Question

1. The primary of a step-up transformer connected to a 120 V source has 200 turns. If the output is 480 V, how many turns does the secondary have?

2. The primary of a step-down transformer connected to a 240 V source has 1500 turns. The secondary has 200 turns.   What is the output voltage of this transformer?

3. A laptop computer requires 24 volts to operate properly. A transformer with 600 turns in the primary needs to have how many turns in the secondary to operate the computer from a 120 V source?

4. If the output current for the transformer in the previous problem is 4.5 A, calculate the input current.

5. A transformer has an input of 24 V and an output of 36 V. If the input is changed to 12 V what will the output be?

6. A model electric train requires 12 V to operate. When connected to a household voltage of 120 V, a transformer is needed. If the primary coil has 250 turns, how many turns must the secondary coil have?

7. A transformer has 25 turns in the primary coil and 200 turns in the secondary coil. If 24 V is connected to the primary coil and a 20-Ω device is connected to the secondary coil, calculate the current in Amperes passing through the device.

8. Neon signs need 12,000 V to operate. If a transformer operates off a 240 V source and has 1000 turns in its primary coil, how may turns must the secondary coil have?

9. A power of 200 kW is delivered by power lines with 48,000 V difference between them. Calculate the current, in amps, in these lines.

10. If the lines in problem #9 above, have a resistance of 100-Ω, calculate the change of voltage along each line.

Answer 1

The governing equation for the number of turns and the voltage of the transformer is,

1. Here the value of N1 is 200 and V1 is 120, then the value of V2 is 480 V. Substituting values we can write,

Therefore the secondary will have 800 turns.

2. Here N1 is 1500 and V1 is 240V, then the value of N2 is 200. Substituting values we get,

Therefore the output voltage is 32V.

3. In this case, V2 is 24 V, and N1 is 600 and V1 is 120V. Substituting values we get,

Hence the required number of turns is 120.

4. Now the power in a transformer will be constant hence we can write,

Multiplying the equation relating the number of turns and the voltage we get,

In our case, N1 is 600, and N2 is 120 and I2 is 4.5 A.

Substituting values we get,

Therefore the input current is 0.9 A.