The speed-torque gradient is one of the most informative pieces of data and allows direct comparison between different motors. The smaller the speed-torque gradient, the less sensitive the speed reacts to torque (load) changes and the stronger the motor. With the maxon motor, the speed-torque gradient within the winding series of a motor type (i.e. on one catalog page) remains practically constant. Current gradient The equivalence of current to torque is shown by an axis parallel to the torque: more current flowing through the motor produces more torque. The current scale is determined by the two points no load current I 0 and starting current I A (lines 3 and 8 of motor data). The no load current is equivalent to the friction torque M R , that describes the internal friction in the bearings and commutation system. M R = k M · I 0 In the maxon EC motor, there are strong, speed dependent iron losses in the stator iron stack instead of friction losses in the commutation system. The motors develop the highest torque when starting. It is many times greater than the normal operating torque, so the current uptake is the greatest as well. The following applies for the stall torque M H and starting current I A M H = k M · I A Efficiency curve The efficiency η describes the relationship of mechanical power delivered to electrical power consumed.
Speed n
U = U N
n 0
Torque M Current I
I A
n · ( M − M R ) U mot · I mot
� 30 000
η =
·
One can see that at constant applied voltage U and due to the propor- tionality of torque and current, the efficiency increases with increas- ing speed (decreasing torque). At low torques, friction losses become increasingly significant and efficiency rapidly approaches zero. Maxi- mum efficiency (line 9 of motor data) is calculated using the starting current and no load current and is dependent on voltage.
n 0
2
I
0 I A
η max = 1 −
Torque M
M H
Maximum efficiency and maximum output power do not occur at the same torque.
Speed n
Rated operating point The rated operating point is an ideal operating point for the motor and derives from operation at nominal voltage U N (line 1 of motor data) and nominal current I N (line 6). The nominal torque M N produced (line 5) in this operating point follows from the equivalence of torque and current. M N ≅ k M · (I N − I 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 the maximum no load speed should be. The nominal current derives from the motor’s thermally maximum permissible continuous current.
n N
I N M N
Torque M Current I
maxon 79
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