maxon

The speed-torque gradient is one of the most informative pieces of dataand allows direct comparison between different motors. The smaller thespeed-torque gradient, the less sensitive the speed reacts to torque (load)changes and the stronger the motor. With the maxon motor, the speedtorquegradient within the winding series of a motor type (i.e. on onecatalog page) remains practically constant.Speed nn 0U = U Nmaxon motorCurrent gradientThe equivalence of current to torque is shown by an axis parallel to thetorque: more current flowing through the motor produces more torque. Thecurrent scale is determined by the two points no-load current I 0 and startingcurrent I A (lines 3 and 8 of motor data).The no-load current is equivalent to the friction torque M R , that describesthe internal friction in the bearings and commutation system.Torque MM R = k M · I 0I ACurrent IIn the maxon EC motor, there are strong, speed dependent iron losses inthe stator iron stack instead of friction losses in the commutation system.The motors develop the highest torque when starting. It is many timesgreater than the normal operating torque, so the current uptake is thegreatest as well.The following applies for the stall torque M H and starting current I AM H = k M · I AEfficiency curveThe efficiency describes the relationship of mechanical power deliveredto electrical power consumed. =30 000·n · ( R)U · IOne can see that at constant applied voltage U and due to the proportionalityof torque and current, the efficiency increases with increasing speed(decreasing torque). At low torques, friction losses become increasinglysignificant and efficiency rapidly approaches zero. Maximum efficiency (line9 of motor data) is calculated using the starting current and no-load currentand is dependent on voltage.n 0 max= 1 I 0I A2M HMaximum efficiency and maximum output power do not occur at the sametorque.Rated working pointThe rated working point is an ideal working point for the motor and derivesfrom operation at nominal voltage U N (line 1 of motor data) and nominalcurrent I N (line 6). The nominal torque M N produced (line 5) in this workingpoint follows from the equivalence of torque and current.M N k M · (I N 0 )Nominal speed n N (line 4) is reached in line with the speed gradient.The choice of nominal voltage follows from considerations of where themaximum no-load speed should be. The nominal current derives from themotor‘s thermally maximum permissible continuous current.May 2012 edition / subject to changeKey information37