Published on August 23, 2013
Prepared by V.BHUVANESWARI TRANSFORMERS
• Transformer works on the principle of electromagnetic induction. • Transformer Is an electrical device having no moving parts, it transfers electric energy by mutual induction from one circuit to another at same frequency with changed values of V and I. • It consists of two winding insulated from each other and wound on a common core made up of magnetic material.
• Alternating voltage is connected across one of the winding called primary winding. • In both the windings emf is induced by electromagnetic induction. • And the second winding is the secondary winding of transformer.
WORKING PRINCIPLE OF TRANSFORMER • When primary winding is connected to an ac source an exciting current flows through the winding. As the current is alternating it will produce an alternating flux in the core which will be linked by both the primary and secondary windings. E1=V1 • Where ,E1 is induced emf in primary V1 is applied voltage in primary • Power transferred from the primary to the secondary circuit by electromagnetic induction. • Flux in the core will alternate at same frequency of the supply voltage. • Frequency of induced emf in secondary is the same as that of the supply voltage. • Magnitude of emf induced in secondary winding will depend upon its number of turns.
In a transformer if the number of turns in secondary winding is less than that in the primary winding it is called step-down transformer. In a transformer if the number of turns in secondary winding is higher than that in the primary winding it is called step-up transformer.
CLASSIFICATION OF TRANSFORMERS Transformers are classified on basis of 1. Duty they perform 2. Construction 3. Voltage output 4. Application 5. Cooling 6. Input supply
• Duty they perform I. Power transformer – from transmission and distribution II. Current transformer- instrument transformers III. Potential transformer- instrument transformers • Construction I. Core type transformer II. Shell type transformer III. Berry type transformer • Voltage output I. Step down transformer(Higher to Lower) II. Step up transformer(Lower to Higher) III. Auto transformer(Variable from ‘0’ to rated value)
• Application I. Welding transformer II. Furnace transformer • Cooling I. Duct type transformer II. Oil immersed transformer self cooled Forced air cooled Water cooled Forced oil cooled • Input supply I. Single phase transformer II. Three phase transformer star- star Delta-Star Star-delta Open- Delta Delta-delta Scott connection
Constructional details Transformer is a static device and has no moving parts. Main components of a transformer are: • Magnetic core • Primary and secondary windings • Insulation of windings • Expansion tank or conservator • Tank , oil , cooling arrangement , temperature gauge, oil gauge • Buchholz relay • Silica gel breather
(1) MAGNETIC CORE • Magnetic core consists of an iron core. The core is laminated and made of silicon steel. • Thickness varies from 0.35mm to 0.5mm. • Laminations are insulated from each other by coating then with a thin coat of varnish. • Various types of stampings and laminations employed in the construction of transformers. There are two types of transformer cores ,they are (a) Shell type (b)Core type Shell type – Two windings are carried by central limb. Core is made up of E and I stampings and has three limbs. Has two parallel paths for magnetic flux. Core type- Has two limbs for two windings and is made up of two L-type stampings. Has only one magnetic path.
(2) Winding • There are two windings in a transformer. • They are primary and secondary windings. • Made up of copper. (3) Insulation • Paper is still used as basic conductor insulation. • For power transformers enamelled copper with paper insulation is also used. (4) Insulating oil • The coil used in transformer protects the paper from dirt and moisture and removes the heat produced in the core and coils, • It also acts as insulating medium. Oil must possess following properties: • High dielectric strength • Free from inorganic acid , alkali and corrosive sulphur. • Low viscosity to provide good heat transfer. • Good resistance to emulsion so that the oil may throw down any moisture entering the tank instead of holding it.
(5)EXPANSION TANK or Conservator • A small auxiliary oil tank mounted above the transformer and connected to main tank by a pipe. • Its function is to keep transformer tank full of oil. (6)TEMPERATURE GAUGE • Is to indicate hot oil or hottest spot temperature. • It is self contained weather proof unit made of alarm contacts.
(7) OIL GAUGE • Every transformer is fitted with an oil gauge to indicate the oil level present inside the tank. • It is provided with an alarm contact which gives an alarm when the oil level drops beyond permissible height due to oil leak or due to any other reason. (8) BUCHHOLZ RELAY • First warning that fault is present is given by presence of bubbles in oil. • It gives an alarm in case of minor fault and to disconnect transformer From the supply mains in case of severe faults.
(9) Breather • It is a chamber which prevents entry of moisture inside the transformer tank. • It is filled with drying agent such as calcium chloride or silica gel. This absorbs moisture and allows dry air to enter transformer tank. It is replaced regularly.
EMF EQUATION OF TRANSFORMER Wkt, T=1/f Average emf (e)= dΦ/dt dΦ = Φm dt = 1/4f Average rate of change of flux= Φm / (1/4f) = 4f Φm volts Average emf induced per turn = Average rate of change of flux= 4f Φm volts Form factor = RMS value/ Average value = 1.11 RMS value= Form factor x Average value =1.11 x Average value RMS value of emf induced/turn = 1.11 x 4f Φm = 4.44 f Φm volts Primary and Secondary winding having N1 and N2 turns. RMS value of emf induced Primary winding, E1 = 4.44 f Φm N1 volts RMS value of emf induced Secondary winding, E1 = 4.44 f Φm N2 volts
TRANSFORMER RATIO For an ideal transformer, E1 = V1 and E2 = V2 There is no voltage drop in the windings. V1 I1 = V2 I2 V2 / V1 = E2 / E1 = I1 / I2 = N2 / N1 = K V2 / V1 = K -> Voltage ratio E2 / E1 = K -> Transformation ratio N2 / N1 = K -> Turns ratio I1 / I2 = K -> Current ratio (i) If K > 1 , then the transformer is called step-up transformer. (ii) If K < 1 , then the transformer is called step-down transformer.
RATING OF TRANSFORMER • The rating of transformer is expressed by Volt- Ampere (VA) • Cu loss depends on Current (A) • Iron loss depends on Voltage (V) • Total loss depends upon Volt- Ampere (VA) • It is independent of Load power factor cos Φ.
Ideal transformer Ideal transformer has following properties : • No winding resistance i.e., purely inductive • No magnetic leakage flux • No cu loss • No core loss Ideal transformer secondary is open. Ac supply is connected to primary winding. Current flows through primary winding. This current is called MAGNETISING CURRENT (Iμ). Value of Magnetising current is small. The Magnetising current produces an alternating flux (Φ). Iμ and Φ are in-phase. This changing flux links primary with secondary winding. Due to alternating flux a self-induced emf (E1) is produced in primary winding which is equal to and in opposition with V1. It is known as counter emf or back emf of primary winding. Induced emf E2 is produced in secondary winding because of alternating flux linking with secondary winding. This emf is known as mutually induced emf.
PRACTICAL TRANSFORMER ON NO-LOAD If the primary winding is connected to alternating voltage and secondary winding is left open then transformer is said to be on NO-LOAD. Since secondary is open this current is called no-load primary current (Io). No load input power, P0= V1 I0 cos Φ0 Active or working or iron loss or wattful component (Iw) which is in-phase with ‘V1’ and supplies iron loss and small amount of primary cu loss. Iw = I0 cos Φ0 Where, cos Φ0 is no load power factor. Reactive or magnetizing or wattles component Iμ which is in quadrature with V1 and its function is to sustain flux in core. Iμ= I0 Sin Φ0 I0 =
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