A DC machine needs poles to produce magnetic flux. Armature conductors cut this flux while rotating and produce emf. This emf is used to deliver power into the connected load and the machine becomes a generator.
If the machine is connected to external power, the armature draws current and the current carrying conductors placed in magnetic field rotate, the machine becomes a motor.
The flux producing poles are electromagnets. A coil with the iron core carries DC current to produce poles. Depending upon the current direction, polarity of the pole is decided.
Generated emf (or the back emf in case of a motor) is proportional to the flux per pole. The BH curve for the iron core can be found out performing a simple experiment.
The experiment is performed on a separately excited machine.
- We change the current in the field coil by changing the field circuit resistance.
- Then we measure the emf across the output terminals (because the armature current “Ia” is zero) when the generator is running on no load.
- The emf thus measured can be considered as being proportional to the flux produced.
- So the Ea (induced armature emf) Vs I(field) graph actually represents the BH curve (magnetization curve) for the pole core of the machine.
- This curve can be used to predict performance of the machine. Download the file below to know how to use the magnetization curve to do this.