Principles of Design of High-Speed Reversible Electric Drives with Thyristor Pulse-Width Converters with a Variable Commutation Law

V. A. Beinarovich, L. S. Udut, V. S. Fadeev, A. E. Deev

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

The use of low-inertia direct-current electric motors of the PGT, MIG, and other series, with a smooth armature or a printed armature winding, makes it possible in principle to implement electric-drive systems whose principal quality indicators prove to be substantially higher than when electric motors of general-purpose industrial series P, PBST, and MI are used. Owing to the fact that the time constants of low-inertia motors of the PGT type are several tens of times smaller than those of motors of general-purpose industrial series, and their current overload capacity reaches 8–10, the speed of response of such electric drives is no worse than that of the fastest type of drive itself—an electrohydraulic drive. Taking into account that a hydraulic drive is substantially inferior to an electric drive in operating characteristics and in certain technical indicators, the development of the latter has great prospects.

Full utilization of the dynamic properties of low-inertia electric motors in high-speed electric-drive systems is possible only provided that they are controlled from converters whose overload capacities and dynamic properties are sufficiently high and do not limit the speed of response of the drive. The regulating properties of converters are also of substantial importance; these are determined by the principle of construction of the converter and by the law governing its control.

For high-speed electric-drive systems with low-inertia direct-current motors, among the known converters, under the conditions indicated above, the most expedient is the use of reversible pulse-width converters (RPWC).

Four possible modes of controlling an RPWC are known; their analysis is given in [1]. The requirements imposed on RPWCs for high-speed electric drives are fully satisfied by converters with joint (symmetric and asymmetric) control. However, the practical implementation of such control in thyristor RPWCs proves difficult because of the occurrence of through short-circuit currents as a result of failures of forced commutation. In [2], RPWCs are proposed with an alternating asymmetric commutation law under separate control of the groups; these possess increased commutation reliability, but do not provide effective braking modes of the motor. In [1], for high-speed electric drives, RPWCs with separate control were recommended

and a symmetrical commutation law, ensuring smooth regulation of the drive current and speed in motoring and braking modes. However, because of the increased current pulsations in the armature winding, such RPWTCs provide poorer energy characteristics of the electric drive.

In the present work, questions are considered concerning the construction of RPWTCs with separate control and a variable commutation law: asymmetrical in the motoring mode of operation (in quadrants I and III of the plane of mechanical characteristics) and symmetrical in the braking mode (in quadrants II and IV). In such systems it is possible to substantially improve the energy characteristics and to obtain effective braking modes with increased commutation reliability of thyristor RPWTCs.

Fig. 1.

With separate control of an RPWTC, the simplified schematic diagram of the power section of which is denoted in Fig. 1 by PC, for each direction of rotation of the electric motor only one group of valves operates: the odd group (T1, T3) or the even group (T2, T4); control pulses are not applied to the thyristors of the other group. The forced-commutation unit PK1 operates at the selected control frequency and switches off thyristors T1 or T2 in each commutation period. The forced-commutation unit PK2 is switched on only at the instants when the sign of the control action changes, i.e., during reversal; consequently, thyristors T3 or T4 remain continuously switched on over the entire interval of motion in one direction.

With a symmetrical commutation law, the forced-commutation units PK1 and PK2 operate synchronously and switch off thyristors T3 (or T4) simultaneously with T1 (or T2).

In the general case, control of the converter with a variable commutation law on the interval with asymmetrical and symmetrical commutation laws is described in relative units, respectively, by the equations

\[ \tau = E^{*} + I^{*}_{\text{a dop}}; \]

\[ \tau = 0.5 \left( 1 + E^{*} + I^{*}_{\text{a dop}} \right) \]

and is presented in Fig. 2. The equation of the switching line for the commutation law has the form \(\Omega^{*} + I^{*}_{\text{a}} = 0\), which makes it possible to obtain the following law for changing the type of commutation:

a) if \(U_{y1} > 0\) and \(\Omega^{*} + I^{*}_{\text{a}} \ge 0\), or \(U_{y1} < 0\) and \(\Omega^{*} + I^{*}_{\text{a}} \le 0\), then the asymmetrical commutation law is implemented;

b) if \(U_{y1} > 0\) and \(\Omega^{*} + I^{*}_{\text{a}} < 0\), or \(U_{y1} < 0\) and \(\Omega^{*} + I^{*}_{\text{a}} > 0\), then the symmetrical commutation law is implemented.

Fig. 2.

In order to simplify the implementation of a converter with a variable commutation law, it is possible to restrict oneself to satisfying the following simplified condition:

a) if \(U_y>0\) and \(\Omega>0\), or \(U_y<0\) and \(\Omega<0\), then the asymmetrical commutation law is implemented;

b) if \(U_y>0\) and \(\Omega<0\), or \(U_y<0\) and \(\Omega>0\), then the symmetrical commutation law is implemented.

In this case the line at which the commutation law changes passes along the current coordinate axis.

The block diagram of the control device (CD) of the reversible thyristor pulse-width converter (RTPWC), implementing the proposed variable commutation law, is shown in Fig. 1. Two pulse-width modulators, PWM I and PWM II, are controlled through two separate inputs \(U_{y1}\) and \(U_{y2}\) from the feedback block FB and the current-limiting block CLB. Modulator PWM I operates at \(U_{y1}>0\), and PWM II at \(U_{y1}<0\).

In principle, the CLB may be made according to a leading current-limiting circuit or a current cut-off circuit with nonlinear elements. The operation of all elements is synchronized by a master oscillator MO of constant or adjustable frequency. The pulse shaper PS and unit FC1 operate at the same frequency. The converter thyristors of the odd (T1, T3) and even (T2, T4) groups are controlled from the corresponding modulator through switches SW1, SW2 and pulse shapers F1, F2. The logic block LB, through switches SW1 and SW2, controls the operation of shapers F1 and F2 when the sign of the control voltage \(U_{y1}\) changes, and through switch SW3 controls the operation of unit FC2 in accordance with the condition specified above for switching the converter commutation law. To increase the reliability of converter operation, two protection blocks PB1 and PB2 are provided; they monitor the readiness of the forced-commutation units FC1 and FC2 for the process of turning off the power thyristors.

Experimental studies of electric-drive systems with an RTPWC-D drive and a converter having separate control and a variable commutation law have shown that the proposed principles for constructing reversible pulse-width converters make it possible to improve the energy and dynamic characteristics of the drive. A method for calculating the static characteristics of such converters and electric drives based on their application is set forth in [3].

References

1. V. A. Beinarovich, L. S. Udut, V. S. Fadeev, N. G. Volkov. Methods of controlling reversible pulse-width converters. Proceedings of the VII Scientific and Technical Conference. Static Converters in Automation and Electric Drives. Tomsk, 1971.

2. A. M. Alter, M. E. Golts, L. A. Shpigler. Reversible electric drives with thyristor pulse-width converters, with an ultra-wide range of speed regulation. Elements of Electrical Equipment and Automation for Metal-Cutting Machine Tools. Moscow, NIIMASH, 1968.

3. V. A. Beinarovich, L. S. Udut, V. S. Fadeev, A. E. Deev. A method for calculating the static characteristics of electric drives with thyristor pulse-width converters with a variable commutation law. The present collection.