Self-Reversing Electric Drive Systems

L. I. GANDZHA

In recent years, our national science has achieved major successes in the theoretical and experimental development of the principle of electric-machine automation of electric drives. At the same time, research is being conducted chiefly in the direction of automatic regulation of electric drives, forcing of transient processes, and the development of control circuits built on the principle of electric-machine amplification; the questions of applying electric-machine automation to reversal of the driving motor have remained completely untouched. Meanwhile, there exist circuits in which reversal can be obtained as a function of the drive operating mode and which thus make it possible to carry it out according to the principle of electric-machine automation.

Here the properties of a number of such circuits are discussed, since they are of interest in the sense indicated above.

Reversal of a direct-current driving motor supplied from a network with constant voltage is at present carried out by changing the direction of current either in the motor armature or in its field winding. The latter method has found limited application for motors with series excitation. An elementary circuit of the power circuit for the case of a motor with series excitation is shown in Fig. 1, from which it follows that reversal is carried out according to the principle of using relay-contactor apparatus, namely by contactors 1B, 2B, 1H, 2H. If it is taken into account that in the control circuit of a reversible motor, in addition to the four contactors indicated, it is also necessary to have a number of devices acting on the coils of the contactors mentioned, it becomes clear that such a circuit, as regards motor reversal, proves to be rather complex.

Fig. 1

Fig. 2

If the driving motor is supplied directly from a generator, then its reversal is carried out by changing the direction of current in the generator field winding according to Fig. 2. In this case the number of reversing contactors remains the same as in Fig. 1, if one does not take into account

attention to devices for providing forced excitation of the generator.

Reversing control circuits can be considerably simplified if the principle of electromachine automation of reversal is taken as the basis of their design. Such systems are presented below.

1. The “series-excited generator—separately excited motor” system

This system is shown in Fig. 3. Some of its properties are described in [1—6]. The principal properties of the system reduce to the following:

1). If the load current of the motor in steady-state operation is less than the critical current $I_{\mathrm{cr}}$ (Fig. 4), then undamped oscillations arise in the system, both of electrical and of mechanical quantities (current, emf, torque, speed). Here the current $I_{\mathrm{cr}}$ is determined by point $D$, formed by the tangent to the generator magnetization curve

\[ e_g = f(I), \]

and the parallel straight line

\[ I R = \varphi(I), \]

representing the voltage drop in the power circuit of the system.

2). If the load current of the motor in steady-state operation is greater than the current $I_{\mathrm{cr}}$, then the system is not prone to oscillations and the drive operates stably.

Thus, if the drive is operating on some reversing mechanism and is under a load satisfying condition 2, then after the mechanism and the drive go into no-load operation, at which the drive load satisfies condition 1, the motor reverses owing to the tendency of the system toward oscillations.

Fig. 3

Fig. 4

If, after reversal, a load satisfying condition 2 is again applied to the motor, the motor will operate stably, rotating in the reverse direction.

2. The “mixed-excitation generator—separately excited motor” system

The schematic diagram is shown in Fig. 5. Some properties of the system are described in [7, 8]. With the parallel winding of the generator connected, the drive motor operates in steady-state mode at a constant speed corresponding to the load. If, however, a short-time impulse is applied to the circuit to disconnect the parallel winding of the generator, then the circuit of Fig. 5 is for a short time reduced to the circuit of Fig. 3; in this case the motor reverses, and since the polarity of the generator brushes, due to the series field winding, changes to the opposite one, upon subsequent connection of the parallel field winding

of the generator, the direction of the current in it will change. The generator will then be excited up to normal voltage with reverse polarity at the brushes, and the motor will acquire normal speed with the reverse direction of rotation.

Self-reversal of the motor can also be obtained in a system with a shunt-excited generator provided with additional poles. In the latter case, the excitation winding of the additional poles will play the role of the series winding in the circuit of Fig. 5.

3. The “shunt-excited generator—separately excited motor” system

The schematic diagram is shown in Fig. 6. Here it is assumed that the generator has only one shunt excitation winding.

If the generator brushes are located on the neutral line, then when the shunt excitation winding of the generator is briefly disconnected, self-reversal of the motor does not occur.

Fig. 5                Fig. 6

To obtain self-reversal of the motor, a small displacement of the brushes from the neutral line is sufficient; in this case a longitudinal component of the generator armature-reaction field appears, acting along the poles. If the excitation winding of the generator is now briefly disconnected from the brushes, then, with the brushes shifted in the appropriate direction, the longitudinal component of the armature-reaction field, when the current in the power circuit reverses, will induce at the generator brushes an emf from the residual magnetism of opposite polarity, by which the generator will be excited up to the normal voltage of reverse polarity upon subsequent connection of the shunt excitation winding. The reverse-polarity voltage of the generator will also cause reversal of the motor.

4. The “generator with exciter—separately excited motor” system

This system was found by S. S. Kropanin and is shown in Fig. 7. With appropriate parameters of the circuit of the generator excitation winding, undamped oscillations of the generator excitation current also arise in it, as a result of which the generator is periodically remagnetized. Remagnetization of the generator causes a periodic change in the polarity of the voltage at its brushes, and this, in turn, gives rise to periodic changes in the direction of rotation of the motor.

It is easy to see that the resistance of the generator excitation winding must be such that the generator excitation current, calculated as

\[ i_{в.г.к.}=\frac{U_{в}}{r_{о.в.г.}}, \]

would be greater than the current \(i_{\max c}\), corresponding to point “\(a\)” of the external characteristic of the exciter (Fig. 8). In this formula:

\(U_{\mathrm{в}}\) is the exciter voltage, and
\(r_{\mathrm{о.в.г.}}\) is the resistance of the generator field winding.

When the exciter is connected to the generator field winding, the field current of the latter tends toward the value \(i_{\mathrm{в.г.к.}}\); in this case the exciter

Fig. 7

is in a regime close to a short circuit, and its voltage is almost equal to zero. Since, as the generator field current passes through the value \(i_{\max c}\), its field winding has stored magnetic energy, the self-induction e.m.f. of the field winding

Fig. 8

of the generator becomes a source of current, sending it in the reverse direction. In this process the exciter is remagnetized and, when the current from the self-induction e.m.f. becomes equal to zero, it will send current in the direction opposite to that which existed at the beginning of the preceding period.

5. System 1 with Shunting of the Motor Armature

System 1, described above, does not give a steady-state regime at motor no-load, since the no-load current is less than the critical current. Therefore, motor no-load operation will be accompanied by undamped oscillations, which in a number of cases is inadmissible. This can be avoided at no-load by using a parallel field winding of the generator. In this case systems 2 or 3 are obtained. A steady-state regime without oscillations can also be obtained by shunting the motor armature at no-load with an ohmic resistance, according to Fig. 9.

In this case the shunting resistance, in magnitude, must be such that the resulting generator current, equal to the sum of the current

the motor no-load current and the current in the shunting resistance, would be greater than the critical current (Fig. 10), that is,

\[ I_2 = I_{\mathrm{m.n.l.}} + I_{\mathrm{sh}} > I_{\mathrm{cr}}. \]

Under this condition, the generator regime is determined by a point on the magnetization characteristic located to the right of the critical point \(D\), that is, in the region corresponding to stable operation of the circuit (Fig. 10).

The self-reversing electric-drive systems described above by no means exhaust all the questions touched upon; they are far from perfect and require further profound theoretical and experimental development.

Fig. 9

Fig. 10

However, from the foregoing it is clear that the self-reversal of the motor in these systems, caused by their tendency to oscillate, may be interpreted as an element of electromechanical automation of electric drives.

References

1. E. Arnold. Direct-Current Dynamo Machines, Vol. I, p. 794. St. Petersburg: “Printed Labor,” 1909.

2. E. Arnold, Prof. I. L. La Cour. Direct-Current Machines, Vol. I, p. 492. Gostekhizdat, 1931.

3. K. I. Shenfer. Direct-Current Dynamo Machines and Motors, p. 383. Gosenergoizdat, 1934.

4. N. N. Rukavishnikov. Collection of Problems on Direct-Current Electrical Machines, p. 455. KUBUCH, 1933.

5. M. V. Shuleikin. “Self-Excitation of Electrical Oscillations in the Circuits of Commutator Dynamo Machines.” Separate offprint from Proceedings of the St. Petersburg Polytechnic Institute, Vol. XX, p. 108, 1913.

6. M. V. Shuleikin. “On Free Oscillations in the Circuit of a Series Generator and Motor.” Proceedings of the St. Petersburg Polytechnic Institute, Vol. XI, Issue 2, p. 735, 1909.

7. V. K. Popov. The Application of Electric Motors in Industry, Part II, Issue I, p. 398. Leningrad: KUBUCH, 1935.

8. D. P. Morozov. “On Oscillatory Processes in Electric Drives.” Electricity, No. 8, 1945.