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

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

• Calculate Voltage drop in Transformer ,1000KVA,11/0.480KV,impedance 5.75%, due to starting of  300KW,460V,0.8 Power Factor, Motor code D(kva/hp).Motor Start 2 times per Hour 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 Locked Rotor Current =Multiplier x Motor Full Load Current

Locked Rotor Current (Kva/Hp)
Motor Code	Min	Max
A	3.15
B	3.16	3.55
C	3.56	4
D	4.1	4.5
E	4.6	5
F	5.1	5.6
G	5.7	6.3
H	6.4	7.1
J	7.2	8
K	8.1	9
L	9.1	10
M	10.1	11.2
N	11.3	12.5
P	12.6	14
R	14.1	16
S	16.1	18
T	18.1	20
U	20.1	22.4
V	22.5

• Min Motor Locked Rotor Current (L1)=4.10×471=1930 Amp
• Max Motor Locked Rotor Current(L2) =4.50×471=2118 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 2118x471x1.732 / 1000=1688 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) =1688/17391 =10%
• Voltage Drop at Transformer Secondary is 10% which is within permissible Limit.
• Motor Full Load Current<=65% of Transformer Full Load Current
• 471 Amp <=65%x1203 amp = 471 Amp<= 781 Amp
• Here Voltage Drop is within Limit and Motor Full Load Current<=TC Full Load Current.
•  Size of Transformer is Adequate.