How temperature affects a servomotor´s electrical and mechanical time constants

This paper shows that a servomotor´s dynamic motion response is controlled by both its electrical and mechanical time constants. As shown, both time constants depend on temperature. As a minimum, the increase in every motor´s electrical resistance with increasing winding temperature causes the motor´s mechanical time constant to increase in value while its electrical time constant decreases in value. In all permanent magnet motors, including the widely used brushless DC, there´s the added affect of reversible thermal demagnetization that reduces the motor´s K/sub E/ and K/sub T/, thereby further increasing the mechanical time constant only. Ignoring these time constant changes can result in a significant error when calculating the motor´s dynamic motion response. In turn, this error can result in the wrong motor being selected for the application thereby leading to costly production delays and unhappy customers. To avoid making this mistake, it is recommended that the motor´s dynamic motion response be calculated using the motor´s electrical and mechanical time constant values adjusted to the motor´s maximum continuous or rated operating temperature.