EXPERIMENTAL STUDY OF THE MAIN CHARACTERISTICS OF AN ELECTRIC MACHINE AMPLIFIER WITH A CONTROLLED SEMICONDUCTOR SWITCH

Sh. S. Roiz, B. E. Trofimenko, A. I. Skorospeshkin

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

According to the circuit proposed by the authors in [1], an experimental prototype of an adjustable-frequency amplifier (in a contact version) with a controlled semiconductor switch (CSS) at the input was manufactured.

The electric-machine part of the EMA–RFC is made in a single-frame design on the basis of the standard EMA-12A amplifier.

The control, output, and rotor windings are made with the number of pole pairs equal to 4. The speed of the prime mover (asynchronous) is 2980 rpm.

Capacitors of constant capacitance are connected into the stator and rotor circuits \((C_s = 7\ \mu\text{F},\ C_r = 10\ \mu\text{F})\). As the output frequency increases, the current through the capacitance in the stator circuit increases, while the current through the capacitance in the rotor circuit decreases; when the frequency decreases, the opposite occurs. Thus, the capacitive current, remaining approximately constant over the entire frequency range and being magnetizing, reduces the required power at the input of the CSS, i.e., increases the gain of the EMA–RFC.

At the same time, connecting capacitances to the rotor and stator windings reduces commutation overvoltages on the transistor cells of the CSS (by 30%) and significantly improves the waveform of the current and voltage at the output of the EMA–RFC.

The CSS is made according to the circuit [2] with minor changes. The number of phases of the control winding and of the CSS cells is 6.

The purpose of these studies is the experimental acquisition of the main characteristics of the manufactured EMA–RFC: the external characteristic, frequency characteristic, gain, response speed, and its efficiency.

As the amplifier load, an active load and a high-speed DAT-600 motor were used.

Figure 1 presents typical frequency characteristics of the EMA–RFC under active load \(I_n = \text{const}\) (curve \(a\)) and when operating on a motor \(M = \text{const}\) (curve \(b\)). Curves \(a, b\) in Fig. 1 show that the EMA–RFC without additional devices has a change in output voltage, with

Fig. 1. Dependence of the output voltage, k.p.d. of the EMU-RCh on the output frequency:
curve (a) with an active load \(I_n = 1a\);
— curve (b) with a motor load \(M = 5\ \text{kgcm}\)

changes in output frequency, ensuring the basic law of frequency regulation.

Figure 2 gives the external characteristics \(U_{\text{out}} = f(I_n, M)\) at \(U_y = \text{const}\) with active load \((a, б, в)\) and motor load \((г, д)\). The EMU-RCh has sufficiently stiff characteristics (15–25% at \(I_n = 0—1a\)) over the entire operating frequency range. Obtaining stiffer external characteristics is associated with increasing the ampere-turns of the premagnetizing winding.

In Fig. 1 curve в shows the nature of the change in the gain coefficient over the entire frequency range of the EMU-RCh. In the range 30–170 cps the gain coefficient reaches 15–60.

The dip in the characteristic \(K_y = f(f_{\text{out}})\) at the middle frequencies of the operating range is due to the absence of effective action of the connected capacitances of the stator and rotor at these frequencies.

The speed of response of the EMU-RCh can be determined from the oscillograms given in Figs. 3, 4. With a step change in the control voltage (Fig. 3), the settling time of the output voltage from 7% to \(U_{\text{n}}\) is \(\approx 15\ \mu\text{s}\) (with an accuracy of 10%). Fig. 4, a and b, shows that the EMU-RCh can be regarded as a practically inertialess element when determining the response speed of the establishment of the output frequency at ...

Fig. 2. External characteristics of the EMU-RCh. \(a, б, в\)—active load; \(г, д\)—motor load.

with its stepwise variation. In Fig. 4, \(a\) shows the change in output frequency from 140 to 70 Hz, and Fig. 4, \(б\) shows the oscillogram during the reverse switching of the frequency.

The efficiency of the EMU-RCh, determined at nominal load in the operating frequency range, is not less than 0.35–0.55.

Thus, tests of the prototype have shown that the EMU-RCh, possessing the data obtained, can be used as an alternating-current source of adjustable frequency in an automated electric drive. The investigation of the prototype is continuing.

Fig. 3. Oscillogram of the transient process when switching the control voltage.

Fig. 4. Oscillogram of the transient process when switching the output frequency of the EMA-RC.

REFERENCES

1. Sh. S. Roiz, B. E. Trofimenko, A. I. Skorospeshkin et al. Electric-machine amplifier with a controlled semiconductor commutator. Izvestiya TPI, vol. 212, 1969.

2. B. E. Trofimenko, A. I. Skorospeshkin. Controlled semiconductor commutator with a split control winding. Izvestiya TPI, vol. 189, 1969.