Troubleshooting in collector motors using a generator and an oscilloscope. Encyclopedia of radio electronics and electrical engineering

Encyclopedia of radio electronics and electrical engineering / Electric motors

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Collector electric motors form the basis of portable power tools: electric drills, electric punchers, electric saws, grinders, as well as household machines: vacuum cleaners, coffee grinders, etc., they all operate on single-phase alternating current. In these power tools, the excitation windings (stator) are connected through brushes in series with the armature winding (Fig. 1).

Such electric motors are called serial. Their use in portable power tools is due to the ability to regulate their rotation frequency over a wide range by a large starting torque, as well as their relatively small weight and dimensions. However, due to the presence of a complex armature winding and a collector with brushes, collector motors are laborious, expensive and less reliable electric machines compared to brushless AC machines.

Collector motors do not withstand prolonged overloads. The main reasons for their damage are prolonged overloads and frequent jamming of the working tool, for example, drills in a concrete wall. It is for these reasons that a large alternating current passes through the motor windings, which, firstly, overheats (burns through) the wire, and secondly, by a strong alternating magnetic field, causes vibration of the turns of the stator and armature windings. As a result of all this, the enamel insulation of the wire is shed and the turns in the windings are shorted or broken.

External signs of damage to the collector motor are sparking in a circle (under the brushes); reduction in the number of revolutions; power drop; rapid heating (overheating) of the stator and armature windings and the appearance of a burnt smell.

Examining the disassembled electric motor, it is possible to identify traces of burning of the armature and stator windings. But the true picture (what burned down) can be obtained by taking measurements. Both excitation windings (stator) are checked with an electronic ohmmeter, since it has a measurement accuracy of up to tenths of an ohm. If the difference in resistance of the windings is greater than 0,2 ohms, then in the winding with less resistance, there is a short circuit of the turns, and it should be replaced. But in practice, the stator winding is rarely damaged.

An electronic ohmmeter can sometimes detect damage to the armature windings, but most often this cannot be done due to the low resistance of its windings (few turns and thick wire). Therefore, on the instructions of a workshop for the repair of power tools, the author of this article developed a scheme and a method for checking the health of the armatures of collector electric motors using a generator and an oscilloscope. A similar scheme, apparently, is used somewhere, but the author had to start everything from scratch. This method can be used to check the armatures of any collector electric motors, including the armatures of automobile starters. It has high sensitivity and detects the short circuit or open circuit of the armature windings.

Figure 2 shows the connection diagram of the generator, oscilloscope and armature windings. The principle of operation of the installation is as follows. From the generator, an alternating voltage with a frequency of 50 ... 200 kHz and a stable level is supplied to the sockets for the vertical deflection of the oscilloscope beam Y (Fig. 2).

(click to enlarge)

From another socket of the same generator, alternating voltage with the same frequency, but with an adjustable level, is supplied through makeshift sliding contacts to the collector lamellas (Fig. 2, 3) and then to one of the armature windings, where it is transformed into all windings of the same armature.

(click to enlarge)

With the help of sliding contacts, this voltage is removed from the adjacent collector lamella (adjacent winding) and fed to the horizontal deflection sockets of the oscilloscope beam X. The horizontal sweep switch must be in the "External synchronization" or "External sweep" position (Fig. 2).

By changing the frequency of the generator (50 ... 200 kHz), its level (0 ... 0,25 V), the position of the oscilloscope switch V / div. (1 or 2 V), it is necessary to achieve an image of an ellipse figure on the oscilloscope screen (Fig. 4, a). Further, slowly, rotating the armature installed on the machine by hand (Fig. 3), moving the sliding contacts along the collector lamellas, observe the figures on the oscilloscope screen.

When moving one step, an ellipse of a slightly smaller size will appear on the oscilloscope screen (Fig. 4, b), while the previous size of the ellipse in this figure is shown by a dotted line. In a working anchor, all ellipses should be of only these two sizes, alternating one after the other. If there are closed turns or a break in the armature windings, then when sliding contacts pass through these damaged windings, a wide variety of figures will appear on the oscilloscope screen (Fig. 5), which are very different from the figures in Fig. 4.

(click to enlarge)

If a faulty armature is detected in this way, then the electric motor can be restored in two ways: either replace the armature with a new, factory-made one, or completely rewind all armature windings (only in a specialized workshop). In the first case, it will cost more, but more reliably (it will work longer), in the second, it will be cheaper, but less reliable.

One of the options for the design of the machine with the anchor installed is shown in Fig.3. Sliding contacts can be made from relay plates fig.3 or spring wire (from a pin) fig.6.

In the author's installation, a G3-106 generator with a tunable frequency of 20 Hz ... 200 kHz and an adjustable level and an S1-114 oscilloscope with a rectangular screen 15 cm diagonal were used. With this screen size, it is more convenient to observe the measured figures.

To eliminate the influence of an external alternating electromagnetic field on the measurement results, all connections in the installation are made with a shielded cable.

The author expresses his gratitude to N. V. Kompanets, head of the workshop for the repair of power tools, for his assistance in preparing this article.

Author: N.P. Vlasyuk

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