Why do we prefer frequency domain instead of time domain analysis?

Abstract: 
The big reason for using frequency domain analysis rather than time domain analysis: 
i) Acquire a measurement
ii) Identify the noise of the signal in the frequency domain
iii) Filter the unwanted signal. 

If we have a noise in our measured signal, we can move from the time domain into the frequency domain to isolate the disturbance in your measurement.

Description:

If we want to describe it elaborately Fourier’s theorem states that any waveform in the time domain can be represented by the weighted sum of sines and cosines. In the frequency domain, we can separate conceptually the sine waves that add to form the complex time-domain signal. 

Figure 1 shows single frequency components, which spread out in the time domain, as distinct impulses in the frequency domain.

Example:

Some measurements, such as noise or harmonic distortion, are difficult to quantify by inspecting the time waveform. When the same signal is displayed in the frequency domain by an FFT, you easily can measure the harmonic frequencies and amplitudes.

Frequency domain techniques can be use in conjunction with the S-domain (Laplace transform),whereas the time domain response of some circuits cannot be represented by Laplace functions. One example is the skin effect losses in coax cables, because the loss is proportional to √f.

AC & DC MACHINES

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.

Generator

 

 

 

 

 

 

 

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:

  (i) Electrical port(=armature terminals);receiving/delivering electrical energy
(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..