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March 20, 2015

Give a list of the factors involved for selecting a factory drive.

Factors involved in selection of factory drives:

  - Supply system        
:  DC or AC

  - Rating                  
:  kW, MW

  - Drive                    
:  constant speed, Variable speed

- Special Condition    
 :  High starting torque, Hazardous location, Traction duty etc.

   - User’s requirement
: Duty cycle, noise level, load      characteristics etc 

  - off load, load condition         

-  Enclosures & cooling requirement.

  - Environment condition

-  Applicable standards

    -  Cost considerations 

What are the various losses occurring in a transformer?

Various losses in a transformer
(i). Core or iron losses:
Core flux in transformer remains constant for all load. Its variation is 1 to 3%. This losses are due to eddy current and hysteresis in  the core of transformer. 

(i). Copper losses :
These losses are due to ohmic resistance of transformer windings.

Explain the construction and working principle of a transformer.

Transformer Construction:
The simple element of a transformer consists of two coils having mutual inductance and a laminated steel core. The two coils are insulated from each other with steel core. Other necessary parts are : some suitable container for assembled core windings from its container: Suitable bushings (either of porcelain, oil- filled or capacitor type) for insulating and bringing out the terminals from the tank. Core is constructed of transformer sheet steel laminations, assembled to provide a continuous magnetic path with a minimum of air gap included. Steel used is of high silicon content, sometimes heat treated to produce a high permeability and low hysteresis loss at the usual operating flux-densities. Lamination is of varnish or an oxide layer, which reduces eddy current loss. Thickness of lamination varies with frequency inversely.

Transformers are of 2 general types distinguished by the manner in which primary and secondary coils are placed around the laminated core :
1. Core Type – winding surrounded a considerable portion of the core.
2. Shell Type – core surrounds a considerable portion of windings.

Working principle :
A transformer is a static piece of apparatus by means of which electric power in one circuit is transformed into electric power of the same frequency in other circuit. It can raise or lower the voltage in a circuit but with a corresponding decrease or increase in current. The physical basis of transformer is mutual induction between two circuits linked by a common magnetic flux. It consists of two inductive coils which are electrically and magnetically linked through a path of low reluctance. The 2 coils possess high mutual inductance. If one coil is connected to a source of alternating voltage, an alternating flux is set-up in laminated core, most of which is linked  with other coil in which it produces mutually induced emf (according to Faraday’s law of EMI e = MdI/dt). If the second coil circuit is closed, a current flows in it and so electric energy is transferred  (entirely magnetically) from the first coil to the second coil. The first coil in which electric energy is fed from A.C. supply mains is called primary winding and other from which energy is drawn out, is called secondary winding. 

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