LOCKED ROTOR CURRENT OF A MOTOR
The Locked Rotor Torque or Starting Torque is the torque the electrical motor develop when its starts at rest or zero speed. The magnitude of the Locked rotor current is determined by motor horsepower and design characteristics. Normally it will be 6 to 7 times of the Motor full Load current.
Normally Motor Name plate contains KVA code or Design Letter.
We can calculate the Locked Rotor Current Using the following Formula
Single Phase = (577 x HP x KVA/HP)/V
Three Phase= (1000 x HP x KVA/HP)/V
Find KVA/HP using the Following chart
CODE
LETTER |
KVA/HP
RANGE |
CODE
LETTER |
KVA/HP
RANGE |
A
|
0.00 – 3.14
|
J
|
7.10 – 7.99
|
B
|
3.15 – 3.54
|
K
|
8.00 – 8.99
|
C
|
3.55 – 3.99
|
L
|
9.00 – 9.99
|
D
|
4.00 – 4.49
|
M
|
10.00 – 11.19
|
E
|
4.50 – 4.99
|
N
|
11.20 – 11.49
|
F
|
5.00 – 5.59
|
O
|
12.50 – 13.99
|
G
|
5.60 – 6.29
|
R
|
14.00 – 15.99
|
H
|
6.30 – 7.09
|
Example:
Motor Name plate indicates phase 3,300 HP,4160 Volts,KVA Code G
LRC will be
(577 X 300 X 5.60)/4160 to (577 X 300 X 6.29)/4160
=232.02 to 261.73 Amps
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warm Greeting
ReplyDeleteHave you been thinking about the power sources and the tiles whom use blocks
I wanted to thank you for this great read!! I definitely enjoyed every little bit of it and I have you bookmarked to check out the new stuff you post.
Single Phase Motor
Hi your example calc states phase as 3 but you have used the value 577 as for single...therefor 403A to 453A would be the answer ?
ReplyDeletethe relation is (kva/hp × hp × 1000/root(3)×volt)
ReplyDeletethe relation for calculating the locked rotor current for three phase motors is LRC=(kva/hp × hp ×1000 / root(3) × v(line voltage).
ReplyDeletewhat is the full meaning of Kva
ReplyDeleteYOU HAVE SINGLE (SHOULD BE 1000) AND THREE (SHOULD BE 577) PH SWITCHED
ReplyDeleteOkay guys your help needed:
ReplyDeleteQ) A 4000kW 6.6kV 50Hz MOTOR with Torque load is 5 times the rated load.
I need its Locked Rotor/Starting Current as well as full load current to arrange a well-sized power system sufficient for its operation.
you must have the NEC charts 430.251(a) for single phase or 430.251 for polyphase to make accurate calculations
ReplyDeletePLEASE CORRECT THE FORMULA AS SAID ABOVE!!!!!
ReplyDeleteWhy using 577 for 3 phase and 1000 for single phase. Where are these numbers coming from
ReplyDeleteit is correct 3 phase for 1000 and single phase for 577 please confirm
ReplyDeleteThanks