In the figure above, a single rectangular conductor loop is positioned between the opposite poles of a magnet.
Let the rectangular loop be labeled as ABCD, rotating within the magnetic field around its own axis, abab. As the loop rotates from a vertical to a horizontal position, it intersects the magnetic flux lines of the field. During this rotation, two sides of the loop, ABAB and CDCD, cut through the flux lines, resulting in an e.m.f. being induced in both sides ABAB and CDCD of the loop.
Since the loop is closed, a current will flow through it. The direction of this current can be determined using Fleming’s right-hand rule. According to this rule, if you stretch your thumb, index finger, and middle finger of your right hand so that they are mutually perpendicular, then:
- The thumb indicates the direction of the conductor’s motion.
- The index finger points in the direction of the magnetic field (from the North pole to the South pole).
- The middle finger shows the direction of the induced current in the conductor.
Applying this rule to the loop in its horizontal position, the current will flow from point AA to BB on one side of the loop, and from point CC to DD on the other side.
If the loop continues to rotate, it will return to a vertical position. However, in this new vertical position, the upper side of the loop will now be CDCD, and the lower side will be ABAB, opposite to the previous vertical position. At this point, the tangential motion of the loop’s sides is parallel to the magnetic flux lines. As a result, no flux cutting occurs, and no current is induced in the loop.
As the loop rotates further, it reaches the horizontal position again. This time, the ABAB side of the loop faces the North pole, and the CDCD side faces the South pole, opposite to their orientations in the previous horizontal position, as illustrated in the accompanying figure.
At this position, the tangential motion of the loop’s sides is perpendicular to the magnetic flux lines, resulting in the maximum rate of flux cutting. According to Fleming’s right-hand rule, the current flows from BB to AA on one side and from DD to CC on the other side.
As the loop continues to rotate about its axis, the current direction alternates based on the position of the sides relative to the magnetic poles. When side ABAB faces the South pole, the current flows from AA to BB, and when it faces the North pole, the current flows from BB to AA. Similarly, when side CDCD faces the South pole, the current flows from CC to DD, and when it faces the North pole, the current flows from DD to CC.
Observing this phenomenon differently, it can be concluded that whenever a side of the loop is in front of the North pole, the current through that side flows in the same direction—downward relative to the reference plane. Conversely, when a side is in front of the South pole, the current flows upward relative to the reference plane.
This brings us to the principle of a DC generator. To convert this alternating current into a unidirectional current, the loop is opened and connected to a split ring, as shown in the figure. The split ring is made from a conducting cylinder that is divided into two insulated halves or segments. These segments are in contact with two carbon brushes, which rest on the split ring and are connected to the external load terminals.