JUSTIFICATION FOR THE SELECTION OF AN ELECTRIC MOTOR AND ITS CONNECTION CIRCUIT FOR A PRECISION SPEED CONTROL SYSTEM

Yu. M. ACHKASOV, B. A. ZAKHAROV

(Presented by the Scientific and Technical Seminar of the Research Institute of AEM)

Precision machine building and instrument making, and new developments in geophysical, seismic, and video-and-sound-recording equipment, impose stringent requirements on the accuracy of maintaining the rotational speed of actuator electric motors. In precision speed control systems (PSC) using alternating current, various types of synchronous electric motors are used. Owing to astatic self-regulation, their rotational speed is determined only by the frequency of the supply current, which makes it possible to obtain an accuracy of maintaining the average speed of \((0.01 \div 0.001)\%\).

The electric motors of PSC systems for tape recorders, for example, are subject to the following requirements [1]:

instability of the rotor rotational speed must be within the limits \((10^{-3} \div 10^{-4})\);

the possibility of switching speeds in the range \(2:1,\ 4:1\);

the possibility of supply from a single-phase alternating-current network;

a low noise and vibration coefficient, and a minimal stray field;

heating not more than \(80^\circ\text{C}\) relative to the ambient environment.

In PSC systems where accurate stopping is required for the braking mode, the following requirements are imposed:

minimal braking and release time;

constancy of the braking torque during operation;

the possibility of regulating braking torques;

reliability in operation.

Such requirements are characteristic of most low-power PSC systems with stepwise speed control.

According to [2], the greatest uniformity of rotor rotational speed is possessed by synchronous motors with permanent magnets (SPM) and asynchronous starting. Hysteresis motors (HM) have poorer indicators of rotational uniformity than SPM. Because of the absence of a short-circuited winding on the rotor, they have greater rotor oscillations when the load changes. Synchronous reluctance motors (SRM), in terms of uniformity of rotor rotation, are inferior to SPM and HM, which is due to the complex spectral composition of the field in the machine gap. However, SPM and SM are produced for powers not exceeding tens of watts, have low energy indicators, a complex manufacturing technology, and require the use of expensive materials. Improved low-power SRM have energy-

…performance indicators close to those of asynchronous motors of the same frame size [3]. In addition, SRM have a simple manufacturing technology and are cheaper than other types of synchronous motors.

Fig. 1. Starting mechanical characteristics of synchronous electric motors.

Let us compare the starting characteristics of the motors under consideration, shown in Fig. 1. In the starting characteristic of an SDPM (curve 1, Fig. 1) there is a dip caused by the braking torque due to the action of the permanent magnets, and their demagnetization during starting is possible. In addition, SDPM have a high starting-current ratio. The starting torque of a hysteresis motor (curve 2, Fig. 1) decreases during starting, which increases the acceleration time. An advantage of the HM is the high pull-in torque and the smoothness of entry into synchronism; however, HMs have a wide scatter of characteristics. Three-phase SRM

Fig. 2. Circuit for connecting the field winding of an SRM to a single-phase network.

have a starting characteristic (curve 3, Fig. 1) similar to that of an asynchronous motor. Because of the presence of a critical torque, the SRM enters synchronism abruptly, which leads to speed overshoot. The characteristic of an SRM of the DRS150M type, connected according to the capacitor circuit (Fig. 2), is shown in Fig. 1 as curve 4. Starting is carried out with a constant starting capacitance, which is disconnected at the moment the motor enters synchronism. Characteristic features are acceleration of the motor with a constant starting torque and the presence of sufficient torque for entering synchronism, which ensures a minimum starting time.

In precision speed-control systems, as a rule, three-phase SRM powered from thyristor frequency converters (TFC) [4] are used. A block diagram of such a system is given in Fig. 3, a. The SRM is supplied from a three-phase controlled rectifier (CR) through a filter (F) and a three-phase inverter (A, B, C). The rectifier control device (RCD) and the inverter control device provide the required magnitude and waveform of the output voltage.

Fig. 3. Block diagrams of speed frequency-control systems.

In precision speed-control systems it is often necessary to have two or three levels of stable speeds with a smoothness of regulation of 1:2. In this case, simplification and cost reduction of the system, with a simultaneous increase in its reliability, are achieved by connecting a single-phase capacitor synchronous reluctance motor to a single-phase frequency converter (SPC), according to Fig. 3, b.

The single-phase capacitor synchronous reluctance motor (OK SRM) receives power through a rectifier, a filter, and a single-phase inverter controlled by the control device for the single-phase inverter (UUO I). In this case the number of power elements is reduced by two thirds, and the control circuit is substantially simplified.

It is necessary to select an autonomous single-phase inverter with a pulse-width method of stabilizing the output voltage and with a transformer output, which, in addition to the main function of frequency conversion, performs the role of a voltage regulator. If feedbacks are used in such a system, the smoothing filter is not included in the control loop, which increases the speed of response of the system.

Since in precision speed-control systems the motor most often operates with a constant load on the shaft and with large flywheel masses, the use of a capacitor SRM, with a properly selected capacitance, ensures uniformity of rotor rotation no worse than in the three-phase mode.

It is most expedient to use capacitor SRMs with three-phase stator windings, since for them, as for asynchronous motors [3], the utilization factor of the dimensions is equal to 1, and the total manufacturing costs are lower than for single-phase motors with two-phase asymmetrical stator windings, for which the utilization factor is 0.5–0.7. In addition, in the latter the starting-current multiple is 1.5–2 times higher.

Of the entire variety of circuits for connecting three-phase motors with capacitors to a single-phase network, only one circuit is acceptable for an SRM—the stator windings are connected in a star with a series phase-shifting capacitance (Fig. 2). In such a circuit there are no zero-sequence currents or third-harmonic and multiple-of-three currents.

harmonics, which improves the energy performance of the machine in comparison with the delta winding-connection circuit. In addition, with delta connection the required phase-shifting capacitance is 1.7 times greater than with star connection [5].

When the operating motor is disconnected from the power source (Fig. 2), automatic asymmetrical capacitor braking of the SRM occurs (the capacitance is connected to two phases). Studies of the DRS-150M motor have shown that, when the starting capacitance is connected simultaneously, the initial braking torque reaches 8–10 times the rated value. Since the capacitors of the capacitor-winding SRM constitute an integral part, the need for special braking capacitors is eliminated.

As experiment shows, when the capacitor-winding SRM is supplied with sinusoidal voltage, the motor delivers 100% of the power of the three-phase mode without overheating; at the same time the overload capacity of the motor is somewhat reduced.

However, the overload capacity of the motor can be increased in comparison with the three-phase mode by using an overload capacitance stage connected during motor overload.

When supplied from a frequency converter with a rectangular output-voltage waveform, the motor current increases by 6%, and the energy efficiency decreases by 9% in comparison with the symmetrical three-phase mode.

If the line voltage from the output of the frequency converter has a half-wave duration of 120 electrical degrees, then the third and its multiple harmonics are absent in such a voltage. Taking into account the filtering action of the phase-shifting capacitance, the waveform of the phase voltages is improved; therefore, the reduction in energy efficiency is 3–5%. To obtain a symmetrical operating mode of the motor at constant shaft load, it is necessary to choose the value of the capacitive reactance or capacitance according to the relations [6]:

\[ X_c = \frac{2 \sqrt{3}\,(R_1 + x_1)}{1 + \sqrt{3}};\quad C = \frac{10^6 (1 + \sqrt{3})}{4 \sqrt{3}\,\pi f (R_1 + x_1)}, \]

where

\(R_1;\ x_1\) are the equivalent active and inductive resistances of the equivalent circuit of the capacitor-winding SRM for the positive sequence.

A major advantage of the capacitor-winding SRM, as of the capacitor asynchronous motor, is its high power factor, which may practically be taken as equal to unity [7]. Consequently, the motor draws from the mains a current proportional only to the active power, since the source of reactive power for it is the capacitor. This is of considerable importance when supplied from an industrial network and especially from a high-frequency converter, since the problem of returning reactive energy from the motor to the power source is eliminated.

Thus, the use of mass-produced low-power three-phase SRMs as single-phase capacitor motors makes it possible to create a simple and economical precision drive system. The selected connection circuit of the capacitor-winding SRM meets the requirements imposed on executive motors of precision drive systems and is recommended for practical application.

REFERENCES

1. V. K. Kachan et al. Dispatcher Tape Recorders for Civil Aviation. Moscow, “Transport,” 1972.

1. E. A. Lodochnikov, F. M. Yuferov. Micromotors for Automation Systems. Moscow, “Energiya,” 1969.

2. V. V. Ralle. Study of a method for calculating synchronous reluctance motors. In: Contactless Electrical Machines. Part 1. Synchronous Machines. Riga, “Zinātne,” 1966.

3. V. N. Sharakhin. Measures for limiting oscillations of the instantaneous speed of an SRM. Proceedings of the Leningrad Polytechnic Institute, 1969, No. 303.

4. A. I. Adamenko, V. I. Kislenko. Conversion of Single-Phase Current into Polyphase Current. Kiev, “Tekhnika,” 1971.

5. Yu. M. Achkasov, B. A. Zakharov. Condition for obtaining symmetry of the magnetomotive forces of a three-phase SRM operating in a single-phase capacitor mode. Papers of the VII Scientific and Technical Conference, “Static Converters in Automation and Electric Drive.” Tomsk, Tomsk Polytechnic Institute Publishing House, 1971.

6. N. D. Toroptsov. A Three-Phase Asynchronous Motor in a Single-Phase Connection Circuit with a Capacitor. Moscow, “Energiya,” 1970.