Torque M0 in case of continuous operation S1, which meaning does it have?

Question:
Why an increasing torque of asynchronous motors causes a decreasing continuous torque from M0 to MN?

Answer:
The iron losses and losses due to friction rise proportionally to the field frequency / speed. These losses lead to an additional motor heating. Therefore, the continuous torque in the speed-torque-characteristic for the duty type S1 (continuous operation, see characteristic 1) decays.
This proportion can also be found in the field weakening range, which is, however, not obvious due to the torque reduction caused by the field weakening.
The maximum torque's sequence is constant below the rated speed (see characteristic 2), as this duty type can be operated only for a short while and the thermal time response of the motor must not be considered.

• MN = Continuous torque in case of rated speed
• M0 = Standstill continuous torque
• Mmax = Maximum torque (independent e. g. from the maximum current of the drive controller)