Investigation of a reversible AC electric machine amplifier
Abstract Generated abstract
This paper examines a commutatorless reversible AC electric machine amplifier intended as an alternative to commutator amplifier and DC motor systems in automation applications. The authors describe the operating principle of a three-stage two-phase amplifier with increased, unregulated output frequency, including an inverted synchronous generator, an asynchronous frequency converter, and a single-phase synchronous generator, and report tests on a 480 W model under single-phase and symmetrical three-phase loading. Particular attention is given to the effects of the reverse synchronous field on rotor overvoltages, control-winding emf, damping, output-voltage waveform, and self-excitation. The study finds that rotor windings can damp the reverse synchronous field sufficiently, that induced control-winding emf is small, and that low resistance in the rotor winding circuit improves damping and output-voltage quality.
Full Text
STUDY OF A REVERSIBLE AC EMU
A. I. Skorospeshkin, M. L. Kostyrev, A. P. Felzing
(Recommended by the seminar of the departments of electrical machines and general electrical engineering)
In automation, the system comprising a commutator electric-machine amplifier—a direct-current motor (EMU–DPT)—is widely used. A substantial drawback of such a system is the presence of two commutators, which is sometimes undesirable and, in some cases (strict requirements on the level of radio interference, etc.), altogether impermissible.
In this respect the advantage lies with commutatorless electric-machine amplifiers. It should be noted, however, that in the technical literature there is almost a complete absence of information on questions of theory, design, and investigation of characteristics. Only certain circuit solutions are available [1].
We have designed, built, and investigated a model of a commutatorless reversible two-phase EMU with an output at an increased, unregulated frequency. The present article gives some results of the investigation concerning the use of a reverse synchronous field (r.s.f.) and the output voltage.
Figure 1 shows the circuit of the amplifier, explaining the principle of its operation. The circuit consists of three stages: 1) an inverted-type synchronous generator, 2) an asynchronous frequency converter, 3) a single-phase synchronous generator. All three stages, or any two of them,
Fig. 1. Circuit of a reversible two-phase AC amplifier.
can be combined in one magnetic core provided the compatibility conditions are fulfilled [2].
The operation of the amplifier may be explained as follows.
When a signal is applied to the control winding CU, an emf \(E_1\) with frequency
\[ f_1=p_1\cdot n_p, \tag{1} \]
is induced in the rotor winding \(W_{p1}\), where \(p_1\) is the number of pole pairs of the first stage, and \(n_p\) is the rotational speed of the rotor.
In the rotor winding \(W_{p2}\), connected to \(W_{p1}\), an alternating current will flow, creating a field rotating relative to the stator at the speed
\[ n_c = n_p \pm \frac{f_1}{p_2}, \tag{2} \]
where \(p_2\) is the number of pole pairs of the second stage.
The sign \((+)\) refers to the case when the rotor and the field of the second stage rotate in the same direction (the most expedient mode); the sign \((-)\), in opposite directions.
From the flux of the second stage, an emf \(E_2\) is induced in the output winding, with frequency
\[ f_{\mathrm{out}}=p_2\cdot n_c=(p_2\pm p_1)n_p. \tag{3} \]
To increase the stiffness of the external characteristics of the second stage, positive current feedback is used, implemented by means of a current transformer (CT), a block of controlled valves (V), and a compensating winding (CW).
The third stage is an ordinary single-phase synchronous generator. Moreover, in order to obtain at its output an emf of the same frequency as at the output of the second stage, the following condition must be fulfilled:
\[ p_3=p_1+p_2, \tag{4} \]
where \(p_3\) is the number of pole pairs of the third stage.
The excitation of the generator may be supplied from the first stage through rectifiers [1]. If, however, under the operating conditions of the regulated two-phase motor connected to the amplifier output, frequent and rapid reversal is required, then it is better to make the third stage self-excited.
To provide self-excitation and to increase the stiffness of the external characteristic of the third stage, a reverse synchronous field (r.s.f.) and a capacitor connected at the output of the third stage are used. In this case, a special winding along the transverse axis \(W_q\) is placed on the rotor; it is closed through a two-half-period rectifier onto the winding \(W_d\), located along the longitudinal axis. The alternating current flowing in winding \(W_q\) has a good damping effect on the r.s.f. along the transverse axis. A direct current flows in winding \(W_d\), creating the excitation flux of the generator.
The amplitude of the first harmonic of the mmf of winding \(W_d\) may be taken from [3] as
\[ F_d=0.45\,\frac{I_{v3}\cdot W'_{v3}}{p_3}\cdot \frac{W'_d}{W'_q}, \tag{5} \]
where
\[ \begin{aligned} W'_{v3}&\text{ is the number of turns of the output winding, taking into account the winding factor,}\\ W'_d \text{ and } W'_q&\text{ are the numbers of turns of the longitudinal and transverse rotor windings of the third stage, taking into account the winding factors.} \end{aligned} \]
By changing the ratio \(\dfrac{W_d}{W_q}\), one can obtain external characteristics with different degrees of stiffness.
The required m.m.f. of excitation, taking the capacitance into account, is
\[ F_{\mathrm{v}}=F_c+F_d=0.45\,\frac{I_c W_{\mathrm{v3}}}{p_3}\left(1+\frac{W_d}{W_q}\right); \tag{6} \]
here \(I_c\) is the current flowing through the capacitance.
The value of the capacitance is selected either from the condition of compensating the inductive component of the load current, or from the condition of permissible losses in the output winding \(W_{\mathrm{v3}}\) under a purely capacitive load.
To exclude possible oscillations of the output voltage due to a certain scatter in the capacitance parameters, the magnetic circuit of the generator should be made saturated.
To ensure guaranteed self-excitation and a stable phase of the output voltage, a magnetic star is pressed into the middle of the rotor stack; it occupies \((8\text{--}10)\%\) of the stack volume.
The amplifier circuit is reversible, since when the polarity of the control signal is changed the phase of the output voltage of the second stage changes by \(180^\circ\), whereas the phase of the output voltage of the third stage remains unchanged. The controlled two-stage asynchronous motor will change its direction of rotation, i.e., reversal will occur.
The principal feature when the amplifier operates into a single-phase load consists in the presence of an r.m.f. The installation on the rotor of a special damping winding does not give the required effect, since the direct field also rotates relative to the rotor.
The investigations were carried out on a model of a three-phase AC electric-machine amplifier operating into a single-phase load.
Model data:
\[ P_{\mathrm{n}}=480\ \mathrm{W},\qquad n_r=50\ \mathrm{rev/s},\qquad U_{\mathrm{n}}=127\ \mathrm{V},\qquad 2p_1=2,\qquad f_{\mathrm{out}}=200\ \mathrm{Hz}, \]
\[ 2p_2=6. \]
The r.m.f., rotating relative to the rotor at the speed
\[ n_{\mathrm{rosp}}=\frac{f_{\mathrm{out}}}{p_2}+n_r =\left(\frac{p_2+p_1}{p_2}+1\right)n_{r1}, \tag{7} \]
induces in the rotor winding \(W_{r2}\) an e.m.f. \(E_{\mathrm{rosp}}\) with frequency
\[ f_{\mathrm{rosp}}=(2p_2+p_1)n_r=350\ \mathrm{Hz}, \tag{8} \]
which is superimposed on the main rotor e.m.f. \(E_r\) and causes overvoltages. The latter amount to \(80\text{--}90\%\) of the amplitude of the main rotor voltage. This must be taken into account when selecting the insulation of the rotor winding.
The current from the r.m.f. (Fig. 2), flowing through the winding \(W_{r2}\), creates a field that damps the r.m.f. The quality of damping depends on the magnitude of the resistance of the circuit formed by the rotor windings. The smaller this resistance, the better the r.m.f. is damped.
Since the current from the r.m.f. also flows through the winding \(W_{r1}\), a field is formed that rotates relative to the stator at the speed
\[ n_{\mathrm{uosp}}=\frac{f_{\mathrm{rosp}}}{p_1}+n_r =\frac{2p_2+2p_1}{p_1}\cdot n_r. \tag{9} \]
This field induces in the control winding an e.m.f. with frequency
\[ f_{\mathrm{uosp}}=p_1\cdot n_{\mathrm{uosp}}=400\ \mathrm{Hz}, \tag{10} \]
whose amplitude is \((4\text{--}5)\%\) of the supply voltage of the control winding. Increasing the active resistance of the control wind—
Fig. 2. Oscillogram of the current in the rotor under single-phase load.
...in order to reduce its time constant is undesirable, since it leads to an increase in the amplitude of the e.m.f.
To evaluate and compare the quality of the output voltage, oscillograms were taken under single-phase and symmetrical three-phase loads (Figs. 3, 4). From a comparison of the oscillograms it follows that the presence
Fig. 3. Oscillogram of the output voltage under single-phase load.
Fig. 4. Oscillogram of the output voltage under symmetrical three-phase load.
the reverse synchronous field (Fig. 3) slightly distorts the shape of the output-voltage curve.
On the whole, based on the results of the investigations carried out, the following conclusions may be drawn:
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The rotor windings damp the reverse synchronous field with sufficient effectiveness.
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The emf induced in the control winding is small in magnitude and has no substantial effect on the operation of the amplifier.
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In order to increase the effectiveness of damping the reverse synchronous field and, consequently, to improve the quality of the output voltage, it is necessary that the circuit formed by windings \(W_{p1}\) and \(W_{p2}\) have the lowest possible resistance.
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The reverse synchronous field in the third stage can be used for self-excitation and for increasing the stiffness of the external characteristics of this stage.
REFERENCES
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E. Mishkin, Poly-field Alternating-Current Induction Machines. United States Patent Office, Dec. 27, 1960, No. 2966523.
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V. S. Novokshenov. Study of an asynchronous brushless frequency converter. Dissertation, Tomsk, 1960.
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M. V. Antonov, V. I. Radin. Single-phase synchronous generator using a reverse-synchronous field. Elektrotekhnika, No. 5, 1965.