Electrical Machine - IES Academy

More documents

Recommendations

Info

India’s No. 1 IES Academy Case II: Unequal Voltage Ratios Transformer Chapter-1 1. In case of unequal voltage ratios, a circulating current will flow which will boost lower voltage magnitude and buck higher voltage magnitude so that the two transformers have same terminal voltage. 2. The currents in the two transformers will be unequal: 1 1 Ia = + Icand Ib = − Ic 2 2 3. The two transformers will operate at different power factors. The effect of circulating current is to reduce the power factor and increase the current shared by the transformer having greater no-load induced e.m.f. At the same time, the power factor of the second transformer with smaller no-load induced e.m.f. is increased and the current shared by it is decreased. 4. In case of unequal voltage ratios, a circulating current will flow which will boost lower voltage magnitude and buck higher voltage magnitude so that the two transformers have same terminal voltage. 5. The currents in the two transformers will be unequal: 1 1 Ia = + Icand Ib = − Ic 2 2 6. The two transformers will operate at different power factors. The effect of circulating current is to reduce the power factor and increase the current shared by the transformer having greater no-load induced e.m.f. At the same time, the power factor of the second transformer with smaller no-load induced e.m.f. is increased and the current shared by it is decreased. Auto - Transformers When the primary and secondary windings are electrically connected so that a part of the winding is common to the two, the transformer is called an auto-transformer. Unlike a two winding transformer, in an auto- transformer the two windings are not electrically isolated. They are used particularly economical where the voltage-ratio required is small (less than 2) and electrical isolation of the two windings is not essential. The major applications are www.iesacademy.com E-mail: iesacademy@yahoo.com Page-22 25, 1 st Floor, Jia Sarai, Near IIT. New Delhi-16 Ph: 011-26537570, 9810958290
India’s No. 1 IES Academy Transformer Chapter-1 induction motor starters, interconnection of HV systems at voltage levels with ratio less than 2 and in obtaining variable voltage power supplies. Advantages of auto- transformer over two-winding transformer They require less copper in winding, hence are more economical. Further, they have lower reactance, lower losses, smaller exciting current, better voltage regulation and higher operating efficiency because in an auto-transformer a part of energy transfer is through conduction process. The given auto-transformer has N1 turns primary with N2 turns tapped for a lower secondary voltage. The winding section BC is common to both primary and secondary circuits. For a two-winding transformer, turn-ratio a = (V1 – V2)/V2 = (N1 – N2)/N2 where, (N1 > N2) = N1/N2 – 1 Auto- transformer turn ratio, a' = V1/V2 = N1/N2 (>1) Therefore, a' = 1 + a VA Ratings For two-winding transformer: (VA)TW = (V1 – V2)I2 = (I2 – I1) V2 For auto- transformer: (VA)auto = V1 I1 = V2 I2 From above, (VA)auto = 1/[1– (1/a')] (VA)TW ; a' > 1 Therefore, (VA)auto > (VA)TW; (since part of VA is transferred conductively in autotransformer). www.iesacademy.com E-mail: iesacademy@yahoo.com Page-23 25, 1 st Floor, Jia Sarai, Near IIT. New Delhi-16 Ph: 011-26537570, 9810958290
Page 1 and 2: Electrical Machine Contents Chapter
Page 3 and 4: 1. Transformer Theory at a Glance (
Page 5 and 6: India’s No. 1 IES Academy Transfo
Page 9 and 10: India’s No. 1 IES Academy On Load
Page 17 and 18: India’s No. 1 IES Academy Efficie
Page 23: India’s No. 1 IES Academy Transfo
Page 41 and 42: India’s No. 1 IES Academy (c) Fla

Electrical Machine - IES Academy

Create successful ePaper yourself

Delete template?

Save as template?