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1808 results in Circuits and systems

Page 34 of 91

  • 3 - Three-Phase Transformers

  • S. K. Sahdev
  • Book:
    Electrical Machines
    Published online:
    03 August 2019
    Print publication:
    04 October 2017, pp 224-300

  • Summary

    Introduction

    Three phase system is invariably adopted for generation, transmission and distribution of electrical power due to economical reasons. Usually, power is generated at the generating stations at 11 kV (or 33 kV), whereas, it is transmitted at 750 kV, 400 kV, 220 kV, 132 kV or 66 kV due to economical reasons. At the receiving stations, the voltage level is decreased and power is transmitted through shorter distances. While delivering power to the consumers, the voltage level is decreased to as low as 400V (line value) for safety reasons.

    Thus to increase the voltage level at the generating stations step-up transformers and to decrease the voltage level at the receiving stations, step down transformers are employed.

    Merits of Three-phase Transformer over Bank of Three Singlephase Transformers

    The voltage level in three-phase system at the generating stations and at the receiving stations can be changed either by employing a bank of three single-phase transformers (inter connecting them in star or delta) or by employing one three phase transformer. Generally, one three phase transformer is preferred over a bank of three single phase transformers because of the following reasons.

  • (i) It requires smaller quantity of iron and copper. Hence, its cost is nearly 15% lesser than a bank of three single phase transformers of equal rating.

  • (ii) It has smaller size and can be accommodated in smaller tank and hence needs smaller quantity of oil for cooling.

  • (iii) Because of smaller size, it occupies less space; moreover it has less weight.

  • (iv) It needs less number of bushings.

  • (v) It operates at slightly better efficiency and regulation.

    • These transformers suffer from the following disadvantage.

  • (i) It is more difficult and costly to repair three-phase transformers.

  • (ii) It is difficult to transport single large unit of three-phase transformer than to transport three single phase transformers individually.

    • The advantages of three-phase transformer (such as lower cost, lower weight, lower space requirement etc.) over weighs its disadvantages and hence are invariably employed in the power system to step-up or step-down the voltage level.

  • 12 - Special Purpose Machines

  • S. K. Sahdev
  • Book:
    Electrical Machines
    Published online:
    03 August 2019
    Print publication:
    04 October 2017, pp 893-932

  • Summary

    Introduction

    So for, we have discussed about synchronous machines, induction machines, fractional kW machines etc. These machines are the basic machines which are employed at various places as per requirement. However, in industries, at commercial places and in houses there are variety of jobs for which these general purpose basic machines are not suitable. But these machines with suitable amendments in their design can be employed to perform these jobs. Such specially designed machines are called special purpose machines.

    The machines which are designed specially to perform a particular job are called special purpose machines.

    Feedback Control System

    A control system which tends to maintain a prescribed relationship between a controlled variable and a reference input by comparing relationship between a controlled variable and a reference and difference as a means of control is call a feedback control system.

    Electrical feedback control systems rely upon mainly electrical energy for operation. The essential of this type of control are (i) an error detecting device, (ii) an amplifier and (iii) an error-correcting device as shown in Fig. 12.1. Each one of them serves a functional purpose in matching the controlled variable to the reference input.

    The error-detecting device determines when the controlled variable is different from the reference input. It then sends out on error signal to the amplifier, which is turn supplies power the error correcting device changes the controlled variable so that it matches the reference input. The closed loop at feedback control system is shown in Fig. 12.1.

    Thus, a system in which output is compared with the input and error is used to control the operating system is called feedback control system.

    Servomechanism

    The mechanism in which the control variable is adjusted by the error served by comparing output and input is called servomechanism.

    Any quantity e.g., voltage, speed, temperature, position, direction or torque be controlled by providing appropriate feedback. Any quantity can be used as a reference input. Moreover, the reference input need not have the same units as the controlled variable provided that the proper error detecting device is employed.

Page 34 of 91