JPS5927194B2 - DC motor control device - Google Patents

Description

DETAILED DESCRIPTION OF THE INVENTION The present invention relates to an improvement in a speed control device for a DC motor, and particularly to a speed control device that allows current to flow in only one direction through the armature and in both directions through the field.

When controlling a DC motor, there is a so-called stationary Leonard device in which two sets of controllable large-capacity rectifier power supplies are connected in antiparallel to the armature circuit, and the field is controlled in one direction, but the armature power supply device is expensive. There are drawbacks to it. Therefore, a circuit as shown in FIG. 1 is known as a method of controlling the rotation speed by controlling the armature current of a DC motor in one direction and changing the magnitude and polarity of the field current. The operation of this circuit device is as follows: First, a speed command is input to the command terminal 1, and a field current control device 2 consisting of a positive thyristor device 21, a negative thyristor device 22, an amplifying phase shifter 23, and a field current detector 24 is input. A magnetic command is given to cause field current to flow through the field winding 3 in both positive and negative directions.

On the other hand, an armature current command is given from the command terminal 4 to an armature current control device 5 consisting of a thyristor device 51, an amplifying phase shifter 52, and an armature current detector 53 to cause the armature current to flow in one direction. As a result, torque is generated, the armature 6 rotates, and the output of the speed generator T is fed back to control the rotational speed to match the command. In this case, in order to obtain good speed control performance, the response of the field current must be quick, so the field current is detected by the field current detector 24 and negatively fed back to the amplification phase shifter 23.

In this type of speed control system, the rate of change in armature voltage is extremely large compared to the commonly used method in which the field current is held constant and the armature current is controlled to control torque. .

Since a change in armature voltage causes a disturbance in the constant current control system, the armature current changes steadily and transiently from the commanded value. When the armature current changes from the command value, the torque naturally also changes from the command value, resulting in poor speed control performance. Steady changes in the armature current can be reduced to a sufficiently small value by positive feedback of the armature voltage, so the problem is rather transient changes in the armature current, but this is because the rate of voltage change is Since the larger the value, the larger the value becomes, it is necessary to significantly improve the characteristics especially in this control method in which speed control is performed by changing the magnitude and polarity of the field current. On the other hand, in a method in which positive and negative torques are controlled while switching the field current to the positive or negative rated value, the armature voltage changes faster than in this control method, but in this case, the field current The switching period is considered an out-of-control period, and since the armature current is typically reduced to a relatively low level, even if the armature current changes transiently from the commanded value, However, it can be said that the effect on speed control performance is small.

The present invention is a system in which the armature current is controlled in one direction and the rotational speed is controlled by changing the magnitude and polarity of the field current. The purpose is to obtain better speed control performance by making the change as small as possible.

Figure 2 shows a block diagram of the armature current control system.

The armature current index +1ca is compared with the armature current 1a fed back through the feedback circuit constant KF and is applied to the input of a thyristor amplifier with a gain KPl time constant Taa (S) and a dead time τ (S), and the output voltage of the thyristor is The back emf of the motor is subtracted and added to the impedance formed by the motor's resistance R1 and inductance L. In this case, the transfer function to the armature current variation Δ1a with respect to the motor back electromotive force Em, which is a disturbance, is as shown in Fig. 3a,
Assuming that the electrical time constant of the armature is Te, this can be simplified as shown in FIG. 3b.

From this, ignoring the dead time τ, the loop gain H is K
Using the fact that p-KF/R is about 100, an approximate solution for the armature current change Δ1a is found as follows. It becomes Hrl.

In this equation, an informal expression up to the first-order term of S is found as follows. Here, H is 100 or more, Taa is the number S, T
Since e is about 0.1S and τ is about 5mS, this equation can be simplified as follows.

In this equation (1), the steady component 1/HR can be made sufficiently small by taking measures such as negative feedback of the armature voltage, so if we do not consider it, the armature current affected by the change in armature voltage ΔEm The transient component of Δ1a is 11
It is H.

Here, the resistance R of the armature circuit cannot be changed, and (1-=)
Since H is about 100, it is sufficiently close to 1. Therefore, it can be seen that in order to reduce the transient change in the armature current, Taa/H should be made as small as possible. However, as is clear from the fact that the open-loop transfer function G in FIG.
Therefore, even if the time constant Taa of the thyristor amplifier is decreased or the loop gain H is increased, the intersection angle frequency on the Bode diagram will increase and the phase margin will decrease. , the system becomes unstable and the armature current becomes oscillatory.

Therefore, it is necessary to employ a braking circuit that does not vibrate even if Taa/H is reduced. The present invention provides this braking circuit with negative feedback of the differential value of the armature current.

Negative feedback of the differential value of the armature current increases the phase margin, and the system does not become unstable even if Taa/H is increased. A speed control circuit for a DC motor according to the present invention is shown in FIG.
The armature current is detected by a detector 53, differentiated by a capacitor 54, and negatively fed back to the amplifying phase shifter 52. The transfer function when negative feedback is given to the differential value of the armature current with a constant KD is expressed as
As shown in the figure. The amount of change Δ1a in the armature current in this case is as follows. In other words, the transient component of the armature current is 1 (1-HR KD/KF-1/H) TaaS.

When the differential value of the current is negatively fed back in this way, the loop gain H
It is possible to increase the time constant Taa and decrease the time constant Taa, and at the same time, if K and ball F approach 1, (1-KD/K
Since F-1/H) can be made sufficiently small, transient changes in armature current due to changes in armature voltage can be greatly reduced, and speed control performance can also be greatly improved, making it possible to improve industrial performance. The effect is large.

FIG. 6 shows another embodiment of the invention.

A differential current transformer 55 is used instead of a capacitor to detect the differential value of the armature current. By doing so, more complete differential values can be obtained. Another embodiment of the invention is shown in FIG.

In Fig. 4, the armature current was controlled regardless of the difference between the speed and the speed command, but in this method, the difference between the speed and the speed command is determined by a preamplifier consisting of an amplifier 85 and resistors 81, 82, and 84. By applying this output to the function generators 91 and 92, the armature current is kept constant in a range where the required torque is small, and the field current is controlled according to the difference between the speed and the speed command, and when the required torque is large, the field current is The current is kept constant at a positive or negative rated value and the armature current is controlled according to the difference between the speed and the speed command. In this way, the armature current is increased only when torque is required, so that the heat generation and power consumption of the armature can be reduced.

The present invention is also effective in such a circuit.

Furthermore, by making the characteristics of the function generator 92 in FIG. 7 such that the absolute value of the input is proportional to the output,
The present invention is also effective in a control system that makes the armature current proportional to the absolute value of the field current in a small torque region.

[Brief explanation of the drawing]

Figure 1 is a circuit diagram of a conventional speed control device for a DC motor, Figure 2 is a block diagram of its armature current control system, and Figure 3 is a diagram of a transfer function from armature voltage to armature current fluctuation. Fig. 4 is a circuit diagram showing one embodiment of the present invention;
The figure is a block diagram of the armature current control system, and FIGS. 6 and 7 are circuit diagrams showing other embodiments of the present invention. Explanation of symbols, 1...Command terminal, 2...
Field current control device, 3...Field winding, 4...
... Command terminal, 5 ... Armature current control device,
6... Armature, 7... Speed generator, 8
......Preamplifier, 91...Function generator, 92...Function generator, Lca...Armature current command, Em...・Motor voltage, 1a...
... Armature current, Kp ... Gain, Taa
... Time constant, τ ... Det time, R.
...Resistance, L...Inductance, KF
...Feedback circuit constant, Te...Electrical time constant, 54...Capacitor, KD...
Differential feedback constant, 55... Differential current transformer.

Claims (1)

[Claims] 1 Equipped with a device that controls the armature current of a DC motor in one direction and a device that controls the field current in both positive and negative directions, the magnitude of the field current is controlled according to the difference between the speed and the speed command, and the armature In those that have a section where the current is controlled at a constant level,
A control device for a DC motor, comprising means for differentiating the armature current, and means for negatively feeding back the differential value of the armature current to the armature current control device.