# Transformers Questions and Answers – Variable Frequency Transformer

This set of Transformers Multiple Choice Questions & Answers (MCQs) focuses on “Variable Frequency Transformer”.

1. Output transformers are also called for __________
a) Maximum efficiency
b) Maximum power
c) Maximum output
d) Maximum efficiency, power and output

Explanation: In electronic circuit applications maximum power is given more preference than maximum efficiency unlike in power system operations. So, all the transformers working for maximum power are called as output transformers.

2. The output transformers are also named as _________
a) Audio transformers
b) Power transformers
c) Industrial transformers
d) Electric transformers

Explanation: in electronic circuit applications the performance criterion is the maximum power unlike the maximum efficiency in power system applications. Such transformers are known as output transformers while in audio applications these are known as audio transformers.

3. Which of the following is the major requirement for the transformers used for electronic purposes?
a) Perfect DC isolation
b) Maximum efficiency
c) Constant amplitude voltage gain
d) Perfect DC isolation , maximum efficiency and constant voltage gain

Explanation: An important requirement of these transformers is that the amplitude voltage gain (ratio of output/input voltage amplitude) should remain almost constant over the range of frequencies (bandwidth) of the signal.

4. Why frequency scale is logarithmic?
a) Frequency range is very small for these transformers
b) Frequency range is very large for these transformers
c) Frequency used is negligible
d) Can’t say

Explanation: When we investigate the gain and phase frequency characteristics of the transformer. This would include the effect of the output impedance (resistance) of the electronic circuit output stage. In these characteristics as the frequency range is quite large the frequency scale used is logarithmic.

5. When series leakage inductances are ignored then the region is called as _____________
a) Low-frequency region
b) High-frequency region
c) Mid-frequency region
d) Ultra high-frequency region

Explanation: In this region the series leakage inductances can be ignored (as these cause negligible voltage drops) and the shunt inductance (magnetizing inductance) can be considered as open circuit. With these approximations the equivalent circuit as seen on the primary side can be drawn and gain is then calculated.
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6. What is the gain of the transformer in mid-band region?
a) [N2/N1] [RL ‘/(R+RL’)].
b) [N2/N1] [RL ‘/(R+RL’+jωL)].
c) AH=A0/ [1+(ω/ ωH)2]1/2
d) AL=A0/ [1+(ω/ ωL)2]1/2

Explanation: It immediately follows from the circuit analysis of transformer in mid-frequency region that VL and VS are in phase, the circuit being resistive only. As for the amplitude gain, it is given as
VL’=VS [RL ‘/(R+RL’)] [N2/N1]VL=VS [RL ‘/(R+RL’)] A0= VL/VS= [N2/N1] [RL ‘/(R+RL’)]

7. When series leakage inductances are not ignored but shunt inductance is an effective open circuit then the region is called as _____________
a) Low-frequency region
b) High-frequency region
c) Mid-frequency region
d) Ultra high-frequency region

Explanation: In this region the series inductances must be taken into account but the shunt inductance is an effective open circuit yielding the approximate equivalent circuit. Amplitude and phase angle as function of frequency are derived below.

8. What is the gain of the transformer in high-frequency region?
a) [N2/N1] [RL ‘/(R+RL’)].
b) [N2/N1] [RL ‘/(R+RL’+jωL)].
c) AH=A0/ [1+(ω/ ωH)2].
d) AL=A0/ [1+(ω/ ωL)2]1/2

Explanation: It immediately follows from the circuit analysis of transformer in high-frequency region that AH is equal to [N2/N1] [RL ‘/(R+RL’+jωL)]. If recognised the term [N2/N1] [RL ‘/(R+RL’)] as the gain of transformer in mid-band region then AH=A0/ [1+(ω/ ωH)2]1/2.

9. When series leakage inductances are not ignored but shunt inductances are considered in parallel then the region is called as _____________
a) low-frequency region
b) high-frequency region
c) mid-frequency region
d) ultra high-frequency region

Explanation: In this region the series effect of leakage inductances is of no consequence but the low reactance (ωLm) shunting effect must be accounted for giving the approximate equivalent circuit and in the calculations of gain of a transformer.

10. What is the gain of the transformer in low-frequency region?
a) [N2/N1] [RL ‘/(R+RL’)].
b) [N2/N1] [RL ‘/(R+RL’+jωL)].
c) AH=A0/ [1+(ω/ ωH)2].
d) AL=A0/ [1+(ω/ ωL)2]1/2

Explanation: The corner frequency ωL of this circuit is obtained by considering the voltage source as short circuit. This circuit is Lm in parallel with R||R￠L. Thus, ωL = [R||RL’/Lm].
Complex gain can be expressed as AL=A0/ [1+(ω/ ωL)2]1/2.

11. Relative voltage ratio curve for an output transformer is _______________
a) Circular
b) Straight line
c) Bell shaped
d) Irregular curve

Explanation: The complete amplitude and phase response of the transformer (with source) on log frequency scale. At high frequencies, the interturn and other stray capacitances of the transformer windings need to be considered. The capacitance-inductance combination causes parallel resonance effect thus, amplitude peak shows up in the high-frequency region of the frequency response.

12. Phase angle characteristic of an output transformer is ____________
a) Curve increasing towards high-frequency region
b) Curve increasing towards low-frequency region
c) Curve increasing towards mid-frequency region then decreasing
d) Can’t define

Explanation: The phase-angle characteristics of an output transformer starts from below of relative voltage ratio characteristics drawn on the same axis. Further it goes on increasing towards high-frequency region non-linearly.

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