control - Mechanical time constant of PMDC Motor in starting and after switching off the supply is same or different?

In my MATLAB/SIMULINK model of the PMDC motor the mechanical time constant has same value in the case starting of motor and after switching off the supply but in experimental setup there is a difference? I have taken many experimental readings, in which starting mechanical time constant is around 0.05 seconds and after switching off the mechanical time constants is around 0.254 seconds. .

If any one have any idea, please reply me.