Made for students, by students Motor Selection Guide


Figure 8: Constant Voltage Speed-Torque line vs. Torque Curve



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Figure 8: Constant Voltage Speed-Torque line vs. Torque Curve

The speed-torque line and the motor data sheet show the operating characteristics typically only for one voltage setting. If the ModBot only needed to operate along this line, this part of the analysis would be complete. However, the ModBot is required to run at multiple different speeds for the same load; your system may even require sets of different loads and different speeds. To determine what the speed-torque line looks like for a different applied voltage, the equations below can be used. The new no-load speed (n0) is calculated using the new voltage (U) and the speed constant (kn) which is a property of the motor and can be found on the motor’s data sheet.8






( 7 )

For example, if the motor has a no-load speed of 10,000 rpm at 24 Volts and a speed constant of 416 rpm/V, then when 15 Volts is applied, the no load speed is only 6250 rpm. Because the speed constant (kn) is a property of the motor, the speed torque lines for all different voltages are parallel offsets with the same slope equal to the speed constant (kn). On some data sheets, instead of listing the slope of the line as the speed constant (kn), the slope of the line is reported as the speed/torque gradient or the curve gradient of the motor. The results of this calculation are shown in Figure 9.





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