Basic Physics

# Step-up and step-down transformer – problems and solutions

1. A transformer has a primary coil and a secondary coil with the number of loops are 500 and 5000. Input voltage is 220 V. What is the output voltage?

Known :

Primary coil (NP) = 500 loops

Secondary coil (Ns) = 5000 loops

Primary voltage (VP) = 220 Volt

Wanted : Secondary voltage (VS)

Solution :

Vs / Ns = Vp / Np

Vs / 5000 = 220 / 500

Vs / 5000 = 0.44

Vs = (0.44)(5000)

Vs = 2200 Volt

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2. A transformer has primary coil with 1200 loops and secondary coil with 1000 loops. If the current in the primary coil is 4 Ampere, then what is the the current in the secondary coil.

Known :

Primary coil (NP) = 1200 loops

Secondary coil (NS) = 1000 loops

The current in the primary coil (IP) = 4 Ampere

Wanted: The current in secondary coil (IS)

Solution :

IS/IP = NP/NS

The current in the secondary coil :

IS/4 = 1200/1000

IS/4 = 1.2

IS = 1.2 (4)

IS = 4.8 Ampere

3. The secondary voltage is 220 Volt and primary voltage is 110 volt, then a comparison of the secondary coil and primary coil is…

Known :

Secondary voltage (VS) = 220 Volt

Primary voltage (VP) = 110 Volt

Wanted : NS/NP

Solution :

VS/VP = NS/NP

220/110 = NS/NP

22/11 = NS/NP

2/1 = NS/NP

12/6 = NS/NP

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4. Based on figure below, what is the primary voltage of the transformer.

Known :

Secondary voltage (VS) = 24 Volt

Primary loops (NP or N1) = 11 N

Secondary loops (NS or N2) = 1 N = N

Wanted : Primary voltage (VP)

Solution :

Vs / Ns = Vp / Np

24 / N = Vp / 11 N

24 / 1 = Vp / 11

24 = Vp / 11

Vp = (24)(11)

Vp = 264 Volt

5. Based on figure below, what is the input voltage of the transformer.

Known :

Primary coil (N1) = 21 N

Secondary coil (N2) = 3 N

Secondary voltage (V2) = 20 Volt

Wanted : Primary voltage (V1)

Solution :

V2 / N2 = V1 / N1

20 / 3 N = V1 / 21 N

20 / 1 = V1 / 7

20 = V1 / 7

V1 = (7)(20)

V1 = 140 Volt

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6. According to figure below, what is the amount of the secondary loops of the transformer.

Known :

Primary voltage (VP) = 220 Volt

Primary loops (NP) = 200 loops

Secondary voltage (VS) = 1100 Volt

Wanted : Secondary loops (NS)

Solution :

Vs / Ns = Vp / Np

1100 Volt / Ns = 220 Volt / 200 loops

1100 / Ns = 220 / 200

1100 / Ns = 1.1

Ns = 1100 / 1.1

Ns = 1000 loops

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7. According to figure below, what is the amount of the secondary loops of the transformer.

Known :

Primary voltage (VP) = 220 Volt

Primary loops (NP) = 200 loops

Secondary voltage (VS) = 1100 Volt

Wanted : Secondary loops (NS)

Solution :

Vs / Ns = Vp / Np

1100 / NS = 220 / 200

1100 / NS = 22 / 20

1100 / NS = 1.1

NS = 1100 / 1.1

NS = 1000

8.

 Transformer Np Ns Vp Vs Ip Is 1 1000 120 12 0.4 2 2 80 8 20 2 2.5

Based on the above table, if the efficiency of both transformers is the same, 50%, Determine Ns and Ip.

Solution :

The equation of transformer’s efficiency :

Calculate Ns of the transformer 1 :

Calculate Ip of the transformer 2 :

9.

Based on the above figure, which of the following statements about the figure above is correct.

A. Figure 1 is a step-up transformer and figure 2 is a step-down transformer

B. Figure 1 is a step-down transformer and figure 2 is a step-up transformer

C. Figure 1 and 2 are a step-down transformer

D. Figure 1 and 2 are a step-up transformer

Solution

Step-up transformer serves to raise the electrical voltage while the step-down transformer to reduce electrical voltage. In Figure 1, the primary voltage is greater than the second voltage (20 Volts > 5 Volts) so that Figure 1 is a step-down transformer. In Figure 2, the primary voltage is smaller than the second voltage (5 Volts < 40 Volts) so that Figure 2 is a step-up transformer.

The correct answer is B.

10.

If the primary coil has 800 loops, then determine the secondary coil.

Known :

Primary loops = Np = 800 loops

Primary voltage =VP = 220 Volt

Secondary voltage = VS = 55 Volt

Wanted : Secondary loops (NS)

Solution :

Vs / Ns = Vp / Np

55 / Ns = 220 / 800

55 / Ns = 22 / 80

NS = (80)(55) / 22

NS = 4400 / 22

NS = 200 loops

11.

The correct statement about transformer P and Q is…

A. P is a step-down transformer because of Is < Ip

B. P is a step-up transformer because of Vp < Vs

C. Q is a step-up transformer because of Vp > Vs

D. Q is a step-up transformer because Is > Ip

Solution

P is step-up transformer because primary voltage (Vp = 110 V) < secondary voltage (Vs = 200 V).

Q is step-down transformer because primary voltage (Vp = 220 V) > secondary voltage (Vs = 110 V).

The correct answer is B.

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