Introduction
"Energy can neither be created nor be destroyed". We can only
change its forms, using appropriate energy-conversion process.Energy conversion takes place between well known pairs of forms of energy. Electrical↔ Chemical ,Electrical↔ Thermal, Electrical ↔ Optical, Electrical ↔Sound, and Electrical ↔ Mechanical.
Electrical ↔ Mechanical conversion is the focus of discussion in this chapter.
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Electromechanical energy-conversion finds applications in following categories of systems:
(a) Transducers : Devices for obtaining signals for measurement/control,
(b) Force-producing devices : Solenoid-actuators,relays,electromagnets,
(c) Devices for continuous -energy-conversion : Motors/Generators.
Salient Aspects of Conversions
Purpose of Electro-Mechanical conversion device is to change the form of energy.Here, for simpler discussion,only rotary systems will be dealt with.When it is converting mechanical input to electrical output the device is "generating". With electrical input,when Mechanical output is obtained,the device is motoring.
Some simple aspects of an electrical machine (motor/generator) have to be noted at this place:
(1) Electrical Machine has a Stator, a Rotor and an air-gap in between the two.For a flux path,the magnetic circuit has these three parts in series.In general,magnetic poles are established in Stator and in Rotor.
(2) Magnetic effects of following types can be categorized...(a)Electromagnetic(b)Permanent Magnets(c) Reluctance variation.
(3) Equal no.of poles of such devices must be created on the two sides.
(4) Out of stator,rotor and air-gap,maximum energy-storage at any angular position takes place in the air-gap,since its reluctance is highest out of the three members.
(5)Ideal output of a motor is a constant unidirectional torque with given currents through its windings. In some cases,the output torque (as a compromise ) is an average value of a cyclieally varying torque.
(6) A device can work either as a generator or as a motor,provided pertinent conditions are satisfied for the concerned mode of operation
Energy-Balance
For an electro-mechanical system,following terms are important:
(ii) Mechanical port(=shaft);delivering/receiving mechanical energy.
(iii) Coupling field: Magnetic field or electrical field.
It is obvious that an electrical motor receives energy at the electrical port and delivers it at the mechanical port. While an electric generator receives the energy at the mechanical port and delivers it at the electrical port. It is also known that the following losses take place in such systems and are dissipated away as heat;(losses in the windings of the machines ,(ii)friction and windage losses,(iii) core-losses.
Change in Electrical Energy =Change in Mechanical Energy + Change in Field-energy
.dWelec = dWmech + dWfld
And
dWelec = dWfld..
