Commutation of a Cross-Field Electric Machine Amplifier During Operation with an Alternating Control Signal
Abstract Generated abstract
This paper examines commutation in transverse-field electromachine amplifiers operating with an alternating control signal, where transformer electromotive forces induced by time-varying magnetic fluxes can substantially worsen brush commutation. The authors analyze the dominant effect of the main transverse flux on the longitudinal commutating sections and present experimental current curves showing large commutation currents and sparking under both no-load and loaded conditions. To compensate this transformer emf, they propose using a shunt winding on the auxiliary pole connected through a capacitor, which preserves compensation over a regulated frequency range and opens the circuit under direct-current operation. Tests on an EMU-12A over 25 to 50 Hz showed substantially improved and nearly uniform commutation, with reduced sparking across the tested frequencies.
Full Text
COMMUTATION IN A TRANSVERSE-FIELD EMA OPERATING WITH AN ALTERNATING CONTROL SIGNAL
A. I. Skorospeshkin, B. I. Kostylev
(Recommended by the scientific seminar of the Departments of Electrical Machines and General Electrical Engineering.)
Transverse-field EMAs are widely used in automated electric drives and in various special circuits.
On direct current, EMAs operate mainly in a dynamic mode, when the control signal varies according to one or another law of regulation. In addition, EMAs are used as alternating-current generators adjustable within a certain frequency range.
When an EMA operates with an alternating control signal, time-varying magnetic fluxes act along its axes. Along the longitudinal axis there is the control flux, and along the transverse axis—the main flux of armature reaction. These magnetic fluxes induce transformer e.m.f.s in the circuits located along the corresponding axes; these e.m.f.s substantially affect the commutation conditions and the characteristics of the machine.
In the commutating sections of the transverse circuit, a transformer e.m.f. is induced by the control flux. Since the control flux is small in magnitude owing to the requirement of obtaining a large amplification factor, the transformer e.m.f. is insignificant and its influence may be neglected.
In the commutating sections of the longitudinal circuit, a transformer e.m.f. is induced by the main magnetic flux, which is considerably greater than the control flux. The influence of this e.m.f. on the commutation conditions and on the characteristics of the amplifier can no longer be neglected. Since the resistance of the commutating circuit, short-circuited by the brushes, is very small and is determined mainly by the resistance of the brush contact, the current due to the transformer e.m.f. reaches a significant value. This leads to an increase in losses in the brush contact and to a change in the parameters of the transverse circuit as the primary winding of an autotransformer [1]. In addition, strong sparking occurs under the brushes of the longitudinal circuit, even when the amplifier is operating at no load.
Thus, Fig. 1 gives the current curves of four commutating sections of the longitudinal circuit at no load of the EMA-12A amplifier. The current of the transverse circuit is 2 A, which is 55% of the rated value; the frequency of the control signal is 50 cps; the transformer e.m.f. of a section is 2.54 V. The curves were taken at the amplitude value of the section current. In the figure shown and below, curve 1 corresponds to the section last completing commutation in the first slot, while curves 2, 3, 4 refer to the 1st, 2nd, and 3rd sections of the second slot.
Figure 2 shows the current curves at a load current of \(5.3\ a\) and under the same remaining conditions as in Fig. 1.
The curves shown in Figs. 1 and 2 confirm that the nature of commutation is determined mainly by the current due to the transformer emf. The current of the commutating sections reaches a considerable value—\(10\)–\(15\ a\), which leads to strong sparking and burning of the trailing edge of the brushes of the series circuit.
Fig. 1. Current curves of commutation in the series circuit
at no-load operation of the amplifier and a frequency of \(50\) Hz.
In order to increase commutation reliability and improve the characteristics of the amplifier, we used the shunt winding of the auxiliary pole recommended in [2].
Fig. 2. Current curves of commutation in the series circuit
at a load current of \(5.3\ a\) and a frequency of \(50\) Hz.
The shunt winding is connected to the output of the amplifier and is placed on the stator in the same slots as the series winding of the auxiliary pole. The purpose of the shunt winding is to create in the commutating sections a rotational emf equal in magnitude and opposite in direction to the transformer emf.
The transformer emf lags the main magnetic flux by \(90^\circ\), while the emf at the output of the amplifier is in phase with it. Therefore
To provide compensation it is necessary that the current of the shunt winding and the e.m.f. at the amplifier output be in quadrature; that is, the winding must have a predominantly reactive resistance.
Direct connection of the shunt winding provides compensation of the transformer e.m.f. only at a constant frequency. If, however, the amplifier operates in a mode with adjustable control-signal frequency, the compensation of the transformer e.m.f. is disturbed. This is explained by the fact that the laws governing the variation of the transformer e.m.f. and of the shunt-winding current as functions of frequency are opposite. If the transformer e.m.f. is directly proportional to the frequency, then the shunt-winding current is inversely proportional to the frequency. Therefore such a circuit for connecting the shunt winding, when the frequency is being regulated, may even worsen the commutation conditions in the longitudinal circuit. In addition, when the amplifier operates on direct current it is necessary to disconnect the shunt winding, since there is a danger of short-circuiting owing to the insignificant active resistance of the winding.
In order to eliminate this drawback we used a circuit in which the shunt winding is connected through a capacitor. If the shunt winding is designed for full compensation of the transformer e.m.f. at the frequency \(f_{\mathrm{n}}\), then the capacitance of the capacitor is chosen from the condition
\[ \frac{1}{2\pi f_{\mathrm{n}} C} = 4\pi f_{\mathrm{n}} L, \]
where \(L\) is the inductance of the shunt winding \([\mathrm{H}]\);
\(C\) is the capacitance of the capacitor \([\mathrm{F}]\).
In this case the current of the shunt winding will vary with frequency according to the law
\[ I_{\mathrm{sh}} = \frac{E}{L}\cdot \frac{f}{4\pi f_{\mathrm{n}}^{2} - 2\pi f^{2}}, \]
where \(E\) is the e.m.f. at the amplifier output;
\(f\) is the current frequency.
In this case the uncompensated part of the transformer e.m.f., especially when the frequency is regulated downward from \(f_{\mathrm{n}}\), is small, and commutation proceeds satisfactorily over the entire frequency range. With direct current, the shunt-winding circuit is opened by the capacitor.
The proposed circuit was tested experimentally on an EMU-12A over the frequency range 25–50 Hz. In order to obtain a sufficient amplification factor and to attenuate the control flux, a resonant capacitance was connected into the transverse circuit of the amplifier according to the circuit proposed in [3, 4]. If the capacitance in the transverse circuit is absent,
Fig. 3. Commutation-current curves in the longitudinal circuit of an EMU with a shunt winding at a load current of 6 A and a frequency of 50 Hz.
it is necessary to increase the control flux, which leads to a deterioration of the commutation conditions in the transverse circuit and affects commutation in the longitudinal circuit. This is explained by the fact that the sections commutating in the longitudinal circuit are located in the field of the control winding, and a considerable rotational emf may be induced in them.
In Figs. 3 and 4 the commutation-current curves in the longitudinal circuit are given, respectively, at frequencies of 33 cps and 50 cps and with a capacitance in the shunt-winding circuit of 30 μF. The transverse-circuit current is 2 A, the load current 6 A; sparking under the brushes does not exceed 1½ points on the GOST scale.
Fig. 4. Commutation-current curves in the longitudinal circuit of a transverse-field EMA with a shunt winding at a load current of 6 A and a frequency of 33 cps.
The curves show that the commutation conditions at these frequencies are practically identical. Similar curves were also obtained at other frequencies. Sparking is the same over the entire frequency range.
Conclusions
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To improve commutation and the characteristics of an EMA when operating with an alternating control signal, it is necessary to compensate the transformer emf of the commutating sections.
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For more complete compensation of the transformer emf when regulating the frequency of the control signal, it is advisable to connect the shunt winding of the additional pole through a capacitor.
References
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A. I. Skorospeshkin, B. I. Kostylev. Transformer couplings and parameters of a transverse-field EMA when operating on alternating current. Proceedings of OMIIT, Omsk, 1964.
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F. A. Goryainov. Electromachine Amplifiers. GEI, 1962.
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V. V. Rudakov. Electromachine Amplifiers in Automation Systems. GEI, 1961.
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Yu. M. Aleksandrov. Application of a transverse-field electromachine amplifier in drive systems as a variable-frequency current generator. Collected Works on Problems of Electromechanics, issue 2, 1958.