Investigation of a DC and AC Cross-Field Electric Machine Amplifier
Abstract Generated abstract
This paper describes the development and experimental study of a transverse-field electric-machine amplifier based on the EMA-12A that can supply direct current and multiphase alternating current either separately or simultaneously. The authors explain the machine as combining ordinary transverse-field DC amplifier operation with synchronous-generator behavior on the AC side, requiring compensation of armature reaction through separate or adjusted feedback windings for DC and AC loads. They derive relations for output frequency, DC and AC emf ratio, external characteristics, feedback compensation, and gain, and report experimental characteristics showing broadly similar DC and AC behavior. The tested 1 kW, 220 V, 3000 rpm machine achieved gain coefficients of about 3000 on DC and 2000 on AC, rising to about 7000 with parallel capacitors, with a response time of 0.12 s.
Full Text
STUDY OF AN ELECTRIC-MACHINE AMPLIFIER WITH A TRANSVERSE FIELD FOR DIRECT AND ALTERNATING CURRENT
A. I. Skorospeshkin, B. I. Kostylev, Sh. S. Roiz
Presented by the scientific seminar of the Department of Electrical Machines
The transverse-field EMA finds wide application in various regulation and control circuits, and in special circuits. In this connection, amplifiers are required that generate direct and alternating current.
Usually a transverse-field EMA operates in generator mode on direct current. In some cases they also operate as alternating-current generators. In this case, with regulation (change in the frequency of the voltage supplied to the control winding), the frequency of the output voltage changes correspondingly.
It is very attractive to obtain both direct and alternating current from one and the same transverse-field EMA. This will make it possible to broaden the range of application of the amplifier.
We developed and investigated such an amplifier based on the EMA-12A, making it possible to obtain direct and alternating \(m\)-phase current either separately or simultaneously.
Figure 1 presents the circuit of the amplifier.
Designations:
1 — armature winding,
2 — commutator,
3 — compensation winding,
4 — shunting resistance of the compensation winding,
5 — additional winding,
6 — load on direct current,
7 — control winding,
8 — slip rings,
9 — current transformers,
10 — capacitors,
11 — rectifier,
12 — feedback winding,
13 — shunting resistance of the feedback winding,
14 — load on alternating current,
15 — drive motor.
The operating principle of the amplifier is explained quite fully by the figure. Here, on the one hand, the amplifier operates as an ordinary EMA
with a transverse constant-current field, whose properties and characteristics have been studied to a certain extent.
On the other hand, a number of taps are taken from the armature winding and brought out to slip rings. As a result, an \(m\)-phase alternating voltage can be obtained with frequency
\[ f=\frac{Pn}{60}. \tag{1} \]
The alternating-current side of the amplifier may be regarded as a synchronous generator with its inherent properties (the action of armature reaction, parameters, characteristics).
In order that the machine under consideration may be regarded as an amplifier on the alternating-current side, compensation of the armature reaction is necessary when the amplifier operates into a load.
In our case, compensation is carried out by introducing positive current feedback. The feedback is introduced through current transformers (9), a rectifier (11), and winding (12). The latter is shunted by a variable resistance (13) for better adjustment of the compensation. With unsaturated magnetic systems of the current transformers, the direct current flowing through winding (12) will be directly proportional to the load current, and the flux produced by the m.m.f. of the winding will compensate the action of the armature reaction.
Simultaneous operation of the amplifier as a direct-current and alternating-current generator is possible. In this case it is necessary to have two windings for compensation of armature reaction on direct current (3) and on alternating current (12). To eliminate the influence of the loads, separate compensation for direct and alternating current must be adjusted. In separate operation, one winding with the corresponding compensation adjustment is sufficient.
The e.m.f. relations in the amplifier may be considered in the same way as in a single-armature converter, i.e.
\[ E_{\sim}=k_e E_d, \tag{2} \]
where
\(E_d\) — no-load e.m.f. of the EMU on direct current;
\(E_{\sim}\) — no-load e.m.f. of the EMU on alternating current;
\[ k_e=\frac{\sin \frac{\pi}{m}}{\sqrt{2}} \]
— coefficient indicating the ratio of the e.m.f. of direct and alternating current;
\(m\) — number of phases.
Figure 2 presents experimental no-load characteristics for direct and alternating current. They differ only in the magnitudes of the e.m.f.
External characteristics were obtained (Fig. 3). For direct current this will be the dependence \(U_d=f(I_d)\), for alternating current — \(U_\phi=f(I_\phi)\). In form they are identical and differ only in their inclination to the abscissa axis. On alternating current, the change in output voltage will be determined by the influence of armature reaction and by the voltage drop in the active and reactive resistances.
On alternating current the characteristic was obtained for the case of an active load and with single compensation. Consequently, the form of the characteristic will be determined by the expression
\[ U_\phi=\sqrt{E_\phi^{2}-I_\phi^{2}x_s^{2}}-I_\phi r_\phi, \tag{3} \]
Fig. 2. No-load characteristics.
where
\[ E_{\phi},\ I_{\phi} \]
are the phase emf and current;
\[ x_s \]
is the leakage reactance;
\[ r_{\phi} \]
is the active resistance.
The positive feedback with respect to the load current, implemented in the amplifier, is linear for unsaturated magnetic systems of current transformers. Therefore one may write
\[ I_{\mathrm{o.s.}} = k_{\mathrm{o.s.}} I_{\phi}, \tag{4} \]
where
\[ I_{\mathrm{o.s.}} \]
is the feedback current;
\[ k_{\mathrm{o.s.}} \]
is the current-feedback coefficient.
The degree of compensation is determined by the ratio of the ampere-turns of the armature winding and of the feedback winding, i.e.,
\[ \varepsilon = \frac{F_{\mathrm{o.s.}}}{F_{a}} . \tag{5} \]
where
\[ F_{\mathrm{o.s.}} = I_{\mathrm{o.s.}} W_{\mathrm{o.s.}} k_{w\mathrm{o.s.}}; \]
\[ F_{a} = m \cdot 0.9 I_{\phi} \frac{W_{a}}{m} k_{w a}; \tag{6} \]
$W_a$, $W_{\mathrm{f.b.}}$ — the number of turns of the armature and feedback windings;
$k_{wa}$, $k_{w\mathrm{f.b.}}$ — winding coefficients.
From (4, 5, 6) we obtain
\[ \varepsilon = \frac{k_{\mathrm{f.b.}} W_{\mathrm{f.b.}} k_{w\mathrm{f.b.}}} {0.9 W_a \cdot k_{wa}} . \tag{7} \]
For $\varepsilon = 1$
\[ k_{\mathrm{f.b.}} = 0.9 \left(\frac{W_a}{W_{\mathrm{f.b.}}}\right) \left(\frac{k_{wa}}{k_{w\mathrm{f.b.}}}\right). \tag{8} \]
Thus, with linear feedback and unity compensation, the feedback coefficient is determined by the ratio of the numbers of turns and the winding coefficients of the corresponding windings.
The gain coefficients for direct and alternating current are determined by the expressions
\[ \begin{aligned} k_y &= k_U k_I,\\ k_{y\sim} &= k_{U\sim}\cdot k_{I\sim}. \end{aligned} \tag{9} \]
where
$k_U$ and $k_{U\sim}$ — voltage gain coefficients;
$k_I$ and $k_{I\sim}$ — current gain coefficients.
In our case, the gain coefficient in operation on direct current was obtained of the order of 3000, and on alternating current 2000.
Fig. 3. External characteristics.
The increase in the amplification coefficient can be obtained by connecting capacitors in parallel. In this case the armature reaction becomes magnetizing, and the output voltage will be greater than without capacitors.
Figure 4 presents the no-load characteristic and the volt-ampere characteristic of the capacitors, on the scale of the m.m.f. of the control winding. The dotted line shows the volt-ampere characteristic of the capacitors under self-excitation. In the figure \(OC=\Sigma F\), \(OB=F_{\text{cap}}\), \(BC=F_y\). It is obvious that, when capacitors are connected, the m.m.f. of the control winding is reduced by as many times as the m.m.f. \(F_y\) is less than \(\Sigma F\). In practice the control current is reduced by a factor of \(2 \div 3\), and the amplification coefficient increases by a factor of \(4 \div 9\). In our case the maximum amplification coefficient obtained was of the order of 7000.
Fig. 4. Volt-ampere characteristic of the capacitor and no-load characteristic of the amplifier.
The response speed of the amplifier is determined by the parameters of the control and armature windings. Therefore, when operating on direct and alternating current it will be the same. In the amplifier under study \((P = 1\ \text{kW}, U = 220\ \text{V}, n = 3000\ \text{rpm})\), it is \(0.12\ \text{s}\).
Thus, the transverse-field electric-machine amplifier for direct and alternating current of this type has certain advantages, due to the possibility of obtaining direct and alternating current from a single machine with the required characteristics. These advantages make it possible to expand the fields of application of the amplifier.