HIGH-PRECISION AC ELECTRIC DRIVE SYSTEM

Yu. M. Achkasov, B. A. Zakharov, V. N. Sorokin

(Presented by the scientific and technical seminar of the Research Institute of AEM at TPI)

The development of technology and the appearance of new equipment in a number of industries impose increased requirements with respect to the accuracy of maintaining a specified speed. In particular, in the chemical industry, in the production of thin films and fibers, geophysical apparatus, instrument making, motion-picture equipment manufacturing, machine-tool building, and paper production, it is required that over a long period of time the deviation of the speed from the specified value not exceed \(0.01 \div 0.001\%\). For high-precision drives, the most expedient approach is the use of AC electric motors with frequency speed control.

Fig. 1.

A system for high-precision stabilization of the speed of an electric drive using a synchronous-reactive motor (SRD), Figs. 1, 2, has been developed at the Research Institute of AEM.

In this system, the specified frequency value is formed with high accuracy by an inverter using thyristors \(T_{16}, T_{17}\), under the action of signals arriving from a highly stable master quartz generator (KG). The required discreteness of the signals is obtained in a frequency divider (FD) based on flip-flops. The inverter control unit (ICU) forms control pulses of the required amplitude and duration \(180^\circ\). The single-phase inverter (I) and the control circuit are powered from a stabilized DC voltage source (SDVS). To maintain the constancy of the overload capacity of the motor, the output voltage of the inverter is changed by means of taps on the secondary winding of transformer \(TR_3\), according to the relation

\[ \frac{U}{f} = \text{const}. \]

The actuator SRD is made on the basis of a standard single-phase asynchronous motor of type AOLB-31-2.

The inverter control circuit provides alternating voltages of three frequencies: \(f = 100\) Hz, \(f' = 50\) Hz, \(f' = 25\) Hz, correspondingly—

Fig. 2.

… corresponding to the required motor rotational speeds: \(n = 6000\ \text{rpm}\); \(n = 3000\ \text{rpm}\); \(n = 1500\ \text{rpm}\). The operating principle of the inverter control circuit can be explained by considering the timing diagrams in Fig. 3. The rectangular voltage of frequency \(1000\ \text{Hz}\) from the output of the frequency divider of the quartz oscillator (a) is divided by five by a multivibrator using transistors \(T_1, T_2\), as a result of which the output frequency is \(200\ \text{Hz}\) (b). The output…

Fig. 3.

Fig. 4.

the voltage pulses of the multivibrator, with the aid of three transistor flip-flops \(T_3, T_4, T_5, T_6, T_7, T_8\), are converted into pulses of frequency \(100\ \text{Hz}\) (в), \(50\ \text{Hz}\) (г), \(25\ \text{Hz}\) (д). The required frequency is set by switch \(P\); in this case the output pulse, through an emitter follower on transistor \(T_9\), synchronizes the rectangular-voltage amplifier (transistor \(T_1\)), which controls a push-pull power amplifier (transistors \(T_{11}, T_{12}\)) with transformer output. Control pulses (ж, з) are applied to the control electrodes of the thyristors from the output windings of transformer \(TP_2\). Of definite interest is the multivibrator (\(T_1, T_{10}\)); the voltage waveforms in it when operating in the self-oscillating mode are shown in Fig. 4, and when operating in the frequency-division mode in Fig. 5.

Fig. 5.

The divide-by-five frequency divider is implemented on the basis of a multivibrator synchronized by a quartz generator. The multivibrator in the self-oscillating mode has increased frequency stability owing to the fact that the timing capacitor \(C_2\) is connected in the emitter circuit; therefore the magnitude of the transistor’s uncontrollable current in this case has a significantly smaller effect on the recharge rate of capacitor \(C_2\). The natural frequency of the multivibrator can be changed by changing the values of resistances \(R_3, R_8\).

The multivibrator without the synchronizing generator operates as follows.

Through resistance \(R_5\) a negative bias is applied to the base, and \(T_2\) is in saturation mode. The emitter current of transistor \(T_2\) creates a voltage drop across resistances \(R_8\) and \(R_7\), which cuts off \(T_1\). As soon as the negative potential on the negative plate of \(C_2\) becomes greater than the negative potential of the emitter of transistor \(T_1\), the latter will open and capacitor \(C_2\) will discharge through the circuit: \(R_8, R_7\), the emitter-base junction of \(T_1\), \(R_2\). Transistor \(T_2\) will be cut off by the voltage drop across \(R_6\). After \(C_2\) discharges, \(T_1\) will cut off, and \(T_2\) will open, and the cycle will repeat again.

The timing diagrams (Fig. 4) explain the operation of the multivibrator, where

\(U_c\) — change of voltage on capacitor \(C_2\),

\(U_{b1}\) — change of voltage at the base of transistor \(T_1\),

\(U_{b2}\) — change of voltage at the base of transistor \(T_2\),

\(U_1(R_0, R_7)\) is the voltage drop across \(R_8\) and \(R_7\) with transistor \(T_1\) open,

\(U_2(R_8, R_7)\) is with \(T_2\) open,

\(U_{\text{out}}\) is the output voltage from transistor \(T_2\).

A necessary condition for operation of this frequency divider is that the natural frequency of the multivibrator \(f_m\) be less than the frequency of the quartz generator \(f_g\); then, under the action of the synchronizing pulse (Fig. 5), forced leading of the circuit occurs. Thus, the differentiated input pulses are superimposed on voltage \(1 < \delta_1\) (B), as a result of which at time \(t_1\) \(T_1\) opens and \(T_2\) closes. The circuit is adjusted by means of \(R_3, R_8\) so that cutoff of \(T_2\) occurs after the fifth synchronizing pulse. Thus, in this frequency divider, synchronized opening and closing of \(T_1\) occurs, thereby achieving high stability of the division frequency.

The frequency of the divider output pulses can be varied by changing the recharge duration of \(C_2\) and the amplitude of the input pulses. It is most expedient to vary the division factor \(K = \frac{f_{\text{out}}}{f_g}\) (where \(f_{\text{out}}\) is the frequency of the divider output pulses) from \(1 \div 6\) by changing the charging duration of \(C_2\), and, for larger \(K\), it is necessary to change not only the charging time of \(C_2\), but also the amplitude of the input pulses.

The value of capacitance \(C_2\) is determined by

\[ C_2=\frac{K}{f_g(R_3+R_4)}. \]

The duration of the output pulse \(t_i\) is determined by the discharge duration of \(C_2\)

\[ t_i=C_2(R_8+R_7+R_2). \]

The temperature instability in this divider depends on the values of the resistances \(R_8, R_7, R_6\); therefore they should not be taken as less than 100 ohms.

The use of a highly stable power supply, a multivibrator with a timing capacitor in the emitter circuit, complete synchronization of the opening and closing of the transistors contribute to the stability of frequency division, and the absence of storage capacitances makes the output pulses have steep fronts of no more than 5 μsec for MP16 transistors.

The accuracy of maintaining speed for the SED is determined by the formula:

\[ \vartheta_3=\frac{\Delta \omega}{\omega_3}=\frac{2\pi \Delta f}{P}:\frac{2\pi f_3}{P}=\frac{\Delta f_3}{f_3}, \tag{1} \]

where

\(f_3\) is the specified (true) value of the motor supply frequency;

\(\Delta f_3\) is the deviation of the frequency from the specified value;

\(P\) is the number of pole pairs of the motor.

The frequency \(f_3\) is determined by the expression \(f_3=\frac{f_g}{K_d}\), where \(K_d\) is the overall frequency-divider factor.

Then the relative error of the open-loop system will be expressed as:

\[ \vartheta_3 \leqq \pm \frac{\Delta f_g}{f_g} \mp \frac{\Delta t}{T_{\min}}=\pm \delta_g \mp \delta_{\text{km}}. \tag{2} \]

The relative error of the master generator is equal to \(1\cdot 10^{-6}\). The maximum relative error of the commutation path, including the inverter, is calculated by the formula

\[ \delta_{\text{km}}=\frac{\Delta t}{T_{\min}}=\Delta_3\cdot f_{3M} \tag{3}, \]

\(\Delta t\) is the maximum scatter in the switching time of one of the inverter thyristors; according to tests, \(\Delta t = 1\) μsec.

\(f_{3M}\) is the maximum value of the motor supply frequency.

Table 1

The calculated values of the errors, computed by formulas (2–3), are given in Table 1. It is evident from the calculation that, for a control range equal to 4, the system ensures maintenance of the average speed value with a relative error $v_z \leq 0.01\%$. Practical verification of the accuracy of speed maintenance confirmed the validity of the calculation by formulas (2–3).

REFERENCES

1. Yu. M. Achkasov, B. A. Zakharov. AC electric drive with precision speed stabilization. Reports of the VI Scientific and Technical Conference on Problems of Production Automation. Vol. II. Edited by A. I. Zaitsev. TSU, Tomsk, 1969.

2. L. M. Regelson, A. V. Kuznetsov, A. P. Pyatibratov. Pulse Technology. MSU, 1967.

3. F. Gentri, F. Gutswiller, N. Golonyak, E. von Zastrov. Controlled Semiconductor Valves. Mir, Moscow, 1967.