Transformers worksheet with answers

Free printable Transformers worksheet: 6 problems from easy to exam level, with an answer key. Print it or save it as a PDF.

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1.Easy

An ideal transformer has 120 turns on its primary winding and 60 turns on its secondary winding. The primary is connected to an 800 V AC supply. Find the secondary voltage V₂.

N₁ = 120N₂ = 60V₁ = 800 V

V₂ = V

2.Easy

An ideal transformer must step 9600 V down to 240 V. Its primary winding has 800 turns. How many turns N₂ does the secondary winding need?

V₁ = 9600 VV₂ = 240 VN₁ = 800

N₂ =

3.Medium

An ideal transformer with a turns ratio N₁:N₂ = 20:1 has 4800 V applied to its primary winding. A 20 Ω resistor is connected across the secondary winding. Find the primary current I₁.

N₁:N₂ = 20:1V₁ = 4800 VR_L = 20 Ω

I₁ = A

4.Medium

A 4 Ω resistor is connected across the secondary winding of an ideal transformer with a turns ratio N₁:N₂ = 5:1. Find the resistance R_1′ seen looking into the primary winding.

N₁:N₂ = 5:1R_L = 4 Ω

R_1′ = Ω

5.Hard

A single-phase 200 kVA transformer has a core loss of 1200 W and a copper loss of 2400 W at rated load. At what load S_η,max, in kVA, is its efficiency highest?

S = 200 kVAP_core = 1200 WP_cu,fl = 2400 W

S_η,max = kVA

6.Hard

A single-phase 25 kVA transformer has a core loss of 150 W. At rated load and a power factor of 0.9 lagging its efficiency is 98.2%. Find the full-load copper loss P_cu,fl in watts.

S = 25 kVAP_core = 150 Wpf = 0.9η = 98.2%

P_cu,fl = W

Answer key

1. V₂ = 400 V
2. N₂ = 20
3. I₁ = 0.6 A
4. R_1′ = 100 Ω
5. S_η,max = 141.4 kVA
6. P_cu,fl = 262.4 W

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