INDEPENDENT INVERTER WITH AN AUTONOMOUS COMMUTATION UNIT FOR SUPPLYING SYNCHRONOUS-REACTIVE MOTORS

Yu. M. ACHKASOV, B. A. ZAKHAROV

(Presented by the Scientific-Technical Seminar of the Research Institute of AEM at TPI)

One of the principal areas of application of static inverters is the drive with synchronous motors, in which precise speed regulation within a certain range is required. Such drives are necessary for geophysical equipment, seismic stations, telegraph apparatus, motion-picture equipment, and others. To preserve the overload capacity of the motors, it is necessary to regulate the supply voltage proportionally to the frequency. Thyristor inverters make it possible to obtain an output voltage with a frequency determined by the accuracy of a quartz oscillator. For the indicated drives, the inverter control circuit must ensure that the voltage-to-frequency ratio is kept constant when the load current varies from zero to the maximum permissible value at any value of the load power factor. Fig. 1 shows the circuit of the power section and the basic electrical control circuit of an independent inverter with an autonomous commutation unit.

In this circuit the specified value of the frequency is formed with high accuracy by an inverter using thyristors UV1, UV2, UV3 and transformer TR3, under the action of control signals arriving from the highly stable quartz oscillator KG. The specified average value of the output voltage is maintained by the method of pulse-width stabilization (PWS) according to formula (1).

\[ U_c = 2n\alpha U_{1\min}\cdot \gamma, \tag{1} \]

where

\[ n=\frac{w_2}{w_1} \]

is the transformation ratio of the power transformer of the inverter;

\[ \alpha \]

is the coefficient of voltage increase when the supply voltage \(U_1\) changes, whose instantaneous value is equal to \(\alpha U_{1\min}\).

The minimum value \(\alpha_{\min}=1\), since the maximum value is

\[ \alpha_{\max}=\frac{U_{1\max}}{U_{1\min}}. \]

The duty factor is

\[ \gamma=\frac{t_u}{T}=\frac{1}{2\alpha}, \]

Fig. 1. Schematic circuit diagram of an independent inverter with an autonomous commutation unit.

where

\(t_i\) is the pulse duration,
\(T\) is the pulse repetition period.

The effective value of the rectangular voltage is determined by the formula:

\[ U_{\mathrm{eff}\, i}= n\alpha U_{1\min}\sqrt{2\gamma}; \]

and the effective value of the first harmonic is

\[ U_{\mathrm{eff}\,1}= \frac{2\sqrt{2}}{\pi}\cdot n\alpha U_{1\min}\sin \pi\gamma . \]

The control circuit of the VFC is built according to the vertical control method and provides modulation of the pulse width by shifting the trailing edge.

Such an inverter combines both a converter and an output-voltage regulator simultaneously, which is an advantage compared with continuous voltage regulation by means of a controlled rectifier with a power filter at the inverter input.

The autonomous commutating unit (chokes DR1, DR2, DR3, diodes D15, D31, commutating capacitance \(c_1\) and thyristor UV3) is made according to a circuit with direct recharging of the commutating capacitor, which provides self-regulation of the commutating voltage as a function of the load current and eliminates the phenomenon of “swinging” of the charging circuit [2]. The inverter load is a three-phase synchronous reactive motor connected according to a single-phase supply circuit.

The operation of the control circuit is illustrated by the time diagram shown in Fig. 2.

The input voltage \(U_1\), of rectangular form with a frequency of 100 Hz, from the frequency divider of the quartz oscillator (Fig. 2, a) is fed through the differentiating circuit \(C_1R_2\) to a self-biased trigger (transistors \(T_1T_2\)) and simultaneously through an emitter follower (transistor \(T_8\)) and a phase-inverting amplifier (transistor \(T_9\)) controls the operation of the sawtooth-voltage generator (SVG) on transistor \(T_{10}\). From each positive differentiated pulse the trigger is flipped, and therefore its output voltage has a frequency half as large, 50 Hz (Fig. 2, b). The output voltage from the trigger, through an emitter follower (\(T_3\)), is fed to the input of the rectangular-voltage amplifier (transistor \(T_4\)), whose load is the primary winding of transformer TR1. Bipolar pulses (Fig. 2, c), controlling the operation of the pulse-duration modulator (PDM1), made on transistors \(T_5\), \(T_6\) and transformer TR2, are taken from the output windings of transformer TR1. From the output windings of transformer TR2, positive pulses are applied to the control electrodes of the working thyristors of the inverter UV1, UV2 (Fig. 2, d). The duration of the control pulses is regulated by the comparison circuit, made on a Schmitt trigger (transistors \(T_{11}\), \(T_{12}\)) and transistor switch \(T_{13}\).

The control voltage \((U_y)\), equal to the difference between the rectified feedback voltage \((U_{\mathrm{os}})\), taken from an additional winding of the inverter power transformer (Fig. 2, g), and the reference voltage \((U_e)\), is applied to the input of the comparison circuit, i.e. \(U_y = U_{\mathrm{os}} - U_e\).

The voltage of the sawtooth generator, taken from capacitor \(C_{10}\) \((U_p)\), is summed with the control voltage. The sum of the voltages \(U_y + U_p\) is compared with the triggering voltage of the Schmitt trigger \(U_{\mathrm{sr}}\). Under the condition \(U_y + U_p > U_{\mathrm{sr}}\), the trigger switches to the other stable state. During the negative half-wave of the voltage at the input of transistor \(T_{10}\), it is open, and the voltage at the input of the Schmitt trigger

Fig. 2. Timing diagrams of the inverter control circuit.

is equal to zero. The emitter-coupled trigger is in such a state that a collector current flows through transistor \(T_{12}\), which cuts off transistor switch \(T_{13}\). In the absence of an input pulse, transistor \(T_{10}\) is cut off by a positive bias through transistor \(T_9\) and resistor \(R_{27}\); capacitor \(C_{10}\) begins to charge from the sum of the control voltages and the supply voltage of the sawtooth-voltage generator. The internal resistance of the first source is small; therefore capacitor \(C_{10}\) is charged almost instantaneously to the level \(I_{\mathrm{u}}\), i.e., a step of sawtooth voltage is formed. Next, capacitor \(C_{10}\) charges with the time constant \(\tau = R_{28}\cdot C_{10}\); the voltage source \(I_{\mathrm{u}}\) is disconnected from the circuit by diode \(D_{15}\), to which a reverse voltage is applied.

When the voltage on capacitor \(C_{10}\) reaches the triggering threshold \(I_{\mathrm{sr}}\), the trigger flips into the other stable state; transistor switch \(T_{13}\) is opened by a current determined by resistance \(R_{41}\), and a bias voltage of \(+5\text{ V}\) relative to their emitters is applied to the bases of the MDI transistors \(T_5, T_6\) through diodes \(D_3, D_4\). This leads to cutoff of the transistor \(T_5\) (or \(T_6\)) that is open at this time and, consequently, to removal of the control signal from the control electrode of the operating thyristor UB1 (or UB2) that is open at this time. The form of the voltage at the base and collector of transistor \(T_{13}\) is shown in Fig. 2, з and 2, и. At the same time, the output pulse from the Schmitt trigger output, differentiated by the circuit \(C_{11}, R_{35}\) (Fig. 2, к), triggers the waiting blocking-

a generator on transistor \(T_7\), from whose output winding a short voltage pulse is applied to the control electrode of the quenching thyristor UB-3 (Fig. 2, l).

Thus, simultaneously with the removal of the control pulse from the open working thyristor, an unlocking pulse is applied to the quenching thyristor. It is unlocked, and, owing to the commutation process, the working thyristor is quenched at the required instant. When the load on the inverter changes, the control voltage will change (Fig. 2, zh), which will lead to a change in the half-wave duration of the inverter output voltage pulses (Fig. 2, m) and, consequently, to maintenance of the specified voltage level. The control circuit provides a range of variation in the output-voltage pulse duration from 90 to 180 electrical degrees.

By means of potentiometer \(R_{42}\), the control voltage is set to such a value that, in the steady-state mode, the duration of the half-waves of the output voltage is 120 electrical degrees. An output voltage of this form contains no even harmonics, no third harmonic, and no multiples of the third, which facilitates filtering of the higher harmonics of the output voltage.

However, when the supply voltage or the load on the motor shaft changes, the pulse duration of the output voltage will change, which will cause a change in the composition of the low-frequency harmonics. The influence of all harmonics can be taken into account by the nonlinear distortion coefficient, which for the pulse-width method of regulation is determined by the expression [3]

\[ K_{\mathrm{ni}} = \sqrt{ \frac{\pi^2 \beta}{4} \cdot \frac{1}{1 - \cos \pi \beta} - 1 }, \]

where

\[ \beta = \frac{2t_4}{T} \]

is the relative angle of regulation, and the dependence of \(K_{\mathrm{ni}}\) on the relative angle of regulation is given in [4], where it is shown that \(K_{\mathrm{ni}}\) assumes its minimum value at \(\beta = 0.76\), equal to 0.28.

In the present case, for \(t_i = \dfrac{T}{3}\), \(\beta = 0.66\), which corresponds to \(K_{\mathrm{ni}} = 0.31\).

When \(t_i\) changes from \(\dfrac{T}{4}\) to \(\dfrac{T}{3}\) and from \(\dfrac{T}{3}\) to \(\dfrac{T}{2}\), the nonlinear distortion coefficient changes from 0.31 to 0.5, which demonstrates the expediency of using the pulse-width method of regulation.

The developed control circuit was implemented in the sector for automation of oil and gas exploration processes of the Scientific Research Institute of AEM and provides inertia-free and sufficiently reliable inverter control.

References

1. A. G. Gontsov. Stabilized parallel voltage inverters. Elektrotekhnika, 1969, No. 10.

2. A. I. Zaitsev, M. A. Zhitkov. A device for forced commutation of thyristors with direct recharge of the commutating capacitor. Proceedings of the Sixth Scientific and Technical Conference on Problems of Production Automation. Tomsk State University Publishing House, 1969.

3. S. I. Korolev et al. A low-frequency converter for supplying SRM. Proceedings of the Sixth Scientific and Technical Conference on Problems of Production Automation. Vol. 2. Tomsk State University Publishing House, 1969.

4. A. M. Azarov et al. Harmonic composition of the pulsed voltage of an inverter. Ibid.