NOISE IN ELECTRIC ROTATING MACHINES

Author: RNDr. B. Madejewski, CSc., Research Institute of Electric Machines, Brno

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1. INTRODUCTION

At present electric rotating machines are the most frequently used driving units of the machinery in industry, transport, civil engineering and agriculture. They are also commonly used in non-productive fields – in households, schools and offices.
Varying application of these machines is based on their technical parameters, predominantly on the type of the machine (i.e. induction, synchronous, direct-current), the other parameters being their power output, speed and cooling method.
Parasitic phenomena in these machines include, among others, also their noise. In the whole wide assortment of produced electric machines it is necessary to search common sources and causes of their excessive noise so that the problems of its reduction can be dealt with in a complex way.

2. NOISE OF ELECTRIC MACHINE

The analysis of noise of an electric rotating machine shows that its total noise consists of superposition of three basic components, namely::
- noise of electromagnetic origin
- noise of ventilation origin
- noise of mechanical origin
…

3. REDUCTION OF NOISE OF ELECTRIC MACHINES

In the technical practice there are two basic methods of reduction of machinery noise:
- primary method, i.e. reduction of noise directly in its source in the machine,
- secondary method, i.e. absorption of radiated acoustical energy and its transformation into another kind of energy.
The first method is naturally more effective, it solves the very substance of the problem. When it is applied, it must be considered which partial components the total noise of the machinery comprises and the reduction of which of these components will bring about significant reduction of its total noise. As it was already mentioned, in the case of an electric rotating machine there are three partial components of noise. The basic principles that must be complied with in suppressing or total removing of individual noise components are as follows:

3.1. Electromagnetic noise component

- order of electromagnetic power wave of the machine, depending on the ratio of the number of rotor and stator slots, must be chosen as high as possible,
- frequency of power magnetic waves of all orders must be sufficiently different from natural frequencies of vibration, particularly of stator stack and frame,
- recommended ratio of stator and rotor slots must be chosen,
- minimum induction in air gap must be chosen,
- maximum air gap must be chosen,
- if possible, magnetic wedges must be used,
- eccentricity of air gap must be minimized,
- maximum number of slots per pole and phase must be used,
- two-layer winding must be used,
- slot skewing by one slot pitch must be used,
- sufficiently rigid structure of the motor part being exposed to influence of electromagnetic forces must be used,
- in DC motors, tapering of pole pieces must be used,
- non-uniform air gap under poles must be used,
- symmetry of air gap and of the whole magnetic circuit must be observed.

3.2. Ventilation noise component

- minimum required cooling air quantity for ventilation of the machine must be used (ventilation system must not be oversized),
- generation of siren phenomena must be prevented by means of observing recommended values for the distance between the standing part and the rotating one,
- optimum number of fan blades must be chosen,
- resonance phenomena of the fan cover must be prevented.

3.3. Mechanical noise component

- the bearing size must be chosen as small as possible with regard to the requirements being imposed,
- roller bearings are noisier than ball bearings under the same conditions,
- class of accuracy of the bearing is manifested mainly at low frequencies,
- radial gap of the end shield must be chosen with regard to a minimum influence of temperature and load being applied,
- geometry and tolerances of bearing assembly parts must be complied with,
- axial spring for mounting of the bearing must be used,
- proper lubrication of the bearing must be applied,
- mounting of the bearing must be carried out by the prescribed technological procedure.
The second method of noise reduction consists in transformation of acoustic energy into another kind of energy, most frequently into thermal energy. It is achieved in three ways:
- friction of air particles in the course of flowing through pores in absorptive material,
- reduction of potential energy of sonic wave (heat exchange between air and absorptive material),
- non-reversible deformation of the body.
Suppressors of noise of ventilation origin are made in various versions, mainly according to the requirements for their noise attenuation. Frequently they have modular construction.