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# Download How to Calculate/Find the Rating of Transformer in kVA (Single

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Transcript

How to Calculate/Find the Rating of Transformer in kVA (Single Phase and Three Phase)? We know that, Transformer always rated in kVA. Below are the two simple formulas to find the rating of Single phase and Three phase Transformers. Find Rating of Single Phase Transformer Rating of Single Phase Transformer: P = V x I. Rating of a single phase transformer in kVA kVA= (V x I) / 1000 Rating of a Three Phase Transformer Rating of a Three Phase Transformer: P = √3. V x I Rating of a Three phase transformer in kVA kVA = (√3. V x I) /1000 But Wait, A question is raised here… Look at the General nameplate rating of a 100kVAtransformer. Did you notice something????Anyway, I don’t care what is your answer lets me try to explain. Here is the rating of Transformer is 100kVA. But Primary Voltages or High Voltages (H.V) is 11000 V = 11kV. And Primary Current on High Voltage side is 5.25 Amperes. Also Secondary voltages or Low Voltages (L.V) is 415 Volts And Secondary Current (Current on Low voltages side) is 139.1 Amperes. In simple words, Transformer rating in kVA = 100 kVA but Primary Voltages = 11000 = 11kV Primary Current = 5.25 A Secondary Voltages = 415V Secondary Current = 139.1 Amperes. Now calculate for the rating of transformer according to P=V x I (Primary voltage x primary current) P = 11000V x 5.25A = 57,750 VA = 57.75kVA Or P = V x I (Secondary voltages x Secondary Current) P= 415V x 139.1A = 57,726 VA = 57.72kVA Once again, we noticed that the rating of Transformer (on Nameplate) is 100kVA but according to calculation…it comes about 57kVA… The difference comes due to ignorance of that we used single phase formula instead of three phase formula. Now try with this formula P = √3 x V x I P=√3 Vx I (Primary voltage x primary current) P =√3 x 11000V x 5.25A = 1.732 x 11000V x 5.25A = 100,025 VA = 100kVA Or P = √3 x V x I (Secondary voltages x Secondary Current) P= √3 x 415V x 139.1A = 1.732 x 415V x 139.1A= 99,985 VA = 99.98kVA Consider the (next) following example. Voltage (Line to line) = 208 V. Current (Line Current) = 139 A Now rating of the three phase transformer P = √3 x V x I P = √3 x 208 x 139A = 1.732 x 208 x 139 P = 50077 VA = 50kVA