Transformers

medical physics

1. TRANSFORMERS Construction and their working

2. What is transformer? A transformer is an electric device which is used for changing the a.c . voltages and to render the current earth-free A transformer which increase voltages is called as step up transformer A transformer which decrease voltages is called as step down transformer Principle: based on the principle of mutual induction

3. Circuit Symbol for Transformer

4. Construction of transformer Consist of two coils (primary and secondary) of insulated wire wound onto a laminated soft iron frame Two coils may be wound on top of one another on central bar or on opposite side of the frame Frame is usually rectangular in shape One coil usually contain more turns than other

5. Working of Transformer An alternating current is passed through primary coil which sets up magnetic field which cut the secondary coil Emf is induced in secondary coil due to change in magnetic field of primary coil For an emf to be induced, current should be a.c . No electrical connection b/w two coils both are insulated from each other, current is induced via EMI Soft iron core serves to concentrate the mag. lines of forces Soft iron is laminated to prevent eddy currents

6. Step up transformer: No. of turns in primary coil is less than that in secondary coil Converts the low voltages into high voltages Primary coil is made of thick insulated copper wire with less no. of turns Secondary coil is made of thin insulated copper wire with larger no. of turns Step down transformer: No. of turns in primary coil is larger than that in secondary coil Converts the high voltages into low voltages Secondary coil is made of thick insulated copper wire with less no. of turns Primary coil is made of thin insulated copper wire with larger no. of turns

7. Function of Transformer To render the current earth free: The main current passes through primary coil induces emf in secondary coil, as there is no elec. connection b/w primary and secondary coil, which has no connection to earth so render the current earth free If connection b/w earth and sec. coil made, no current passes to earth as circuit is not complete, so no earth shock occur To alter the voltage of an alternating current: via step up and step down transformer Emf induced in secondary coil is directly proportional to no. of turns in this coil

8. Types of transformer Static transformer (step up, step down and even ratio transformer) Variable transformer Autotransformer

9. Variable transformer That can put out different amount of voltage from the same input voltage Tapings may be provided so that either primary or secondary coil can be varied in length To use the same apparatus on supply of different voltages by varying no. of turns Higher the voltage of supply the more turns of wire are included in primary coil Care should be taken to see that control is in right position for supply used Tapings may be taken from different points on secondary coil to supply different voltages to separate circuits in same apparatus

10. Used to speed up and slow down electric motor Make lights dimmer and brighter

11. Autotransformer Consist of a single coil of insulated wire wound over laminated soft iron core Tapped at two points, C and D When used as a step up transformer , coil b/w C and D act as primary and A and B act as secondary and vice versa It has certain disadvantages It can be used only for a small step up and down in voltage Do not make current earth free in secondary coil

12. Choke coil It is included in circuit in order to produce a self induced EMF Low frequency choke coil: consist of many turns of insulated wire wound on soft iron usually on central bar of rectangular frame High frequency choke coil: consist of usually less no. of turns of insulated wire wound on bobbin of non conducting material as current varies rapidly Uses: Even out variations in the intensity of current To prevent the flow of high frequency current and allow the passage of lower one

13. Dynamo It is a device which is used for the production of an EMF via electromagnetic induction Construction A coil of wire lie b/w poles of magnet and by some mean rotation of the coil is provided via motor, steam engine or water power When coil rotates, emf is induced in coil first in one direction than other So current produced is alternating type Collecting device is used to convey current from coil to external circuit

14. Supply both types of currents to external circuit either a.c or by converting a.c into d.c Used on large scale for production of electricity including main supply