Slip rings when connected to a variable resister bank, it allows you to vary the motor starting speed, because of the load characteristics [heavy load]. Doing so reduces the startup current, so when it start to roll, you reduce the resistance gradually and eventually you short the slip rings when it attains its nominal speed.
Osman, you didn't mentioned what is the need for change. As the link from previous post, it is better to change to a VFD rated motor if you are looking for a variable speed options.
Regards