Scheme of inclusion and static characteristics of...

Switching scheme and static characteristics of the DC motor of series excitation

In general, the circuit diagram of the power circuit of the DC motor of series excitation is shown in Fig. 3.14.

Diagram of the DC power circuit of a DC motor of series excitation

Fig. 3.14. Serial Excitation DC Motor Circuit Diagram

The notation adopted in Fig. 3.14, are the same as in Fig. 3.1 for a DC motor of independent excitation.

The equation of the electromechanical characteristics of a sequential excitation motor, as well as for an independent excitation motor, is given by (3.3)

where is the resistance of the armature winding chain, consisting of armature winding resistance, additional poles, excitation winding and additional resistance to the armature winding.

If the load on the motor shaft changes, the angular velocity will vary due to the voltage drop at the anchor circuit resistance, and due to the increased excitation flux F. At the DC motor of series excitation, the excitation winding is connected in series with the armature winding, therefore the armature winding current is simultaneously the current of the excitation winding. The dependence of the excitation flux on the excitation current of the motor is represented by the universal magnetization curve in Fig. 3.12, dependence 1. The magnetization curve does not have an exact analytic expression and in calculations

is usually approximated by straight line segments. Alternatively, in Fig. 3.12 the magnetization curve is approximated by two straight segments 2:

with ; (3.31)

with , (3.32)

where is the proportionality factor between the current and the flux of the magnetization curve, is the saturation flux of the magnetization curve of the motor. Substituting (3.31) in (3.3), we obtain the equation of the electromechanical characteristics of the sequential excitation motor for a linear dependence of the excitation flux on the armature current

(3.33)

The dependence (3.33) is hyperbolic.

With motor armature currents, high rated current and the adopted approximation law, the motor current is stabilized and becomes . In this area of ​​the electric motor, its electromechanical characteristic is described by the equation

(3.34)

Expression (3.34) is the equation of a straight line. Thus, for large loads, the relationship between the angular velocity of the motor and the winding current of the armature is linear.

With the accepted approximation, the graph of the electromechanical characteristics of the DC motor of series excitation is shown in Fig. 3.15, a.

The electromagnetic moment of the DC motor is determined by the dependence (3.4)

Substituting the expression for the flow (3.31) into equation (3.4) and solving the resulting equation for the armature current I , we have

(3.35)

The simultaneous solution of expressions (3.35) and (3.33) allows one to obtain the equation of the mechanical characteristics of a sequential excitation motor for small loads on the motor shaft:

The dependence (3.36) is hyperbolic.

For large loads, when the flow is limited to the level Φ1, the mechanical characteristic of the sequential excitation motor is determined by the expression

(3.37)

With the accepted approximation, the graph of the mechanical characteristic of a DC motor of series excitation is shown in Fig. 3.15, b.

Electromechanical (a) and mechanical (b) characteristics of the sequential excitation motor

Fig. 3.15. Electromechanical (a) and mechanical (b) characteristics of the sequential excitation motor

Analysis of equations (3.33) and (3.36) shows that the electromechanical and mechanical characteristics, although they are hyperbolic in nature at low loads, do not coincide even constructed in relative units.

The peculiarity of the electromechanical and mechanical characteristics of the sequential excitation motor is that:

• they are nonlinear at low loads and become practically linear at loads greater than the nominal ones

• Theoretically, they do not have the ideal idle speed. In practice, due to the residual magnetizing flux the ideal idle speed exists and is determined by the equation

(3.38)

however, it is large enough, so series thrust motors can not be turned on without load to avoid their destruction from centrifugal forces.

Equations (3.33), (3.34) and (3.36), (3.37) do not allow to calculate the static electromechanical and mechanical characteristics of the DC motor of series excitation, since there is no information for determining the design coefficient k motor and proportional coefficient to n between the current and the flux of the motor magnetization curve.

In most practical cases, universal characteristics are used to calculate real natural electromechanical and mechanical characteristics. The universal characteristics of the series excitation engines MP, DP and D up to 10 kW are shown in Fig. 3.16. They represent the dependence of the relative values ​​of the velocity and the moment of the relative armature winding current , where - the nominal values ​​of speed, torque and armature current.

Universal characteristics of series excitation engines MP, DP and D

Fig. 3.16. Universal characteristics of series excitation engines of MP types. DP and D

In the sequential field drive catalog data, the following parameters are usually given:

- rated motor power, kW;

- rated speed, rpm;

- the nominal voltage of the armature winding, V;

- rated armature current, A;

- the nominal efficiency, about. e.

The procedure for calculating the natural electromechanical and mechanical characteristics of a sequential excitation motor is as follows:

• The nominal angular velocity is determined

• The rated torque on the motor shaft is determined

• an arbitrary relative value of the current is set, and is plotted on the graph of the universal characteristic of the serial excitation motor;

• the corresponding current , the relative values ​​of the angular velocity and the moment ;

• Relative values ​​of current, speed and torque are converted to absolute values ​​

• the right-handed decarpic coordinate system is constructed for electromechanical and mechanical characteristics in absolute units;

• the absolute values ​​of the speed and torque current are stored in

corresponding to the coordinate system and ;

• Similar calculations are performed for other current values ​​

• the natural electromechanical and mechanical characteristics of the sequential excitation motor are constructed from the deferred points.

thematic pictures

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