Shield drain wire must be spliced only to mating shield drain wires and not grounded at the junction box. Feedback shields must be passed through pin for pin. Separate junction boxes for power and feedback are required.
The shield strongly attenuates the electric field noise. Core to shield capacitance is added to the stray capacitance, increasing ground currents in the loop. These currents generate a magnetic field. It is important to minimize the area of this loop as far as possible by routing the cable close to grounded metalwork.
A ferrite sleeve around the three power conductors as they leave the drive will help to reduce common-mode noise current. Take all three conductors two or three times through the core. If it runs hot reduce the number of turns.
A simple, experimental motor such as this is not capable of making much power. We can increase the turning force (or torque) that the motor can create in three ways: either we can have a more powerful permanent magnet, or we can increase the electric current flowing through the wire, or we can make the coil so it has many "turns" (loops) of very thin wire instead of one "turn" of thick wire. In practice, a motor also has the permanent magnet curved in a circular shape so it almost touches the coil of wire that rotates inside it. The closer together the magnet and the coil, the greater the force the motor can produce.