3. Calculate TC Size & Voltage Drop due to starting of Large Motor

CALCULATE SIZE OF TRANSFORMER & VOLTAGE DROP DUE TO STARTING OF MULTIPLE NO OF MOTORS

Calculate Voltage drop in Transformer, 1000KVA, 11/0.480KV, impedance 5.75%, due to starting of 300KW Three Phase Motor and 5KW Single Phase Motor, 460V (Line-Line), 0.8 Power Factor, Locked Rotor Current is 450% and The allowable Voltage drop at Transformer Secondary terminal is 10%.

MOTOR CURRENT / TORQUE:

• Motor Full Load Current= (Kwx1000)/(1.732x Volt (L-L)x P.F) • Motor Full Load Current=300×1000/1.732x460x0.8= 471 Amp. • Motor Full Load Current= (Kwx1000)/( Volt (L-P)x P.F) • Motor Full Load Current=10×1000/ (460/1.732)x0.8= 24 Amp. • Total Motor Full Load Current=471+24=494 Amp • Motor inrush Kva at Starting (Irsm)=Volt x locked Rotor Current x Full Load Currentx1.732 / 1000 • Motor inrush Kva at Starting (Irsm)=460 x 2118x494x1.732 / 1000=1772 Kva

TRANSFORMER:

• Transformer Full Load Current= Kva/(1.732xVolt) • Transformer Full Load Current=1000/(1.732×480)=1203 Amp. • Short Circuit Current at TC Secondary (Isc) =Transformer Full Load Current / Impedance. • Short Circuit Current at TC Secondary= 1203/5.75= 20919 Amp • Maximum Kva of TC at rated Short Circuit Current (Q1) = (Volt x Iscx1.732)/1000. • Maximum Kva of TC at rated Short Circuit Current (Q1) =480x20919x1.732/1000= 17391 Kva. • Voltage Drop at Transformer secondary due to Motor Inrush (Vd)= (Irsm) / Q1 • Voltage Drop at Transformer secondary due to Motor Inrush (Vd) =1772/17391 =10.2% • Motor Full Load Current<=65% of Transformer Full Load Current • 494 Amp <=65%x1203 amp = 471 Amp<= 781 Amp • Here Motor Full Load Current<=TC Full Load Current but Voltage Drop is High (10.20%) so Size of Transformer is Not Adequate.

REQUIRED TO INCREASE THE SIZE OF TRANSFORMER.