DC machines with semiconductor switches

 DC machines with semiconductor switches / Information about the material

The collector and brush apparatus of a DC machine constitute a unit that causes difficulties in the design, manufacture and operation of the machine. Hence the desire to replace this unit with a contactless current commutator, which can be accomplished using controlled electric valves, especially semiconductor ones.

It is not difficult to build a DC source for an electric machine without a mechanical collector. A synchronous generator in combination with a semiconductor DC rectifier can be used for this purpose. Therefore, the main task is to create DC motors with semiconductor commutators. There are two types of such motors. In both types, the armature winding together with the semiconductor commutator is located on the fixed part of the machine (stator), and the inductor is the rotor of the machine. In this case, poles in the form of permanent magnets or excited by direct current through contact rings are located on the rotor. In the first case, the motor is completely devoid of sliding electrical contacts (contactless motor).

Figure 1 shows a schematic diagram of a motor using the same closed armature winding 1 as conventional DC machines. For simplicity, Figure 1 shows a two-pole motor with a small number of sections in the armature winding. The role of collector plates and brushes here is played by controlled semiconductor valves - thyristors 1 ', 1 '', 2 ', 2 '' and so on, connecting the armature winding 1 to the busbars 2 . The busbars 2 in turn are connected to the DC network.

Figure 1. Schematic diagram of a DC motor with a semiconductor commutator and a DC type armature winding

In the position of rotor 3 shown in Figure 1, the current should be conducted by thyristors of groups 2 ' –2 '' and 6 ' –6 ''. Let us assume that the current is conducted by thyristors 2 ' and 6 ' '. Then the current I a = 2 × i a will be distributed over the armature winding as shown in Figure 1. Let this create an armature flux Ф a , the direction of which is also shown in Figure 1. Then an electromagnetic moment M arises , under the influence of which the rotor will rotate clockwise. After the rotor has rotated by 1/8 of a turn, it is necessary to turn off thyristors 2 ', 6 '' and turn on thyristors 3 ', 7 '', then after the rotor has rotated by 1/8 of a turn, turn on thyristors 4 ', 8 '', and so on. As a result of such switching of thyristors coordinated with the rotation of the rotor, the machine in question operates like a normal DC machine and has the same characteristics. 

The thyristors are switched on and off by applying electric voltage pulses of the appropriate duration to their control electrodes. These pulses are generated by a special device that reacts to the rotor position (not shown in Figure 1). In the simplest case, such a device consists of an auxiliary permanent magnet mounted on the motor rotor and coils located on the stator, along its circumference, the number of which is equal to the number of armature sections. When the rotor rotates, the permanent magnet induces electromotive forces in the coils in turn, which are applied to the control electrodes.

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With a large number of sections, the motor of the type under consideration has good properties, but it requires a large number of thyristors and a complex device for controlling them. Therefore, motors with the circuit shown in Figure 2 are currently predominantly used.

Figure 2. Schematic diagram of a DC motor with a semiconductor commutator and an AC type armature winding

The top part of Figure 2 shows a diagram of a semiconductor switch, and the bottom part shows a schematic device of a motor with a number of pole pairs 2 p = 2. On the stator of this motor there are three windings ("phases") A , B , C , shifted along the circumference by 120°. The device of these windings is similar to the device of the armature windings of alternating current machines. Each of the windings, when supplied with current, creates a magnetic flux acting along its axis, and therefore the fluxes of individual windings are also shifted by 120°.

All three windings are simultaneously supplied with current, and the directions of the currents in them alternately change in the sequence shown in Figure 3, a . From this same figure it becomes clear how the magnetic field of the anchor winding rotates in space. As a result of the interaction of the magnetic field and the inductor, the latter will rotate following the anchor field. The semiconductor switch is controlled by the same principle as the motor discussed above.

Figure 3. Sequence of current directions in the "phases" of the motor armature winding according to the diagram in Figure 2 ( a ) and idealized shapes of current curves in the "phases" of the armature winding ( b )

Note that the switch shown in Figure 2 is essentially a semiconductor inverter that converts direct current into three-phase alternating current.

Figure 3, b shows idealized current curves in the "phases" of the winding. The numbers 1 - 6 in this figure indicate the time intervals that correspond to positions 1 - 6 in Figure 3, a . In reality, due to the smoothing effect of the winding inductances, the shape of the current curves approaches sinusoidal.

Based on the above, the machine shown in Figure 2 is essentially a three-phase synchronous machine that is fed through a three-phase current inverter. However, it has all the properties of a conventional DC commutator machine due to the fact that its armature winding is fed with current as a function of the rotor rotation angle, just as in a conventional DC machine.

More detailed information about DC machines with semiconductor switches is contained in the book by I.I. Ovchinnikov and N.I. Lebedev, "Contactless DC motors for automatic devices", 1966.

Source: Voldek A. I., "Electrical Machines. Textbook for Technical Educational Institutions" - 3rd edition, revised - Leningrad: Energia, 1978 - 832 p.

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