8/13/2019 Module 8 DC-AC Machines

1/10

Module 8Transformers

Engr. Gerard AngSchool of EECE

8/13/2019 Module 8 DC-AC Machines

2/10

8/13/2019 Module 8 DC-AC Machines

3/10

8/13/2019 Module 8 DC-AC Machines

4/10

Classification of Transformers

1. According to the core useda. Core-typeb. Shell-typec. Berry type

2. According to the purpose for which it is used a. Transmission transformerb. Distribution transformerc. Generator transformerd. Testing transformere. Instrument transformerf. Current transformerg. Potential transformer

8/13/2019 Module 8 DC-AC Machines

5/10

Classification of Transformers

3. According to the method of coolinga. Oil-filled self-cooledb. Oil-filled water-cooledc. Air-blast type

4. According to specific applicationsa. Rectifier transformerb. Regulating transformer

c. Welding transformerd. Traction transformere. Furnace transformer

8/13/2019 Module 8 DC-AC Machines

6/10

EMF Equation of a Transformer

= . = .

= . = .

Magneticcore

V 1

MagneticFlux ( m )

V 2

SecondaryCoil (N 2 )

PrimaryCoil (N 1 )

E 1 E 2

I1

I2

8/13/2019 Module 8 DC-AC Machines

7/10

EMF Equation of a Transformer

Where:E1 = primary induced emfE2 = secondary induced emfV1 = primary applied emfV2 = secondary terminal emfN1 = number of turns of primary coilN2 = number of turns of secondary coilBm = maximum magnetic flux density in Tesla

m = maximum flux in the core in Webers A = cross-sectional area of the magnetic coref = frequency of ac input in HzI1 = primary currentI2 = secondary current

8/13/2019 Module 8 DC-AC Machines

8/10

Ideal (Lossless) Transformer

=

=

=

8/13/2019 Module 8 DC-AC Machines

9/10

Transformation Ratio a)

The transformation ratio of a transformer is derived as:

= = =

Note:If N1 < N 2 (a < 1), then transformer is called step-up transformerIf N1 > N 2 (a > 1), then transformer is called step-down transformer

8/13/2019 Module 8 DC-AC Machines

10/10

Sample Problems

1. The maximum flux density in the core of 250/3000-volts, 50-Hz single- phase transformer is 1.2 Wb/m 2. If the emf per turn is 1.8 volts,determine (a) primary and secondary turns (b) area of the core.

2. The core of a 100-kVA, 11000/550 V. 50-Hz, 1-ph, core-typetransformer has a cross-section of 20 cm x 20 cm. Find (a) the numberof h.v. and l.v. turns per phase and (b) the emf per turn if the maximumcore density is not to exceed 1.3 Tesla. Assume a stacking factor of0.9. What will happen if its primary voltage is increased by 10% on no-

load?

3. A 25-kVA transformer has 500 turns on the primary and 50 turns onthe secondary winding. The primary is connected to 3000-V, 50-Hzsupply. Find the full-load primary and secondary currents, thesecondary emf and the maximum flux in the core. Neglect leakagedrops and no-load primary current.