The emf equation

DC generators are still used for applications like battery chargers, power supply for electroplating, dc welding machines etc. The emf produced by the rotating conductors on the armature produce emf across the winding. This emf is finally used to deliver load power.

To understand the mathematics behind the derivation, we must first know the concept of electrical frequency and the mechanical frequency.

The above diagram shows a four pole machine.

  • Consider the armature rotating with N rotations per minute.
  • Now when the armature completes only a half cycle (From the upper N pole to the lower N pole), one complete electrical cycle is complete.  Because the coil AB goes from (upper) N to S and again from S to (lower) N current reversal takes place and one electrical cycle is complete.
  • Thus, when the armature completes one (mechanical) rotation, it goes through 2 such electrical cycles.
  • This simply means that the electrical cycles and the the mechanical cycles have the following relation:

electrical frequency=mechanical frequency x (Poles/2)

Once you understand this concept, it is easy to understand the emf equation. Here you can download the file emf equation to understand the mathematical expression for the emf, derived from the fundamentals.

Here are some interesting numerical examples which use emf equation for a dc machine.