STARTING AND REVERSING ROLLER-TABLE MOTORS WITH PRE-CONNECTED ACTIVE RESISTANCES

E. V. Kononenko, B. A. Danchinov

(Presented by the scientific seminar of the Departments of Electrical Machines and General Electrical Engineering)

Roller-table asynchronous motors are intended mainly for the individual drive of the rollers of working and transport roller tables at metallurgical enterprises.

In accordance with the requirements of the technological process of metal rolling, they operate in intermittent short-time duty with a large number of switchings per hour, especially in the drives of working roller tables of reversing rolling mills. Under these conditions, starting and reversing, accompanied by transient processes, are among the principal operating modes of roller-table motors.

The large pulsating dynamic torques arising under transient conditions substantially reduce the reliability and service life of the drive, and at times may even prove dangerous. Therefore, the questions of limiting impact torques and currents during transient processes are of great practical importance.

In addition, in a number of drives, in particular in roller-table drives, limitations are imposed not only by the strength of the kinematic links of the drive, but also by the magnitude of the maximum permissible torque and acceleration (from the conditions of obtaining the necessary acceleration). The point is that, during starting and reversing of roller-table motors with rolled metal, the acceleration must not exceed a certain value that ensures the absence of slipping between the metal and the roller. The limiting value of the acceleration is determined by the limiting adhesion force between the metal and the roller.

A rational acceleration value has been established on the order of \(1.47 \div 2.94\ \text{m/sec}^2\) for hot metal and \(0.98 \div 1.47\ \text{m/sec}^2\) for cold metal, excluding its slipping [1].

Exceeding the value of the maximum permissible acceleration leads to slipping of the roller along the rolled product and, consequently, to a reduction in the actual acceleration and an unnecessary loss of time, and to a decrease in the productivity of rolling and of metal transport.

The indicated limitation determines the uniformly accelerated character of the transient processes of roller tables.

Such a course of transient processes in asynchronous squirrel-cage motors can be ensured by known methods of limiting impact currents and torques, for example, by connecting active, inductive, capacitive, active-inductive, and active-capacitive resistances into the stator circuit [2, 3, 4]; by connecting the motor to a network of regulated voltage [3]; by nonsimultaneous switching of phases by means of semiconductor controlled valves [3, 5, 6]; by damping

of the nondecayed rotor field by short-circuiting the stator winding or by connecting it to a discharge resistance (during reversals, repeated switchings-on, and braking of motors with a large time constant of the decaying field) [3, 7].

The simplest and most widely used method for limiting surge currents and torques is the method of using pre-connected resistances during transient conditions.

The limiting effect of inductive resistances is weaker than that of active resistances, since inductance does not help suppress the periodic component of the transient torque [2, 3].

The use of active-capacitive pre-connected resistances has a good limiting effect while preserving the speed of response of the drive. However, in this case the phenomenon of asynchronous self-excitation and unstable operation of the motor near the steady-state speed are possible. In addition, the use, in this method, of capacitors of considerable dimensions requires a techno-economic analysis, especially for the individual drive of roller-table rollers.

It follows from the foregoing that, in most cases, it is preferable to use active resistances as pre-connected elements. However, it should be borne in mind here that an inevitable consequence of the limiting action of additional active resistances is a decrease in the duration of electromagnetic transient processes and an increase in the duration of transient conditions as a whole. It is therefore necessary to determine the optimum value of the additional resistances, at which a sufficient reduction of surge currents and torques can be obtained with only a slight increase in the duration of the transient condition.

This article presents some results of an investigation of the effect, on the course of transient processes, of symmetrically connected additional active resistances in the stator winding of a roller-table motor having the following basic parameters in relative units:

\[ r_s = 0.05,\qquad r_r = 0.2,\qquad x_m = 2.05,\qquad J_{\mathrm{rot}} = 61.5. \]

Since the time of transient processes depends substantially on the moment of inertia of the drive, the investigations were carried out for various total moments of inertia \(J_{\Sigma}\) (with \(k_J = \dfrac{J_{\Sigma}}{J_{\mathrm{rot}}} = 1,\ 2,\ 5,\ 8\)).

The calculations were performed on an analog computer of type MN-14 according to the equations and model circuit given in [8]. In this case, the pre-connected resistances were taken into account by a corresponding increase in the active resistance of the stator winding. In this case the first two equations of system (4) in [8] take the form:

\[ \frac{d\psi'_{s\alpha}}{dt} = u_{s\alpha} - (r_s + r_{\mathrm{add}}) i_{s\alpha} + \psi'_{s\beta}, \]

\[ \frac{d\psi'_{s\beta}}{dt} = u_{s\beta} - (r_s + r_{\mathrm{add}}) i_{s\beta} - \psi'_{s\alpha}. \]

The additional resistances and the total moment of inertia were varied by changing the corresponding transmission coefficients of the computing blocks.

Figures 1 and 2 present oscillograms of starting and reversal, respectively, for various additional resistances in the stator circuit (\(J_{\Sigma}=J_{\mathrm{rot}},\ M_c=0\)), and Figs. 3 and 4 show the dependences of the transient characteristics on the ratio

\[ \frac{r_{s\Sigma}}{r_s} = \frac{r_s + r_{\mathrm{add}}}{r_s} \]

and \(J_{\Sigma}\), also for starting and reversal.

Fig. 1. Oscillograms of starting with various additional resistances in the stator circuit.
a — \(r_{s\Sigma}=r_s\); b — \(r_{s\Sigma}=2r_s\); c — \(r_{s\Sigma}=5r_s\); d — \(r_{s\Sigma}=10r_s\).

Fig. 2. Oscillograms of reversing with various additional resistances in the stator circuit.
a — \(r_{s\Sigma}=r_s\); b — \(r_{s\Sigma}=2r_s\); c — \(r_{s\Sigma}=5r_s\); d — \(r_{s\Sigma}=10r_s\).

As follows from the oscillograms, an increase in \(r_{\text{add}}\) leads to a monotonic reduction in the peak values of torque and current; the number of pulsations in the torque curve at the initial stage of the transient process increases, while the amplitude of the pulsations simultaneously decreases, and at the end of the transient process the pulsations disappear; as a result of the decrease in the amplitude of the torque pulsations, the rotor speed changes more smoothly, with lower acceleration (deceleration), since the average value of the driving (braking) torque decreases.

The magnitude of the drive moment of inertia has practically no effect on the magnitude and variation of the peak values of torque and current.

The reduction of \(M_{\text{ud}}\) and \(i_{\text{ud}}\) and the increase in starting and reversing time (in Figs. 3 and 4, the starting time \(t_p\) and reversing time \(t_r\) are taken as the time required for the rotor to reach a speed equal to 0.95 of the steady-state value) at \(r_{ss} = (2 \div 3)r_s\), in comparison with the case in which no additional resistances are present, are given in the table:

Fig. 3. Dependences of the peak current and torque and of the starting time on the magnitude of the additional resistances in the stator circuit during starting

Fig. 4. Dependences of the impact current and torque and of the reversal time on the magnitude of the additional resistances in the stator circuit during reversal

If the permissible increase in the starting and reversing time when additional resistances are connected into the stator circuit is taken to be 25–30%, then, as is clear from the table, quite satisfactory results can be obtained at \(r_{s\Sigma}=3r_s\) in the starting mode and at \(r_{s\Sigma}=2r_s\) in the reversing mode.

However, the connection of different additional resistances during starting and reversal is undesirable because of the considerable complication of the control circuit. Therefore, in practice, the connection \(r_{\text{add}}=r_s\) may be recommended both for starting and for reversal.

REFERENCES

1. V. P. Bychkov. Electric Drive and Automation of Metallurgical Production. Vysshaya Shkola, 1966.

2. A. D. Gildebrand, N. I. Zenkin, V. M. Kirpichnikov, N. I. Tomashevskii. Investigation of \(r_d\), \(x_d\), \(z_d\) for limiting the torques of an asynchronous motor on an analog computer. Proceedings, 1967, Collection 151, UPI.

3. M. M. Sokolov, L. P. Petrov, L. B. Masandilov, V. A. Ladenzon. Electromagnetic Transient Processes in an Asynchronous Electric Drive. “Energiya,” 1967.

1. M. P. Rashkovich, N. K. Shaparev. Dynamic overloads during starts and reversals of squirrel-cage induction motors. Elektrotekhnika, No. 12, 1967.

2. V. A. Shubenko, I. Ya. Braslavskii, V. M. Kirpichnikov, A. N. Kovshov. Study of the dynamics of starting induction motors under thyristor control. Elektrotekhnika, No. 6, 1969.

3. W. Werner. Übersicht über die Einschaltmomente bei Asynchronmaschinen im Stillstand. Elektrotech. Z., 1967, A88, No. 22.

4. V. A. Shubenko, N. I. Zenkin, A. V. Karochkin. On the question of the influence of electromagnetic transient processes on the principles of constructing automatic-control circuits for squirrel-cage induction motors. Proceedings of UPI, 1960, collection 106.

5. E. V. Kononenko, B. A. Danchikov. Investigation of electromechanical transient processes of roller-table motors on an analog computer, taking into account saturation of the steel by leakage fluxes. Article in the present collection.