A modular top drive system for construction of a wellbore includes a motor unit. The motor unit includes: a drive body; a drive motor having a stator connected to the drive body; a trolley for connecting the drive body to a rail of a drilling rig; and a drive ring torsionally connected to a rotor of the drive motor and having a latch profile for selectively connecting one of: a drilling unit, a casing unit, and a cementing unit to the motor unit.
