CC BY
5УДК 621.3
Sultanov R.Z.
teacher
Azerbaijan State Oil and Industry University (Baku, Azerbaijan)
ANALYSIS OF SCHEMES OF A CURRENT LIMITING UNIT IN AUTOMATIC CONTROL SYSTEMS FOR DC ELECTRIC DRIVES
Abstract: typically, traction and lifting mechanisms operate in a static overload mode -frequent starting, electric braking and speed control in a wide range. The most suitable electric drive for these mechanisms is considered to be a direct current electric drive with thyristor automatic control. Frequency control of asynchronous electric drives finds less application due to the complexity of control. The article considers exemplary electromechanical characteristics for electric drives with existing schemes of current limiting units for various values of the driving speed. Also in this article, a control circuit for a DC electric drive with a new current limiting unit is drawn up. The advantages and disadvantages of this scheme are considered.
Keywords: electric drive, control, current limiting, speed, overload.
In the event of overloads, as well as in transient processes of starting, braking and reversing, the current of the armature circuit of the electric motor increases. If the value of the armature current becomes higher than the permissible values, then the current limiting unit starts to operate, a negative feedback signal is generated, the voltage of the armature circuit decreases, the speed of rotation decreases and thus the increase in the current of the drive armature is limited. In various existing circuits of the current limiting unit, the beginning of the negative feedback signal is taken when the current value of the armature current exceeds the cutoff current value. The value of the cutoff current is selected depending on the operating mode of the electric drive. In most cases, the current limiting value is determined by the degree of deterioration of switching on the collector, i.e. according to the maximum permissible current at the
rated rotation speed for a specific series of the drive motor. For various values of the driving speed, the approximate electromechanical characteristics for electric drives with existing circuits of the current limiting unit have the form shown in fig. 1, a. [1].
Fig.1. Electromechanical characteristics of the drive: a) with the existing current limiting unit; b) with a new current limiting unit
As can be seen from the characteristics of Fig. 1, and the negative feedback signal of the existing circuits of current limiting units, regardless of the value of the driving speed, occurs at the same cutoff current value, since Ic = const. At the same time, it is known that the maximum - permissible value of the armature current, which worsens the switching process for collector motors, depends on the current speed engine. This phenomenon is caused by a change in the reactive e.m.f. armature winding of the electric motor when changing the speed of rotation. With a decrease in the rotation speed, the reactive e.m.f. decreases. armature winding, commutation improves and, accordingly, the maximum permissible value of the motor armature current increases. In many cases, to improve the traction characteristics, as well as the performance of the electric drive, it is advisable to use the maximum overload capacity of the drive motor [2]. Therefore, in order to fully utilize the maximum permissible overload capacity of a DC motor at all speeds, it is necessary to increase the cut-off current value when the rotation speed decreases. However, as can be seen from Fig. 1a, the goal cannot be achieved by using one of the existing circuits of the current limiting unit in the electric drive, since the cut-off current remains constant. Hence it becomes obvious that these current limiting circuits cannot provide full use of the
maximum permissible overload capacity of the drive motor at given reduced operating speeds. This drawback becomes even more significant if the controlled electric drive operates in a short-term mode with maximum overloads, in which there is no possibility of increasing the installed power of the electric drive. To achieve the above goal, because for full use of the maximum permissible overload capacity of the drive motor at all given rotation speeds, it is required to draw up such a current limiting circuit, with the help of which it would be possible to obtain electromechanical characteristics, as approximately indicated in Fig. 1.b. That is, when changing the speed, the value of the cutoff current would not remain constant, but would automatically increase with a decrease in the set rotation speed (according to Fig. 1, b., for example: Ic1 < Ic2 < Ic3 at the values of the given speed > &d2 > A simplified control circuit for a DC electric drive that meets the above requirements, using the new current limiting unit proposed by the author of this article [3], is shown in fig. 2.
Fig. 2. Simplified DC drive control circuit with a new current limiting unit:
A distinctive part of this circuit from the existing ones is that the voltage from the control signal of the setting speed Dd is additionally introduced into the circuit of the current limiting unit. The polarity of the additionally introduced signal is taken according to the polarity of the signal ui taken from the resistance of the shunt Rs included in the motor armature circuit (11,=I Rs). Note that in this case the value the reference voltage Ub existing in the current cutoff circuit must be increased by the value of the additionally introduced voltage ufrom the driving speed. When, for operation at speeds less than the nominal one, the operator lowers the setting signal ui, at the same
time automatically increases the comparison voltage, which leads to a corresponding increase in the cut-off current value and thereby automatically ensures full use of the increasing maximum permissible overload capacity of the drive motor at reduced speeds. It should be noted that the stall current remains constant Ii = const (see Fig.1, b).
(Ub - ud)
kc(uT.) Rc + Rm T
и = —,--ud-----I -
kc(uz)Rs
I-
Rs
D
k • < k • < k • < Note that usually for DC drives, the stop current is determined by the short-time allowable current of the motor when the armature is stationary, which is a constant value. If from equation (1), setting D=0, we determine the stop current, we will see that:
kc(ux)Ub Is t = —---———¡— = c onst
A family of static velocity characteristics constructed according to the equation (1) for different values of the driving signal ud, completely coincide with the approximate characteristics shown in Fig. 1b. Laboratory tests of the model of the new circuit of the current limiting unit gave identical electromechanical characteristics [4]. It should be noted that the slope of the steeply dipping part of the electromechanical characteristic is selected according to the requirements of a particular operating mechanism. For example, for electric drives of metalworking machines, the steeply dipping section of the electromechanical characteristic is adjusted to an almost vertical shape. And for electric traction drives, for example, for the main drive of a rolling mill, for a walking excavator, etc., this section is somewhat flat. The lead operator, who has noticed a decrease in the speed of the feed mechanism, by acting on the setting signal, reduces the feed, prevents further undesirable speed reduction and thereby keeps the performance of the mechanism at the highest possible level.
Conclusion. The use of the proposed current limiting unit in the circuits of automated electric drives of such mechanisms can give tangibly useful results in terms
of increasing their traction capabilities and performance, through the beneficial use of the maximum permissible overload capabilities of the drive motor.
REFERENCES:
1. Tolkunov V.P. Theory and practice of switching DC machines. - M .: Energy, 1979, p. 6-61.
2. Sultanov R.Z. Implementation of the maximum permissible overload capacity of a DC motor. - Izv. Universities of the USSR. Electromechanics 1981, No. 9, p. 1045 - 1049.
3. Sultanov R.Z. Current limiting system with automatic cutoff current control. -Izv. Universities of the USSR. Electromechanics 1985, no. 8, p. 97 - 100.
4. R. Z. Sultanov, G. M. Safarov, E. M. Farkhadzade, and S. D. Osmanov. Implementation of a current limiting unit circuit with automatic change in allowable currents from the value of a given motor speed. Proceedings of the first international conference "Technical and Physical Problems of Power Engineering". - Baku, 2002, p. 235-237.
