Several factors within the design of the motor, pulley, and the mass itself all may limit the speed at which the mass can be pulled along the surface.
Motor design
The torque produced by a motor is given in the formula τ=nIABsinθ. The following factors would limit torque produced by the motor.
n - number of windings
Increasing the number of windings can increase the force produced by the motor. Thus too little windings would limit the max torque and thus speed which the mass can be pulled at.
I - current
Too low a current will limit the torque of the motor and thus max speed at which the block can be pulled. Current is determined by Ohm's Law I=V/R. Consequently, the supply voltage of the motor and associated resistance can also limit the speed be being too low or high respectively, It is also important to note that the opposite, high V and low R can result in motor burn out as well, due to excessively high current and temp, preventing max speed or breaking down the motor altogether.
A - area of coil
This is directly related to the size of the motor. A larger motor can house larger coil which produce greater torque, while too small a motor would limit speed. The area of the coil must also have components perpendicular to the magnetic field to produce torque. This is usually accounted by adding more coils at different orientation so that there is always sufficient area facing the magnetic field to produce torque. Thus, insufficient coils at differing orientations will also limit max speed.
B - magnetic field strength
Too weak a magnetic field will also limit max speed.
Back EMF
Faraday's Law predicts that a changing magnetic flux through a coil will induce an emf oriented to induce eddy currents which flow to induce a magnetic field opposing the original change in flux. (ε=-N Δφ/Δt) Within a motor, this back emf opposes the supply voltage, (ΣV=Vsupply-ε), and its strength is dependant f the rate of change of flux. This directly depends on the motor's speed, and thus the motor will reach a point which max speed is reached, directly limiting the speed. Several functions affect the extent of this limitation such as the load limiting the motor's speed instead of back emf, strength of magnetic field influencing Δφ, and supply voltage which opposes emf. All these factors may play a role in limiting the speed of the block.
Pulley
The radius of the pulley may limit the speed of the mass. A limited torque is produced by the motor, which translates to a force in the string by F=τ/r, where r is the radius of the pulley. If the radius is too great, the force on the rope is reduced. However, if it is too small, much more rotations per minute are required to reel the mass at the same speed (C=2πr). These factors can thus limit the speed of the mass.
Mass
The greater mass will have greater inertia, and thus be more difficult to pull of the motor (F=ma). The friction forces also play a role, counteracting the force applied by the string (Ff=μFN=μFg). This is once again dependant on mass, but also the coefficient of friction between the mas and the surface.
Thus, all these factors could limit the speed of the mass being pulled.
