The most straightforward example would be a constant acceleration torque(see figure3).In reality the acceleration torque is rarely constant. However,static
calculation models as outlined in the Conveyor Equipment Manufacturers Association handbook (CEMA) make this assumption. When using static models the average acceleration torque is estimated over the entire acceleration time and assumed to be linear. Dynamic models,which are beyond the scope of this paper,allow acceleration
torque values to vary in magnitude during the acceleration(or deceleration)Period.
It should be noted that,given a constant load,a larger acceleration torque results in a
faster acceleration time and also higher Peak belt tensions. Conversely,a smaller
acceleration torque results in a longer start time and smaller Peak belt tensions. Across-The-Line AC Motor Start
Technically this is the simplest type of drive used on a belt conveyor. In this drive type an AC squirrel cage induction motor is started by simply throwing the contactor and energizing the motor. The resulting output torque,assuming that rated voltage is
maintained,is strictly a function of the motor design. NEMA has Provided design standards that define the output torque characteristics of the most commonly used 3 Phase motors up to approximately 250 hp(figure4).In sizes larger than 250 hp manufacturers generally use the NEMA design codes in a relative manner(i.e.,NEMA
C has a greater locked rotor torque than a NEMA B motor).
The most critical locations on the AC motor speed/torque curve have been named for definition purposes. These common names are provided in figure 5.
The most rigorous method of determining average acceleration torque,for static
calculations,is to break the curve into several vertical sections,then sum the
individual areas under the curve and finally divide by the number of sections.
The more common way is to apply the following simplified equation:
These static approximation methods work for most belt conveyors but can get the designer into trouble from time to time,especially on long and/or steep and/or fast
conveyors. One item that needs to be examined is breakaway torque. Just because the drive provides enough average torque to accelerate the load doesn’t mean that it
provides enough torque to break it away from zero speed and get it moving. CEMA defines breakaway torque as twice the torque required to overcome the total friction plus the torque required to lift the load vertically. Locked rotor torque (LRT) needs to be greater than breakaway torque! A good static Program makes this check.
In addition to examining the effect that average torque has on the conveyor components the belt designer needs to determine the effect of peak torque. It is not uncommon for the breakdown torque (BDT) of a NEMA C motor to be greater than 2.5 times full load torque (FLT).Generally the belting and Pulley manufacturers allow a transient overload of 1.5 times full load operating load. An across-the-line start can easily cause tensions to exceed these maximums. These higher than normal loads can be designed into the conveyor if they are known up front.
Considering only average starting torque can cause the conveyor designer to undersize the take-up weight. It is not uncommon for conveyors with across-the-line starters to experience intermittent drive slip. This generally happens when Peak torque (BDT) is input by the drive and the take-up has been sized for average torque but not peak torque. The result can be devastating. When the drive pulley slips during this condition,the tension on the Tl and T2 sides (high and low)of the drive Pulley tries to equalize. This can subject a low tension bend or take-up pulley,just behind the drive
pulley,to tensions that approach Tl tension. These Pulleys are rarely,if ever,designed
for this load condition and the result is low tension Pulley failure. This condition is easily demonstrated with dynamic analysis.
Another common Problem with across-the-line starts is caused by voltage dips during starting. If the power distribution system is not stiff enough to handle the huge inrush currents of an across-the-1ine start,the starting torque of the motors can be reduced to
a Point that the conveyor will not start. This is due to the fact that the output torque of
an AC squirrel cage induction motor is reduced by the square of the applied voltage. In other words,a voltage drop of 10%would equate to a torque reduction of 19%. Reduced Voltage Starting
The reduced voltage starting of an AC squirrel cage induction motor is done for two basic reasons:
1 .To reduce the inrush current that naturally occurs when a motor is Started
across-the-1ine. A typical current/speed graph is shown in figure 6.It is not
uncommon for the inrush current to be 6 times or more than it is at full load
torque. As stated above high inrush currents cause the voltage in a power
distribution system to sag. The cost of electrical power distribution equipment
can become very high if it needs to be designed to handle the high inrush
currents.
2 . To reduce Peak motor torque during starting conditions,which subsequently
increases acceleration time. By reducing the Peak torques the conveyor
components can be designed for lower tension loads. This primarily includes
belting,Pulleys and external support structure. This can result in significant cost
savings.
Two common types of reduced voltage starters are the Current Limiting and the Constant Torque devices.
Graphs are included above(figures 7 through 8) that depict the same motor/conveyor application with an Across-The-Line,a limitd Curren, and a constant Torque start.
After studying the graphs it becomes apparent that the best use of the limited torque start is to protect the power distribution system from high inrush currents. The constant torque start reduces the high torque Peaks and Protects the conveyor’s
mechanical components. In both cases the Start time is increased because the over all magnitude of accelerating torque is reduced. However,neither method will make it
easier to start a“hard-to-start conveyor.” Correcting a hard starting conveyor is not a
reason to use a reduced voltage starter!
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