Relay-type servo electric drive with a series-excited motor
Abstract Generated abstract
This paper examines a relay pulse automatic position control system using a low power universal commutator motor with series excitation, motivated by the need for faster and simpler regulated electric drives. The authors describe the system structure, in which proportional control is corrected by an output derivative signal from a tachogenerator, while transistor blocking oscillator comparators and thyristors switch single half period current pulses to the motor. Experimental study indicates that the drive responds faster than an analogous system with a separately excited motor, with a transient process approximating a trapezoidal form and an error voltage of about 40 to 60 millivolts under critical damping. The system is presented as simple, accurate, relatively insensitive to parameter settings, and suitable for tracking drives up to several hundred watts.
Full Text
A Relay-Action Servo Drive with a Series-Excited Motor
V. A. BEINAROVICH, A. E. ALEKHIN
(Recommended by the scientific seminar of the Electromechanical Faculty)
The development of automation technology for industrial installations makes it necessary to improve systems for regulating electric drives in the direction of increasing speed of response and improving the quality of regulation. Relay automatic-control systems, in comparison with continuous-control systems, have a number of advantages, the main ones being simplicity of construction, low weight and low cost, and the possibility of achieving a minimum regulation time by simple means.
Among the large number of relay systems, the systems of electric-drive control in which an optimal form of the transient process is achieved through the introduction of nonlinear links are of greatest interest. It is evident that, in developing such systems, one should proceed not only along the path of using nonlinear links, but also make maximum use of the capabilities of all elements of the system.
The principal element determining the speed of response of a system is the motor. With an increase in the starting and maximum permissible torques of the motor, the speed of response of the system increases, since the greatest accelerations can be obtained. As is known, series-excited motors possess the greatest starting torque. However, in automatic-control systems shunt motors have until now been used most often. Therefore, a comparative study of electric drives with relay automatic control and with separately excited and series-excited motors is of great interest.
In the present article, a relay-pulse system of automatic position control with a universal commutator motor of low power, developed at the EPA department, is considered. The functional diagram of the system is shown in Fig. 1, and the schematic circuit in Fig. 2. In terms of formation of the control signal, the system belongs to proportional-regulation systems with correction by the first derivative of the output quantity.
The control signal is formed from the voltages taken from the command potentiometer $R_z$, the follower potentiometer $R_{oc}$, and the poten-
of the potentiometer \(R_{\mathrm{n}}\), supplied from the correcting device—the tachogenerator 11. The control device contains two transistor regenerative comparators on blocking oscillators. The role of the relay element is performed by two silicon controlled diodes \(T_1\) and \(T_2\).
Fig. 1.
To provide reversal, an actuator motor of universal supply with a split excitation winding is used.
The principle of operation of the system is as follows. In the balanced position the control signal is absent; both comparators are locked by the bias voltage taken from potentiometers \(P_1\) and \(P_2\); thyristors \(T_1\) and \(T_2\) are not excited and the motor \(D\) receives no power. When a mismatch appears, a dc control signal of one polarity or the other excites one of the comparators and “locks” the other. From the excited blocking oscillator, a voltage is fed to the control electrode of the power thyristor; the thyristor opens on the positive half-wave of the supply voltage, and the motor receives power and eliminates the mismatch. Diodes \(D_1\) and \(D_2\) serve to shunt the self-induction emf. Diode \(D_9\) is used as a fast-acting fuse to protect thyristors \(T_1\) and \(T_2\) from overload; its presence is not obligatory.
Fig. 2.
The distinctive features of the system developed are the supply of the motor by single-half-period current pulses at the line frequency, high sensitivity to the control signal, and the possibility of adjusting the relay characteristic by changing the dead zone by means of potentiometers $P_1$ and $P_2$. Studies have shown that the speed of response of the system is always greater than for an analogous system with a separately excited motor. The transient process is approximated by a trapezoidal graph (Fig. 3).
However, the magnitude of the current and acceleration during starting is always smaller than during counter-current braking, owing to the constant parameters of the motor circuit, the unchanged supply voltage, and the absence of adjustment of the firing angle of the thyristors. The advantages of the system are simplicity, high accuracy, and the noncritical nature of the parameters and settings.
Fig. 3.
The error voltage has a magnitude on the order of 40–60 millivolts under critical damping.
The developed system may be recommended for a variety of tracking drives with power up to several hundred watts.
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