In homopolar motor, What do you mean by the absence of polarity change?

Moving electric charges (an electric current) in a magnetic field experience a force that is perpendicular to both their direction of movement and the magnetic field, called the Lorentz force. In the homopolar motor shown on the right, the electric current produced by the battery moves radially through the disk magnet, which has a magnetic field along its longitudinal axis. The resulting Lorentz force in the tangential direction produces a torque in the magnet, which is free to rotate with the attached screw.

It is not necessary for the magnet to be electrically conductive, or to move. One can attach the magnet to the battery and allow the wire to rotate freely while closing the electric circuit even at the axis of rotation. Again, where at some point along the electric loop the current in the wire is not parallel to the magnetic field, there occurs a Lorentz force that is perpendicular to both. This Lorentz force is tangential and produces a torque in the wire, so that the wire rotates.

In contrast to other electrical motors, both the orientation and magnitude of the magnetic field and the electric current do not change.

Like most electro-mechanical machines a homopolar motor is reversible so that when electrical energy of a suitable kind is put into its terminals, mechanical energy can be obtained from its motion and vice versa.

Sir Beltrano? wahaha...=P