What would happen to a motor without a split-ring commutator? I realise that a similar question has already been asked (Why does the coil in this apparatus reverse its direction of oscillation?), however after reading the answers, I am still unclear on certain aspects of the concept, and I thought that it would be best put as a seperate question rather than a comment.
My main issue is this; without a split-ring commutator, what happens to the coil of wire after a 90 degree turn? Does it stop or go back on itself only to come back and repeat the process, my textbook says it stops but the answer to the other question said it reverses direction. 
Furthermore, I don't understand why what happens happens. I realise that as the coil rotates the "position" of the direction of the current flips, but I don't understand the impact it would have (what is the 'zero torque point').
My guess is that a certain current direction will cause force in a certain direction (upwards or downwards - depending on the right angles between the current and the magnetic field lines). Therefore, even after the coil of wire rotates 180 degrees, the wire still experience the same force, therefore the coil just goes back on itself and this process continues. The split-ring commutator flip the current so that the forces experienced are also reversed, and it is this that allows the motor to continuously turn in one direction. But this explanation doesn't account for why no forces are experienced when the coil in in a vertical position. Would it be because the coil of wire is experiencing a force, but its just that it cannot push it any more upwards/downwards?
Any help would be much appreciated, thanks.
 A: Here are a series of diagrams to show what happens if there is no friction.

The diagrams 1 to 6 show the coil speeding up and reaching a maximum speed at position 3 and then slowing down until it stops at position 5.
The sequence then repeats itself but with the torques and the angular velocities in the opposite direction until position 1 is reached and then the sequence repeats itself.
In position 1 the coil orientation relative to the magnetic field is such that a maximum torque is exerted on the coil.
If the coil had stated at position 2 then at position 5 it would have been stationary and then it would have returned to position 2, then to repeat the cycle.
Starting at position 3 would have meant that the coil did not move at all.
With friction present the amplitude of the oscillations would have decreased with position 3 being the final resting place of the coil.  
With a lot of friction the coil would have just gone through position 2 and then stopped at position 3.
A: The zero torque point occurs when the magnetic poles of the rotating coil (or armature) are in line with the poles of the field magnet.  At this point there is no torque. The south pole of the field magnet it trying to pull the the north pole or the coil towards it, radially away form the axis of rotation and similarly, the force on the south pole of the coil is radially out from the axis towards the north pole of the field magnet.  This is a point of stable equilibrium. If the coil is rotated slightly from this orientation the torque acts to return it back. You'll see a bit of damped oscillation.
The motor passes through this zero torque point every rotation (or actually twice if you have the split ring commutator).  It can be problem because - at this time the motor is providing no torque - i.e. not doing its job and, if the motor is in this orientation when you try and start it it won't go.  To avoid this any non-demonstration brushed DC motor will have an armature with at least 3 poles (only two of which are live at any given point) so that there is no zero torque point.
