Automatic Control System for the Regenerative Current of a Traction Electric Motor

A. P. Zaitsev, D. M. Kuznetsov, V. V. Bolotov

(Presented by the scientific and technical seminar of the Research Institute of Electrical Machines)

Questions of improving the reliability of control systems determine their application to no lesser degree than the development of new systems with other advantages. Relay automatic control systems are, in principle, more reliable; because of their simplicity they have found wide application in aircraft, and at present are beginning to be used in ground and rail transport.

Below, a relay automatic control system for regenerative braking of a traction motor is considered. Its distinguishing feature consists in the use of blocking against commutation failures of the thyristor converter and of a time delay between pulses when it is opened and closed.

In Fig. 1 the following designations are adopted:

TP — thyristor converter;

Fig. 1

DT — armature-current sensor;
CC — comparison circuit (comparator);
“NE” — inversion circuit;
Tr — trigger;
BG — blocking generator;
BF — blocking shaper;

The magnitude of the regenerative current is set by a reference, which is a standard voltage. The latter is compared with a sawtooth voltage whose amplitude is inversely proportional to the regenerative current.

The beginning of braking may be considered the mode in which the armature current is equal to zero, since, in the transition from traction mode to regenerative mode, the ends of the motor field winding must be switched over.

Fig. 2

In this case the frequency of the current sensor DT will be minimal, and the amplitude of the sawtooth voltage will be maximal, exceeding the standard voltage.

The comparator (CC) will generate pulses, and the power thyristors TP will open. When TP opens, the armature current will reach a valـ

...at which the amplitude of the sawtooth voltage becomes equal to or less than the reference value. The comparator is switched off, and the blocking oscillator is switched on (the “NOT” circuit); control pulses \(U_g\) are applied to the turn-off thyristors. In this case the thyristor converter closes, and, under the action of the self-induction emf, the recuperation current passes through diode \(D\) into the network. Subsequently, as the armature current decreases, the amplitude of the sawtooth voltage again exceeds the reference value, the comparator operates, the thyristor converter opens, and the process is repeated. To implement blocking and time delay, a flip-flop and two blocking oscillators operating in a self-oscillating mode are used. Cases are possible in which the pulses of the comparison circuit \(CC\) and of the inversion circuit “NOT” are issued simultaneously or with a small time shift. This leads to failure of commutation of the thyristor

Fig. 3

converter, because the commutating capacitance does not have time to charge or recharge during these time intervals.

To prevent commutation failure, the pulses from the comparison circuit \(CC\) and from the “NOT” circuit are applied to separate inputs of a symmetrical flip-flop. Thus, a clear separation between the opening and closing pulses of the thyristor regulator is possible. Blocking oscillators \(BG_1\) and \(BG_2\) are connected to each of the flip-flop arms. At any moment in time the flip-flop can be in only one stable state and, consequently, only the blocking oscillator whose input is connected to the closed transistor of the flip-flop will operate.

Usually the charging and recharging time of the commutating capacitance \(U_{ck}\) (Fig. 2) of the thyristor regulator depends both on the parameters of the power circuit and on the parameters of the thyristor regulator. It is therefore important that the signals for opening or closing the thyristor regulator be issued only at the moment when all transient processes in the regulator have ended. This is achieved by means of the timing capacitors of blocking oscillators \(BG_1\) and \(BG_2\). By changing the time constant for charging these capacitors (Fig. 3), one can vary the time delay between the closing and opening pulses of the thyristor regulator.

Thus, failure of the thyristor switch is prevented by creating the necessary delay between the opening and closing pulses for the time required to charge and recharge the commutating capacitance \(C_k\).