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Chapter 2 Construction, operation and failure modes of electrical machines 2. Rotating electrical machines convert electrical to mechanical energy, or vice versa, and they achieve this by magnetically coupling electrical circuits across an airgap that permits rotational freedom of one of these circuits. Mechanical energy is transmitted into or out of the machine via a drive train that is mechanically connected to one of the electric circuits.

An example of one of the largest electromagnetic energy conversion units in the world, at 1 megavolt-amperes MVA , is shown in Figure 2. The construction Figure 2. The generator exciter is on the left, the turbine generator is in the centre of the picture and the low pressure turbine to the right. The purpose of this chapter is to explain their constructional principles and the main causes of failure. The chapter is illustrated with a large number of photographs to demonstrate to the reader the salient features of electrical machines.

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Metals with good magnetic and electrical properties do not necessarily have high mechanical strength. Indeed the atomic structure of a good conductor is such that it will naturally have a low yield strength and high ductility. Yet the magnetic and electric circuits of the machine must bear the mechanical loads imposed upon them by the transfer of energy across the airgap. Furthermore, the magnetic and electrical circuits must be separated by insulating materials, such as films, fibres and resins, which have even weaker mechanical properties.

Rotating Electrical Machines and Power Systems

Table 2. Right from the outset then, there is a conflict between the electrical and mechanical requirements of the various parts of an electrical machine, which the designer must attempt to resolve. The transfer of energy inevitably involves the dissipation of heat, by ohmic losses in the electric circuit and by eddy current and hysteresis losses in the magnetic circuit. The performance of the insulating materials that keep these circuits apart is highly dependent upon temperature, and deteriorates rapidly at higher temperatures. Materials that can sustain these higher temperatures become progressively more expensive and their mechanical and dielectric properties are often worse than lower temperature materials.

Natural fibre materials, cotton, silk, paper and wood impregnated, coated or immersed in dielectric liquid, such as oil. Synthetic-resin impregnated or enamelled wire not containing fibrous materials such as cotton, silk or paper but including phenolics, alkyds and leatheroid. Combinations of mica, glass and paper with natural organic bonding, impregnating or coating substances including shellac, bitumen and polyester resins.

The use of slightly asymmetrical windings for rotating electrical machines

Combinations of mica, glass, film and paper with synthetic inorganic bonding, impregnating or coating substances including epoxy and polyester resins. Combinations mica, paper, glass or asbestos with synthetic bonding, impregnating or coating substances including epoxy, polymide and silicone resins. Combinations of asbestos, mica, glass, porcelain, quartz or other silicates with or without a high temperature synthetic bonding, impregnating or coating substance including silicone.

These can include high-temperature aramid calendared papers like Nomex. Uncertainties about the temperatures within a machine mean that the designer is forced to restrict the maximum measurable operating temperature to an even lower 16 Condition monitoring of rotating electrical machines value than that given in Table 2.

It is clear that the heat dissipated within a machine must be removed effectively if design limits are to be met. For example, in the 1 MVA turbine generator shown in Figure 2.

Electric Power Components and Systems

The problem is exacerbated because the losses are not evenly distributed and in practice at some locations the rise in temperature will be even faster than this. So cooling and its distribution become a vital part of machine design. The health of an electrical machine, its failure modes and root causes, are ultimately related to the materials of which it is made, the mechanical and electrical stresses those materials are subjected to and the temperatures they attain in service. In Chapter 1 we explained how electrical machines are protected by relays, which sense serious disruptions of the current flowing in the windings and operate to trip or disconnect the machine.

However, when fault currents are flowing the machine has already failed as an electrical device. Electrical or mechanical failure modes are always preceded by deterioration of one of the mechanical, electrical, magnetic, insulation or cooling components of the machine.

This is the case regardless of the type of electrical machine. If this deterioration takes a significant period of time and can be detected by measurement, then that root cause detection will be a means of monitoring the machine before a failure mode develops. The heart of condition monitoring is to derive methods to measure, as directly as possible, parameters that indicate root cause deterioration activity and provide sufficient warning of impending failure in order that the machine may be taken off for repair or may be tripped before serious damage occurs.

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A degree of protection could be achieved by making the protective relays especially sensitive and providing an alarm indication before tripping occurs. Experience has shown that this is a precarious mode of condition monitoring leading to false alarms and a lack of confidence in the monitoring process. The following sections show how the construction, specification, operation and types of fault can lead to the identification of generic failure mode root causes in the machine.

The rotor, which usually has a relatively high inertia, is normally supported on two bearings, which may be mounted on separate pedestals or incorporated into the enclosure of the machine, as shown in Figure 2. Some larger, slower-speed machines incorporate a single non-drive end bearing and rely on the prime mover or driven plant and its bearings for the remaining support. Rolling element bearings are used Construction, operation and failure modes of electrical machines 17 a b Figure 2. Section through a MVA, 15 kV, 60 Hz, two-pole, air-cooled, brushless excitation turbine generator showing the fabricated main frame of the machine, stator core, winding, rotor and on the right the main exciter of the machine.

Construction of the stator core of a MW, twopole, hydrogen-cooled turbine generator showing the fabricated inner frame of the machine and the segmented laminations being inserted into that frame. Vertically mounted machines will incorporate a thrust bearing usually at the low end of the enclosure. This may be a relatively modest angular contact ball bearing for a small, vertically 18 Condition monitoring of rotating electrical machines mounted pump motor but could be a large hydrodynamic oil film thrust pad Michelltype bearing for a hydro-type generator where the rotor may weigh tonnes or more see Figure 2.

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As Table 2. Furthermore, if the laminated structure is to have the cohesion necessary to transmit the load torque, and have low levels of vibration when carrying the magnetic flux, it must be firmly clamped between cast or fabricated end-plates that are secured to a cylindrical frame into which the core is keyed. The core is constructed within the frame and compressed before the clamping plates are applied. The frame structure and its clamping are clearly visible in Figures 2. On larger machines the clamping plates are tightened by large bolts see Figure 2.

In a DC machine the laminated stator field poles are bolted to a rolled-steel yoke that has much greater inherent strength than a laminated core Figure 2. AC induction and DC motors have laminated rotors where the laminations are clamped together and shrunk onto the steel shaft Figure 2. Turbine-type generators have large, solid, forged-steel rotors that are long and thin Figure 2.

Where air or gas cooling is necessary an axial or radial fan may be fitted at either or both ends of the rotor shaft. However, smaller machines rely solely on air circulation as a result of the windage of the rotor itself, which is usually slotted to accept the rotor windings Figure 2. The rotor windings of generators are constructed of hard-drawn copper and are insulated with rigid epoxy or formaldehyde resin and impregnated into a woven material.


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On squirrel cage induction motors the winding may consist of lightly insulated copper bars driven into the slots in the laminated rotor or of aluminium bars cast directly into the rotor. The rotor windings of a DC machine or wound rotor induction motor are rather similar to a conventional AC stator winding that is described later. Typical induction motor and generator rotors are shown in Figures 2. Stators for 2 MW, two-pole, hydrogen-cooled turbine generators.

The stator nearest the camera is wound. The stator furthest from the camera is awaiting winding.

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Rotor for a MW, two-pole, hydrogen-cooled turbine generator showing rotor forging and rotor winding before the fitting of end bells. Individual subconductors are covered with a paper or glass-based tape and the assembled bar is overtaped with a similar material impregnated on older designs with bitumen but nowadays with epoxy resins see Figure 2.

In the portion of the conductor bar embedded in the stator slot the insulation system is compacted by being heated and pressed or it may be impregnated under vacuum and pressure. In the end-winding portion, where one coil is connected to another, the insulation system is not compacted and may be slightly altered, containing less impregnant, so that it is more flexible and therefore better able to withstand the large electromagnetic forces that part of the winding experiences. An important part of the construction is the manner of the bracing of these end windings. They are usually pulled back onto rigid insulated brackets made of impregnated laminate or steel using nylon or Terylene lacing cord.

On the largest machines Figure 2. The exact nature of the bracing depends upon the machine rating and the relative length of the end winding, as determined by the number of pole pairs. The yoke or stator core is fitted into a frame and enclosure. On smaller machines and those of standard design, the stator core is secured directly into a simplified design of a machine main frame Figure 2. Where a pressurised gas system of cooling is used the enclosure will be a thick-walled pressure vessel but for simple air-cooling with an open-air circuit the enclosure will consist of thin-walled ducting.

Typical enclosures are shown Construction, operation and failure modes of electrical machines 21 a b Figure 2. End region of a MW, two-pole, hydrogen-cooled turbine generator with water-cooled stator windings showing the end winding bracing structure and the hoses carrying water to the winding. Stator of a MW, two-pole, hydrogen-cooled turbine generator showing stator core, frame and windings being inserted into its stator pressure housing preparatory to factory testing. Installation of the stator of a 75 MVA, pole, 50 Hz, Section through a W, wound-field DC motor showing stator frame and bearing housings, armature and commutator on the left.

Note the drive shaft on left and the water-cooled heat exchanger on top of machine. There is an increasing demand nowadays to reduce the noise level from electrical machines and apart from affecting the basic design of the stator and rotor cores, this will require specially designed noise-proof enclosures. View of a 20 MW advanced induction motor for ship propulsion showing low speed drive shaft and heat exchangers on the machine flanks. This is a large multi-phase, multipole, variable speed motor fed by a current-fed inverter. Source: Alstom, France.

View of a combined 4 kW motor and inverter used in a small Nissan full-electric-vehicle, the Hyper-Mini. Induction motor, V line, 40 kW. The main three phase busbars of the 1 MVA generator are visible rising from the stator frame in the centre of Figure 2. The busbars may be lightly insulated to protect them against the environment.


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The bushings usually consist of the busbar embedded into an epoxy resin casting, although wound paper bushings may be used on older machines. The electrical connections are well braced to withstand the large electromagnetic forces that are developed when fault currents flow. The terminal enclosure allows the proper termination of the supply cables or busbars, and must be specially designed to suit the environment in which the machine works.

For example, special enclosures are required for motors that operate in inflammable areas and these incorporate baffles and seals to ensure that any flashover in the enclosure does not ignite gas or vapour outside the terminal box. Many machines incorporate brushgear for connection to the rotor windings either through steel or copper sliprings or through a copper commutator Figure 2.