CONTROLLED DISCRETE CONVERTER—MOTOR SYSTEM WITH IMPROVED POWER FACTOR

A. I. ZAGITSEV, V. N. MISHIN, A. A. KUVSHINOV

(Presented by the scientific-technical seminar of the Department of EPP of the Electromechanical Faculty)

Controlled discrete converter—motor (CDC–M) systems are widely used in various branches of technology for regulating the rotational speed of drives with direct-current motors. Owing to the appearance of silicon controlled valves, which have a high efficiency and a long service life, CDC–M systems are becoming still more widespread, since the reliability of drives will increase and their technical-economic indices will improve.

One of the principal practical requirements for the modern electric drive is the creation of drives that are fast-acting and sufficiently simple in installation and operation. This has found expression in the development of very simple relay CDC–M systems, in which complicated phase-shifting, amplifying, and correcting circuits are absent. However, these systems have a number of substantial shortcomings:

1) the presence of pulsating currents in the supply network and in the motor, which leads to a reduction in efficiency;
2) a small range of speed regulation;
3) a sharp decrease in the permissible load of the motor as the regulation range is increased.

The CDC–M systems with silicon controlled valves now appearing [1] are, in their principle of construction, analogous to systems with ionic valves, in which the motor speed is regulated by changing the firing angle of the valves. However, the presence of silicon controlled valves with a short deionization time opens broad prospects for the creation of various CDC–M systems with forced commutation, which are considered in the present work.

Pulse and relay CDC–M systems with forced commutation, possessing high speed of response and sufficient simplicity, are free from the stated shortcomings of relay circuits; they have a high efficiency and a wide range of speed regulation. In addition, they allow the valves to operate with an advanced firing angle, which improves the power factor of the drive.

As is known, in order to restore controllability of a discrete valve it is necessary to reduce the current through it to zero. This role in the proposed

in the systems considered below is performed by the forced-commutation unit (Fig. 1 and Fig. 2), which operates as follows.

To extinguish the power valve \(B_1\), the discharge of capacitor \(C\) into the load through the forced-commutation valve \(B_2\) is used. Capacitor \(C\) is preliminarily charged to a voltage greater than the anode voltage of valve \(B_1\). If the load has an active-inductive character, it is shunted by valve \(B_{\text{sh}}\), through which, during the pause period, the energy stored in the magnetic field is discharged. At the appropriate commutation frequency of the power valve, the current in the load will be continuous.

Fig. 1.

Fig. 2.

The energy for the preliminary charging of the commutating capacitor may be taken from an additional low-power direct-current source \(U_2\) (Fig. 1), or resonant charging through an inductance (Fig. 2) from the network feeding the load may be used. The inductance coil must have a high quality factor and have a value from several tens to hundreds of microhenries, that is, be sufficiently small for rapid charging of the capacitor. When the system (Fig. 2) operates from an alternating-current network, a controlled valve is used instead of the uncontrolled charging diode \(B_3\), since in this case the capacitor should be charged immediately before extinguishing the valves of the power circuit.

When designing systems with forced commutation, the problem reduces to selecting the value of the commutating capacitance as a function of the load, the voltage, and the power of the additional source, if the circuit of Fig. 1 is used.

We shall assume that, under forced commutation, the current of the main valves instantaneously falls to zero, that is, the influence of the anode inductance of the supply circuit is not taken into account; the parameters of the capacitor-discharge circuit are constant; and there is no voltage drop in the valve arc.

In this case, the differential equation describing the discharge process will have the form:

\[ \frac{d^2 U_c}{dt^2}+\frac{1}{T}\frac{dU_c}{dt}+\frac{1}{LC}U_c=-\frac{E}{LC}, \tag{1} \]

where
\(U_c\) — voltage across the capacitor;
\(T\) — electromagnetic time constant of the load circuit;
\(L\) — load inductance;
\(E\) — counter-e.m.f. of the motor.

The roots of the corresponding characteristic equation will be

\[ p_{1,2}=-\frac{1}{2T}\pm \sqrt{\frac{1}{4T^2}-\frac{1}{LC}}. \tag{2} \]

The solution of expression (1) in the case of real roots has the form

\[ U_c=\frac{1}{p_1-p_2}\left[\frac{1}{C}I_{я0}\left(e^{p_1t}-e^{p_2t}\right)-(E-U_{c0})\left(p_2e^{p_1t}-p_1e^{p_2t}\right)\right]-E, \tag{3} \]

where
\(I_{я0}\) — load current at the moment the capacitor discharge begins;
\(U_{c0}\) — capacitor charge voltage.

Expanding the exponential functions in (3) in a Taylor series, we obtain

\[ U_c=\frac{1}{C}I_{я0}t-U_{c0}. \tag{4} \]

The power consumed in charging the capacitor is

\[ P_c=\frac{U_{c0}^{2}C}{2}\cdot f, \tag{5} \]

where \(f\) is the frequency of artificial commutation.

The initial charge of the capacitor and its capacitance must be selected from the condition of ensuring the required time for artificial locking of the valve going out of operation. At the same time, the power of the extinguishing circuit must be minimal, in order to broaden the speed-control range and increase the efficiency of the system by reducing losses in the double conversion of the energy supplied from the artificial-commutation circuit.

Examining expression (5), taking (4) into account, for a minimum we obtain the conditions for the optimal selection of the parameters of the artificial-commutation circuit

\[ C=\frac{I_{я0}\cdot t_{\delta}}{U}; \tag{6} \]

\[ U_{c0}=2\cdot U, \tag{7} \]

where
\(t_{\delta}\) — time for recovery of controllability of the valves going out of operation;
\(U\) — load supply voltage at the moment of recovery of controllability of the power valve.

When operating on a motor, the capacitance of the commutating circuit must be selected from the condition of the permissible current overload

\[ C \geq \frac{I_{н}\cdot \lambda_I\cdot t_{\delta}}{U}, \tag{8} \]

where
\(\lambda_I\) — coefficient of permissible motor current overload;
\(I_{н}\) — rated motor current.

When an artificial-commutation unit with resonant charging of the capacitor is used (Fig. 2), the initial voltage on the capacitor is known; in this case the capacitance is found from (4) as

\[ C \geq \frac{I_{\text{n}}\lambda_1 t_{\delta}}{U(\gamma-1)}, \tag{9} \]

where \(\gamma\) is a coefficient characterizing the excess of the capacitor charging voltage over the supply voltage and depending on the parameters of the charging circuit. With the quality factor of the charging circuit equal to 10, which is readily achievable, \(\gamma = 1.7 \div 1.8\).

In the case of complex roots, the solution of expression (1) is written in the form

\[ U_c = e^{-\frac{t}{2T}} \left[ (E-U_{c0})\cos \beta t + \frac{1}{\beta} \left( \frac{I_{g0}}{C} + \frac{E}{2T} - \frac{U_{c0}}{2T} \right) \sin \beta t \right] -E, \tag{10} \]

where

\[ \beta = \sqrt{ \frac{1}{LC} - \frac{1}{4T^2} } \]

is the coefficient at the imaginary part of the roots of the characteristic equation.

The expression obtained, (10), is transcendental and can be solved only by numerical-graphical methods.

The advantage of the above-described artificial-commutation circuits is their universality, since they are applicable when the load is supplied both from a DC network and from an AC network; in the latter case they do not depend on the rectifier circuit. It should be noted that the energy of the commutating capacitor is delivered completely to the load, undergoing only a twofold conversion.

On the basis of the artificial-commutation circuits considered, UDP-D systems can be created with various methods of regulating the drive rotation speed. Below is presented a continuous UDP-D system with improved \(\cos\varphi\) (Fig. 3), with converters connected according to a three-phase star circuit with a neutral lead.

The possibility of creating valve circuits for generating leading capacitive energy was pointed out by G. I. Babat [2]; a number of circuits were developed by I. L. Kaganov [3], I. M. Chizhenko, and others. The idea of the circuit in Fig. 3 is that, when the average rectified voltage is reduced, the firing angle of the next valve is taken not as lagging, as in ordinary circuits, but as leading. For this purpose, the valve going out of operation is first artificially turned off, and then the next one is turned on. In this case the fundamental harmonic of the anode current is shifted with respect to the voltage by a certain leading angle.

The control circuit of the UDP-D system with improved \(\cos\varphi\) consists (Fig. 3) of a synchronization unit, a PWM pulse-width-modulator circuit, a device for switching on the power valves, and an artificial-commutation circuit. The system provides negative speed feedback and current cutoff. The time diagram explaining the operation and the purpose of the individual elements of the system is presented in Fig. 4.

The synchronization unit is used to synchronize the control circuit from the AC network and includes a low-power three-phase transformer Tr, connected according to the \(\Delta/Y\text{—}11\) circuit, valves \(B_1—B_6\), fast-saturating transformers \(\mathrm{BNT}_1—\mathrm{BNT}_6\), and triggers \(T_1—T_3\).

Each of the triggers is controlled by means of the corresponding BNTs, the pulses from which are taken at the instants of natural commutation of the valves \(B_1 — B_6\). Owing to the phase shift of the secondary voltage \(U_2\) of transformer Tr relative to the primary (Fig. 4), the trig-

Fig. 3.

gers switch from one stable state to another at instants corresponding to the beginning of the positive half-waves of the anode voltage \(U_1\), and return to the initial state by pulses from the PWM, and, in their absence, by signals from \(BNT_1 — BNT_3\), arriving 150 electrical degrees after the pulses of \(BNT_4 — BNT_6\). Thus, as the time delay of the corresponding PWM units is changed, the triggers will be in the excited state during the time interval between 0 and 150 electrical degrees of the positive half-waves of the anode voltage \(U_1\). Since the starting of the power and commutating valves is synchronized from triggers \(T_1 — T_3\), changing the PWM time delay will change the conduction duration of the corresponding power valves \(KUV_1 — KUV_3\) from 0 to 120 electrical degrees; in this case the firing angle will vary from 0 to 30 electrical degrees, remaining leading at all times.

Pulse-width modulators (PWMs), which set the conduction duration of the main valves, are made on the basis of a monostable multivibrator with separate collector voltages of the transistors. The duration of the pulses generated by the PWM depends on the algebraic sum of the reference voltage and the voltage of the tachogenerator TG, by means of which negative speed feedback is implemented in the system.

Fig. 4.

To turn on the silicon controlled valves, blocking oscillators BG\(_1\)—BG\(_6\) are used; the start pulses for them arrive from the synchronization unit through the logic circuits AND and OR.

When the triggers T\(_1\)—T\(_3\) return to their initial state with the arrival of signals from the PWM or BNP\(_1\)—BNP\(_3\), the blocking oscillator BG\(_4\), which controls the charging valve KUV\(_4\), is started through the OR logic circuit; through this valve the capacitance \(C_k\) is resonantly charged. At the same time, the signal from the OR circuit is applied to the monostable multivibrator ZhM\(_1\), which, after a time delay sufficient for charging the capacitance and restoring controllability of the valve KUV\(_4\), turns on

the artificial-commutation valve KUV\(_5\). The capacitance \(C_k\), discharging through KUV\(_5\) into the motor, turns off the power valve carrying current.

Synchronization of the switching-on of the power valves KUV\(_1\)—KUV\(_3\) is performed by means of logic circuits I\(_1\)—I\(_3\), to one of whose inputs signals are applied from triggers T\(_1\)—T\(_3\), and to the other—from the one-shot multivibrator ZhM\(_2\) through logic circuit I\(_4\). The one-shot multivibrator ZhM\(_2\) is started simultaneously with the artificial-commutation valve KUV\(_5\) and serves to create a delay for the time of discharge of the capacitance \(C_k\); otherwise the next power valve may fail to turn on, since the voltage at its cathode will be higher than the anode voltage. A signal appears at the outputs of circuits I\(_1\)—I\(_3\) when the signal of the one-shot multivibrator ZhM\(_2\) coincides with the corresponding triggers T\(_1\)—T\(_3\), which synchronize the sequence of switching on the power valves. Thus, the firing angle of the power valves can be varied from zero to 30 electrical degrees, depending on the prescribed motor speed and the load.

To make possible forced starting and to limit the motor current within permissible limits, delayed current feedback is used in the system.

When the motor current reaches the limiting value, the voltage taken from the shunt \(R_{\text{sh}}\) proves sufficient to actuate a semiconductor relay made on the basis of the Schmitt trigger T\(_{\text{sh}1}\). On operating, the Schmitt trigger sends a pulse to the artificial-commutation circuit to extinguish the conducting power valve. When the motor current decreases by a certain amount determined by the hysteresis of the Schmitt trigger, the latter returns to its initial state, applying a signal for switching on the next power valve through circuit I\(_4\).

To prevent breakdowns in the operation of the system due to incomplete discharge of the capacitor \(C\), a special circuit is provided which carries out its discharge. The circuit consists of the Schmitt trigger T\(_{\text{sh}2}\), logic circuit I\(_5\), blocking generator BG\(_6\), and valve KUV\(_6\).

After discharge of capacitor \(C_k\) into the motor, the motor current for a certain interval of time flows through valve \(B_{\text{sh}}\). In this case the voltage drop from the valve is applied to trigger T\(_{\text{sh}2}\); it switches from one stable state to the other, and the pulse from the one-shot multivibrator ZhM\(_2\) for starting valve KUV\(_6\) does not pass through circuit I\(_5\). If, after a time delay corresponding to the discharge of the capacitor, there is no current through valve \(B_{\text{sh}}\), then the pulse from the one-shot multivibrator ZhM\(_2\) turns on valve KUV\(_6\), and capacitor \(C\) is discharged through the resistance \(R_c\).

To estimate \(\cos \varphi\) in the drive system under consideration, let us assume that the load inductance is infinitely large, i.e., an ideally smoothed current flows through the motor, and let us neglect the influence of the anode inductance on the commutation process. In this case the anode current will have the form of rectangular pulses. After expanding the anode-current curve into a Fourier series, we obtain

\[ I=\frac{\lambda}{2\pi} I_d+ \sum_{\kappa=1}^{\kappa=\infty}\frac{2}{\kappa\pi} I_d \sin \kappa\frac{\lambda}{2} \sin\left[\kappa\vartheta+\frac{\pi}{2} -\kappa\left(\vartheta_3+\frac{\lambda}{2}\right)\right], \tag{11} \]

where \(\lambda\) is the duration of conduction of the main valves;
\(I_d\) is the load current;
\(\vartheta_3\) is the firing angle of the valves.

The first harmonic of the anode current is

\[ I_{1(1)}=\frac{\sqrt{2}I_d}{\pi}\cdot \sin \frac{\lambda}{2}, \tag{12} \]

the displacement angle of the first current harmonic is

\[ \varphi_{1(1)}=\frac{\pi}{2}-\left(\vartheta_3+\frac{\lambda}{2}\right). \tag{13} \]

The capacitive reactive power generated by the system is

\[ Q=3U_1\cdot I_{1(1)}\cdot \sin \varphi_{1(1)} =\frac{3}{\sqrt{2}\pi}U_1 I_d\left[-\sin \vartheta_3+\sin(\vartheta_3+\lambda)\right], \tag{14} \]

where \(U_1\) is the rms value of the supply voltage.

Let us estimate the effectiveness of using capacitors in a circuit with artificial commutation to create a leading displacement factor, as compared with connecting them to an ac network.

The installed capacity of the capacitors, taking expression (8) into account, is

\[ Q_c=2\cdot \pi\cdot f U_1^2\cdot C =\sqrt{2}\cdot \pi\cdot f\cdot U_1\cdot I_{\mathrm{n}}\cdot \lambda_I\cdot t_\delta, \tag{15} \]

where \(f\) is the frequency of the ac network.

The utilization factor of the capacitors is

\[ K_c=\frac{Q}{Q_c} =\frac{1.5}{\pi^2 f}\cdot \frac{I_d}{I_{\mathrm{n}}\lambda_I t_\delta} \left[-\sin \vartheta_3+\sin(\vartheta_3+\lambda)\right]. \tag{16} \]

If \(I_d=I_{\mathrm{n}}\), \(\lambda_I=2.5\), \(t_\delta=20\ \mu\text{sec}\), \(\vartheta_3=0\), \(\lambda=\frac{2\pi}{3}\), then, under the assumptions adopted, the utilization factor is \(K_c=52.7\). That is, in order to obtain an analogous leading displacement factor, it would be necessary to connect to the ac network a capacitance almost 50 times greater than the capacitance of the artificial-commutation circuit.

The experimental investigations carried out on the considered circuits with artificial commutation showed their good operability. The principles set forth may serve as the basis for creating similar systems both with silicon controlled valves and with ionic valves.

UDP-D systems with silicon controlled valves using artificial commutation make full use of such a distinguishing feature of semiconductor valves as high speed. The systems developed make it possible to create fast-acting, sufficiently simple drives with high technical and economic performance.

REFERENCES

1. S. N. Veshenevskii, B. S. Zamaraev, Ya. Yu. Solodukho. Power silicon controlled valves and their application in electric drive. GOSINTI, 1963.

2. G. I. Babat, Ya. A. Katsman. Thyratron converters with an improved power factor and thyratron compensators. Elektrichestvo, No. 4, 1937.

3. I. L. Kaganov. Electronic and ionic converters, Part III, GEI, 1956.