Commutation Features of a Transverse-Field EMU Operating on Alternating Current

B. I. Kostylev, A. I. Skorospeshkin

(Presented by the scientific seminar of the Department of Electrical Machines and General Electrical Engineering)

At the present time, transverse-field EMUs operating with an alternating or pulsating control signal are finding ever broader application in various automatic-control and electric-drive circuits [1, 2].

A transverse-field EMU is a specific direct-current machine. This specificity is due to the presence of two sets of brushes, the nonuniform distribution of current over the armature winding, the substantial influence on the characteristics of the reaction of the commutated and eddy currents, and the properties of the magnetic system.

When an EMU operates on alternating current, in addition to the circumstances indicated above, new features arise that are caused by the alternating current. First of all, these include transformer couplings. Thus, along the longitudinal axis of the amplifier (Fig. 1), the control winding, compensating winding, armature winding, additional-pole windings, and the sections commutated in the transverse circuit are transformer-coupled. In this case the control winding, supplied by alternating current, may be regarded as the primary winding of a transformer with respect to the remaining windings. The compensating winding, additional-pole windings, and armature winding are connected in series with the load resistance. These windings are connected in such a way that the resultant transformer emf is very small and may be neglected. But the compensating winding and the sections commutated in the transverse circuit are closed respectively through the shunt resistance and the small resistance of the brush contact. As a result of the appearance of currents in these circuits from the induced

Fig. 1. Amplifier circuit.

the emf of transformation, commutation of the transverse brushes is worsened, the power of the control winding increases, and the amplification factor decreases. In addition, in the circuit of the compensation winding, when the EMU operates on alternating current, the current distribution is disturbed, which leads to impairment of compensation and deterioration of the characteristics.

In the transverse circuit an autotransformer coupling should be considered. In this case the armature winding, short-circuited through the transverse brushes, may be regarded as the primary winding of an autotransformer, and the commutated sections of the longitudinal circuit as the secondary. The sections commutated in the longitudinal circuit lie in a plane perpendicular to the axis of the transverse flux and are short-circuited through the small resistance of the brush contact. From the transverse flux a considerable emf is induced in these sections and large currents arise, which leads to a sharp deterioration of commutation of the longitudinal brushes even in the absence of load current, to an increase of losses, and to an increase of the equivalent active resistance of the transverse circuit, which has a strong influence on the amplification factor of the EMU.

For example, in an EMU-12A rated at 1.2 kW and with the number of turns in an armature section \(W_c = 4\), at a transverse-circuit current \(I_q = 3\) A, which is 75% of the rated value, and at a frequency of 50 Hz, the transformer emf of the commutated section is equal to 3.5 V. In this case, strong sparking with burning of the trailing edge of the brushes is observed under the longitudinal brushes. The equivalent resistance of the transverse circuit in the absence of longitudinal brushes is equal to 5.27 ohms, and with them to 9.3 ohms; that is, the resistance increases almost twofold.

Thus, when an EMU operates on alternating current, transformer emfs and currents arise in the commutated sections of the longitudinal and transverse circuits; these worsen the commutation conditions and adversely affect the amplifier characteristics. Therefore, the problem of studying the commutation process and improving it in a transverse-field EMU operating on alternating current is a pressing one. If the unfavorable influence of the compensation winding with a shunting resistance can be eliminated by adjustment of the compensation without shunting, then the influence of commutation can only be weakened by thoroughly studying this process with the aim of finding effective methods of improving commutation that are acceptable in practice.

For a transverse-field EMU, a peculiar interaction of the emfs in the commutated sections is characteristic. If the commutated sections of the transverse circuit are considered, then in them the reactive and transformer emfs interact depending on the frequency and the parameters of the circuit. For this case one may present the vector diagram shown in Fig. 2. The control flux \(\Phi_y\) induces in the transverse circuit an emf of rotation \(E_q\), coinciding with it in phase, and in the commutated sections a transformer emf

Fig. 2. Vector diagram of the emfs of the commutated sections of the transverse circuit.

\(E_{tq}\), lagging behind it by an angle \(\dfrac{\pi}{2}\). Under the action of the rotational emf, an alternating current \(I_q\) flows in the short-circuited transverse circuit, lagging behind \(E_q\) by the angle

\[ \Psi_q = \operatorname{arctg}\frac{X_q}{R_q}, \]

where \(R_q\) and \(X_q\) are the active and inductive resistances of the transverse circuit.

The reactive emf of the section \(E_{Rq}\) is in phase with this current. The angle \(\Psi_q = \dfrac{\pi}{2}\) even at low frequencies, since the active resistance of the transverse circuit is very small in order to obtain a high amplification factor. In this case the commutation conditions in the transverse circuit will be the worst, since the reactive and transformer emf’s are added algebraically.

For widening the frequency pass band of the amplifier, a compensating capacitance is connected into the transverse circuit, creating a resonant circuit. In this case the current of the transverse circuit coincides in phase with the rotational emf, while the reactive and transformer emf’s of the sections being commutated are in quadrature; that is, the commutation conditions are somewhat improved.

Fig. 3. Vector diagram of the emf’s of commutated sections of the longitudinal circuit.

The commutation conditions in the longitudinal circuit will depend both on the magnitude and on the nature of the load. This is seen from the vector diagram shown in Fig. 3. The main transverse flux of the amplifier \(\Phi_q\) induces in the armature winding a rotational emf \(E_d\) coinciding with it in phase, and in the commutated sections of the longitudinal circuit a transformer emf \(E_{td}\) lagging by an angle \(\dfrac{\pi}{2}\). When a load is connected, a current \(I_d\) flows in the longitudinal circuit, shifted in phase relative to \(E_d\) by the angle

\[ \Psi_d = \operatorname{arctg}\frac{X_d}{R_d}, \]

where \(R_d\) and \(X_d\) are the active and reactive resistances of the longitudinal circuit and the load.

From the load current, a reactive emf \(E_{Rd}\) appears in the commutated sections of the longitudinal circuit, coinciding with it in phase. In the general case the angle of shift between \(E_{Rd}\) and \(E_{td}\) is equal to \(\alpha\), and the resultant emf of the commutated section \(E_{cd}\) is written as

\[ E_{cd} = \sqrt{E_{Rd}^{2} + E_{td}^{2} + 2E_{Rd}E_{td}\cos\alpha}. \tag{1} \]

Since, depending on the nature of the load current, \(\alpha = \dfrac{\pi}{2} \pm \Psi_q\), expression (1) can be written in the form

\[ E_{cd}=\sqrt{E_{Rd}^{2}+E_{\tau d}^{2}\pm 2E_{Rd}E_{\tau d}\sin\Psi_d}, \tag{2} \]

where the plus sign corresponds to an inductive load,
the minus sign corresponds to a capacitive load.

Expression (2) shows that the resultant emf of the sections commutated in the longitudinal circuit depends not only on the magnitudes of the components \(E_{Rd}\) and \(E_{\tau d}\), but also on the nature of the load. Moreover, more severe commutation conditions arise with an inductive load of the amplifier.

Studies of the commutation process in a transverse-field EMA operating on alternating current are carried out on actual amplifiers of the EMA-12A type by the method proposed in [3]. This method reduces to an initial separate investigation of commutation in the transverse and longitudinal circuits, followed by transition to joint operation of the circuits, that is, a method with transition from the simple to the complex. The first stage makes it possible to determine how alternating current affects the commutation process in the absence of transformer emfs in the commutated sections.

The studies carried out at this stage showed that, in the absence of transformer emfs, alternating current, as compared with direct current, does not introduce any substantial change into the commutation process. This is explained by the fact that during the commutation period the instantaneous value of the current remains practically constant, and the commutation conditions, as in direct current, are determined by the parameters of the commutating sections and of the brush contact.

Fig. 4. Static volt-ampere characteristics of EG-8 brushes: 1 — alternating current of frequency 50 Hz, 2 — direct current.

Figure 4 gives the static volt-ampere characteristics of EG-8 brushes, obtained experimentally on a short-circuited commutator at

on direct and alternating current. They show that the resistance of the brush contact on direct and alternating current is practically the same. The identical nature of commutation on direct and alternating current is also confirmed by oscillograms of the current curves of the commutated sections, shown in Figs. 5 and 6, obtained respectively for the transverse and longitudinal circuits.

Thus, the commutation conditions in a transverse-field EMA on alternating current are determined by the magnitude of the reactive and transformer e.m.f.’s and by the nature of their interaction. Therefore, the problem of improving commutation is closely connected with the compensation of these e.m.f.’s.

Fig. 5. Current curves of the commutated sections of the transverse circuit, fed by direct and alternating current:
1—alternating current of frequency 50 Hz,
2—direct current.

Fig. 6. Current curves of the commutated sections of the longitudinal circuit, fed by direct and alternating current, with the auxiliary poles disconnected:
1—alternating current of frequency 50 Hz,
2—direct current.

Conclusions

1. The commutation conditions and characteristics during operation of a transverse-field EMA on alternating current deteriorate because of transformer couplings of the commutated sections with the amplifier windings.

2. The commutation conditions during operation of a transverse-field EMA on alternating current depend not only on the parameters of the sections and of the brush contact, but also on the parameters of the amplifier circuits and on the nature of the load.

3. In the absence of transformer e.m.f.’s in the commutated sections, alternating current does not introduce substantial changes into the commutation process.

References

1. Yu. M. Aleksandrov. Application of a transverse-field electric-machine amplifier in drive systems as a variable-frequency current generator. Collection of papers on problems of electromechanics, issue 2, 1958.

2. V. V. Rudakov. Electric-machine amplifiers in automatic-control systems. GEI, 1961.

3. A. I. Skorospeshkin. Commutation of electric-machine amplifiers with a transverse field. Dissertation, Tomsk, 1960.