Transformers Questions and Answers – Three Phase Transformer Construction

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This set of Transformers Multiple Choice Questions & Answers (MCQs) focuses on “Three Phase Transformer Construction”.

1. Which type of winding is used in 3-phase shell-type transformer?
a) Circular type
b) Sandwich type
c) Cylindrical type
d) Rectangular type
View Answer

Answer: b
Explanation: In core type of the transformer, winding is done by normal method, while in the shell type transformer, winding is sandwiched between corresponding shells of core material. Hence, it is sandwich type.
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2. 3-phase transformers compare to 1-phase transformers ________
a) More economical
b) Easy in construction
c) Easy to construct
d) Easy to handle
View Answer

Answer: a
Explanation: 3-phase transformers are also used to power large motors and other heavy loads. A three-wire three-phase circuit is more economical than an equivalent two-wire single-phase circuit at the same line to ground voltage because it uses less conductor material to transmit a given amount of electrical power.

3. How 3-phase transformers are constructed?
a) A bank of 3 single phase transformers
b) A single 3-phase transformer with the primary and secondary of each phase wound on three legs of a common core
c) Single 3-phase transformer or a bank of 3 1-phase transformers
d) By different method
View Answer

Answer: c
Explanation: #-phase transformers can be either constructed by 2 methods. One is a bank of 3 single phase transformers and another method is a single 3-phase transformer with the primary and secondary of each phase wound on three legs of a common core.

4. Three phase transformer compare to a bank of 3 single phase transformers is ____________
a) Cheaper
b) Costlier
c) More in space
d) Can’t be determined
View Answer

Answer: a
Explanation: 3-phase transformers can be manufactured by 2 methods. The three-phase transformer unit costs about 15% less than that of a bank and furthermore, the single unit occupies less space. There is little difference in reliability.

5. In mines we use _______________
a) a single unit of 3-phase transformer
b) a bank of 3 single phase transformers
c) use of 3-phase transformer is avoided
d) a single unit or a bank
View Answer

Answer: b
Explanation: It is cheaper to carry spare stock of a single-phase rather than a three-phase transformer. In underground use (mines) a bank of single-phase units may be preferred as it is easier to transport these units. The bank also offers the advantage of a de-rated open-delta operation when one single-phase unit becomes inoperative.
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6. In three phase bank phases are ___________________
a) the phases are electrically not connected and magnetically independent
b) the phases are electrically not connected and magnetically dependent
c) the phases are electrically connected and magnetically independent
d) the phases are electrically connected and magnetically dependent
View Answer

Answer: c
Explanation: The 3-phase load bank is constructed by so that all three phases are made to be connected with together electrically. By the same time these banks form three magnetic circuits which are independent.

7. Where the tappings are provided in a transformer?
a) At the phase end of LV side
b) At the phase end of HV side
c) At the neutral side end of the HV side
d) At the middle of HV side
View Answer

Answer: d
Explanation: Tappings are provided in order to maintain the terminal voltage across the loads within the permissible specified limits with respect to load variations. Tappings can be provided at phase end or neutral end of HV side. But to maintain magnetic symmetry tappings are preferred in the middle of the winding only.

8. Tappings are on _____________
a) LV side of a transformer
b) HV side of transformer
c) Not on any side
d) On both sides
View Answer

Answer: b
Explanation: Generally, tappings are preferred on HV side, because the HV side is low current-high voltage side, tap changer involves low current interruption and large variations or small steps are possible due to more no of turns.

9. In core type 3-phase transformer flux path chooses how many paths to return?
a) 2
b) Single
c) 3
d) Many
View Answer

Answer: a
Explanation: Flux of each limb in core type uses the other two limbs for its return path with the three-magnetic flux’s in the core generated by the line voltages differing in time-phase by 120 degrees. Thus, the flux in the core remains nearly sinusoidal, producing a sinusoidal secondary supply voltage.
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10. Why shell type 3-phase transformer is used in large power transforming applications?
a) Can be made with more height
b) Can be made with less height
c) More height and less height flexibility
d) Due to other reasons
View Answer

Answer: b
Explanation: The shell-type five-limb type three-phase transformer construction is heavier and more expensive to build than the core-type. Five-limb cores are generally used for very large power transformers as they can be made with reduced height.

11. A three-phase transformer generally has the three magnetic circuits interlaced.
a) True
b) False
View Answer

Answer: b
Explanation: A three-phase transformer generally has the three magnetic circuits which are interlaced to give a uniform distribution of the dielectric flux between the high and low voltage windings. The exception is a three-phase shell type transformer. In the shell type of construction, even though the three cores are together, they are non-interlaced.

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Manish Bhojasia, a technology veteran with 20+ years @ Cisco & Wipro, is Founder and CTO at Sanfoundry. He is Linux Kernel Developer & SAN Architect and is passionate about competency developments in these areas. He lives in Bangalore and delivers focused training sessions to IT professionals in Linux Kernel, Linux Debugging, Linux Device Drivers, Linux Networking, Linux Storage, Advanced C Programming, SAN Storage Technologies, SCSI Internals & Storage Protocols such as iSCSI & Fiber Channel. Stay connected with him @ LinkedIn