Sulaim b. Ab Qais 2012

DESIGN A TRANSFORMER

This paper guides a simple and practical calculation in designing a transformer. The hands-on work can

be understood from this video http://www.youtube.com/watch?v=s6NyTprQCBI

MATERIALS

Masking tape, wax paper, copper cable, magnet copper wire, sandpaper, silicon-iron sheet, formwork,

soldering wire

CALCULATION

1. Determine the load power, P.

Let say,

P=300W

2. Find the primary winding current. Malaysia using 230 VAC, +10%,-6%, thus the current would be;

Ip = P/V

Ip = 200W/230V

= 0.87 A, thus the wire that we will using for primary winding is 0.26 mm2

Let say the secondary winding voltage, Vs = 40 VAC

Is = 200W/40V

= 5 A, thus the wire that we will using for secondary windings is 1.31 mm2

3. Find the size of the cable for primary windings and secondary windings.

By referring to the table below we can determine the size of primary and secondary coil (magnet wire).

Table: cable size for safe current carrying capacity

American Wire Gauge (AWG) Area size (mm2) Current (max)

30 0.05 0.22

29 0.06 0.29

28 0.08 0.3

27 0.1 0.4

Sulaim b. Ab Qais 2012

26 0.14 0.5

25 0.16 0.6

24 0.25 0.8

23 0.26 1.0

22 0.34 1.2

21 0.38 1.6

20 0.5 2

19 0.65 2.6

18 0.75 3.2

17 1.04 4.1

16 1.31 5.2

15 1.65 6.6

14 2.5 8.3

13 2.6 5.1

12 4 5.13

11 4.15 6.16

10 6 2.21

9 6.62 5.26

8 8.36 33.3

7 10.6 44.2

4. Find the section kernel

Section kernel = 𝑃

= 200

= 14.4 cm2

Thus, the size of the centre core is 14.4 cm2. We can use any combination of length and width to get the

area. For example, 3.8 cm x 3.8 cm = 14.4 cm2

L= 3.8 cm

W= 3.8 cm

Sulaim b. Ab Qais 2012

5. Calculate the number of turn for primary windings.

Turn per volt = 42/section kernel

*42 is a constant

= 3 turns/volt

No of turn (primary) , Np = 3 x 230

= 690 turn

6. Calculate the no. of turn for secondary windings (refer table above)

No of turn (primary) , Ns = 3 x 40

= 120 turn

SELECTED Q & A

1. Why primary windings and secondary windings of the transformer didn’t short circuit?

The windings will act as an inductor when AC current flow in it, blocking the high current. Though

voltage is applied, current flowing, impedance also exists. That's why the transformer doesn't get short

circuited. Insufficient number of coil due to miscalculation will result in short circuit.

Width,W

Length,L

Sulaim b. Ab Qais 2012

2. Is a transformer is obeying ohm's law or not?

Ohms law is for determining the behavior of electricity through a load (a resistor). Transformer is

nothing more than a pair of inductor. A transformer by itself is not a load. You apply Ohms law

separately to the primary circuit and to the secondary circuit of a transformer. You do not use Ohms law

to learn how the primary induces current in the secondary. The current that we calculate is the

maximum (short circuit) or ideal transformer current. If we put resistor at any side it still obeys ohm’s

law.

3. Why we usually use magnetic copper wire for the windings?

Magnetic copper have insulation. To build a transformer, solenoid or inductor each turning wire cannot

short with the other from the side.

REFERENCE

1. http://www.construyasuvideorockola.com/construc01.php

2. http://answers.yahoo.com/question/index?qid=20090705145110AAjSE33

3. Suruhanjaya Tenaga Malaysia (ST).