Transformers - Study Mode

[#76] The use of higher flux density in the transformer design
Correct Answer

(A) reduces weight per kVA

[#77] A transformer has a 1:6 turns ratio and a secondary coil load resistance of 470 Ω. The load resistance as seen by the source is
Correct Answer

(D) 13 Ω

Explanation

Solution: Here's an explanation to help you understand how to solve this transformer question: The question involves the concept of the turns ratio of a transformer and how it relates to the impedance transformation . Turns Ratio (N1:N2): In this problem, the turns ratio is given as 1:6. This means for every 1 turn in the primary coil, there are 6 turns in the secondary coil. We can represent this as N1/N2 = 1/6. Impedance Transformation: A transformer changes the apparent impedance (resistance in this case) seen by the source (primary side). The relationship is: Z1 = (N1/N2)^2 * Z2 Where: Z1 is the impedance seen by the source (primary side). Z2 is the impedance of the load connected to the secondary side. N1/N2 is the turns ratio. In our problem: N1/N2 = 1/6 Z2 = 470 Ω Now, let's calculate Z1: Z1 = (1/6)^2 * 470 Z1 = (1/36) * 470 Z1 ≈ 13.05 Ω Therefore, the load resistance as seen by the source is approximately 13 Ω. So the correct answer is Option D: 13 Ω

[#78] In a transformer the tappings are generally provided on
Correct Answer

(C) low voltage side

Explanation

Solution: In a transformer, tappings are usually provided on the low voltage side of the transformer. This is because the low voltage side is more flexible and easier to adjust, allowing for more precise control of voltage variations. These tappings help in controlling the output voltage in certain applications where the exact output voltage is needed for specific devices or systems. Tapping on the low voltage side is common in transformers because it provides easy access to change or regulate the voltage as needed. In most transformers, the low voltage side is where adjustments are made to either step up or step down the voltage. On the other hand, the primary side and secondary side are usually involved in the primary voltage transfer and do not typically have adjustable tappings. The high voltage side also doesn't need tappings as adjustments are not typically required here in normal operation. Thus, the correct answer is Option C: low voltage side , as the low voltage side is where tapping is commonly provided in transformers.

[#79] No-load current in a transformer
Correct Answer

(A) lags behind the voltage by about 75°

[#80] The purpose of providing iron core in a step-up transformer is
Correct Answer

(C) To decrease the magnitude of magnetizing current