DC Buyer's Guide

Application Information

The load torque of a driven machine can be measured by wrapping a rope or cable around their input shaft or by using a torque arm and applying a steady pull through a spring scale. The horsepower can then be calculated using the following relation: 7? , RPM x Torque 5,252 Stalled Current Capability The Continuous Load section defines the torque capability at all speeds considering the thermal limits. While the curves may seem to indicate an intersection at zero speed, the data only applies at speeds other than zero, but may be very low, such as 10 RPM. At zero speed, localized heating of the segments under the brushes may produce segment A common example of a constant horsepower drive is a center driven winding reel. The material is wound on the mandrel at constant linear speed and constant tension using the following relation: 7? , Linear Speed (FPM) x Tension (lb) 33,000 The horsepower is constant . At the start of the winding process, the torque requirement is low because of the small radius and the high rotational (motor) speed. As the roll builds up, the radius increases with a resulting increase in torque. The rotational speed must decrease in order to maintain constant linear speed. CWT \PX] SaXeTb ^U \TcP[ f^aZX]V \PRWX]Tb aT`dXaT P__a^gX - mately constant horsepower because an optimum cutting speed is maintained for particular types of material regard- less of the diameter of the surface being machined. When \PRWX]X]V b\P[[ SXP\TcTa bc^RZ cWT c^a`dT aT`dXaT\T]c Xb [^f P]S cWT a^cPcX^]P[ b_TTS Xb WXVW Bc^RZ ^U [PaVTa SXP\ - eters require higher torque and decreased rotational speed. DC motors operated by field control and a constant arma- ture voltage have a constant horsepower capacity over the speed range. (See Fig. 2 on the previous page) In some applications, constant horsepower may be required over a wider speed range than is obtainable by field control. The additional speed range must be obtained by armature voltage control. Horsepower and speed are approximately proportional to the applied armature voltage, and the distortion and resulting brush vibration. Constant Horsepower Drives

horsepower requirements of the load must be available at the lowest operating speed obtained by armature voltage control. Combination of Constant Horsepower and Constant Torque Applications such as the center-driven winding reel often require a combination of constant torque and constant horsepower. (See Fig. 2 on the previous page) The horsepower required for a given linear speed of material is constant during the build-up of a roll. However, it is often desirable to change the surface speed when reeling different material. Since horsepower is proportional to the surface speed, armature voltage control will provide for the surface speed adjustments while the field control will provide the constant horsepower requirement during the build-up of the roll.

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