Intro to transformers

1. LESSON 01:
INTRODUCTION TO TRANSFORMERS
Engr. Onofre E. Algara, Jr.
BS Electrical Engineering (DLSU-D)
MS Electrical Engineering (Mapua University)
Course Instructor
COLEGIO DE MUNTINLUPA
ELECTRICAL ENGINEERING DEPARTMENT

2. Working Principle of a Transformer
• A static or stationary piece of apparatus
• Power in one circuit is transformed to another circuit (electric power)
but with the same frequency.
• Can raise or lower the voltage in circuit corresponding to a decrease
and increase in current
• Physical basis: Mutual induction between two circuits linked by
common magnetic flux
• Two inductive coils which are electrically separated but magnetically linked
through a path of low reluctance

3. Working Principle of a Transformer
• A static or stationary piece of apparatus
• Power in one circuit is transformed to another circuit (electric power)
but with the same frequency.
• Can raise or lower the voltage in circuit corresponding to a decrease
and increase in current
• Physical basis: Mutual induction between two circuits linked by
common magnetic flux
• Two inductive coils which are electrically separated but magnetically linked
through a path of low reluctance

4. Simple Transformer Circuit
• The two coils passes high mutual
inductance
• If one coil is connected to a source
of sinusoidal voltage, an
alternating flux is set up in
laminated core, most od which is
linked with the other coil in which
it produces mutually induced emf
(FLEI)
𝑒 = 𝑀
𝑑𝐼
𝑑𝑡

5. Simple Transformer Circuit
• If the second coil circuit is closed, a current flow in it and so electric
energy is transferred (magnetic) from the first to second coil.
• The first coil is called primary winding which energy is fed from AC
supply.
• The second coil is called secondary winding which energy is drawn out.
• Points to Remember:
• transfers electric power from one circuit to another
• it does so without a change of frequency
• it accomplishes this by electromagnetic induction
• where the two electric circuits are in mutual inductive influence of each other

6. Transformer Construction
• two coils having mutual inductance
and a laminated steel core
• the two coils are laminated from each
other and the steel core
• Other parts:
• container for assembled core and
windings
• medium for insulating the core and its
winding from the container
• Bushings (porcelain, oil filled and
capacitor type
• The core is constructed in a form of
sheet steel laminations to provide
continuous magnetic path with
minimum air gap.

7. Transformer Construction
• Steel lamination
• High silicon content
• For high permeability
• Low hysteresis loss
• General Types Based on Construction
• Core Type
• windings surround a considerable part of the core
• primary and secondary are on opposite legs of the core
• Shell Type
• core sorrounds a considerable portion of the windings
• Spiral Core/ Wound Core/ Spirakore

8. Transformer Construction
• Individual laminations

9. Core-Type Transformers
• Coils used are form-wound and cylindrical type
• Coils may be circular, oval or rectangular

10. Shell-Type Transformers
• Advantages:
• More rigid core
• lesser weight and size per kVA rating
• lower iron losses at higher operating flux density
• lower manufacturing cost

11. Transformers

12. Transformers

13. Ideal Transformers
• has no losses
• Its winding have no ohmic resistance
• No magnetic leakage
• No (I^2)(R) and core losses
• Consists only of two purely inductive coils
wound on a loss-free core

14. EMF Equation of a Transformer
• flux increases from its zero value to maximum value in one
quarter of the cycle
• The rate of change of flux per turn means induced emf in volts
• If flux varies sinusoidally, the rms value of induced emf is
obtained by multiplying the value with form factor

15. EMF Equation of a Transformer
• The RMS value of the induced emf in the whole primary
winding

16. Voltage Transformation Ratio (K)
• If number of turns on secondary winding is greater than
primary, it is a step up XF.
• If number of turns on secondary winding is less than primary, it
is a step down XF.
• For ideal XF, input VA = output VA
• Currents are in inverse ratio of the voltage transformation ratio

17. Problems
1. The maximum flux density in the core of a 250/3000 V, 50 Hz single phase transformer is 1.2 Wb/
square meter. If the emf per turn is 8 volts, determine, the primary and secondary turns and the
area of the core.
2. The core of a 100 kVA, 11kV/550 V 50 Hz, single phase core type transformer has a cross section of
20 x 20 cm. Find the number of HV and LV turns per phase and the emf per turn if the maximum
core density is not to exceed 1.3 Tesla. Assume a stacking factor of 0.9.
3. A single phase XF has 400 primary and 1000 secondary turns. The net cross sectional area of the
core is 60 square centimeters. If the primary winding be connected to a 50 Hz supply at 520 V,
calculate the peak value of flux density in the core and the voltage induced in the secondary
winding.
4. A 25 kVA XF has 500 turns on the primary and 50 turns on the secondary winding. The primary is
connected to 3000 V 50 Hz supply. Find the full load primary and secondary currents, the
secondary emf and the maximum flux in the core. Neglect leakage drops and no load primary
current.
5. A 25 kVA single phase XF has 250 turns on the primary and 40 turns on the secondary winding.
The primary is connected to 1500 V, 50 Hz mains. Calculate the primary and secondary currents
on full load, secondary emf, maximum flux in the core.