EFFECT OF INTERFERENCE ON THE STATIC AND DYNAMIC PROPERTIES OF MUS-D ELECTRIC DRIVES WITH PPU

V. A. SEVASTYANOV, A. P. INESHIN, V. P. SIBILEV

(Presented by the scientific seminar of the Department of “Electric Drive and Automation of Industrial Installations” of the Ulyanovsk Polytechnic Institute)

Wide-range adjustable electric drives are subject to the following requirements: high static accuracy of regulation, stability of characteristics, optimal speed of response, and reliable operation.

These qualities are partly combined in systems built on the basis of a DC actuator motor (ID), a power magnetic amplifier (MUS), a preliminary amplification stage using semiconductor elements (PPU), and a speed sensor—the tachogenerator (TG).

Among drives of this type that have found application in metal-cutting machine tools, the best known are the PMUP ENIMS drives. They combine the best qualities of existing amplification stages, the problem of coordinating which in an automatic control system (ACS) is of primary importance.

A general disadvantage of MUS-D drives with PPU is their sensitivity to interference, which has a substantial effect on the magnitude of the overall gain factor, the static characteristics, the speed of response, and the reliability of drive operation. Therefore, when designing automatic control systems (ACS) with MUS-D, it is necessary to have a complete understanding of the cause of interference, the composition of the spectrum of its frequencies, the magnitude of its amplitude, etc.

An analysis of the composition of interference and of its influence in EMU-D systems with PPSU was carried out in [2]. However, as applied to MUS-D drives with PPU, this question has not been studied sufficiently and requires additional investigation.

Production tests of the PMUP-9M on UZTS machines [3] and its investigation in the branch laboratory of the UPI, aimed at determining the possibility of using UMZP in them and expanding the range of regulation of the speed of the actuator motor, made it possible at the same time to reveal the influence of interference in MUS-D drives with PPU on their static and dynamic characteristics, as well as on the reliability of drive operation.

The schematic diagram of the input circuit of the MUS-D drive with PPU is given in Fig. 1. Here, for stabilization, the well-known RC circuit is used, transmitting the correction signal to resistor \(R_8\). Forced feedback ...

this converter circuit (the links with the largest gain) leads to the appearance of a considerable level of interference at the input of the PPU.

Table 1 gives experimental data for the amplitude \(U_{ya}\) and rms \(U_{yad}\) values of the variable component of the armature voltage \(U_y\sim\) as a function of the current \(I_y\), when the ED operates at a speed of 30 rpm, i.e., \(D=100\). The magnitude of the pulses \(U_{ya}\), occurring at a frequency of 300 Hz, increases rapidly when the motor is loaded and reaches more than ten volts.

Fig. 1. Input circuit of the MUS-D drive with PPU.

Fig. 2. Variants of correcting circuits and their frequency characteristics.

Table 1

The RC circuit does not have the desired frequency selectivity at the parameters selected according to the stability conditions of the automatic-control system, \(R_k=1.2\ \mathrm{kOhm}\), \(C_k=40\ \mu\mathrm{F}\), as is seen from the amplitude-frequency characteristic in Fig. 2b (curve 1) of this link. In the pass band of the automatic-control system (0-5 Hz), transmission of the useful signal is insignificant, while \(U_{ya}\), as the principal interference, is transmitted by the RC circuit into the comparison circuit practically without phase shift, with a maximum transmission coefficient equal to

\[ K_{RC}=\frac{R_v}{R_v+R_k}=\frac{160}{160+1200}=0.11, \tag{1} \]

where the capacitive reactance at a frequency of Hz may be neglected.

Thus, at the PPU input, even when the drive is operating in the steady-state mode, in addition to the error signal \(U_y=U-U_{tach}\), there acts a considerable alternating interference voltage \(U_p=K_{RC}U_{ya}\).

The simplified input-output characteristic of a single-cycle PPU with saturation, shown in Fig. 3 in relative units, allo-

makes it possible to reveal the influence of an interference of fixed frequency with amplitude \(U_{\mathrm{p}}\). An interference of the selected triangular form is almost equivalent in its action to a sinusoidal one, but it considerably simplifies the analysis of the combined action of the interference and the dc signal \(U_c\).

When \(U_c=0\), interference of any amplitude is detected as a result of limiting, opening the PPU. At small interference amplitudes \((0<U_{\mathrm{p}}\leq 1)\), its limiting occurs only from below, along the zero line (Fig. 3), and the mean value of the output signal is determined by the expression (in relative fractional units):

\[ U_{\mathrm{out}}=\frac{U_{\mathrm{p}}}{4}. \tag{2} \]

Fig. 3. Input–output characteristic of a single-cycle PPU with saturation.

At large values of the interference amplitude \((U_{\mathrm{p}}>1)\), two-sided limiting is observed, and the mean value of the output signal is

\[ U_{\mathrm{out}}=\frac{2U_{\mathrm{p}}-1}{4U_{\mathrm{p}}}. \tag{3} \]

As \(U_{\mathrm{p}}=\infty\), \(U_{\mathrm{out}}\) tends to the maximum value, equal to 0.5.

Figure 4 shows the equivalent input–output characteristics of the PPU for different values of the interference \(U_{\mathrm{p}}\). In a real drive, the interference level \(U_{\mathrm{p}}\gg 1\), and the equivalent characteristics are then almost linear.

In accordance with Fig. 3, by specifying an increment \(\Delta U_c\), one can find the dependence of the value of the equivalent gain coefficient \(K=f(U_{\mathrm{p}})\). For this, the size of the shaded areas \(S\) (Fig. 3), corresponding to the increment of the output signal, should be related to the time \(T\) of the interference action period:

Fig. 4. Equivalent input–output characteristics for different values of the interference.

\[ K=\frac{\Delta U_{\mathrm{out,\,avg}}}{\Delta U_{\mathrm{in}}} =\frac{2S}{\Delta U_c T} =\frac{\left(\dfrac{T\Delta U_c}{4U_{\mathrm{p}}}\right)}{T\Delta U_c} =\frac{1}{2U_{\mathrm{p}}}. \tag{4} \]

Consequently, the gain coefficient of the PPU, as a result of the action of the interference and the subsequent saturation, decreases inversely proportional to the amplitude \(U_{\mathrm{p}}\).

Figure 5 gives, in relative units, the final dependences \(K=f(U_c,U_{\mathrm{p}})\). The initial characteristic \(K=f(U_c)\) for \(U_{\mathrm{p}}=0\) (bold), plotted according to expression (5) [4], valid in our case for \(U_c>0\), is also shown there:

\[ K=\frac{2}{\pi}\left(\arcsin \frac{1}{U_c}+\frac{1}{U_c}\sqrt{1-\left(\frac{1}{U_c}\right)^2}\right). \tag{5} \]

According to [1], in PMUP drives with a range of up to 100:1, in order to ensure stability of the rotational speed when the load, mains voltage, ambient temperature during drive operation, etc. change, the overall gain of the open-loop ACS must be no less than 1000, while \(K_{\text{ppu}}=75\div100\). A reduction of \(K_{\text{ppu}}\) due to the action of interference (5 instead of the initial \(75\div100\)) leads to a decrease in the stiffness of the static characteristics, a reduction in the speed-control range, and gives rise to additional nonlinearities in the characteristics.

Fig. 5. Dependence of the PPU gain coefficient on the amplitude of the interference.

The detection phenomenon in the closed-loop ACS of interference manifests itself in practice in the form of undesirable “self-running” of the drive (rotation of the drive electric motor when the command signal is removed). Elimination of “self-running” can be achieved, for example, by introducing a locking voltage \(U_d\) (Fig. 1), by using an additional stage of the speed setter, or by increasing the bias current, as provided in the PMUP under consideration (400 instead of 250 mA). The latter variant leads to an additional reduction in the overall gain coefficient and does not exclude the self-running mode when there is a load on the electric-motor shaft.

In the most probable fault—an open circuit in the input circuit (tachogenerator brush, contact of the speed-setter slider), for example at point 1, uncompensated interference can cause “runaway” of the drive, which is especially likely when the correction signal is introduced in parallel (point 3).

The action of interference also causes distortion of the static characteristics [3], leads to nonuniformity of the static error, unstable operation of the drive at low speeds, and makes it necessary to adjust the ACS gain coefficient over the range.

In order to evaluate the dynamic properties of the MUS when operating under real conditions, the transient process was investigated for a step change in the control signal in the open-loop system. For this purpose, in the input circuit of the PPU an interference level of frequency 300 Hz was created, with an amplitude corresponding to the operating conditions of a real drive. During the rise of the current in the control winding, the output triode of the PT amplifier period-

...periodically switched by the disturbance. The control current $I_y$ during operation of the PT in the switching mode cannot reach the steady-state value during the time $t_0$ of the open state of the PT, since the time constant of the control circuit $T_{oy} > t_0$. During the time $t_3$, despite the fact that $t_3 < t_0$, $I_{oy}$ decreases considerably, since the time constant of the control circuit when the PT is opened is much greater than when it is blocked. This results in an additional increase in the total opening time of the MUS.

The above is confirmed by comparing oscillograms of the transient process, taken in an open-loop ACS with a step change of the control signal without disturbances at the input of the PPU (Fig. 6a) and with disturbances present in the input circuit (Fig. 6b), which corresponds to the operating conditions of a real drive.

Fig. 6. Oscillograms of transient processes in an open-loop ACS:
a) without disturbances at the PPU input,
b) with disturbances present in the PPU input circuit.

Improvement of the performance of MUS-D drives should be pursued in two directions: by modernizing the existing version (improving interference immunity and operating reliability) and by developing a new version of the drive for a wider speed-control range of the actuator electric motor, 200:1 and higher. In doing so it is necessary to proceed from the following considerations:

a) the effect of disturbances on the control law must be reduced to a minimum;

b) the use of the PPU must help reduce the inertia of the power MUS;

c) the reliability of the PPU must approach that of the MUS, since otherwise the replacement, in regulated drives of the EMU, of the more reliable MUS loses its purpose;

d) the PPU elements must be reliably protected against the influence of overvoltages arising in the control winding of the MUS when it is connected to the network.

A reduction in the effectiveness of the interference arriving from the armature side at the input of the PPU can, in principle, be achieved by improving the selectivity of \(R_k C_k\) through the use of the simplest \(RC\) filters. Figure 2 shows circuits and amplitude-phase frequency characteristics of \(R_k C_k\), obtained experimentally in the presence of single-section (curve 2) and two-section (curve 3) filters. It follows from this that their installation reduces the sensitivity of the correction circuit to interference signals with a frequency of 300 Hz. Moreover, a single-section filter reduces the interference by a factor of two, and a two-section filter by a factor of 25. Unfortunately, the introduction of filters weakens the effectiveness of the differentiating action of the \(R_k C_k\) circuit with respect to the main pass frequency of the SAR. An increase in the stability margin at the upper end of the range can be achieved by using nonlinear stabilization or by regulating the gain coefficient of the SAR as a function of the speed setting [3].

In our opinion, a more correct approach is the construction of MUS-D circuits with a class-D PPU with electromagnetic correction [3].

Conclusions

1. The pulsating nature of the voltage at the armature of the electric motor, in the absence of the desired selectivity in the \(R_k C_k\) circuit, can lead to the appearance of an “interference” signal in the control law of the SAR. In the presence of saturation this leads to detection of the interference and a reduction of \(K_c\), distortion of the static characteristics, deterioration of static accuracy, and is the principal cause of “self-running” and emergency “runaway” of the drive.

2. In an actual drive, interference increases the transient-process time, reducing the speed of response of the SAR.

Literature

1. A. Ya. Petrovskii, Ya. B. Rozman. Regulated Electric Drive with Magnetic Amplifiers. Energiya, 1964.

2. Ya. S. Brovman, F. D. Kochubievskii, A. V. Feldman. Amplifiers on semiconductor triodes in a regulated electric drive. Elektrichestvo, No. 5, 1962.

3. V. A. Sevast’yanov, A. P. Ineshin. Book: Electric-Drive Systems with Magnetic-Semiconductor Converters MUS-D with PPU. Privolzhskoye Book Publishing House, 1966.

4. E. P. Popov, I. P. Pal’tov. Approximate Methods for the Study of Nonlinear Automatic Systems. Fizmatgiz, 1960.