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.
No comments:
Post a Comment