Starting and Reversing Roller Table Motors at Different Initial Speeds

E. V. Kononenko, B. A. Danchinov

AR-series roller table motors are intended for the individual drive of the rollers of working and transport roller tables at metallurgical enterprises. In the practice of roller table operation, modes of repeated switching-on of rotating motors are encountered, as a result of which the motor rotor either accelerates (starting), or decelerates (braking), followed by acceleration in the opposite direction (reversing). In this case, at the moment of repeated switching-on, the rotor speed may have different values, depending on the speed at the moment when the motor is disconnected from the network, on the duration of the power interruption, on the parameters of the motor and the drive, and on the load resistance torque.

Studies of similar modes for general industrial asynchronous motors have shown that these modes are among the most severe [1, 2]. This is due to the fact that general industrial motors have a relatively large time constant of the rotor winding, as a result of which repeated switching-on occurs with a significant magnitude of the decaying magnetic field of the rotor.

Since roller table motors, unlike general industrial asynchronous motors, have certain specific features, the questions of their investigation under repeated switching-on are of definite interest.

Roller table motors operate mainly in intermittent-duty modes with a large number of switchings per hour. For such operating modes, as is known [3], it is expedient to use motors with increased slip, possessing an increased active resistance of the rotor. In roller table motors, the active resistance of the rotor is increased by a factor of 4–6 by reducing the cross section of the rotor bars and casting them with an aluminum–magnesium alloy of increased specific resistance.

Taking into account the influence of the undamped magnetic field of the rotor on repeated-switching modes, we shall analyze the decay of this field for roller table motors.

After the motor is disconnected from the network, the rotor flux linkage at the first instant remains approximately unchanged and is maintained by the free aperiodic rotor current, i.e.,

\[ \psi_r(-0) \approx \psi_r(+0), \]

where \(\psi_r(-0)\) is the rotor flux linkage before disconnection;

\(\psi_r(+0)\) is the rotor flux linkage after disconnection.

During the disconnected state (during the time between disconnection and reconnection), the rotor flux linkage decays with the no-load time constant \(T_{ro} = \dfrac{x_r}{\omega_1 r_r}\) according to the following law:

\[ \psi_{ro} = \psi_r(+0)\exp\left\{-\frac{t}{T_{ro}}\right\}\exp\{-j(\omega_k-\omega)t\}, \]

where
\(x_r\) is the total inductive resistance of the rotor winding;
\(r_r\) is the active resistance of the rotor winding;
\(\omega_1\) is the angular velocity of rotation of the stator field;
\(\omega_k\) is the angular velocity of rotation of the coordinate axes;
\(\omega\) is the angular velocity of rotation of the rotor.

In a coordinate system stationary with respect to the rotor \((\omega_k=\omega)\), we have

\[ \psi_{ro} = \psi_r(+0)\exp\left\{-\frac{t}{T_{ro}}\right\}. \]

Calculations performed for the time constant for the entire series of roller-table motors showed that \(T_{ro}\) (at the operating-temperature condition, \(120 \div 160^\circ\text{C}\)) lies, for different motors, within the range \(0.005 \div 0.035\) sec.

The values of \(\psi_{ro}\) in % for \(T_{ro}=0.005\) sec and \(T_{ro}=0.035\) sec and for different values of the supply interruption time (with \(\psi_r(+0)=100\%\)) are presented in the following table:

With the inherent operating time of the starting equipment, including the dropout time and pickup time, on alternating control current of \(0.08\)—\(0.13\) sec, the magnetic field of the rotor by the moment of reconnection either decays completely or decays to an insignificant value. Therefore it may be considered that the influence of the decaying rotor field on the transients of reclosing roller-table motors is insignificant.

The investigation of reclosing of roller-table motors was carried out by the method of mathematical modeling on an MN-14 type AVM.

Figures 1 and 2 present calculated oscillograms of starting and reversing, respectively, for different values of the initial rotational speed.

As can be seen from the oscillograms (Fig. 1), during restarting the value of the initial rotational speed of the rotor substantially affects both the nature of the course of the transient process (in its initial stage) and the quantitative values of the electromagnetic torque. The transient torque reaches its largest values at small \((n_{\text{init}} = 0 \div 0.2\ \text{p.u.})\) and large \((n_{\text{init}} > 0.9\ \text{p.u.})\) initial speeds; moreover, at small speeds positive peaks arise, while at large speeds negative peaks of torque arise.

The occurrence, at a high initial rotational speed, of large negative (braking) first half-waves of torque leads to a certain initial decrease in the rotor speed; subsequently, positive (driving) half-waves predominate in the torque curve, as a result of which the rotor accelerates with small oscillations to subsynchronous speed. This also explains the fact that the time to reach the steady-state speed depends very little on the magnitude of the initial speed.

(Fig. 1, a). It should also be noted that the nature of the change in the transient torque at the final stage remains almost the same. The peak values of the transient stator current are practically independent of the magnitude of the initial speed.

Fig. 1. Oscillograms of starting at different values of the initial rotor speed.

a—family of curves \(n=f(t)\) for different values of \(n_{\text{init}}\); b—\(n_{\text{init}}=0\); c—\(n_{\text{init}}=0.4\); d—\(n_{\text{init}}=0.6\); e—\(n_{\text{init}}=0.8\); f—\(n_{\text{init}}=1.0\).

Figure 3 gives the dependences of the transient characteristics on the initial speed. From Figs. 1 and 3 it is evident that the most favorable restart occurs at \(n_{\text{init}}=0.4\div0.55\), when the impact torques are small and have no negative peaks, and the rotor speed increases smoothly.

In contrast to the case considered above, during reversal at any initial speed the transient torque does not change its sign (Fig. 2); the number of pulsations of significant amplitude arising in the initial stage of reversal in the transient-torque curve decreases as the initial speed decreases, and within the range \(n_{\text{init}}=1.0\div0.6\) the magnitude of the impact torque remains almost unchanged; a further decrease \((n_{\text{init}}<0.6)\) of the initial—

speed leads to a decrease in the impact torques (Fig. 3). As the initial speed decreases, the impact values of the transient stator current decrease.

During reversal from high initial speeds, the mean braking torque and, upon change in the direction of rotation of the rotor, the driving torques turn out to be greater than during reversal from low initial speeds. Consequently, braking and the subsequent acceleration to subsynchronous speed at high initial speeds proceed more intensively. This

Fig. 2. Oscillograms of reversal at different values of the initial rotor speed of rotation.
a — family of curves \(n=f(t)\) for different values of \(n_{\text{init}}\);
b — \(n_{\text{init}}=1.0\); v — \(n_{\text{init}}=0.6\); g — \(n_{\text{init}}=0.2\).

can be explained by the fact that, despite the different path of rotor displacement during reversal from different initial speeds, the reversal time changes only very slightly (Fig. 2, a).

In the final stage of the transient process, as in repeated starting, the change in the torque curve is almost independent of the magnitude of the initial speed and has the character of damped oscillations.

In general, speaking of repeated switching-on, it may be said that these modes for roller-table motors do not pose a great danger to the motor and drive, since the impact torques of starting and reversing at any initial rotor speed do not exceed the values of the impact torques that arise, respectively, when starting a stationary motor and reversing a synchronously rotating motor. The absence of the influence of the rotor damping field on the transient processes accounts for the relatively high stability of their course.

Fig. 3. Dependences of \(M_{\mathrm{ud}}\), \(i_{\mathrm{ud}}\), and \(t\) on the initial rotational speed under repeated switching-on: \(p\) — starting; \(r\) — reversing; \(t\) — braking.

References

1. V. A. Shubenko. Some problems in the dynamics of automated asynchronous electric drives. Elektrichestvo, 1960, No. 1.

2. M. M. Sokolov, L. P. Petrov, L. B. Masandilov, V. A. Ladenzon. Electromagnetic transient processes in an asynchronous electric drive. Moscow, Energiya, 1967.

3. V. P. Andreev, Yu. A. Sabin. Fundamentals of Electric Drive. Moscow–Leningrad, GEI, 1963.