SPEED STABILIZATION SYSTEM FOR A SHUNT-WOUND MOTOR

V. A. Beinarovich, A. I. Sapozhnikov, Yu. P. Serdyukov,
V. K. Smirnov

(Presented by the Scientific and Technical Seminar of the Research Institute of Automation of Electric Machines)

In a number of installations powered from direct-current sources, it is necessary to ensure stabilization of the rotational speed of shunt-wound drive motors with an accuracy on the order of one percent when the source voltage fluctuates within the range from \(+50\%\) to \(-25\%\) and the load varies from no-load operation to 1.5 times the rated load. An additional condition is the absence in the system of other electric-machine devices, including a tachogenerator. Proceeding from these requirements, as well as from operating conditions, at the Research Institute of Automation of Electric Machines, jointly with the Kharkov “Elektromashina” plant, a speed stabilization system was developed that can be applied under any conditions of a similar purpose. The stabilization system is constructed on the principle of monitoring the rotational frequency of the motor with speed regulation by excitation.

The frequency method of measuring speed eliminates the possibility of introducing additional errors by the sensor and the transmission line. Of the known sensors for measuring speed, the developed system uses a simple inductive transformer sensor, supplied with increased-frequency voltage and built into the electric motor. The sensor consists of a stationary U-shaped laminated magnetic core with primary and secondary windings and a toothed armature without windings, mounted on the shaft of the electric motor. The sensor converts the rotational speed of the motor into the frequency of the envelope that modulates the output voltage of the sensor. The modulated voltage is converted by a sensitive element into a control voltage that depends on the magnitude of the deviation of the rotational speed (modulation frequency) from the specified speed (specified frequency). As the sensitive element—the frequency discriminator—the developed system uses a frequency-selective amplifier with a capacitor metering device and a low-pass filter [2].

The use of digital and phase measuring devices with quartz oscillators and frequency dividers in the stabilization systems under consideration, with an accuracy of \(\pm 1\%\), is inexpedient because of their complexity, high cost, and excessive accuracy.

The motor speed is changed by regulating the current in the excitation winding by means of a thyristor switch [3], which shunts an additional permanently connected resistance in the circuit of the exci-

the field winding by means of pulse-width modulation. Such a configuration of the field-current control circuit ensures high operating reliability and eliminates the possibility of emergency motor conditions in the event of failure of the thyristor regulator (in the case of breaks and short circuits in the thyristor-switch circuit).

The functional diagram of the developed stabilization system is shown in Fig. 1. The carrier-frequency generator (CFG) supplies a transformer inductive sensor (TrS); the output voltage of the latter, modulated by the rotational frequency, is fed to a selective amplifier (SA). The control voltage from the output of the SA is fed through

Fig. 1

a summing unit to the input of the motor-voltage regulator (MVR), for the purpose of stabilizing its rotational speed. The MVR regulator is supplied by the same voltage \(U_c\) as the motor. A signal from the time-delay device (TD) is also fed to the input of the summing unit; this device ensures that the full voltage is obtained at the output of the MVR regulator for a certain time from the moment the supply voltage is switched on (the motor supply and the stabilization system are switched on simultaneously by one common device), which is necessary for starting the motor at full field flux and for its subsequent transition to operation with a weakened field.

The developed stabilization system is implemented using semiconductor elements. As the thyristor voltage regulator (MVR), it employs an RNT-type regulator, mass-produced by the Kharkov “Elektromashina” plant [1]. The basic electrical circuit of the control device of the stabilization system is shown in Fig. 2. The carrier-frequency generator is built on transistor \(T_1\), in the emitter circuit of which the primary winding \(W_1\) of the transformer inductive sensor TrS is connected. The modulated voltage from the secondary winding \(W_2\) of the sensor is detected by the diode bridge \(D_1\)–\(D_4\), filtered by capacitors \(C_4\), \(C_5\), and fed through the matching emitter follower \(T_2\), \(T_3\) to the amplifier \(T_4\), \(T_5\), \(T_6\), at whose output rectangular pulses are formed, having a modulation frequency proportional to the rotational speed. The rectangular pulses are then fed to the selective amplifier, built on transistors \(T_7\), \(T_9\) and containing the metering device \(D_5 C_9 R_{18} R_{19}\), the low-pass filter \(C_{10} R_{20}\), and the threshold element \(D_7\). As the speed increases, the voltage across \(R_{20}\) rises, the Zener diode \(D_7\) breaks down, and transistor \(T_9\) begins to turn off under the action of the difference between the voltage across \(R_{20}\) and the voltage of Zener diode \(D_7\). The voltage on capacitor \(C_{12}\) then increases and, through the isolating diode \(D_9\), acts on the thyristor voltage regulator.

Fig. 2.

(RNT) in the direction of increasing the excitation current, which leads to a decrease in the motor’s rotational speed.

To ensure the full excitation flux of the motor during starting, a time relay based on transistor \(T_8\) is used; it shunts the input of \(T_9\) for the duration of the charging of capacitor \(C_{11}\) when the power is switched on, which causes transistor \(T_9\) to be cut off and the maximum voltage to appear at the circuit output (at \(C_{12}\)). Contacts \(P_1\), \(P_2\) in Fig. 2 belong to the motor’s magnetic starter.

The required stabilization accuracy is achieved with an open-loop system gain greater than 100. The supply voltage of the metering device (\(+50\) V) must be stabilized to the maximum degree and, in any case, not below the specified speed-stabilization accuracy. The carrier frequency of the generator on \(T_1\) is chosen to be an order of magnitude higher than the highest modulation frequency, determined by the product of the speed in revolutions per second and the number of teeth of the speed-sensor armature. The device supply voltages are supplied from stabilizers connected to the motor supply-source voltage through damping resistors.

REFERENCES

1. Yu. L. Serdyukov, M. M. Feldman, K. A. Shuklin. Regulation of the excitation of DC electric machines using semiconductor amplifiers based on thyristors. Izv. vuzov, “Elektromekhanika,” No. 1, 1968.

2. L. A. Baranov, G. S. Gershenzon, V. I. Dmitriev, A. E. Knyazhinskii. Capacitor Converters in Automation and Control Systems. Automation Library. Moscow–Leningrad, “Energiya,” 1969.

3. M. I. Kraitsberg, E. V. Shikut. Pulse Methods for Regulating DC Circuits Using Thyristors. Automation Library. Moscow–Leningrad, “Energiya,” 1969.