A single - phase 50 – kVA, 250V/ 500 V two winding transformer

A single - phase 50 – kVA, 250V/ 500 V two winding transformer has an efficiency of 95% at full load, unity power factor. If it is reconfigured as a 500 V/ 750 V auto-transformer, its efficiency at its new rated load at unity power factor will be

Efficiency , η = 0.95 =	50
	50 + Losses

⇒ Losses = 50 – 47.5 = 2.5 kW

Now kVA = 50 ×	1
	1 - K

K =	500	=	2
	750		3

= 50 ×	1	= 50
	{1 - (2 / 3)}

∴ New efficiency =	150	× 100 = 98.36%
	150 + 2.5