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25 questions from the electrical theory and calculations practice test.
10 questions, graded as you go, with the reason for every answer. No sign-up.
A transformer has a 480 V primary and a 120 V secondary. What is its turns ratio, primary to secondary, expressed as a number to 1?
Answer in to 1.
4 to 1
Turns ratio = Ep ÷ Es = 480 ÷ 120 = 4, so the ratio is 4:1.
Reference: Turns ratio: Np ÷ Ns = Ep ÷ Es
A transformer has 1,200 turns on its primary and 300 turns on its secondary. If 240 V is applied to the primary, what is the secondary voltage?
Answer in V.
60 V
Es = Ep × Ns ÷ Np = 240 × 300 ÷ 1,200 = 240 × 0.25 = 60 V.
Reference: Turns ratio: Ep ÷ Es = Np ÷ Ns
A step-down transformer has a 10:1 turns ratio and its secondary delivers 50 A. Ignoring losses, what is the primary current?
B. 5 A
Current is inversely proportional to the turns ratio: Ip = Is ÷ 10 = 50 ÷ 10 = 5 A. 500 A multiplies by the ratio, which is what happens to voltage going from secondary to primary, not current.
Reference: Transformer ratios: Ip ÷ Is = Ns ÷ Np