Proceedings
of the Tomsk Order of the October Revolution
and Order of the Red Banner of Labor
Polytechnic Institute named after S. M. Kirov

Vol. 284
1974

STUDY OF THE MAGNETIC FIELD IN THE AIR GAP OF A DC MOTOR OPERATING UNDER LOAD

K. A. KHORKOV, V. N. DMITRIEV, E. E. DUTLOV

(Presented by the scientific seminar of the Departments of Electrical Machines and Apparatus and General Electrical Engineering)

In order to study the alternating component of the magnetic field under the pole during the motion of a toothed armature, an experimental installation was created on the basis of a G-40 motor. The results of studies during operation of the motor in no-load mode were set out by us in [1]. In the present work, results are given of studies of the magnetic field during operation of the DC motor under load. The armatures of the machi-

Fig. 1. Arrangement of measuring sensors

nes have 27 and 31 slots with a skew of one tooth pitch. During the tests, the motor speed was maintained constant and equal to 1500 rpm; the excitation current was equal to 1 A. The armature current was varied from the no-load current of 2.3 A to the rated current of 20 A.

The measuring e.m.f. sensors were installed on the pole shoes of the machine according to Fig. 1. The arrangement of the sensors along the width of the pole shoe makes it possible to trace the change in flux on the surface of the pole during the motion of the toothed armature. On pole 1 the width of the sensor frames is equal to the width of an armature tooth; on pole 2, to the width of the open armature slots. The placement of the sensors on the edges of the poles (pole 3) makes it possible to analyze the change in flux along the length of the leading and trailing edges of the pole shoe. Frame 4–10

encompasses the entire pole shoe along its perimeter and makes it possible to judge the radial pulsations of the magnetic flux as a whole under the entire pole. A typical e.m.f. curve of a sensor located under a pole is shown in Fig. 2, curve 3. Curve 3 is obtained as the sum of the e.m.f. curves of the sides of the measuring frame (curves 1 and 2). It is obvious that the amplitude of the sensor e.m.f. characterizes the magnitude of the alternating component of the magnetic field under the pole. The width of the measuring coil must not be equal to the width of the armature tooth pitch, since in this case the resultant e.m.f. of the sensor is zero. The width of the measuring coil must lie within the limits from \(b_{\mathrm{p}}\) to \(b_{\mathrm{z}}\), and with increasing frame width the amplitude of the sensor e.m.f. decreases because of the influence of the skew of the armature slots.

Fig. 2. Formation of the e.m.f. curve of the measuring sensor

In the machine under test, the width of the pole shoe is 4.7 tooth pitches for an armature having 31 teeth, and 4.1 pitches for an armature with 27 teeth. Therefore the e.m.f. curves of the measuring coils contain an entire spectrum of higher harmonics, the largest of which is the tooth harmonic.

In the investigations, the armature current was increased from the no-load current to the rated current of the machine. Accordingly, owing to the influence of armature reaction, redistribution of the main flux of the machine occurs under the pole: the flux at the leading edge of the pole increases, while that at the trailing edge decreases. The influence of the redistribution of the main flux on the magnitude of the tooth pulsations under the pole can be traced from the measurement results presented in Tables 1 and 2. The tables give the e.m.f. of each sensor in millivolts and the average e.m.f. values for sensors of the same type when the armature rotates in both directions, as a function of the armature current.

As was to be expected, the amplitude of the tooth pulsations of the total flux under the poles varies practically in proportion to the magnitude of the armature current. Of interest is the pattern of change in the e.m.f. of the sensors placed at the leading (sensors 12a—12d) and trailing (sensors 1a—1d) edges of the pole shoe when the armature rotates in the forward direction. When the load is increased to rated, for the sensors at the leading edge of the pole (magnetized by the flux of the armature reaction) a uniform threefold increase in their e.m.f. is observed. At the same time, the e.m.f. of the sensors at the trailing edge of the pole, demagnetized by armature reaction, remains practically unchanged.

When the direction of rotation of the armature is changed, the leading and trailing edges of the pole change places, and the e.m.f. of sensors 12a—12d becomes unchanged, while the e.m.f. of sensors 1a—1d grows in the same proportion as before for the leading edge of the pole. For both armatures, the pattern of change in the sensor e.m.f. is analogous.

An increase in the amplitude of the magnetic-field pulsations under the leading edge of the pole should correspondingly increase the forces of the magnetic

Table 1

The armature has 27 tooth divisions

Table 2

The armature has 31 tooth divisions

Continuation of Table 2

attraction; the inequality of the magnetic forces at the leading and trailing edges of the poles will lead to the appearance of uncompensated radial and tangential forces. Thus, an increase in the load of the machine is accompanied by an increase in the inequality of the forces applied to the leading and trailing edges of the pole shoes, which is accompanied by an increase in the vibrations of the machine.

References

1. K. A. Khorkov, V. N. Dmitriev, E. E. Dutlov. Study of the magnetic field in the air gap of a direct-current machine. Izv. TPI, vol. 265, 1973.