Standard way to tension a string using a motor?
Use a motor driving a worm-gear reduction with the string locked to the output shaft.

In order to tension the string, turn the motor ON in the proper direction until the required tension is achieved, then turn the motor OFF. The worm gear will then keep the string under tension with no power applied to the motor.  In order to release the tension, turn the motor ON again, for a while,  but in the opposite direction.