The effect to a DC generator from armature reaction

by Generator D. Diesel Generator

What is armature reaction?

In a DC generator, armature reaction means the effect of armature magnetomotive force on the fundamental wave of the main pole magnetic field under symmetrical load.

It is assumed that the only magnetomotive force acting on the DC generator is generated by the stator magnetic field. However, the magnetic field generated by the current in the armature winding is called the armature magnetic field, and the axis of the armature magnetic field intersects the axis of the main magnetic field vertically. No matter in the motor or generator, the distortion and weakening of the magnetic field occur. The reaction caused by armature magnetomotive force is usually called armature reaction.

The effect of armature magnetic field on the main magnetic field will distort the main magnetic field and produce armature reaction:

a.Armature reaction under pure resistive load.

The electromotive force of the armature magnetic field is the same as the phase of the current. The armature magnetic field distorts the main magnetic field, half strengthens and half weakens.

b.Armature reaction under pure inductive load.

The current of armature magnetic field lags 90 degrees behind electromotive force, the electromotive force generated by armature magnetic field is opposite to that generated by main magnetic field. Therefore, the EMF of the main magnetic field is weakened, this is why the voltage drops when there are inductive elements in the three-phase circuit. This is called longitudinal axis demagnetization armature reaction.

c.Armature reaction under pure capacitive load.

The current of armature magnetic field is 90 degrees ahead of electromotive force, because the armature magnetic field is 90 degrees to the main magnetic field, The electromotive force generated by the armature magnetic field is in the same direction as that generated by the main magnetic field. Therefore, enhanced EMF of main magnetic field. This is why the terminal voltage rises when there are capacitive elements in the three-phase circuit. This is called longitudinal axis auxiliary magnetoarmature reaction.

Generator

Armature reaction classification

a.Quadrature armature reaction.

The quadrature axis armature reaction is the influence of the quadrature axis armature magnetomotive force on the main pole magnetic field. Here, in order to analyze the problem simply, we assume that ① the magnetic field is unsaturated, ② the armature steering of the generator is counter clockwise, and the motor is clockwise. It can be seen that:

(1)the cross axis armature magnetic field can demagnetize the main pole magnetic field in half pole, increase magnetic action in the other half of the pole, which causes air gap magnetic field distortion, offset the geometric neutral at a position where the flux density on the armature surface is equal to zero.

(2) when saturation is not considered, there is neither demagnetization nor demagnetization in the cross axis armature reaction. When saturation is considered, it plays a role of demagnetization.

b. Direct axis armature reaction.

When the brush is not on the geometric neutral line, the direct axis armature reaction occurs.

(1)In case of generator, the brush moves an included angle in the direction of rotation, for the magnetic field of the main pole, the direct axis reacts to demagnetize. If the brush moves an included angle against the rotation direction, the direct axis armature reaction will be magnetized.

(2)In the case of motors, the opposite is true.

The effect of armature reaction on DC generator

Armature reaction has a great influence on the operation of DC generator, which makes the magnetic field of half pole strengthen. When the magnetic field on the other half weakens, will cause the larger the load and the stronger the magnetic field distortion caused by armature reaction. The result will destroy the normal commutation of armature winding elements, easily cause sparks and worsen the working conditions of the generator. At the same time, the armature reaction will make the flux density at the pole shoe tip, cause the maximum voltage between the commutators to be too high, and also easily cause sparks or even generator ring fire.

Add additional poles to compensate for the distorted flux. For large generator, adding compensation winding on the top of the main pole can make the flux distribution abnormal and correct.

The effect of armature reaction on commutation of DC generator

The armature reaction will distort the air gap magnetic field, which will have an adverse effect on the commutation of the DC generator:

(1)Because the air gap magnetic density at the geometric neutral is no longer zero, therefore, the induced electromotive force will inevitably be generated in the reversing element (the brush is in contact with the reversing piece connected to the element at the geometric neutral), which makes reversing difficult.

(2)The distortion of air gap magnetic field makes the voltage on commutator uneven, especially when the load of generator changes suddenly, it forms strong armature reaction and air gap magnetic field is serious distortion. It is possible to make the potential difference between two adjacent commutators exceed a certain limit, resulting in potential difference spark. And ring fire may occur as the arc lengthens. Commutation is a special problem of DC generator. If the commutation is not good, sparks will be generated between the commutator and the brush. If the spark exceeds a certain degree, the brush and commutator will be burnt out, which will seriously affect the operation of the generator. In addition, sparks will produce electromagnetic waves, which will interfere with wireless communication.

There are many reasons for sparking. In addition to electromagnetic reasons, there are also mechanical reasons. In the reversing process, there are also factors such as electrochemistry and electrothermal. They are intertwined with each other and the process is quite complex.