What effect does the armature reaction have on the motor?
Magnetic field of DC motor under load - armature reaction
After the DC motor is loaded, the armature winding has a current, which is referred to as the armature magnetic field, and the influence of the armature magnetic field on the main magnetic field is called the armature reaction. The specific analysis is as follows: When the motor is loaded, the current in the armature winding passes, and the armature current will generate the armature magnetomotive force. At this time, the air gap magnetic field of the motor is determined by the two magnetic fields of the main magnetic field and the armature. The appearance of the armature magnetomotive force causes the air gap magnetic field to be distorted, that is, the armature reaction. In a DC motor, regardless of the type of armature winding, the current of each branch is drawn through the brush, so the brush is the boundary of the current distribution on the armature surface. The axis of the armature magnetic potential always coincides with the brush axis.
1. Cross-axis armature magnetic potential Faq
The armature magnetic field is as shown on the left. If the current in the upper half of the armature is outflow and the lower half is inflow, according to the right-handed screw rule, the magnetic field established by the armature magnetomotive force is indicated by the dotted line. It can be seen from the figure that the axis of the armature magnetomotive force always coincides with the brush axis. The axis orthogonal to the main pole axis is usually called the intersection axis, and the axis coinciding with the main pole axis is called the straight axis; so when the brush is on the geometric neutral line, the armature magnetomotive force is the armature armature magnetism. Potential.
2, the straight axis armature magnetic potential Fad
3, straight axis armature reaction
If the motor is a generator, the brush moves toward the beta angle. The direct-axis armature reaction only exists when the brush is not in contact with the conductor at the geometric centerline. At this time, there is also a cross-armature armature reaction (later analysis), and the influence of the direct-axis armature magnetic potential is now analyzed separately. DC generator:
If the motor is a generator, the brush moves toward the beta angle.
★★ When the brush of the DC generator is shifted by a small angle, the effect of the direct-axis armature reaction on the main pole magnetic field will be demagnetized.
★★When the DC generator's brush is offset by a small angle, the effect of the direct-axis armature reaction on the main pole magnetic field will be magnetized.
DC motor: The armature reaction of a DC motor is exactly the opposite of that of a DC generator. In the case of DC motors, students can prove it themselves.
4. Cross-axis armature reaction
Regardless of whether the brush of the DC motor is in contact with the conductor at the geometric centerline, the cross-axis magnetic potential Faq and its armature reaction are present.
Here, the normal axial position of the brush is taken as an example to illustrate the reaction of the armature armature. The idea is to synthesize the Bo(x) wave of the magnetic dense distribution wave and the Ba(x) wave of the armature magnetic density separation wave during the load, and obtain the air gap synthetic magnetic density wave B(x). Ie: bo(x)+ba(x)=b(x)
s:
(1) For a DC generator, the cross-armature armature reaction will cause the front extreme to have a demagnetization effect, and the latter extreme has a magnetization effect.
(2) For the DC motor, the front extreme - extreme after magnetization - demagnetization
So what is the front extreme and the last extreme? For the generator, the extremes seen in the past are the extremes of the front, and the extremes of the latter are called the extremes. However, for this schematic diagram, if the direction of the magnetic field and the direction of the current of the motor are unchanged, when the generator is changed to a motor, the steering must be changed, that is, the front extreme for the DC generator and the extreme for the DC motor. The electromagnetic relationship inside the motor is unchanged, so the conclusion of the AC armature reaction of the DC motor should be that the front extreme has a magnetization effect, and the latter extreme has a demagnetization effect.
(3) For DC motors, the cross-armature armature reaction has a certain saturation demagnetization. Due to the influence of magnetic saturation, the magnetization will increase the saturation of the core, and the magnetoresistance will increase, so that the actual air gap magnetic field is slightly lower than that of the unsaturated state; while the demagnetization is not saturated, the demagnetization is more magnetic. The magnitude of the value is large, so it is concluded (3).
5, DC motor armature reaction summary
(1) Cross-axis armature reaction
1 Distort the waveform of the main magnetic field,
2 has a certain degree of saturation demagnetization
(2) Straight-axis armature reaction (only exists when the brush is offset from the theoretical position)
Influence of armature reaction on motor
The changing magnetic field generated by the energized stator winding causes the rotor in the closed loop state to generate a current, and then the rotor generates a magnetic field, which has a superposition effect on the magnetic field generated by the original stator winding, so that it exhibits magnetization or demagnetization. Phenomenon, this phenomenon is reflected by the armature. Due to the reason reflected by the armature, the magnetic field changes, resulting in a decrease in the quality of the air gap magnetic field, which adversely affects its mechanical properties.
