DC Motor Speed: Does it Reach Constant Rotation?

[steve0606]

For a simple DC motor like this:

http://i1-news.softpedia-static.com/images/news2/How-Brushless-DC-Motors-Work-3.jpg

Does it reach a constant rotational speed?

I know that when a current flows through it, Flemmings Left HAnd rule causes a couple of forces that results in rotation. As this happens, its flux linkage changes and Lenz's and Faradays law says that this causes an EMF to be induced as to oppose the current produced from the p.d. from the dc power supply.

What I don't understand is how the motor would move? would it accelerate then slow down, then accelerate then slow back down etc. or would it reach an equilibrium speed?

Thanks!

 

[sophiecentaur]

When stationary, there is no back emf and the armature current is defined by the supply volts and the resistance of the winding. As it speeds up, the back emf increases, gradually reducing the effective voltage to drive current through the armature and limiting the torque. In the end, the maximum speed will be reached when the armature current is just enough to overcome the friction forces and the back emf will be 'almost' the same as the supply volts (but in the opposite sense, of course).