Synchronous Generator with Frequency Control by Means of a Semiconductor Commutator

E. F. OBERGAN, A. I. SKOROSPEShKIN

(Recommended by the seminar of the departments of electrical machines and general electrical engineering.)

The broad development of automation requires the improvement and enhancement of the characteristics of AC electric drives. It is known that the best technical-and-economic characteristics are possessed by an AC electric drive with frequency control of the rotational speed of the actuator motor.

There exist many devices used as sources of alternating current of variable frequency. These include synchronous generators, asynchronous generators, commutator and unipolar converters, compensated commutator generators excited from the stator, various circuits of electronic converters, mechanical contact converters, and converters using semiconductor devices.

AC commutator generators can in principle be used as generators of alternating current of variable frequency; in this case, frequency variation can be achieved by setting the brushes into rotation simultaneously with the rotation of the commutator. In practice, such generators have not found application because of the design and technological complexity of manufacturing the rotating brush apparatus, and also because of its low reliability.

Synchronous generators possess the best technical-and-economic characteristics and high reliability; however, changing their output frequency is possible only by changing the rotational speed of the rotor, which is practically not used because of the large inertia of the rotor.

Changing the frequency of the output voltage of a synchronous generator at constant rotor speed can be accomplished if the stator winding is made of the armature type, with leads from each section. Instead of a mechanical commutator and rotating brushes, semiconductor relays are connected to the leads from the winding sections.

Figure 1 shows a part of the stator winding and the method of connecting the leads from the sections to semiconductor relays PR. As is seen from the circuit of Fig. 1, each winding lead is connected to three PRs, connected respectively to the three buses of a three-phase network. The open state of a PR is characterized by a very small resistance between the winding lead and the corresponding phase \((A, B, C)\) of the three-phase network.

Fig. 1. Connection diagram of the stator winding of a synchronous generator to the semiconductor relays of a controlled semiconductor commutator.

The closed state of the SR causes the presence of a large resistance between the terminal and the phase, which is practically equivalent to disconnecting the terminal from the phase.

Figure 2 shows the amplifying part of the SR, intended for commutating the operating current. A control voltage \(U_{\text{ctrl}}\) from the trigger is applied to the input of triode \(T_5\). The operating current is commutated by a composite triode consisting of triodes \(T_1, T_2, T_3\). Diodes \(D_1\) and \(D_2\) provide the best saturation conditions for triodes \(T_1\) and \(T_2\). Diode \(D_5\) forms the blocking voltage of the composite triode, which is produced as a direct voltage drop across \(D_3\) when current flows through the circuit \(R_3, D_3, R_1\), under the action of the voltage \(U_{\text{off}}\). Diodes \(D_5\) and \(D_6\) serve to limit the blocking voltages of triodes \(T_3\) and \(T_2\). Diode \(D_4\) serves to decouple the operating circuits of the SR groups whose emitters of triodes \(T_4\) are joined together. Switching on of the composite triode occurs when triode \(T_4\) is switched off under the action of the voltage \(U_{\text{on}}\). The winding terminal and the line phase are connected to terminals \(a\) and \(b\).

Fig. 2. Electrical circuit of the amplifying part of the semiconductor relay.

The frequency-control circuit of the synchronous generator is shown in Fig. 3. Each section (or group of sections) of the stator winding has a terminal connected to three SRs, combined in a circuit in the form of an SC cell of the semiconductor commutator. The set of semiconductor commutators SC, looped along the stator winding, constitutes a semiconductor collector. At any given instant only three SRs are switched on in three SCs, located at an angle of \(120^\circ\) along the collector and connected to different buses of the three-phase network. In the absence of switching of the SRs, a three-phase voltage taken from three points of the stator winding and having the synchronous frequency will be supplied to the buses through the open SRs,

\[ f_c = \frac{p \cdot n}{60}, \]

where

\(p\) is the number of pole pairs of the stator winding,
\(n\) is the number of revolutions of the rotor of the synchronous generator.

With constant \(n\) and switching of the SRs, the frequency of the voltage on the buses of the three-phase network will vary in proportion to the switching speed of the SRs.

Figure 4 shows part of the collector’s electrical circuit, namely: the electrical connection and control circuit of three PR in three PC, connected to three terminals of the stator winding located at an angle of 120°, and the circuit for supplying three-phase voltage from the stator winding to the network buses.

Fig. 4. Circuit for regulating the frequency of the output voltage of a synchronous generator with a controlled semiconductor collector

If the number of winding terminals is taken equal to \(S\), then in one of the groups of PC connected to terminals located at an angle of 120° there will be PC having the numbers \(PC_1\), \(PC_{\frac{s}{3}+1}\), \(PC_{\frac{2s}{3}+1}\).

Any other three PC connected to terminals located at an angle of 120° are connected analogously to those shown in Fig. 4 and form semiconductor three-phase relays. The three-phase relays are controlled by voltage from a ring step distributor, made according to the circuit of R. A. Lipman [4].

The common bases of the triodes \(T_1\) are connected to the collectors of the triodes \(T_2\), which are the output triodes of the cells of the step distributor. The common emitters of the triodes \(T_1\) are connected to the “ground” bus of the circuit’s power supply. The cutoff and saturation modes of the component triodes are ensured by the voltages \(U_{\text{on}}\) and \(U_{\text{off}}\).

When the triode \(T_2\) of a step-distributor cell is turned on, the base and emitter circuits of the triodes \(T_1\) connected to the collector of triode \(T_2\) by terminal \(P_1\) are short-circuited. Under the action of the voltage \(\pm E_b\), the triodes \(T_1\) close, and the component triodes PR open.

The ring three-phase circuit is connected to the system of three-phase voltage of the stator winding.

Fig. 4. Electrical connection diagram of the semiconductor relays of the controlled commutator for regulating the frequency of the output voltage.

When the triode \(T_2\) of the next cell of the step distributor is switched on, by means of output \(P_2\) the next group of built-in PRs (the next three-phase relay) is switched on; these will be located in the PCs designated \(PK_2,\ PK_{\frac{s}{3}+2},\ PK_{\frac{2s}{3}+2}\).

Then three PRs operate in the groups \(PK_3,\ PK_{\frac{s}{3}+3},\ PK_{\frac{2s}{3}+3}\), and so on.

Fig. 5. No-load characteristic \(E_{\mathrm{g}}=f(i_b)\) of an experimental specimen of a synchronous generator with a controlled semiconductor commutator.

Fig. 6. External characteristic \(U_{\mathrm{g}}=f(i_g)\) of an experimental specimen of a synchronous generator with a controlled semiconductor commutator at \(f_{\mathrm{ctrl}}=\mathrm{const.},\ i_b=\mathrm{const.},\ \cos\varphi=\mathrm{const.}\)

When the triode \(T_2\) of the cell having output \(P_{\frac{2s}{3}}\) is switched on, the composite triodes in \(PK_{\frac{s}{3}},\ PK_{\frac{2s}{3}}\), and \(PK_s\) will be switched on.

The frequency \(f_{\mathrm{out}}\) of the voltage at the output of the synchronous generator with a semiconductor commutator will be equal to

\[ f_{\text{out}} = f_c \pm \frac{f_{\text{ctrl}}}{s}, \]

where \(f_{\text{ctrl}}\) is the frequency of the control signals at the input of the step distributor,
\(s\) is the number of taps of the stator winding.

The frequency of the output voltage can be regulated from 0 to the upper values determined by the limiting frequency characteristics of the triodes. The magnitude of the voltage does not depend on the switching speed of the collector cells and is determined only by the rotor speed and the excitation of the synchronous generator.

Continuity of the current in the load during switching of the collector PR is achieved because the disconnection of the three PR previously switched on in three PK located at \(120^\circ\) occurs only after the complete switching-on of the next three PR, in sequence, in three PK connected to adjacent taps of the winding.

Fig. 7. Dependence of the frequency \(f_n\) of the voltage at the load on the frequency \(f_{\text{ctrl}}\) of supply to the control circuits of the semiconductor collector cells, with \(i_v = \text{const.}\), \(\cos\varphi = \text{const.}\), \(n = \text{const.}\)

In accordance with the description given, a mock-up model of a synchronous generator with a semiconductor collector was built, at whose output a three-phase voltage of adjustable frequency was taken. The stator winding of the constructed model has 12 terminals connected to 36 PR. The triodes \(T_1\) in the circuit of Fig. 2 were selected as type P4. Fig. 5 shows the no-load characteristic of the generator. Fig. 6 shows the external characteristic at a fixed switching frequency of the collector cells and \(i_v = \text{const.}\), \(\cos\varphi = \text{const.}\)

Fig. 7 shows the dependence of the output frequency—the current frequency in the load—on the frequency of the control voltage effecting the switching of the collector PR. The characteristic \(f_{\text{out}} = f_n = f(f_{\text{ctrl}})\) was obtained with \(i_v = \text{const.}\), \(\cos\varphi = \text{const.}\), \(n = \text{const.}\) From the expression for \(f_{\text{out}}\) and from the characteristic in Fig. 7 it is seen that, by changing the direction of switching of the ring circuit for switching the collector cells, it is possible to obtain a smooth reduction of the frequency down to 0, with a stable value of the output frequency from several hertz to fractions of a hertz. With the corresponding synchronization of the frequency \(f_{\text{ctrl}}\) with the rotational frequency of the ro-

... of the generator \(f_c\), it is possible to operate stably at the point \(f_{\mathrm{out}} = 0\), i.e., to draw direct current from the synchronous generator.

Conclusions

1. A synchronous generator with a controlled semiconductor collector as the frequency regulator of the output voltage can be used as a power source for the actuating motor in an AC electric drive with frequency control. In addition, the indicated generator can be used in various automatic-control and regulation systems.

2. The waveform of the output voltage taken from the cells of the semiconductor collector is sinusoidal in its envelope if a sinusoidal emf is induced in the stator winding. In this case, the waveform of the voltage at the collector output approaches the waveform of the voltage on the stator winding the more closely, the greater the number of leads from the winding sections. The magnitude of the voltage taken from the collector does not depend on the switching frequency of the collector cells, but depends on the emf induced in the stator winding.

3. The characteristics of a synchronous generator with a controlled semiconductor collector, at any fixed switching frequency of the collector cells, correspond to the characteristics of an ordinary synchronous generator. When the switching frequency of the collector cells is changed, the frequency of the current in the load changes, and, for \(\cos\varphi \ne 1\), so does the total resistance of the load circuit. Under this (principal) operating mode, the specified characteristics are obtained by regulating the generator excitation current.

4. The operating features of the semiconductor controlled collector—in particular, the low control power and low inertia—broaden the possibilities for its application in other circuits and purposes, for example, for frequency conversion, for producing the emf of alternating current, etc.

References

1. Sh. I. Lutidze. Semiconductor commutation of electrical machines. In the collection Operating Modes of Electric Power Systems and Regulation of Synchronous Machines, Nauka Publishing House, 1964.

2. L. M. Piotrovsky. Electrical Machines. Gosenergoizdat, 1956.

3. Yu. I. Konev, E. V. Mashukov. Power transistor switching relays. In the collection Semiconductor Devices and Their Application, ed. by E. A. Fedotov, issue 8, Sovetskoe Radio Publishing House, 1962.

4. R. A. Lipman. Semiconductor Relay. Gosenergoizdat, 1963.