A Pulse Excitation System for Electrical Machines Using Thyristors with Forced Commutation

A. I. ZAITSEV, S. A. KITAEVA

Recommended by the scientific seminar of the Electromechanical Faculty

Pulse systems for regulating the voltage of direct- and alternating-current generators have recently begun to attract attention because of their high speed of response and high accuracy. The appearance of controlled semiconductor valves—thyristors—and their use for supplying the excitation windings of electrical machines open up new possibilities for pulse technology.

Thyristors have a number of advantages in comparison with thyratrons and other ionic devices. In a number of cases thyristors are also superior to silicon transistors: they have high operating voltage and current, a small voltage drop across open devices, and much less power is required to turn on a thyristor at a significantly greater switched power in the pulse.

The advantages of thyristors explain the great interest that is currently being shown in these devices when they are used in automatic control systems [1].

To control thyristors, circuits for forced commutation of the current in the thyristor circuit are used.

The use of forced-commutation circuits makes it possible to create fast-acting automatic control systems with relatively high technical and economic indices. Forced-commutation circuits may be used when supplying a load both from a direct-current network and from an alternating-current network. In creating systems with forced commutation, the problem is reduced to selecting the values of the capacitance, inductance, and the charging voltage of these elements [1, 2].

This article considers an automatic voltage-control system for a direct-current generator using a forced-current-commutation circuit in the power circuit.

In the control system under consideration (see Fig. 1), the excitation winding of the generator is supplied by voltage pulses of increased frequency. By changing the duty ratio of the voltage pulses feeding the excitation winding, it is possible to regulate the average value of the voltage at the terminals of the generator armature. During the pause period, there occurs—

Fig. 1.

discharges the energy accumulated in the magnetic field of the field winding through the shunting valve \(D_1\).

The control circuit consists of a pulse generator—PG, a pulse-width modulator circuit—PWM, devices for switching on the power valves—SD, and an artificial-commutation circuit.

The PWM sets the duration of switching on the power valve \(KUV_1\). The duration of the pulses generated by the PWM depends on the magnitude of the error signal coming from the measuring element—ME, connected to the generator armature. To switch on the power valves, starting devices \(SD_1\) and \(SD_2\) are used in the form of blocking oscillators, whose starting is carried out by pulses coming from the control circuit.

The artificial-commutation circuit for extinguishing the power valve \(KUV_1\) uses the discharge into the field winding of the capacitor \(C\), which is charged to a voltage greater than the anode voltage of \(KUV_1\). The capacitor \(C\) is charged from the power circuit through the inductance \(L\) and the uncontrolled valve \(D_2\). When the capacitor \(C\) discharges through \(KUV_2\), the voltage at the cathode of \(KUV_1\) becomes higher than the anode voltage, and the valve \(KUV_1\) is extinguished. The power valve \(KUV_1\) is not switched on again after the discharge of the capacitor \(C\).

The circuit operates as follows. With the power valve \(KUV_1\) switched on, resonant charging of the capacitance \(C\) takes place; a pulse from the pulse generator—PG is sent to start the blocking oscillator BO, which switches on the artificial-commutation valve \(KUV_2\). The capacitance \(C\), discharging through \(KUV_2\) into the field winding, locks \(KUV_1\). Simultaneously with the switching on of \(KUV_2\), a signal is applied to the PWM and to the blocking oscillator BO of the valve \(KUV_1\). Then the process is repeated.

When the voltage at the generator armature decreases, the error signal supplied to the PWM increases, which leads to an increase in the current in the field winding, and the generator voltage is restored to the specified level.

Let us consider the processes of current establishment in the generator field winding. Rectangular voltage pulses with amplitude \(u\), repetition period \(T\), and duty factor \(\gamma\) are applied to the generator field winding. The pulse repetition period is determined—

with the frequency of the wipe-off pulse applied to the CU and can be regulated.

Thus, the proposed excitation-current regulation system may be considered as a pulse system, when the excitation winding is supplied from a dc-voltage source through a periodically opening and closing switch.

The processes by which the excitation current is established in such systems are considered over the time intervals

\[ n\tau \le t \le \tau(n+\gamma) \quad \text{and} \quad \tau(n+\gamma) \le t \le \tau(n+1), \]

where \(n\) is an integer.

\[ i[n,\varepsilon]= \frac{u}{R}+ \left[ \frac{u e^{-\beta(1-\gamma)}(1-e^{-\beta\gamma})(1-e^{-\beta n})} {R(1-e^{-\beta})} -\frac{u}{R} \right]\cdot e^{-\beta\varepsilon}, \tag{1} \]

\[ 0 \le \varepsilon \le \gamma . \]

According to [3], the equations of current variation in the form of shifted lattice functions have the form:

\[ i[n,\varepsilon]= \left\{ \frac{u}{R} + \left[ \frac{u e^{-\beta(1-\gamma)}(1-e^{-\beta\gamma})(1-e^{-\beta n})} {R(1-e^{-\beta})} -\frac{u}{R} \right]\cdot e^{\beta\gamma} \right\} \cdot e^{-\beta(\varepsilon-\gamma)} . \tag{2} \]

\[ \gamma \le \varepsilon \le 1 . \]

In these equations:

\[ \beta=\frac{\tau}{\tau_{\mathrm{н}}}; \qquad \tau_{\mathrm{н}}=\frac{L}{R}. \]

\(\tau_{\mathrm{н}}\) is the time constant of the excitation winding;

\(R\) and \(L\) are the resistance of the excitation winding and its inductance.

From equations 1 and 2, the maximum and minimum steady-state current values can be determined:

\[ i_{\max}= \frac{u(1-e^{-\beta\gamma})} {R(1-e^{-\beta})} \tag{3} \]

\[ i_{\min}= \frac{u(1-e^{-\beta\gamma})e^{-\beta(1-\gamma)}} {R(1-e^{-\beta})}. \tag{4} \]

The current ripple coefficient, characterizing the ratio of the maximum steady-state current to the minimum steady-state current, is equal to

\[ \chi=e^{\beta(1-\gamma)}. \tag{5} \]

The total current-ripple amplitude in steady state is

\[ \Delta i=i_{\max}-i_{\min} = \frac{u}{R} \frac{(1-e^{-\beta\gamma})\,[1-e^{-\beta(1-\gamma)}]} {1-e^{-\beta}} . \tag{6} \]

For a negligible current-ripple amplitude, the process by which the current in the excitation winding is established is characterized by a smooth component.

According to [3], the equation of the smooth component can be represented in the form:

\[ i_{\mathrm{av}}(t)=I\gamma\left(1-e^{-\frac{t}{\tau_{\mathrm{n}}}}\right). \tag{7} \]

When the generator operates on the linear part of the magnetization curve, the dependence of the emf on the excitation current is linear; then the equation of the smooth component of the emf, determined by the smooth component of the excitation current, will have the form

\[ E_{\mathrm{av}}(t)=E_{\mathrm{st}}\gamma\left(1-e^{-\frac{t}{T_{\mathrm{n}}}}\right). \tag{8} \]

\[ E_{\mathrm{st}}=ku, \]

where
\(K\) is a constant quantity;
\(u\) is the supply voltage.

With pulse regulation of the excitation current, the magnitude of the current pulsations is of great importance. The presence of significant current pulsations in the excitation winding leads to the generator emf having an oscillatory character. To reduce current pulsations, as well as oscillations of the generator emf, it is necessary to choose a sufficiently high commutation frequency of the thyristors. For a given coefficient of current pulsations and relative duration of switching on, the commutation period is determined [3] as

\[ \tau=\frac{\ln x}{1-\gamma}\,T_{\mathrm{n}}. \tag{9} \]

where \(T_{\mathrm{n}}\) is the time constant of the excitation winding.

CONCLUSION

Pulse systems for automatic regulation of the excitation of electric machines using thyristors with artificial current commutation in the power circuit have been developed and are being studied at the Department of Electric Drive and Industrial Installations of the Tomsk Polytechnic Institute. The systems have proved to be fast-acting, with high tracking accuracy and a wide regulation range. The advantages of thyristors make it possible to create reliable excitation systems of medium and high power.

The proposed system for regulating the voltage of a direct-current generator can also be used for the excitation of alternating-current generators. When the regulation system is supplied from an alternating-current network, the uncontrolled valve \(D_2\) must be replaced by a controlled one in order to charge capacitor \(C\) immediately before the extinction of the power valve.

REFERENCES

1. Silicon controlled valves—thyristors. Technical handbook. Energiya Publishing House, 1964.

2. A. I. Zaitsev, V. I. Minin, A. A. Kuvshinov. Pulse regulation of speed in drives according to the controlled discrete converter–motor system (CDCM) with artificial commutation. Means of Automation of Electric Drives. Reports to the Fourth All-Union Conference on Automatic Drive, VNIIEM, 1964.

3. A. P. Zaitsev. Development and investigation of certain pulse devices for controlling electric machines. Dissertation, TPI, 1964.