DEVELOPMENT OF CERTAIN ELEMENTS OF RELAY CONTROLLERS

V. V. BOLOTOV, L. V. VOLYNETS, R. K. GACHIK, A. P. ZAITSEV, Yu. F. MIKHEEV

(Presented by the scientific-technical seminar of the Scientific Research Institute of EEM at TPI)

The principal elements determining the magnitude of current pulsations and the quality of the control process in relay ACS are the current sensor and the comparison device. The electromagnetic inertia and delay of the sensor, if the latter is a pulse device, as well as the width of the hysteresis loop of the characteristic of the comparison device, determine the magnitude of the pulsation of the controlled current. Current sensors and comparison devices produced serially and known in practice, successfully used in continuous and pulse systems, cannot always be used in relay-action systems because of the poor values of the above-mentioned indicators. In order for the pulsations of the controlled current to be insignificant, the comparison devices must be inertia-free and have a narrow loop of the output characteristic.

The principal requirements imposed on current sensors of relay ACS are:

1) maximum speed of response and minimum delay;
2) simplicity of the schematic circuit;
3) stability of the sensor transfer coefficient under temperature fluctuations and other external disturbances.

When regulating current in high-current circuits with reversal, a number of additional requirements arise for the sensors:

1) insensitivity of the sensor to the direction of current flow;
2) galvanic isolation of the sensor from the power circuit.

In addition, when using a sensor in circuits having many operating modes alternating in a definite sequence as a function of the current, the sensor must:

1) have several galvanically isolated outputs;
2) permit control by the sum or difference of the currents of several mutually unconnected circuits.

Of several variants of current sensors developed at the Scientific Research Institute of EEM, the sensor based on a multivibrator with magnetic coupling most fully satisfies the stated requirements (Fig. 1). A distinctive feature of the device is the use, in the multivibrator transformer, of a toroidal ferromagnetic core with an air gap. The power cable whose current is being monitored is passed through the window of the core. In the absence of current in the power cable, the sensor has an initial switching frequency. As the monitored current increases, the switching frequency increases. The output alternating voltage of rectangular form, taken from a separate winding, is fed to the comparison device. If an amplitude selector is used as the comparison device, then the output of the sensor is supplemented with an integrat-

integrating RC circuit, which converts an alternating voltage of rectangular form and variable frequency into an alternating voltage of sawtooth form and with an amplitude varying inversely proportional to the frequency. The high switching frequency and the mode of forced magnetization of the core by the power current make the sensor practically inertialess.

Fig. 1.

Several selectors of the frequency of an alternating voltage of rectangular form have been developed for use with such a sensor; they serve as comparison devices in relay automatic current-control systems.

Figure 2 shows a frequency selector based on a blocking oscillator. The input voltage of the current sensor controls the operation of transistor switch 1. The timing capacitor 2 of the blocking oscillator is connected by its left plate directly to the emitter of transistor 3. Under the action of the negative half-wave of the input voltage, the transistor switch is in saturation mode, and the timing capacitor, during a short interval of time, is charged from the source \(E_k\) through resistor 4 almost up to the source voltage. With the onset of the positive half-wave, the capacitor is recharged from the source through resistor 5, the secondary winding of the transformer, and the automatic-bias circuit.

In this case, the voltage on the capacitor is applied with its “positive” terminal to the base of transistor 3, and the latter is closed. The time constant for recharging the timing capacitor is set, by appropriate selection of resistor 5, to be large in comparison with the duration of the voltage half-period.

Fig. 3.

Capacitor 2 tends to recharge to a certain voltage of opposite polarity according to an exponential law, the initial portion of which is almost linear. If the frequency of the input voltage is greater than a certain specified value, then during the positive half-period capacitor 2 will not have time to discharge to zero voltage and the blocking oscillator will not generate. When the frequency of the input voltage deviates downward, which will correspond to a decrease in the current in the controlled circuit, the blocking oscillator will generate output voltage pulses at the frequency of the input voltage. In this case the duration of the output pulses does not depend on the frequency of the input voltage, but is determined by the parameters of the blocking oscillator.

Figure 3 shows the circuit of a selector for an alternating voltage of rectangular waveform, in which the duration of the output pulses is determined by the magnitude of the deviation of the half-period of the input voltage from the specified value and is equal to this deviation.

Fig. 4.

The device consists of an input circuit on transistor 1, an integrating RC network on resistor 5 and capacitor 2, a threshold device on transistor 4 and Zener diode 3. As in the previous case, the input voltage controls the operation of transistor switch 1. When transistor 1 is closed, a sawtooth voltage is formed on capacitor 2. If the duration of the half-period of the input voltage is greater than the specified value, then at the moment when the voltage on the capacitor reaches the value of the voltage

after breakdown of Zener diode 3, transistor 4 will open and will remain in the open state until the end of the half-period. The output voltage pulse is taken from the collector of transistor 4. During the negative half-wave of the input voltage, transistor 1 opens and rapidly discharges the capacitor, after which the device is again ready for operation. Increasing the amplitude of the input voltage, over wide limits above a certain minimum value, does not lead to a disturbance of the device operation. The temperature stability of the circuit when silicon transistors are used is sufficiently high and is determined mainly by the temperature stability of the Zener diode.

In the case where the current sensor at the output has a signal in the form of a constant voltage, it is advisable to use a comparison device based on a regenerative comparator (Fig. 4). To reduce the width of the hysteresis loop of the output characteristic, a transistor switch 1, opened by short periodic pulses \(U_3\) from a separate blocking oscillator, is introduced into the positive-feedback winding circuit of the comparator. As a result of interrupting the positive-feedback circuit for a time equal to the period between pulses, the comparator is forcibly switched off, thereby eliminating the influence of the hysteresis of the characteristic. The frequency of the periodic pulses must be sufficiently high in comparison with the ripple frequency of the regulated current.

Fig. 5.

If the electrical pulses of the comparison device are used, for example, to control thyristors and must be powerful, then it is advisable to use, as the comparison device, a comparator whose circuit is shown in Fig. 5. To eliminate the influence of hysteresis of the output characteristic, it can be supplemented by a transistor switch. A feature of this comparator is the exclusion of the negative-feedback winding and the presence of an additional positive-feedback winding. In one device, a comparator and a blocking shaper are combined. Owing to the additional positive-feedback winding, the output pulses are shaped in duration and are comparatively powerful if the device is built with a sufficiently powerful transistor.