Basic Characteristics of a Cross-Field Electric Machine Amplifier with a Smooth Armature
Abstract Generated abstract
The paper examines transverse-field electric machine amplifiers with smooth armatures, focusing on how longitudinal biasing affects self-excitation, amplification, dynamic response, and permissible armature current density. Three experimental models are tested to determine self-excitation thresholds and to measure changes in amplification coefficient, time constant, and quality factor as the biasing magnetomotive force is varied, with additional observations on brush displacement from neutral. The results indicate that operation closer to self-excitation increases amplification and quality factor but also increases the time constant, allowing a tradeoff between gain and speed of response. The study also reports improved cooling in smooth-armature designs, supporting higher armature current densities, and clarifies practical conditions for selecting operating parameters.
Full Text
Main Characteristics of a Transverse-Field EMU with a Smooth Armature
A. I. Skorospeshkin, G. G. Konstantinov
(Recommended by the seminar of the Departments of Electrical Machines and General Electrical Engineering)
The high dynamic indices and good commutation of electrical machines with a smooth armature open broad prospects for their development and application.
We have developed, manufactured, and are studying transverse-field EMUs with a smooth armature \[1, 2\].
When there is a comparatively large nonmagnetic layer, a significant reduction of the amplification factor is obtained. One of the means that makes it possible to substantially increase the amplification factor is the use of a longitudinal biasing winding \[2\]. The magnitude of the m.m.f. produced by the longitudinal biasing winding is determined by the conditions of self-excitation, which depend on the magnitude of the critical resistance of the transverse circuit.
We checked the self-excitation conditions on three models. The data are given in Tables 1, 2, and 3. In the first model, self-excitation does not occur because of the small number of turns of the longitudinal biasing winding; in the second model, the self-excitation threshold occurs when the m.m.f. of the longitudinal biasing winding amounts to 68% of the resultant excitation m.m.f.; in the third, about 33%. In this case the brushes were set on the neutral and unit compensation was adjusted.
At the same time, the dependence of the amplification factor, the time constant, and the quality factor on the percentage ratio of the m.m.f. of the longitudinal biasing winding and the control winding was checked (Tables 1, 2, 3). It is evident from the tables that the closer the regime approaches self-
Table 1.
| Data of model No. 1 | Data of model No. 1 | \(P_n\) | \(U_n\) | \(n\) |
|---|---|---|---|---|
| Data of model No. 1 | Data of model No. 1 | W | V | rpm |
| Data of model No. 1 | Data of model No. 1 | 700 | 30 | 2900 |
| % m.m.f. | \(F_{\mathrm{pod}}\) | \(K_y\) | \(T\) | \(K_d\) |
| 1 | 0 | 26 | 0.09 | 290 |
| 2 | 15 | 33 | 0.1 | 330 |
| 3 | 24 | 38 | 0.105 | 367 |
| 4 | 33 | 43 | 0.11 | 395 |
| 5 | 43 | 67 | 0.115 | 585 |
Table 2.
| Data of model No. 2 | \(P_n\), W | \(U_n\), V | \(n\), rpm | % n. s. | \(F_{\mathrm{pod}}\) | \(K_u\) | \(T\) | \(K_d\) |
|---|---|---|---|---|---|---|---|---|
| Data of model No. 2 | 700 | 60 | 2900 | 1 | 0 | 80 | 0,06 | 1340 |
| Data of model No. 2 | 700 | 60 | 2900 | 2 | 25 | 144 | 0,1 | 1440 |
| Data of model No. 2 | 700 | 60 | 2900 | 3 | 45 | 240 | 0,11 | 2180 |
| Data of model No. 2 | 700 | 60 | 2900 | 4 | 68 | 620 | 0,14 | 4430 |
Table 3.
| Data of model No. 3 | \(P_n\), W | \(U_n\), V | \(n\), rpm | % n. s. | \(F_{\mathrm{pod}}\) | \(K_u\) | \(T\) | \(K_d\) |
|---|---|---|---|---|---|---|---|---|
| Data of model No. 3 | 1000 | 115 | 3500 | 1 | 0 | 580 | 0,11 | 5270 |
| Data of model No. 3 | 1000 | 115 | 3500 | 2 | 12 | 814 | 0,13 | 6260 |
| Data of model No. 3 | 1000 | 115 | 3500 | 3 | 20 | 930 | 0,135 | 6880 |
| Data of model No. 3 | 1000 | 115 | 3500 | 4 | 25 | 1065 | 0,14 | 7610 |
| Data of model No. 3 | 1000 | 115 | 3500 | 5 | 33 | 1340 | 0,15 | 8950 |
the excitation, the higher become the amplification coefficient, the quality factor, and the time constant increases.
Thus, depending on the required conditions, either a rational value of the speed of response or of the amplification coefficient can be selected.
Of definite interest is the operating mode of the amplifier with a smooth armature when the brushes are shifted from the neutral against the direction of rotation of the armature. In ordinary transverse-field electric-machine amplifiers this mode is impossible because of poor commutation and a tendency toward self-excitation. In an amplifier with a smooth armature these limits are extended. In the second model, a brush shift against the direction of rotation by one and two commutator divisions was carried out. The amplification coefficient increased noticeably in this case, while the speed of response deteriorated; accordingly, it is necessary to reduce the n. s. of the longitudinal biasing winding.
In an amplifier with a smooth armature, the cooling conditions are improved. Consequently, the current density in the armature winding can be increased. For the first model, the dependence of the average overheating (Fig. 1) on current density was determined experimentally. It follows from the figure that the current density in the armature winding may be taken equal to \(17\ \mathrm{A/mm^2}\); in the third model, at \(n = 3500\ \mathrm{rpm}\), the current density in the armature winding is \(23\ \mathrm{A/mm^2}\).
Thus, as a result of the work carried out, the conditions of self-excitation have been clarified, and the possibilities have been determined for selecting rational values of the amplification coefficient, speed of response, and current density in the armature winding.
Fig. 1. Dependence \(Q_{\mathrm{avg}} = f(j)\).
REFERENCES
- A. I. Skorospeshkin, Yu. A. Stepanov. A transverse-field EMA with a smooth armature. Electromechanics, No. 10, 1964.
- A. I. Skorospeshkin, G. G. Konstantinov. Some results of an investigation of the characteristics and commutation of a transverse-field EMA with a smooth armature. Proceedings of TPI, vol. 160, 1967.