1. A 100/10, 50 VA double winding transformer is converted to 100/110 V auto transformer. tWhat will be the rating of auto transformer?
a) 500 VA
b) 550 VA
c) 100 VA
d) 110 VA
Explanation: Secondary current of the two-winding transformer at rated voltage supply = 50/10= 5 A
Thus, autotransformer will also carry the same rated current in secondary giving the power output as 5*110= 550 VA
2. . For a 20kVA transformer with turn ratio 0.4 what amount of total power is transferred inductively?
a) 8kVA
b) 12kVA
c) 10kVA
d) 50kVA
Explanation: For an auto transformer power is transferred partially inductively and partially conductively. Thus, out of total power, power transferred inductively is given by (1-k)*total power= 0.6*20= 12kVA
3. For a 20kVA transformer with turn ratio 0.4 what amount of total power is transferred conductively?
a) 8kVA
b) 12kVA
c) 10kVA
d) 50kVA
Explanation: For an auto transformer power is transferred partially inductively and partially conductively. Thus, out of total power, power transferred conductively is given by (k)*total power= 0.4*20= 8kVA
4. 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
5.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
6. 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.
7. 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.
8. 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.
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. 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.