PULSE CONTROL SYSTEM FOR CALENDER SPEED USING THYRATRONS

A. P. ZAÎTSEV, A. I. ZAÎTSEV

(Presented by the scientific seminar of the Electromechanical Faculty)

The proposed system of pulse control of the speed of DC motors has been designed for a calender drive and is a further development of controlled ionic-drive systems. The purpose of creating this system is to extend the range of motor-speed control in pulse systems of ionic drive, to simplify the control circuit as much as possible, and to increase the reliability of the electric drive without any substantial deterioration in the quality of the speed-control process.

The operation of the circuit is based on the principle of regulating the average rectified voltage of an ionic converter by converting an alternating voltage into DC-voltage pulses of definite duration and frequency. The duration and amplitude of the DC-voltage pulses remain unchanged, while their repetition frequency can be varied over wide limits (0–50 pulses per second at a supply-network frequency \(f = 50\) cps). The average rectified voltage is directly dependent on the repetition frequency of the DC-voltage pulses.

In Fig. 1, the half-periods of the voltages of phases \(A\), \(B\), \(C\), during which the power-controlled valves conduct current, are shaded. The conducting half-periods of the phase voltages \(A\), \(B\), \(C\) make up pulses of DC voltage of constant duration and amplitude. The ignited valves

Fig. 1.

through a different number of periods of the alternating voltage, it is possible to vary the pulse repetition frequency, and hence also the average rectified voltage. Depending on the magnitude of the average rectified voltage, the rotational speed of the motor will change.

The rectified voltage is discontinuous in character. Because of this, for economical current limiting during starting, and also at low motor speeds, a choke can be used. The latter substantially broadens the range of motor-speed regulation while the amplitude of the voltage pulse remains unchanged and maximum. The operating principle of the system is explained by Fig. 2.

The frequency-divider converter 4 converts the alternating voltage of the supply network into voltage pulses with repetition frequency \(f\), which can be varied by means of a constant voltage

\[ U = U_z + U_{\text{f.b.}}, \]

where \(U_z\) is the reference voltage,

\(U_{\text{f.b.}}\) is the speed-feedback voltage.

The starting circuit 5 provides at its output a group of three voltage pulses shifted by 120 electrical degrees, with the same repetition frequency, and supplied as firing pulses to the controlled rectifier 1.

Fig. 2.

At the output of rectifier 1, voltage pulses \(U_f\) with repetition frequency \(f\) are obtained and supplied to motor 2.

If, under the action of the load torque, the speed of the motor—and consequently that of the tachogenerator 3—decreases, then the voltage \(U_{\text{f.b.}}\) will decrease, which will lead to an increase in the pulse repetition frequency \(f\). This, in turn, will cause an increase in the average rectified voltage supplying the motor, and consequently in its speed.

By changing the magnitude of the reference voltage \(U_z\), the rotational speed of the motor can be regulated.

In the initial state the control thyratron \(UT_1\) is blocked by the voltage \(U_z\) (Fig. 3). To the grids of the control thyratrons \(UT_2\) and \(UT_3\), through the phase-shifting circuits \(C_1 — R_1R_{k1}R_{k2}\) and \(C_2 — R_2R_{k4}\), respectively, are applied voltages of leading phases, shifted by 60 electrical degrees in the lead direction. The thyratrons \(UT_2\) and \(UT_3\) are closed, since alternating voltages are applied to their grids in antiphase with the anode voltages. A storage capacitor \(C_H\) is included in the grid circuit of \(UT_1\). In parallel with \(C_H\) a discharge thyratron \(RT\) is connected, which in the initial state is blocked by the voltage \(U_{\text{lock}}\). From the moment the circuit is switched on, voltage pulses are applied to the capacitor \(C_H\) through the valve \(B\) and resistance \(R_z\). The character of the charging and discharging of the capacitor \(C_H\) is determined by the charging time constant \(T_z = R_z C_H\) and the discharging time constant \(T_p = R_p \cdot C_H\), and is shown in Fig. 4.

The voltage on the capacitor \(U_c\) is subtracted from the locking voltage \(U_z\). When the resulting grid voltage \(U_g = U_z - U_c\) becomes equal to the firing voltage, the thyratron \(UT_1\) ignites (Fig. 5). Through the resistances \(R_{k1}\) and \(R_{k2}\), during one half-period, there flows

Fig. 3.

current under whose action a voltage pulse is produced and applied as a positive unlocking pulse to the grid of the power thyratron \(T_1\). The resistance \(R_{k2}\) is chosen so that the voltage drop across it, \(U_k\), is equal to or somewhat greater than \(U_{\text{lock}}\). In the grid circuit of the discharge thyratron \(PT\), while \(UT_1\) is conducting, the voltage difference \(U_{\text{lock}} - U_k \approx 0\) acts, and the latter is unlocked. The storage capacitor \(C_H\) rapidly discharges through the thyratron \(PT\) and the small resistance \(R_{k3}\). The thyratron \(UT_1\) is locked, and the circuit returns to its initial state.

After \(UT_1\) is locked, the thyratron \(PT\) is reliably locked by the voltage \(U_{\text{lock}}\) \((U_k = 0,\ U_c = 0)\). The capacitor \(C_H\) “counts off” the specified number of voltage pulses (Fig. 3).

By changing the value of the setting voltage \(U_z\), one can change the number of pulses “counted off” by the capacitor \(C_H\), and consequently also the firing frequency of the thyratron \(UT_1\).

The circuit built on the thyratrons \(UT_1,\ UT_2,\ UT_3\) is a trigger circuit. The resistance \(R_1 \gg R_{k1} + R_{k2}\), and therefore the voltage drop across

Fig. 4.

Fig. 5.

\(R_{k1}\) and \(R_{k2}\) from the current through the circuit \(C_1,\ R_1,\ R_{k1},\ R_{k2}\) at the time when \(UT_1\) is closed is negligibly small and has no effect on the operation of the circuit. When the thyratron \(UT_1\) conducts current, a voltage drop is produced across the resistances \(R_{k1}\) and \(R_{k2}\), positive at the grid of the thyratron \(UT_2\). By the time the conducting half-period of the anode voltage appears on the anode of \(UT_2\), there is no locking potential on the grid of \(UT_2\). \(UT_2\) conducts current; across the resistance \(R_{k4}\) a voltage drop is produced, applied “positive” to the grid of the power thyratron \(T_2\).

The thyratron \(UT_3\) operates in an analogous manner.

The unlocking pulses for the power thyratrons are taken from the cathode resistances of the control thyratrons through the resistances \(R_{g5}, R_{g6}, R_{g7}\). The frequency of these pulses is equal to the firing frequency of the control thyratrons.

To ensure ignition of the thyratron \(UT_1\) at the beginning of the conducting half-cycle of the anode voltage, charging voltage pulses with a leading phase are applied to the storage capacitor \(C_H\).

To stabilize the rotational speed of the drive motor, a negative speed-feedback voltage \(U_{\mathrm{f.b.}}\), taken from the tachogenerator \(TT\), is introduced into the grid circuit of the control thyratron \(UT_1\).

The accuracy with which the stability of the motor rotation is maintained depends on the magnitude and ratio of the resistances \(R_z\) and \(R_p\).

A system with speed feedback permits smooth stepless regulation of the motor rotational speed, since the average number of voltage pulses over a definite unit of time can be arbitrary.

Conclusions

The proposed circuit for pulse speed control has the following advantages:

1. A high power factor of the system, since the ionic devices operate in relay mode, with fully open grids.

2. The absence, in the control circuit, in comparison with known circuits, of peak chokes, which require precise manufacture and a high-quality magnetic core.

3. Simplicity of the control circuit, its high reliability, exceptional ease of adjustment, compactness, low weight and cost.

4. A wider speed-control range in comparison with other known pulse-control systems at unchanged voltage-pulse amplitude.

5. The inertia-free nature of the control circuit and high sensitivity.

6. Universality of the control circuit. The latter can be used without modification for ionic converters of various power ratings; in this case both thyratrons and controlled mercury-arc rectifiers may be used as the controlled valves.

7. The possibility of direct starting of the motor with an anode-ignition delay angle equal to zero.

8. Owing to the dependent operation of the control thyratrons \(UT\), ignition failures of ionic devices connected into different phases of the supply power network are eliminated.

The present pulse speed-control circuit was tested in the laboratory.

As the drive motor, a \(PN\)-145 motor was used, with the following rated data:

\[ P_n = 21\ \mathrm{kW}; \]

\[ I_n = 109\ \mathrm{A}; \]

\[ n_n = 1500\ \mathrm{rpm}; \]

\[ U_n = 220\ \mathrm{V}. \]

Thyratrons of type TR1-85/15 were used as the controlled valves.

When the motor torque was varied from \(0.15\,M_n\) to \(M_n\), the static speed drop was no more than \(2\%\). The motor rotational speed in this case was \(n = 1100\ \mathrm{rpm}\). A speed-control range of \(D = 3\) was obtained.

A broad prospect for wide application of this system opens up when the frequency of the alternating-current supply network is increased, since the supply-network frequency determines the upper limit of the repetition frequency of the rectified-voltage pulses, their duty factor, and their duration. The shorter the pulse duration, the lower the inductance of the choke required to limit the current at a specified level. As the pulse repetition frequency increases, the amplitude of the dynamic pulsation of the motor’s rotational speed decreases, other conditions being equal.

Received by the editors
in June 1962.