Study of Vibrations in a DC Motor with an Armature Having Herringbone-Skewed Teeth
Abstract Generated abstract
This paper investigates whether herringbone skewing of armature teeth reduces vibration in DC motors by improving the symmetry of magnetic pull forces in the air gap. Experimental armatures with active lengths of 63 mm and 138 mm were tested, and vibration amplitudes and frequencies were measured at several points on the frame, bearing shield, and mounting feet over rotational speeds from 1000 to 3000 rpm. The measured vibration frequency corresponded to tooth frequency, indicating a predominantly magnetic origin. For the 138 mm armature, herringbone skewing produced similar vibration levels to linear skewing below 2000 rpm but reduced housing vibrations by about a factor of 4 to 5 at higher speeds, with still larger reductions at the bearing shield and feet, while the 63 mm armature showed little difference between the two designs.
Full Text
Investigation of Vibrations of a DC Motor with an Armature Having Herringbone-Skewed Teeth
K. A. KHORKOV, E. E. DUTLOV, A. A. GAVRIKOV
(Presented by the joint seminar of the Departments of Electrical Machines and Apparatus and General Electrical Engineering)
Studies of the magnetic field in the air gap of a DC machine show that the best design, from the point of view of symmetry in the distribution of fluxes and magnetic pull forces under the edges of the pole shoe, is to make the machine armature with a herringbone skew of the teeth. When such an armature rotates, the tangential, radial, and axial components of the magnetic pull force are applied symmetrically with respect to the center of the pole and, in an ideal machine design, balance one another pairwise. This is a prerequisite for reducing the vibrations of a DC machine with a herringbone armature in comparison with the case of a linear skew of the teeth. In order to confirm the theoretical premises, experimental armature specimens with a herringbone skew of the teeth were manufactured with active machine lengths of 63 mm and 138 mm.
Vibrations were measured at points on the surface of the frame corresponding to the line of attachment of the auxiliary poles (point 1), the line of attachment of the main poles (point 3), the center line between the axes of these poles (point 2), on the end face of the bearing shield on the commutator side, and on the feet of the machine, Fig. 1. The measuring instruments used were an SM-211 vibrometer with a KD-13 transducer and a Marconi frequency-spectrum analyzer. Measurements of the vibration level were made with a smooth variation of the armature rotational speed from 1000 rpm
Fig. 1. Arrangement of vibration measurement points.
up to 3000 rpm. When a maximum or minimum signal was detected with the aid of a frequency-spectrum analyzer, the amplitude of the vibration accelerations of the frame section and the vibration frequency were measured. It was established that the vibration frequency is the tooth frequency and is rigidly connected with the rotational speed of the machine. The latter indicates that vibrations of magnetic origin predominate.
The test results for the motor with an armature having a length of 138 mm are presented in Figs. 2, 3, and 4. For comparison, these figures present vibrograms of the motor with an armature having a linear tooth skew. From the vibrograms it is evident that, at armature speeds up to 2000 rpm, the vibration level of the machine with both types of armatures is practically the same. At rotational speeds above 2000 rpm, the advantages of the herringbone skew of the teeth are indisputable: the vibration level of the machine housing decreased by a factor of 4–5. The vibrations of the bearing shield and of the machine feet decreased to an even greater extent.
Fig. 2. Vibrograms of frame sections corresponding to points 1 and 2: 1 — herringbone tooth skew; 2 — linear skew.
Of interest is the distribution of vibrations along the length of the frame. As an example, Fig. 3 presents vibrograms of three sections of the frame along the axis of the main pole, corresponding to the section on the drive side (point 3a), on the commutator side (point 3k), and in the middle of the machine (point 3s). The greatest vibrations with linear
Fig. 3. Vibrograms of sections located along the length of the frame and of the main poles:
1 — herringbone skew of the teeth; 2 — linear skew.
Fig. 4. Vibrograms of the feet and the bearing shield:
1 — herringbone skew of the teeth; 2 — linear skew.
the skew of the teeth, the end points (3a, 3k) are subject to vibration. With a herringbone skew of the armature teeth, the vibrations over the entire stator are distributed more uniformly, which agrees well with the nature of the application of magnetic-traction forces that we considered in [1]. Particularly important is the reduction in the vibration level of the machine mounting points—the feet.
Tests of a machine with an active length of 63 mm showed that the vibrations of the surface of the machine frame and of the feet, with armatures of both types, are practically identical.
Thus, the investigations carried out on the vibrations of DC motors with armatures having herringbone tooth skew show the advisability of using this armature design for large active lengths of the machine.
REFERENCES
- K. A. Khorkov, V. N. Dmitriev, E. E. Dutlov. Investigation of the magnetic field in the air gap of a DC machine. Izv. TPI. This volume.