PROCEEDINGS
OF THE TOMSK POLYTECHNIC INSTITUTE
NAMED AFTER S. M. KIROV

Volume 262
1973

TWO-POSITION CONTROL SYSTEM FOR THE STARTING CURRENT OF SEPARATELY EXCITED DC MOTORS

R. K. Gachik, N. V. Agapitova, V. M. Zhmurovskii

(Presented by the scientific-technical seminar of the Scientific Research Institute of AEM)

Two-position control of the armature current of separately excited motors, despite its known advantages (speed of response, simplicity, etc.), is hindered by the low inertia of the armature circuits of these motors. The use of smoothing reactors to reduce pulsations of the armature current is undesirable, since it reduces the speed of response of the control systems and increases their dimensions. This article describes a two-position control system built on low-inertia elements, in which the necessary delays are reduced to a minimum; this made it possible, without using smoothing reactors, to obtain acceptable pulsations of the armature current over a wide range of its regulation.

The operating principle of the system is explained by the block diagram in Fig. 1 and by the timing diagrams in Fig. 2, where:

DT — armature-current sensor;
GPN — sawtooth-voltage generator;
T — trigger;
RG — relaxation generator;
CC — comparison circuit;
TU\(_1\) and TU\(_2\) — thyristor-capacitor pulse amplifiers;
TK — power thyristor switch with a forced-commutation unit;
B\(_1\) — power valve of TK;
B\(_2\) — commutating valve of TK;
\(C_k\) — commutating capacitance of TK;
\(I_a\) — armature current;
\(U_1, U_2, U_3, U_4, U_5\) — output voltages of DT, GPN, CC, T, and RG, respectively;
\(U_z\) — reference voltage proportional to the prescribed average value of the armature current.

In the control system a current sensor DT is used, for which the duration of the period \(T_0\) of the output voltage \(U_1\) is inversely proportional to the magnitude of the controlled current [1]; moreover, when the current changes from zero to the maximum, \(T_0\) changes from 300 to 100 microseconds. The GPN is used with stabilization of the charging current \(I_c\) of the capacitor, and the linear part of the sawtooth voltage is formed during a half-

…of the positive half-period \(U_1\). The amplitude of the sawtooth voltage \(U_2\) is determined by

\[ U_2=\frac{T_0}{2C} I_c=\frac{k}{I_{\mathrm{a}}}, \tag{1} \]

where \(C\) is the capacitance of the SVG.

If \(U_2\) reaches the value \(U_{\mathrm{s}}\) (as in the left-hand part of the timing diagrams), a short rectangular pulse \(U_3\) is formed at the output of the CC; through \(TU_1\) it triggers the power valve \(V_1\). At the beginning of the positive half-period \(U_1\), the trigger \(T\) is set to a state that permits operation of the relaxation generator \(RG\), but it can operate only during the negative half-period of \(U_1\). The pulse from the CC switches the trigger to a state that inhibits operation of the \(RG\); therefore, if during the positive half-period \(U_1\) there was a pulse \(U_3\), then during this period of \(U_1\) there will be no pulse at the output of the \(RG\). In this case the power valve will be open, and the current in the armature circuit will increase.

Fig. 1

Fig. 2

Fig. 3.

As the armature current increases, the duration \(T_0\) and the value \(U_2\) decrease. When \(U_2\) becomes less than \(U_{\text{e}}\), in the positive half-period of \(U_1\) there will be no pulse \(U_3\) (the right-hand part of the timing diagrams); flip-flop \(T\) will remain in the state permitting operation of PG, and with a certain delay \(\tau_3\) relative to the beginning of the negative half-period of \(U_1\), PG will issue a pulse which, through \(TU_2\), will start the commutating valve \(V_2\). This will lead to the switching-off of \(V_1\) and to a decrease in the armature current. Subsequently these processes will repeat. As can be seen from the timing diagrams, the largest delay introduced by the thyristor-switch control system will be equal to the sum of the duration of the period \(T_0\) and \(\tau_3\), which, for the selected current sensor, is \(330\ \mu\text{s}\). It should be noted that as the armature current increases the delay decreases and, at maximum current, is \(130\ \mu\text{s}\).

The schematic diagram of the entire system is shown in Fig. 3. To increase the reliability of the forced-commutation unit, connections from the thyristor switch to the thyristor amplifiers \(TU_1\) and \(TU_2\) have been introduced into the system. These are arranged according to a circuit that prevents their firing while the storage capacitors are being charged; the supply for the thyristor amplifiers is taken from the commutating capacitance \(C_k\). Thus, when the processes of charging or recharging \(C_k\) are under way, the possibility of forming control pulses for the thyristors of the thyristor switch is excluded, which substantially increases the reliability of the forced-commutation unit. Such connection of the thyristor amplifiers requires an increase in \(C_k\) by 5–7 percent, but introduces into the control system the smallest delay for the selected type of thyristor switch.

The application of the technical solutions described above made it possible to construct a starting-current control system for series P motors with separate excitation, of sizes 4 to 8, without the use of smoothing reactors in the armature circuit; moreover, the armature-current pulsations do not exceed 20 percent of the average value when the starting current is varied from \(I_n\) to \(2.5 I_n\).

Conclusions

1. Connecting the output amplifiers of the control pulses to the commutating capacitance significantly increases the reliability of the forced-commutation unit.

2. The use of fast-acting pulse elements in two-position current-control systems for separately excited motors makes it possible to obtain acceptable armature-current pulsations without the use of smoothing reactors.

References

V. V. Bolotov and A. P. Zaitsev. DC-to-AC converter. Author’s certificate No. 311354, Bulletin, No. 24, 1971.