Speed Change:
When the terminal voltage of a DC motor is reduced to half, the speed of the motor will decrease. This is because the speed of a DC motor is directly proportional to the back EMF (electromotive force) which is generated in the armature windings. The back EMF is given by the equation:
E = KϕNZ
where E is the back EMF, K is a constant, ϕ is the flux per pole, N is the number of armature conductors, and Z is the total number of armature conductors.
As the terminal voltage is reduced, the back EMF decreases, resulting in a decrease in the speed of the motor. The speed reduction is approximately proportional to the reduction in terminal voltage.

Current Change:
When the terminal voltage of a DC motor is reduced to half, the armature current of the motor will increase. This is because the armature current is given by the equation:
Ia = (V - E) / Ra
where Ia is the armature current, V is the terminal voltage, E is the back EMF, and Ra is the armature resistance.
As the terminal voltage is reduced, the difference between the terminal voltage and the back EMF increases, leading to an increase in the armature current. The increase in armature current is approximately proportional to the reduction in terminal voltage.

Torque Change:
When the terminal voltage of a DC motor is reduced to half, the torque produced by the motor will decrease. The torque produced by a DC motor is given by the equation:
T = KϕIa
where T is the torque, K is a constant, ϕ is the flux per pole, and Ia is the armature current.
As the terminal voltage is reduced, both the back EMF and the armature current decrease. Since the torque is directly proportional to the product of the back EMF and the armature current, the reduction in both quantities will result in a decrease in the torque produced by the motor. The torque reduction is approximately proportional to the square of the reduction in terminal voltage.