--- title: "Reasons: distribution efficiency and LV placement" subject: "Electric Machine Design" chapter: 3 key: "semester_5/electric_machine_design:distribution-efficiency-lv-placement" source: https://www.bctengineer.com/q/semester_5/electric_machine_design/distribution-efficiency-lv-placement site: BCT Engineering --- # Reasons: distribution efficiency and LV placement Electric Machine Design · Chapter 3: Transformer Design - Asked in: 2082 Baishakh, 8 marks - Answer checked by Claude Sonnet 5.5 on 2026-10-14 - Page: https://www.bctengineer.com/q/semester_5/electric_machine_design/distribution-efficiency-lv-placement - Whole chapter: https://www.bctengineer.com/md/semester_5/electric_machine_design/chapter-3 ## Question Give suitable reasons for the following. (i) Distribution transformer are designed to have maximum efficiency at loads much smaller than full loads. (ii) LV winding is kept near to core and HV winding is kept outside the LV winding. ## Short answer Distribution transformers carry iron loss all day, with average load around 50–70 % of rating; maximum efficiency occurs when copper loss equals the low iron loss at that fraction, improving all‑day efficiency. LV is placed near the core to save insulation, reduce mean turn length and leakage reactance; HV outside facilitates tapping and maintenance. ## Answer #### (i) Maximum efficiency of a distribution transformer at loads much smaller than full load - A distribution transformer is connected to the supply for all 24 hours, so its **iron loss** is present all the time. - Its load varies widely: high only for a few peak hours, light at night and during much of the day. The average load is often only 50–70% of the rating. - **Copper loss** varies with the square of load, so it is large only during the short peak periods. - Efficiency is maximum when copper loss equals iron loss, at load fraction $$ x = \sqrt{\frac{P_i}{P_{c,FL}}} $$ - The useful measure is **all-day efficiency** (kWh output / kWh input over 24 h). To make it high, the iron loss is kept small by using a lower flux density and good grain-oriented steel, while a higher copper loss at full load is accepted. - So the designer makes $P_i$ about one-third to one-half of $P_{c,FL}$, which puts maximum efficiency at roughly 50–70% of full load, matching the actual load pattern. Energy loss and running cost over the day are then minimum. Power transformers, in contrast, run near full load and are designed for maximum efficiency near full load. #### (ii) LV winding near the core and HV winding outside ```text core | ins | LV | duct | HV | |thin | | thick| | ``` 1. **Insulation saving:** the core is at earth potential. Placing LV next to it needs only thin insulation between core and LV. If HV were placed next to the core, thick insulation for full HV would be needed there, increasing the core-winding gap, the mean turn length and cost. 2. **Less leakage and copper:** with the smaller gap, the core diameter and winding diameters stay small, giving shorter mean turns, less copper and lower leakage reactance. 3. **Tappings and repair:** tappings for voltage control are taken on the HV winding. On the outside they are easy to bring out, and the HV winding is easy to inspect and repair. 4. **Lower current:** HV winding carries smaller current with thin conductors, which are easier to wind on the outside on a large diameter. ## Check yourself The answer's key points as multiple-choice questions, with the correct option and why. 1. What is all-day efficiency? A. Output power / input power at full load B. kWh output over 24 h / kWh input over 24 h C. Efficiency at no load D. Average of full and half load efficiency Answer: B. Energy ratio over a day is the right measure for varying load. (Key point: All-day efficiency) 2. Why is iron loss minimised in a distribution transformer? A. It varies with load squared B. It is present for all 24 h regardless of load C. It occurs only at peak D. It has no effect on efficiency Answer: B. Iron loss is constant while copper loss only matters in peak hours. (Key point: Loss behaviour) 3. Why is LV placed next to the core? A. LV carries less current B. Core is at earth potential so only thin insulation is needed for LV C. HV needs more cooling D. It reduces core loss Answer: B. HV next to the core would need thick insulation, increasing size and cost. (Key point: Insulation saving) 4. Why is HV placed outside? A. It improves flux B. It lowers iron loss C. Tappings are on HV and are easy to bring out and repair when outside D. It needs no ducts Answer: C. HV carries less current and tappings are easily accessible on the outer winding. (Key point: Tappings on HV) ## Related in Electric Machine Design - [Why LV winding is placed near core](https://www.bctengineer.com/question.md?key=semester_5%2Felectric_machine_design%3Alv-winding-near-core) — Electric Machine Design, chapter 3 - [Distribution transformer efficiency and number of tubes](https://www.bctengineer.com/question.md?key=semester_5%2Felectric_machine_design%3Adistribution-efficiency-cooling-tubes) — Electric Machine Design, chapter 3, asked 2× - [Design: 25 kVA 11000/433 V distribution transformer](https://www.bctengineer.com/question.md?key=semester_5%2Felectric_machine_design%3Adesign-25kva-11000-433) — Electric Machine Design, chapter 3 - [Design: 150 kVA 6600/400 V distribution transformer](https://www.bctengineer.com/question.md?key=semester_5%2Felectric_machine_design%3Adesign-150kva-6600-400-regulation) — Electric Machine Design, chapter 3, asked 5× - [Design: 100 kVA 4000/433 V distribution transformer](https://www.bctengineer.com/question.md?key=semester_5%2Felectric_machine_design%3Adesign-100kva-4000-433) — Electric Machine Design, chapter 3, asked 4× - [Design: 150 kVA 11000/420 V distribution transformer](https://www.bctengineer.com/question.md?key=semester_5%2Felectric_machine_design%3Adesign-150kva-11000-420) — Electric Machine Design, chapter 3, asked 3× ## Related in other subjects - [500 kVA transformer efficiency full/half load](https://www.bctengineer.com/question.md?key=semester_4%2Felectrical_machine%3Atransformer-500kva-efficiency-full-half) — Electrical Machine, chapter 2 - [500 kVA transformer efficiency, full and half load](https://www.bctengineer.com/question.md?key=semester_4%2Felectrical_machines%3Atf-500kva-6600-400-efficiency) — Electrical Machines, chapter 2 - [Full-load and half-load efficiency of 100 kVA transformer](https://www.bctengineer.com/question.md?key=semester_4%2Felectrical_machines_i%3A100kva-1200w-960w-efficiency) — Electrical Machines I, chapter 2 - [100 kVA transformer efficiency at full and half load](https://www.bctengineer.com/question.md?key=semester_4%2Felectrical_machines%3Atf-100kva-efficiency-full-half-load) — Electrical Machines, chapter 2