Introduction to the excitation method and magnetic field basic knowledge of DC motor

1. Excitation method of DC motor

The main excitation modes of DC generators are separately excited, shunt excited and compound excited.

1. Separately excited DC
motor

Figure 1.3.1 Schematic diagram of separately excited DC motor circuit
(II) Shunt excited DC motor

Figure 1.3.2 Schematic diagram of shunt-excited DC motor circuit
(III) Series-excited DC motor

Figure 1.3.3 Schematic diagram of series-excited DC motor circuit
(IV) Compound-excited DC motor

Figure 1.3.4 Schematic diagram of compound-excited DC motor circuit

2. No-load magnetic field of DC motor

1. No-load DC motor

An operating state in which the armature current is zero or very small and its influence can be ignored

2. No-load magnetic field of DC motor The magnetic field established solely by the excitation magnetic flux potential

Taking a four-pole DC motor at no load as an example, the magnetic field distribution established by the excitation current alone is shown in the figure

1) The main magnetic flux Φm interlinks with both the field winding and the armature winding

2) Leakage flux Φ1σ interlinks the field winding itself, not the armature winding

3) No-load magnetic flux density distribution

Ignoring the effect of tooth slots, when the DC motor is unloaded, the magnetic flux distribution waveform of its air gap magnetic field (main magnetic field) is shown in the figure.

Figure 1-23 No-load magnetic flux distribution of DC motor

4) Magnetization curve

The relationship between the main magnetic flux Φm and the excitation magnetic flux potential Ff0 or the excitation current If0 is called the magnetization curve of the motor.

Indicates the characteristics of the motor magnetic circuit

The magnetization curve of the motor can be obtained by calculating the motor magnetic circuit.

(1) Magnetic circuit through which the main magnetic flux passes

It consists of five parts: main pole core, air gap, armature teeth, armature core and yoke.

Because the B-H curve of ferromagnetic materials is nonlinear, the magnetic permeability is not a constant.

The relationship that makes Φm = f (Ff0) is also nonlinear

(2) Magnetization curve

The shape of the motor magnetization curve is similar to the B-H curve of the ferromagnetic material used.

Figure 1.3.6 Motor magnetization curve

3. Magnetic Field and Armature Reaction of DC Motor under Load

1. Magnetic field under load

After the motor is loaded, current flows through the armature winding, and a magnetic flux potential is formed. This magnetic flux is called the armature magnetic flux potential. Therefore, when loaded, the air gap magnetic field in the motor is jointly established by the excitation magnetic flux potential and the armature magnetic flux potential. It can be seen from this that in a DC motor, from no-load to load, its air gap magnetic field changes.

2. Armature reaction

1) The influence of armature magnetic flux potential on the air gap magnetic field generated by the excitation magnetic flux potential is called armature reaction.
To simplify the drawing, only one layer of the component edge is drawn, and the armature is considered to be smooth.
Considering that the current flowing through the component under a certain polarity is in the same direction, the armature magnetic field distribution is obtained.

Figure 1.3.7 Magnetic field distribution generated by armature magnetic potential

It can be seen that the armature reaction magnetic potential is the largest at the geometric neutral line of the magnetic pole.

2) Expanded diagram of magnetic potential distribution of a single armature

Figure 1.3.8 Magnetic field distribution generated by a single armature element

3) Expanded diagram of multiple armature magnetic potential distributions

When discussing, consider that the brush is on the geometric neutral line, and the magnetic potential distribution diagram is

1.3.9 Magnetic Field Distribution Generated by Three Armature Elements

4) Armature reaction

Considering the effect of the main magnetic field, the air gap magnetic field distribution after considering the armature reaction can be obtained:

Figure 1.3..10 Air gap magnetic field distribution after considering armature reaction

It can be seen from this that the position of the armature reaction magnetomotive force axis always coincides with the brush axis

When the brush is on the geometric neutral line, the armature reaction magnetic potential is perpendicular to the magnetic pole axis.

5) The armature reaction distorts the air gap magnetic field. The armature magnetic field weakens half of the main magnetic field and strengthens the other half, and causes the armature surface magnetic flux to be zero and leave the geometric neutral line.

6) Electromagnetic reaction shows demagnetization effect

* When the magnetic circuit is not saturated

The amount by which the main magnetic field is weakened is exactly equal to the amount by which it is strengthened.

* When the magnetic circuit is critically saturated

Increasing magnetization will increase the saturation degree under half of the pole, increase the core magnetic group, and reduce the saturation degree under the other half of the pole, and reduce the core magnetic group. Because the magnetic circuit is critically saturated, the actual synthetic magnetic field curve is slightly lower than when saturation is ignored. The amount of increased magnetic flux will be less than the amount of reduced magnetic flux.

* in conclusion

Another consequence of armature reaction is a drop in the flux per pole when the motor is loaded.