Hello everyone, in this article I will discuss how to calculate the rating of the distribution transformer or how to select a suitable transformer.
If you want an article on some other topics then comment us below in the comment box. You can also catch me @ Instagram – Chetan Shidling.
Also read – Electrical Machines Questions And Answers. and now I will discuss how to calculate rating of distribution transformer.
How To Calculate Rating Of Distribution Transformer
Let’s take an example
Q. Select a suitable transformer for an Industrial area. The load conditions are as follows :
- a. Available voltage = 11 KV
- b. Lighting load = 10 KW
- c. Demand factor = 0.7
- d. Required voltage = 440/230 volts
- e. Industrial load = 50 KW at 0.8 pf
- f. Diversity factor = 1.46
Solution :
Total connected load
= Lighting load + Industrial load
= (10 × 1) ÷ pf + (50 ×1) ÷ pf
= 10 + 62.5
= 72.5
Maximum demand
= Connected load × Demand factor
= 72.5 × 0.7
= 50.75 say 50 KVA
The required capacity of the transformer
= Maximum demand / Diversity factor
= 50 / 1.46
= 34.246 or say 34 KVA
Allowance for future expansion at 20 % of the required capacity
= 34 × 0.2
= 7 KVA
Hence capacity of the transformer = 34 + 7 = 41
(41 KVA, 11 KV/440 V 3 -phase is not available)
Market availability = 50 KVA
Therefore, a transformer of 50 KVA, 11 KV/440 V, 3-phase delta-star, 50 Hz is selected.
Q. A small workshop has the following load to find the transformer capacity required.
- a. A 1 HP, 400 V, 3-phase for drilling machine
- b. A 1/2 HP, 230 V, single phase for Grinding machine
- c. A 3 HP, 400 V, 3-phase motor for lathe machine.
- d. A 5 KVA, 230 V, welding transformer.
- e. A milling machine 5 HP, 400 V, 3-phase
- f. A power Backsaw, 3 HP, 400 V, 3-phase.
- g. Two power sockets
- h. Lamp loads – 25 points
- i. Ceiling fans – 15 nos
- j. Two numbers, high-pressure mercury vapor lamps of 500 W, 1-phase, 230 V, 50 Hz, supply.
Calculate the total connected load. If the demand factor is 0.6 and the diversity factor is 1 %.
Solution :
(a) All three-phase motor loads 12 Hp assuming Pf 0.8 and efficiency 85 %
Therefore, VA = (HP × 735.5) ÷ (efficiency × Pf)
= (12 × 735.5) ÷ (0.85 × 0.8) = 7853 VA
(b) Taking all single-phase loads :
Power sockets – 2 × 1000 = 2000 w
Lamps – 25 × 40 = 1000 w
Ceiling fans – 15 × 80 = 1200 w
Mercury vapour lamps – 500 × 2 = 1000 w
1/2 HP Grinder – (05 × 735.5) ÷ (0.8 × 0.6)
= 766 / 5966 (efficiency = 08 & Pf = 0.6)
(c) 5 KVA welding transformer – 5000 VA
The total load in VA
= 7853 + 5966 + 5000
= 18819 VA
= 18.819 KVA
Actual demand
= connected load × demand factor
= 18.819 × 0.6
= 11.2914 KVA
The required capacity of the transformer
= Actual demand / diversity factor
= 11.2914 / 1.10
= 10 KVA
Allowance for future expansion at 25 % of the required capacity
= 10 × 1.25
= 12.83 KVA.
Market availability = 15 KVA
Hence, a transformer of 15 KVA, 11 KV/440 V, 3-phase delta-star, 50 Hz is selected.
I hope this may help you………
Tag: How To Calculate Rating Of Distribution Transformer.
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