Armature and winding

The coiled, insulated conductors surrounding the armature through which current is run to create a magnetic field. Reversing the current flow through the armature.

There are two types of armature windings in DC machines.

1. LAP winding. 2. WAVE winding

The difference between the two is merely due to the different arrangement of the end connections at the front or commutator end of armature. Each winding can be   progressively or retrogressively and connected in simplex, duplex and so on. The following rules are followed by the armature windings manufacturer.

(i)The front and back pitch are each approximately equal to the pole-pitch i.e windings should be full-pitched. This results in increased e.m.f round the coils (ii)Both pitches should be odd, otherwise it would be difficult to place the coils properly on the armature. For example if YB and YF were both even, then all the coil sides and conductors would lie either in the upper half of slots or in the lower half. Hence, it would become impossible for one side of the coil to lie in the upper half of one slot and the other side of the same coil to lie in the lower half of some other slot.

(iii) The number of commutator segments is equal to the number of slots or coils because the front ends of conductors are joined to the segments in pairs.

(iv) The winding must close upon itself i.e if we start from a given point and move from one coil to another, then all connectors should be traversed and we should reach the same point again without a break or discontinuity in between.

Classification of Generators

The field windings provide the excitation necessary to set up the magnetic fields in the machine. There are various types of field windings . Depending upon the field there are two types of generators.

(a) Separately-excited generators

(b) Self-excited generators.

Self excited generators are classified according to there field connection. There are mainly three types of self excited generator.

I. Series

II. Shunt

III. Compound

Compound generators are classified as

a) cumulative-compound

b) Differential-compound

Series : In the series-connected generator, all of the current from the generator is passed through the field windings because the external circuit is in series.

Shunt : Another way to excite the field magnets is to connect them in parallel with the load . This type of generator is commonly called a shunt generator.

GENERATOR

The system consists of a cantilever beam supported by the housing. The mass on the beam is made up of two magnets (one pole) mounted on a c-shaped core. Arranging the magnets in this way provides a uniform magnetic field in the air-gap. The main purpose of the core is to provide a path and guide the magnetic flux through it with a minimum of flux leakage. The coil is made up of a number of single solid core enamelled copper wires. It is placed in the air-gap between the magnets at right angles to the direction of the movement of the mass.

The operating principle of the device is as follows. As the housing is vibrated, a mechanical input force feeds into a second order mechanical system, the mass moves relative to the housing and energy is stored in the mass-beam system. This relative displacement, which is sinusoidal in amplitude, causes the magnetic flux to cut the coil. This in turn induces a motional electromotive force on the coil due to Faraday’s law. The magnitude of this voltage is proportional to the rate of change of the coil position. The electrical system involved is simply a first-order LR circuit with the inductance of the coil in series with the load resistance and the parasitic resistance of the coil.

POWER GENERATOR (INTRODUCTION)

Over recent years, an interest has developed in micro electromechanical systems (MEMS) and the subject has matured to the point where its applications to a wide range of areas are now clearly feasible. Applications such as medical implants and embedded sensors in buildings and similar structures, are just a few of many examples. The supply of power to such systems has so far been through batteries. However, in long-lived systems where battery replacement is difficult and in applications consisting of completely embedded structures with no physical links to the outside world, generating power from ambient sources becomes imperative. Systems that depend on batteries have a limited operating life, while systems having their own self-powered supply unit have a potentially much longer life. A potential and promising alternative solution to batteries is the use of miniature renewable power supply units. Such devices convert energy from existing sources energy within their environment into electrical energy.

Ambient energy may be available within the environment of a system and is not stored explicitly. The source of such energies, however, depends on the application. The most familiar ambient energy source is solar power (light energy from ambient light such as sunlight). Thermal energy is another ambient energy source (thermoelectric generators generate electricity when placed across a temperature gradient) . Flow of liquids or gases, energy produced by the human body and the action of gravitational fields are other ambient energy source possibilities. Other examples which depend on injected energy rather than naturally occurring ambient energy fields include electromagnetic fields used in RF powered tags , inductively powered smart cards and non-invasive pacemaker battery recharging . Our approach uses mechanical vibration as the ambient energy source for generation of electrical power . Therefore, in this paper a vibration-based magnet-coil power generator is described.

The most important parameters influencing the design of such a system are its physical size and conversion efficiencies. The size is dependent on the energy requirement and must be as small as possible, to be compatible with the general design objectives of MEMS. However as the size of the device is reduced, mechanical resonances tend to increase in frequency and it is the challenge of generating power from comparatively low vibrational frequencies (hundreds of Hz rather than kHz) that is addressed in this work. The ambient energy may be at a premium in a particular environment so the conversion efficiency must be as high as possible. To analyse the transformation efficiency and to assess the input-output relationship of such a generator, full electromechanical and magnetic analyses have been carried out. Finite element (FE) techniques for the magnetic field distribution solution have been employed. Fabrication and test results of a first prototype based on simulation and modelling results are fully discussed. Practical amounts of power within reasonable space (quarter of a cubic centimetre) have been achieved.

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