Excitation method of DC motor_Excitation characteristics of DC motor

Excitation method of DC motor

The excitation mode of a DC motor refers to the way in which the excitation winding obtains the excitation current. Except for the permanent magnet micro DC motor, the magnetic field of the DC motor is established by exciting the excitation winding with current. According to the different excitation modes, it can be divided into four types: separately excited, parallel excited, series excited and compound excited.

(1) Separately excited motor

The field winding and armature winding of a separately excited motor are not connected to each other, as shown in Figure 4-2-1. The field winding of a separately excited motor uses a separate excitation power supply.

(2) Shunt motor

The excitation winding of the shunt motor is connected in parallel with the armature winding, as shown in Figure 4-2-2. The characteristics of the excitation winding of the shunt motor are thin wire, many turns, large resistance and small current. This is because the voltage of the excitation winding is the terminal voltage of the armature winding, which is usually high. The large resistance of the excitation winding can reduce If, ​​thereby reducing losses. Since If is small, in order to generate sufficient main magnetic flux Φ, the number of turns of the winding should be increased.

(3) Series-excited motor

The excitation winding of the series-excited motor is connected in series with the armature winding and then connected to a DC power supply, as shown in Figure 4-2-3. The characteristic of the excitation winding of the series-excited motor is that its excitation current If is the armature current Ia, which is generally large, so the excitation winding wire is thick, the number of turns is small, and its resistance is also small.

(4) Compound-excited motor

This type of DC motor has two excitation windings installed on the main magnetic poles, one connected in series with the armature winding and the other connected in parallel with the armature winding, as shown in Figure 4-2-4a. Therefore, the characteristics of the compound motor have the characteristics of both the series-excited motor and the shunt-excited motor, so it is also widely used. When the magnetic flux generated by the two excitation windings is in the same direction, it is called a product-excited motor, as shown in Figure 4-2-4b. When it is opposite, it is called a differential-excited motor, as shown in Figure 4-2-4c.

Excitation characteristics of DC motor

1. DC separately excited motor

There is no electrical connection between the field winding and the armature, and the field circuit is supplied by another DC power supply. Therefore, the field current is not affected by the armature terminal voltage or armature current.

2. DC shunt motor

The circuit is connected in parallel, shunted, and the voltage across the shunt winding is the voltage across the armature, but the field winding is wound with thin wire and has many turns, so it has a large resistance, which makes the excitation current passing through it smaller.

3. DC series motor

The current is connected in series, the voltage is divided, and the excitation winding is connected in series with the armature, so the magnetic field in this motor changes significantly with the change of the armature current. In order to avoid large losses and voltage drops in the excitation winding, the resistance of the excitation winding should be as small as possible, so DC series motors are usually wound with thicker wires and fewer turns.

4. DC compound motor

The magnetic flux of the motor is generated by the excitation current in the two windings.