Calculate TC Size & Voltage Drop due to starting of Large Motor
June 5, 2013 11 Comments
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 Multiplier x Full Load Currentx1.732 / 1000
- Motor inrush Kva at Starting (Irsm)=460 x 4.5x471x1.732 / 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/(5.75/100)= 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.
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