# Modes of operation of the DCT of independent excitation - Electric drive

## Independent excitation DCS modes

In Fig. 4.4 shows the scheme of inclusion of the DCT of independent excitation, in the armature circuit of which an additional resistance (resistor) is switched on and a resistance R is used to change the excitation current. The impedance of the anchor chain

where - resistance of the armature winding; -resistance of the winding of additional poles; - resistance of the compensation winding; - Bridging contact resistance.

Fig. 4.4. The scheme of inclusion of the DPT HB

Equations of mechanical and electromechanical characteristics

(4.15)

(4.16)

are derived under the following assumptions: the armature reaction is not taken into account, and the torque on the shaft is equal to the electromagnetic moment.

In Fig. 4.5 presents the electromechanical and mechanical characteristics of the DCT of independent excitation for different polarity of the supply voltage and its absence with a constant excitation current (F = const)

In this case, at a certain scale, the mechanical and electromechanical characteristics are the same.

Analysis of equation (4.15) and (4.16) allows you to specify ways of adjusting the coordinates (co, I, M, of the electric drive:

a) by changing the resistance R d in the anchor chain;

b) the change in the excitation current and, as a consequence, the magnetic flux Φ;

c) a change in the voltage applied to the armature.

Fig. 4.5. The electromechanical and mechanical characteristics of DP'G HB

In detail, these methods, as well as their derivatives, will be considered later.

Let's consider the modes of operation of the DCT of independent excitation at various sites of its characteristics.

1. Idle speed mode DPT. The point A or/Г for different polarity of the voltage applied to the armature is characterized by the following parameters:/= 0, M = 0, with = ± co. The engine does not receive electricity from the network, except for the excitation and on friction, and does not give up mechanical energy.

2. The motor mode takes place in the AB section (forward rotation), or A 'B' (backward rotation), where the speed with and the moment M match in the direction. In this mode | 0 | & gt; | & pound; |; 0 & lt; & lt; U and is directed against EMF E, the electric energy comes from the network, is given from the motor shaft to the working machine.

3. Short-circuit mode DPT. The In or In ' on the characteristics (Figure 4.5) has the following parameters: The electrical energy consumed from the grid is dissipated as heat at the resistances in the armature circuit. Mechanical energy is not given to the working machine.

4. The generator mode of operation of the DCT in parallel with the network, which is called the regenerative braking regime, takes place in the second and fourth quadrants of mechanical characteristics. In this case, the speed with is greater than the idling speed of the units for both forward and backward rotation. EMF E becomes more voltage applied to the anchor,/and M change their direction to the opposite. The engine receives mechanical energy from the working machine and gives it (recovers) it to the grid in the form of electric energy generated by the generator.

5. Generator mode in series with the network, which is called the anti-induction braking mode, occurs when with & lt; O (section VS or VS). In this mode, the EMF/(coincides in direction with voltage and DPT is connected in series with the network,/coincides in direction with E and U and is determined

Electric energy comes from the grid, and is also generated by the DPT generator operating in the generator mode and dissipated as heat in the anchor circuit resistance. This mode is the heaviest, because a large current flows through the armature circuit of the DCT.

6. The mode of the generator independent of the network, which is called the dynamic braking mode, occurs when the anchor chain is disconnected from the network (// = 0) and short-circuited or to an additional resistance. The current of the armature/flows under the action of EMF E and coincides in the direction. Electric energy is produced by converting mechanical energy from a shaft connected to a working machine and dissipated, transforming into thermal energy in the resistance of the anchor chain. On the characteristics of this mode is characterized by a straight line passing through the origin.

When starting the DCT, the starting current exceeds the nominal current by a factor of 10-50, since when operating on a natural characteristic at the time of starting the DCT

, then

Limitations to the permissible are carried out by introducing additional starting resistors R, (starting rheostats). The quantity La is determined by

(4.17)

In Fig. 4.6 shows the natural characteristic of the DCT of the independent excitation (1) and the artificial (2) obtained upon inclusion of the LD. At start-up, as the speed increases, E increases and the armature current of the characteristic decreases (2). When the current is reached, it switches to the natural characteristic (1).

Fig. 4.6. The characteristic of the DCT-independent excitation (I) is the artificial (2) obtained by turning on R ,

With dynamic braking, when the DCT armature circuit is disconnected from the mains and closed to the additional resistance R, 2, the braking process proceeds according to characteristic (3) until the motor stops. The quantity A ;; is determined by

(4.18)

Since s, " is close to ak the value of the EMF E is close to voltage.

When the motor is reversed or braked, the polarity of the armature voltage is reversed by reversing the polarity of the armature voltage, and the resistor is connected to the armature circuit simultaneously. The motor goes to characteristic (4). The value of I L1 is determined

(4.19)

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