What is armature reaction in DC machine

Armature reaction in a direct current generator

Definition: The anchor reaction simply shows the effect of anchor field on the main field. In other words, the anchor reaction represents the influence of the anchor flux on the main field flux. The armature field is generated by the armature conductors when current flows through them. And the main field is generated by the magnetic poles.

The anchor flux has two effects on the main field flux.

  • The anchor reaction distorted the main field flux
  • It reduces the size of the main field flow.

The figure below shows the two poles of the DC generator. When there is no load connected to the generator, the armature current becomes zero. In this state, only the MMF of the main poles is present in the generator. The MMF flux is evenly distributed along the magnetic axis. The magnetic axis means the center line between the north and south poles. The arrow in the figure below shows the direction of the magnetic flux ΦM.. The magnetic neutral axis or plane is perpendicular to the axis of magnetic flux.

The MNA coincides with the geometric neutral axis (GNA). The brushes of the DC machines are always arranged in this axis, and therefore this axis is called the commutation axis.

Consider the condition where only the armature live conductor and no current flows through their main poles. The direction of the current remains the same in all conductors below a pole. The direction of the current that is induced in the conductor is given by Fleming's rule of law. The direction of flow that is generated in the conductors is determined by the corkscrew rule.

The current direction on the left side of the anchor ladder goes into the paper (represented by the cross inside the circle). The anchor conductors combine their MMF to create the downward flows through the anchor.

Similarly, the right side conductors are currently carrying and their direction is out of the paper (indicated by dots inside the circle). The ladder on the right also combines its MMF to create flow downwards. Hence, the ladder on either side combine their MMF so that their flow goes down. The flow is induced in the anchor conductor ΦA is given by the arrow shown above.

The following figure shows the state in which the field current and the armature current act on the conductor at the same time.

This happens when machines are running with no loadcondition. Now the machine has two flows, i. H. The armature flux and the field pole flux. The armature flux is generated by inducing the current in the armature conductors, while the field pole flux is induced due to the main field poles. These two rivers combine and give the resulting flux ΦR. as shown in the picture above.

If the field flux enters the anchor, it can be distorted. The distortion increases the flux density in the upper pole tip of the N pole and the lower pole tip of the South Pole. Similarly, the flux density decreases in the lower pole tip of the North Pole and the upper pole tip of the South Pole.

The resulting flux is induced in the generator shifted in the direction of the rotation of the generator. The magnetic neutral axis of the poles is always perpendicular to the axis of the resulting flux. The MNA is continuously shifted with the resulting flow.

Effect of the anchor reaction

The effects of the anchor reaction are as follows:

  • Because of the anchor reaction, the flux density of more than one half of the pole increases and over the other half it decreases. The total flux produced by each pole is slightly less, which decreases the magnitude of the terminal voltage. The effect by which the armature reaction reduces the overall flux is known as the demagnetization effect.
  • The resulting flow is distorted. The direction of the magnetic neutral axis is shifted with the direction of the resulting flux in the case of the generator, and it is opposite to the direction of the resulting flux in the case of the motor.
  • The armature reaction induces a flux in the neutral zone and this flux creates the voltage that causes the commutation problem.

The MNA axis is the axis in which the value of the induced MEF becomes zero. And the GNA divides the anchor core into two equal parts.