This set of Transformers Multiple Choice Questions & Answers (MCQs) focuses on “Noise in Transformers”.
1. If para magnetic core is used in the place of the ferromagnetic core of the transformer, then what will happen to magnetostriction?
a) be vanished
d) not be affected
Explanation: If the core is not ferromagnetic then the transformer will not operate at all. Thus, humming sound which is produced generally, by a transformer of a ferromagnetic core will get vanished.
2. In which of the following category, noise level test of a transformer falls in?
a) Special test
b) Routine test
c) Type test
d) Different test
Explanation: Special Tests of transformer include Dielectric tests, Measurement of zero-sequence impedance of three-phase transformers, Short-circuit test, Measurement of acoustic noise level, Measurement of the harmonics of the no-load current.
3. The noise produced by a transformer is termed as _____________
Explanation: The transformer when operated produces noise which is termed in an electric system as hum. Sound pressure and frequency are the objective characteristics measured by a sound level meter, it is possible to obtain a rating proportional to the loudness of a sound from the appropriate meter readings.
4. The hum in a transformer is in direct proportion with ______________
a) load changes
b) oil in the transformer
d) mechanical vibration
Explanation: The basic cause of transformer noise is due to magnetostriction, which is an expansion and contraction of the iron core (laminations) due to the magnetic effect of alternating current flowing through the transformer coils. This produces an audible sound called hum.
5. A 400 V, 10 KVA transformer at 60 Hz, is operated at the frequency of 40 Hz, then the humming __________
c) remains same
d) increases to very high
Explanation: We know that frequency can be related with core flux density in the inverse proportion. Thus, if the frequency is reduced, the core flux density increases, which will also increase the sound produced by transformer.
6. How the A-weighted sound power level is calculated?
a) LWA= LpA – 10 log10 S/SO
b) LWA= LpA + 10 log10 S/SO
c) LWA= LpA + 10 log10 S*SO
d) LWA= LpA – 10 log10 S*SO
Explanation: The sound power level can be calculated using the sound pressure levels determined above by computing the effective area for the measurement surface according to the relevant method of measurement and relating this to the standard measurement surface, which is one square metre. The A-weighted sound power level is thus: LWA= LpA + 10 log10 S*SO.
7. Noise emitted by the transformers when drawn against frequency shows the ______________
a) increasing curve
b) straight line increasing
c) straight line decreasing
d) decreasing curve
Explanation: Consideration of magnetostrictive strain in the transformer core reveals that magnetostriction can be expected to produce a longitudinal vibration in the laminations at just this measured frequency. Unfortunately, the magnetostrictive strain is not truly sinusoidal in character, which leads to the introduction of the harmonics.
8. Step-lap is _____________
a) special component used in transformer to control sound
b) special construction technique
c) special supply technique
d) graphical nature shown by transformer sound
Explanation: The other main source of noise from the transformer core is due to alternating attractive and repulsive forces between the laminations caused by flux transfer across the air gaps at the leg to yoke and inter-yoke joints. These forces can be reduced by special building and design techniques of which the best known and most widely used is the step-lap form of construction.
9. Main and basic design of the transformer is tested by ___________
a) routine tests
b) special tests
c) type tests
d) different tests
Explanation: To check that transformer meets customer’s specifications and design expectations, the transformer has to go through different tests in manufacturer premises. Some of these tests are carried out for confirming the basic design of that transformer. Type test of transformer confirms main and basic design criteria of transformer production lot.
10. The most carried method of attenuation of transformer is ___________
a) changing core material
b) increasing oil tank size
c) building walls around transformer
d) adding some another fluid in oil tank
Explanation: The most obvious method of attenuation is by the provision of a suitable barrier between the transformer and the listener. The simplest form of barrier is a screening wall, the effectiveness of which will vary with height and density as well as with the frequency of the noise.
11. Transformers are located ______________
b) in the direction of down wind
c) in the direction of up wind
d) can be located in any wind direction according to the application
Explanation: Topographical features of the site should be exploited to the full in order to reduce noise. Where possible the transformer should be located in the prevailing down-wind direction from houses. Existing walls and mounds should, if possible, be kept between dwellings and the transformer.
Sanfoundry Global Education & Learning Series – Transformers.
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