Semiconductor Collector in Circuits for Electromechanical Inversion and Conversion

E. F. OBERGAN, A. I. SKOROSPESHKIN

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

In automated AC electric drives, the best techno-economic indicators are possessed by systems built on the use of the frequency method of regulating the speed of the actuating motor.

As sources of alternating current of regulated frequency in such systems, inverters and converters using transistors and thyristors are widely known.

One of the most advanced inverter circuits is shown in Fig. 1; however, their widespread practical application is limited by the following inherent shortcomings:

Fig. 1. Schematic electrical diagram of an independent inverter.

1. In the starting mode of motor \(D\), as well as in the braking mode, reliable operation of the thyristors is not ensured. In a number of cases these modes are emergency conditions for the circuit.

2. The nonsinusoidal form of the voltage and current curve at the inverter output, with a large waveform distortion factor.

1. A reduction in efficiency due to the need to include in the dc circuit a choke with high inductance.

2. The range of frequency variation is limited because the supply-voltage variation must be proportional to the frequency variation.

3. To ensure operation of the circuit over a practically acceptable range of variation of the load \(\cos \varphi\), it is necessary to use an additional reverse-current rectifier.

When it is necessary to obtain an alternating-current source of adjustable frequency in systems fed from a three-phase network, converter circuits are used; a version of the thyristor power section of these circuits is shown in Fig. 2.

Fig. 2. Schematic electrical diagram of the power stage of a frequency converter supplied from an ac network.

The principal shortcomings of such circuits include the shortcomings of inverter circuits listed in items 1, 2, and 4, and, in addition, the following:

1. A bulky and complex thyristor control system.

2. The impossibility—without considerable complication of the circuit and design, consisting in the implementation of intermediate converters—of obtaining a practically acceptable range of output-frequency regulation, since the upper limit of the adjustable output frequency is no more than \(0.2 \div 0.25\) of the supply frequency.

To a considerable extent, the indicated shortcomings can be avoided by using combined electronic-machine circuits of inverters and converters.

In this case, the electromechanical part used is a transformer made on a ring-shaped magnetic core, one part of which (Fig. 3) is toothed, and the other is in the form of a smooth cylinder.

Two windings are placed in the slots of the magnetic core; one of them, the primary, is made in the form of an armature winding with leads from sections, and the other, the secondary, is made in the form of a three-phase (in the general case, \(m\)-phase) winding of an electrical machine.

In some cases it is possible to limit oneself to a single primary winding; then the output voltage, for example three-phase, is taken from three leads arranged at an angle of \(120^\circ\). The electronic part is a semiconductor collector (commutator) connected to the winding leads.

As examples, let us consider electron-machine circuits of a converter from alternating current of fixed frequency to alternating current of regulated frequency, and of an inverter with regulated output frequency.

Fig. 3. Diagram of the construction of an annular transformer with an electromagnetic winding.

Example 1. The basic electrical circuit of the converter is shown in Fig. 4. Through terminals \(A_1B_1C_1\) the winding is supplied from a three-phase source of alternating current of fixed frequency. The currents flowing through the winding create a rotating magnetic field, which crosses the turns of the winding and induces in it a three-phase system of e.m.f.

The terminals of the winding are connected to groups of thyristors \(PV\). Each group \(PV\) includes thyristors, each connected to a definite phase of the \(m\)-phase system of output voltage.

In the present example each group \(PV\) consists of three thyristors, since a three-phase voltage is taken from terminals \(A_2B_2C_2\).

The method of connecting the thyristors to the winding terminals and to the busbars of the three-phase output circuit is shown in Fig. 5.

For ensuring a symmetrical three-phase voltage in the output network \(A_2B_2C_2\), at each instant of time only three thyristors must be open, each of which is connected to one of the busbars \(A_2B_2C_2\) and to one of the three winding terminals located at an angle of \(120^\circ\).

Fig. 4. Basic electrical circuit of a converter with a semiconductor collector.

The thyristors are controlled from a ring counting circuit powered by a variable-frequency master oscillator. Each cell of the counting circuit simultaneously feeds the control electrodes of three thyristors.

Fig. 5. Electrical circuit for connecting the cells of the semiconductor collector of the converter with the winding and buses of the output network.

If the first control pulse from the counting circuit opens the thyristors

\[ PV_{1A};\quad PV\left(\frac{n}{3}+1\right)B \text{ and } PV\left(\frac{2n}{3}+1\right)C, \]

where \(n\) is the number of winding terminals, then the next control pulse from the adjacent cell of the counting circuit opens the thyristors

\[ PV_{2A};\quad PV\left(\frac{n}{3}+2\right)B \text{ and } PV\left(\frac{2n}{3}+2\right)C \]

and so on.

The locking of an open thyristor when the next thyristor in sequence is opened will be considered using the thyristors \(PV_{1A}\) and \(PV_{2A}\) as an example.

When a control pulse is applied to thyristor \(PV_{2A}\), the latter opens; and, if there is voltage between the winding points corresponding to the terminals \(PV_2\), \(PV\left(\dfrac{n}{3}+2\right)\), and \(PV\left(\dfrac{2n}{3}+2\right)\), the primary winding of the commutating transformer \(TK_{2A}\) is energized.

From the secondary winding of transformer \(TK_{2A}\), voltage is applied to the primary winding of transformer \(TK_{1A}\) in such a way that the negative terminal of this voltage is applied to the anode of the thyristor.

Taking into account that, with thyristor \(PV_{1A}\) open, the voltage on the plates of the capacitor connected in the bridge diagonal in parallel with thyristor \(PV_{1A}\) and the primary winding of transformer \(TK_{1A}\) is considerably less than the e.m.f. induced in the secondary winding of transformer \(TK_{2A}\), the indicated e.m.f. is applied through the capacitor to thyristor \(PV_{1A}\) in the reverse direction, i.e., in the direction of locking this thyristor.

With an appropriate design of transformer \(TK_{2A}\), ensuring a value of the e.m.f. induced in the secondary windings greater than the e.m.f. induced in the part of winding \(W_s\) enclosed between adjacent terminals, after thyristor \(PV_{2A}\) is switched on, thyristor \(PV_{1A}\) is locked.

All the other thyristors operate similarly.

As a result of the sequential switching of groups of thyristors at a frequency proportional to the frequency \(f_{\mathrm{ctrl}}\) of the master oscillator, the frequency \(f_2\) of the output voltage at terminals \(A_2B_2C_2\) is equal to

\[ f_2 = f_1 \pm \frac{f_{\mathrm{ctrl}}}{n}, \]

where \(f_1\) is the frequency of the supply network \(A_1B_1C_1\).

The amplitude of the output voltage at any frequency is equal to the amplitude of the input voltage.

Example 2. The circuit diagram of the inverter is shown in Fig. 6.

In this case, each group \(PV\) consists of two thyristors \(B\), one of which is connected to the “+” bus of the supply voltage, and the other to the “−” bus.

The control circuit for thyristors \(B\) is constructed so that at any given moment in time, excluding commutation intervals, only two thyristors \(B\) are open; these are connected to buses of opposite polarity and are located in \(PV\) groups connected to diametrically opposite terminals of winding \(W_1\).

When voltage \(U_{\mathrm{sup}}\) is applied, a direct current flows through winding \(W_1\), producing in the magnetic core a magnetic flux that is constant in time and oriented in a definite manner.

The operation of the circuit will be considered using the operation of two pairs of \(PV\), connected to diametrically arranged sections \(W_s\) of winding \(W_1\), as an example; their principal electrical circuit is shown in Fig. 7.

If, at the initial instant of time, thyristors \(B_1\) and \(B_2\) are open and conduct current, then through winding \(W_1\) there flows a current

\[ i = \frac{U_{\mathrm{sup}}}{R}. \]

Here \(R\) is the ohmic resistance of the two parallel branches of winding \(W_1\), measured at points \(a\) and \(b\).

Owing to the constancy in time and in space of the magnetic flux created by the current \(i\), no e.m.f. will be present in winding \(W_2\), and consequently no currents will flow.

The thyristors are also controlled by current pulses taken from a ring recalculating circuit supplied by the reference generator.

Fig. 6. Schematic electrical diagram of an electric-machine inverter with a semiconductor collector.

When a control pulse is applied to the next thyristors in sequence, i.e., to thyristors \(B_3\) and \(B_4\), adjacent to \(B_1\) and \(B_2\), a period begins in which the current is commutated from terminals \(a\) and \(v\) to terminals \(a^1\) and \(v^1\) of winding \(W_1\).

At the same time, capacitors \(C_{13}\) and \(C_{24}\), discharging through the open thyristors \(B_1, B_3\) and \(B_2, B_4\), provide the conditions for turning off thyristors \(B_1\) and \(B_2\). The presence of diodes \(D_1D_2D_3\) and \(D_4\) prevents the discharge of capacitors \(C_{13}\) and \(C_{24}\) into the sections \(W_s\).

When the next control pulse arrives, the next thyristors in sequence, adjacent to thyristors \(B_3\) and \(B_4\), open; the latter are thereby turned off, and so on.

The successive switching-on of closed thyristors and the turning-off of adjacent, previously open thyristors leads to displacement of the points at which the supply voltage is connected to winding \(W_1\) along this winding.

Since the direction of the axis of the magnetic flux formed by the current flowing through winding \(W_1\) coincides with the axis passing through the points where the voltage \(U_{\text{supply}}\) is connected to winding \(W_1\), displacement of the connection points causes rotation in space of the constant magnetic flux.

The speed of rotation of the magnetic flux is determined by the switching speed of the \(PV\) groups, which in turn is determined by the frequency of the reference generator and by the number of \(PV\) groups, equal to the number of terminals of winding \(W_1\).

Rotation of the constant magnetic flux in space causes an e.m.f. to be induced in winding \(W_2\) and provides at the inverter output a three-phase voltage \(U_{\text{out}}\) of sinusoidal form.

Fig. 7. Electrical circuit for connecting the cells of the semiconductor collector of an inverter with the winding and power-supply sources.

Conclusions

The use of a semiconductor collector in circuits for electromechanical inversion and conversion makes it possible to obtain limits for regulating the frequency of the output voltage from zero to several hundred hertz. The upper limit of the output frequency is restricted by the time required to restore the controlling properties of the thyristors and does not depend on the power source of the circuit.

Regulation of the output frequency is considerably simplified for any type of load.

Operation of the inverter circuit is exceptionally reliable; in particular, even with a complete short circuit of the secondary winding, the current in the primary winding and, consequently, through the thyristors will not exceed the nominal value determined by the ohmic resistance of the winding.

Literature

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

2. Sh. I. Lutinidze. Controls and circuits of electrical machines with a controlled semiconductor commutator. Proceedings of the Academy of Sciences of the USSR, No. 6, 1964.

3. I. L. Kaganov. Electronic and Ionic Converters. Part III, GEI, 1956.

4. Sh. I. Lutinidze. Electrical machines with a controlled semiconductor commutator. Academy of Sciences of the USSR, Electric Power Engineering, issue 5, 1962.