EXPERIMENTAL STUDIES OF THE DYNAMICS OF SELF-BRAKING ELECTRIC MOTORS WITH ELECTROMAGNETIC INSERTS

L. K. BURULKO, E. M. GUSELNIKOV, E. V. KONONENKO, O. G. MAKARCHENKO

(Presented by the scientific seminar of the departments of electrical machines and apparatus and general electrical engineering)

Increasing the productivity of actuating mechanisms is associated primarily with increasing speed and reducing the unproductive time spent on stopping the electric motor. Therefore, electrical-machine designers face the task of creating improved designs of electric motors for rapid and precise stopping, equipped with braking devices.

Such electric motors include the so-called self-braking electric motors (SED) with electromagnetic inserts [1]. A distinctive feature of the SED design is that brake control is carried out by means of the magnetic flux of the electric motor itself; that is, the stator magnetic field is used both to create the brake-release force on the brake and to create torque on the rotor.

The principal operating modes of SEDs with electromagnetic inserts are starting, reversing, and braking. Therefore, for the rational design and operation of such motors, knowledge of their dynamics is necessary.

At present, two directions have emerged in the study of the dynamics of electrical machines: experimental investigation and mathematical modeling. In the present work, the study of electromechanical transient processes in asynchronous self-braking electric motors 4A132M6E, 4A132M4ES, and AO-21-2-E was carried out experimentally.

The requirements imposed on self-braking electric motors include ensuring optimal brake-release and braking times and smooth operation of the braking device [1]. Therefore, the experimental studies of transients that were conducted pursued the goal of obtaining dynamic characteristics for various flywheel masses and resistance torques and determining the influence of the braking torque created by the spring of the braking device on the starting and stopping times of SEDs with electromagnetic inserts. The studies were carried out using an instrument for recording the mechanical characteristics of electrical machines in transient modes [2].

The principle of operation of this instrument is based on continuous measurement of rotational speed followed by its differentiation.

The dynamic characteristics of the electric motors under study were recorded with an N-102 loop oscillograph. To record the dependences of torque and speed as functions of time, MOO 1:4 vibrators were used, each of which was connected, respectively, to the speed and acceleration channel of the PMKh-2 instrument.

To investigate the influence of flywheel masses on transient processes, their magnitude was varied for the 4A132M6E motors as follows:

\[ \frac{GD_{\Sigma}^{2}}{GD_{\mathrm{rot}}^{2}} = 1;\ 2{,}02;\ 4{,}08, \qquad \text{AO2-21-2-E}\ \frac{GD_{\Sigma}^{2}}{GD_{\mathrm{rot}}^{2}} = 1;\ 2{,}39, \]

and for the 4A132 M4EC:

\[ \frac{GD_{\Sigma}^{2}}{GD_{\mathrm{rot}}^{2}} = 1;\ 2{,}44;\ 3{,}39. \]

Structurally, the 4A132M6E, AO2-21-2-E motors and the 4A132M4EC motor differ in that the first two are made with inserts on the rotor, while the last is made with an insert on the stator.

Figure 1 gives oscillograms taken for the 4A132M6E and 4A132M4EC motors. Here \(t_p\) is the starting time; \(t_{\mathrm{oc}}\) is the stopping time; \(t_{\mathrm{sr}}\) is the operating time of the electromagnetic insert when the motor is disconnected from the supply, measured from the moment of disconnection to the moment when the brake shoe comes into contact with the fixed part of the brake; \(t_t\) is the action time of the brake shoe during stopping, measured from the moment the brake shoe comes into contact with the fixed part of the brake to the moment when the motor speed reaches zero; \(M_{\mathrm{ud}}\) is the impact dynamic torque; \(M_T\) is the braking torque; \(i\) is the phase current of the stator winding.

The duration of the operating time of the electromagnetic insert is determined by the decay time of the magnetic flux when the motor is disconnected from the supply and by the magnitude of the holding tractive force.

Analysis of the oscillograms shows that, in motors with electromagnetic inserts on the rotor, under the action of the braking device the speed change initially follows a quadratic dependence and then a rectilinear one. In motors with an electromagnetic insert on the stator, the speed during braking changes rectilinearly. In addition, under completely identical conditions, the operating time \(t_{\mathrm{sr}}\) for electric motors with inserts on the stator is less than for motors with inserts on the rotor. All this can be explained by the fact that, after the motor is disconnected from the supply, the holding tractive forces in motors with electromagnetic inserts on the stator and on the rotor differ in magnitude. Thus, in motors with an insert on the rotor, the tractive force in this case is created by the flux linkage of the rotor winding and by the leakage fluxes of the short-circuited ring, whereas in motors with inserts on the stator it is created only by the flux linkage of the stator winding.

The results of the investigations are presented in the form of the dependences

\[ t_p = f(M_T);\quad t_{\mathrm{oc}} = f(M_T);\quad t_T = f(M_T);\quad t_p = f(M_c);\quad t_{\mathrm{oc}} = f(M_c);\quad t_T = f(M_c) \]

in Figs. 2 and 3. Analysis of the investigation results shows that the transient processes when the motors are switched on are accompanied by impact dynamic torques reaching three times the rated torque and higher. As the braking-torque values increase, the starting time remains approximately constant, whereas the decrease in stopping time reaches significant values. With an increase in flywheel masses, a proportional increase in the duration of the transient processes is observed both during starting and during brak-

Fig. 1. Oscillograms of current, speed, and dynamic torque versus time:
a) for a motor with an insert on the rotor;
b) for a motor with an insert on the stator.

coasting. With an increase in the load torque on the shaft, however, the duration of the transient processes during starting increases, while the braking time decreases. A significant increase in the braking torque, which is determined by the amount of spring compression, leads to braking during motor operation in the steady-state mode. This also explains

Fig. 2. Dependences of \(t_n;\ t_T;\ t_{oc}\) on \(M_c\) for values
\[ \frac{G D_{\Sigma}^{2}}{G D_{\mathrm{rot}}^{2}} = 3.39,\qquad M_T = 0.380 \]
and \(t'_n;\ t'_T;\ t'_{oc}\) for values
\[ \frac{G D_{\Sigma}^{2}}{G D_{\mathrm{rot}}^{2}} = 2.44,\qquad M_T = 0.380 \]
for the motor 4A132M4EC

Fig. 3. Dependences of \(t_n;\ t_T;\ t_{oc}\) on \(M_T\) for values
\[ \frac{G D_{\Sigma}^{2}}{G D_{\mathrm{rot}}^{2}} = 3.39, \]
\(t'_n;\ t'_T;\ t'_{oc}\) on \(M_T\) for
\[ \frac{G D_{\Sigma}^{2}}{G D_{\mathrm{rot}}^{2}} = 2.44, \]
and \(t''_n;\ t''_T;\ t''_{oc}\) on \(M_T\) for
\[ \frac{G D_{\Sigma}^{2}}{G D_{\mathrm{rot}}^{2}} = 1.00 \]
for the motor 4A132M4EC

some increase in the starting time when the braking torque is increased.

In conclusion, the following conclusions should be drawn:

1. As the flywheel masses of the drive increase, the magnitude of the braking torque must be increased.

2. The magnitude of the resisting torque on the shaft must not exceed the braking torque.

3. Smoother operation of the braking device is observed in self-braking electric motors with inserts on the rotor.

4. The braking system of asynchronous self-braking electric motors with electromagnetic inserts has no significant effect on transient processes during starting.

References

1. E. M. Guselnikov, B. S. Tsukerman. Self-braking electric motors. Moscow, “Energiya,” 1971.

2. A. I. Zaitsev, M. P. Tabinsky, A. S. Bobrovsky. Removal of mechanical characteristics of asynchronous motors by measuring the rotational speed and acceleration of motors in transient modes. Izvestiya TPI, vol. 117, 1963.