Frequency Converter Based on a Semiconductor Collector
Abstract Generated abstract
This paper proposes a frequency converter in which the mechanical commutator of an alternating-current commutator machine is replaced by a semiconductor collector. It describes a stationary winding with multiple taps connected through transistor or thyristor switching cells to a three-phase output circuit, controlled by a ring step distributor so that successive groups of taps spaced 120 degrees apart are connected. The output frequency is determined by the input frequency and the control-signal frequency divided by the number of winding terminals, enabling electronic regulation without armature rotation, slip rings, or mechanical commutation. The authors report experimental operation with output frequencies from zero to several hundred hertz or several kilohertz, sinusoidal output envelope under sinusoidal supply, and continuous switching currents up to 10 A using composite transistor cells.
Full Text
PROCEEDINGS
OF THE TOMSK ORDER OF THE RED BANNER OF LABOR POLYTECHNIC
INSTITUTE named after S. M. KIROV
Volume 152 1966
FREQUENCY CONVERTER BASED ON A SEMICONDUCTOR COLLECTOR
E. F. OBERTAN, A. I. SKOROSHPESHKIN
Recommended by the seminar of the Department of Electrical Machines
and General Electrical Engineering
The property of an alternating-current commutator machine to convert frequency while preserving the magnitude of the voltage at the brushes is well known. A commutator frequency converter is known that is based on this property and is made in the form of an electrical machine whose armature winding, in addition to the commutator, has slip rings serving to supply to the armature winding a primary voltage of fixed frequency.
The frequency of the secondary (output) voltage taken from the brushes mounted on the commutator is changed by changing the speed and direction of rotation of the armature; in this case,
\[ f_2 = f_1 \pm \frac{pn}{60}. \]
where
\(f_2\) — frequency of the output voltage at the brushes;
\(f_1\) — frequency of the primary voltage supplied to the armature winding;
\(n\) — number of revolutions of the armature.
The stator of the indicated electrical machine, shown in Fig. 1, plays the role of a magnetic core, and the winding on it may be absent.
Frequency regulation by changing the number of revolutions and the direction of rotation of the armature does not make it possible, owing to the inertia of the armature, to obtain the proper speed of response of regulating systems; moreover, commutation in alternating-current commutator machines takes place under severe conditions. All this extremely limits the use of commutator converters.
The present article considers a machine converter in which the mechanical commutator is replaced by a semiconductor one.
The schematic diagram of a frequency converter based on a semiconductor collector is shown in Fig. 2.
Designations in the diagram:
\(PY\) — semiconductor cell;
Fig. 1. Schematic diagram of a frequency converter with a mechanical collector.
\(U_1, f_1\) — primary voltage and frequency;
\(U_2, f_2\) — output voltage and frequency.
The primary (supply) voltage with frequency \(f_1\) is applied to the armature winding at three points spaced at an angle of \(120^\circ\). The armature winding has taps from the sections, just as in the version with a mechanical collector. Each tap is connected to a semiconductor cell “SC,” consisting of three semiconductor switches. Each semiconductor switch is connected to the corresponding phase of the output three-phase circuit, which has three output terminals from which the secondary frequency \(f_2\) is taken.
The semiconductor switch may be implemented using transistors or thyristors; this makes no fundamental difference and is determined by technical and economic requirements and by the specified range of variation of the output frequency \(f_2\). Let us consider the more complicated circuit, implemented using transistors.
Figure 3 shows part of the basic electrical circuit of the converter for three taps of the armature winding spaced at an angle of \(120^\circ\).
Designations in the circuit:
SC — semiconductor cell;
CT — composite triode;
\(W\) — converter winding;
ShR — step distributor;
$f_1$ — frequency of the input voltage;
$f_2$ — frequency of the output voltage;
$T_1$ — triode controlling the semiconductor switch;
$T_2$ — output triode of a cell of the step distributor.
Fig. 2. Schematic diagram of a frequency converter with a semiconductor collector.
The semiconductor cells PYa, connected to the terminals of the armature winding, each consist of three composite triodes “ST,” which are semiconductor switches and are connected into different phases of a three-phase ring circuit. The composite triode is controlled by a low-power triode $T_1$.
The circuit is made with composite triodes in order to increase the power of the converter. The number of semiconductor cells is equal to the number of terminals of the armature winding and is equal to “$n$.”
To provide a symmetrical three-phase voltage at the output, at any given instant three terminals of the armature winding, arranged at an angle of $120^\circ$, must be connected to the ring three-phase circuit. Accordingly, at any given instant three semiconductor switches “PK,” connected to the above-mentioned terminals, must be switched on.
In Fig. 3 the schematic electrical circuit is shown for three switches
\[ PK_1;\quad PK_{\frac{n}{3}+1}\quad \text{and}\quad PK_{\frac{2n}{3}+1}, \]
connected to leads arranged at an angle of 120°. Any three other switches connected to other leads arranged at an angle of 120° are connected analogously to the switches shown in Fig. 3.
To ensure operation of the circuit, the composite triodes of three switches are combined through the control circuits in such a way that the composite triode of the first switch \(PK_1\), connected into phase \(A\), operates with the composite triode of switch \(PK_{\frac{n}{3}+1}\), connected into phase \(B\), and with the composite triode \(PK_{\frac{2n}{3}+1}\), connected into phase \(C\). For this purpose, the bases and emitters of the triodes \(T_1\) of the indicated composite triodes are combined and connected to the source of the control voltage.
Fig. 3. Schematic electrical diagram of the operating cells and control of the frequency converter.
Built-up composite triodes are controlled by a voltage taken from a ring recalculating circuit, which can be implemented by any known methods. In Fig. 3 the semiconductor collector is controlled from a ring step distributor constructed according to the circuit of R. A. Lipman [1].
The combined 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 stepping distributor.
The combined emitters of the triodes \(T_1\) are connected to the “ground” bus of the circuit power supply. The cutoff and saturation modes of the composite triodes are provided by the voltages applied to terminals 1, 2, 3.
Description of the Operation of the Converter
When the triode \(T_2\) of a cell of the stepping distributor is switched on, the base and emitter circuits of the triodes \(T_1\), connected to the collector of the triode \(T_2\) by the terminal “\(P_1\),” are short-circuited. Under the action of the voltage \(+E_{\delta}\), the triodes \(T_1\) are blocked, and the composite triodes are opened. The three-phase ring circuit is connected to the three-phase voltage taken from the armature winding. When the triode \(T_2\) of the next cell of the stepping distributor is switched on by means of terminal “\(P_2\),” the next group of triple composite triodes is switched on, which will be located in the cells
\[ PK_2;\qquad PK_{\frac{n}{3}+2};\qquad PK_{\frac{2n}{3}+2}. \]
Then three switches operate in the cells
\[ PK_3;\qquad PK_{\frac{n}{3}+3};\qquad PK_{\frac{2n}{3}+3}, \]
and so on.
When the triode \(T_2\) of the cell having the terminal “\(P_{\frac{n}{3}}\)” is switched on, the composite triodes in the switches
\[ PK_{\frac{n}{3}};\qquad PK_{\frac{2n}{3}}\ \text{and}\ PK_n \]
will be switched on.
The frequency \(f_2\) of the voltage at the converter output will be equal to
\[ f_2 = f_1 \pm \frac{f_{\mathrm{ctrl}}}{n}; \]
where
\(f_{\mathrm{ctrl}}\) is the frequency of the control signals (the frequency of the voltage at the input of the stepping distributor);
\(n\) is the number of winding terminals.
CONCLUSIONS
Replacing a mechanical collector with a semiconductor one eliminates the need for rotation of the machine armature, makes it possible to place the armature winding on a stationary stator, and eliminates contact rings.
Thus, a converter based on a semiconductor collector can be made static, making it possible to convert a voltage \(U_1 = \mathrm{const}\) with frequency \(f_1 = \mathrm{const}\) into a voltage \(U_2 = U_c = \mathrm{const}\) with frequency \(f_2 = \mathrm{var}\). The indicated converter has a substantial advantage over known static converters, confirmed experimentally, namely: the frequency of the output voltage can be regulated from zero to
upper values determined by the limiting frequency characteristics of transistors or thyristors. In the experimental check, the upper limit of the frequency \(f_2\) ranged from several hundred hertz to several kilohertz.
In addition, when the winding is supplied with a sinusoidal voltage, the output voltage is also sinusoidal in its envelope. The circuit with composite triodes shown in Fig. 3, using a P4B transistor, made it possible to switch currents of up to 10 A during continuous operation.
REFERENCES
- R. A. Lipman. Semiconductor Relay. Gosenergoizdat, 1963.