May 2008 Electrical Apparatus

May 2008 Electrical Apparatus

This is a summary of the Electrical Apparatus May 2008 featured technical article,  by Richard L. Nailen, P.E.  

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.

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