ELECTRICAL THUMB RULES
Current Rating of Capacitor
KVAr X 1.35
Capacitor recommended for 3 Phase Motor
HP X 0.4 KVAr
Motor Power Factor
No-Load P.F = 0.6
Load P.F = 0.8
Transformer Power Factor
No-Load P.F = 0.2
KVAr X 1.35
Capacitor recommended for 3 Phase Motor
HP X 0.4 KVAr
Motor Power Factor
No-Load P.F = 0.6
Load P.F = 0.8
Transformer Power Factor
No-Load P.F = 0.2
Load P.F = 0.8
Capacitor required for No - Load Transformer P.F compensation
% Impedence X 10
Note : These Rules can only used for getting approximate values. For actual value use appropriate formulas.
ELECTRICAL THUMB RULES
Current rating of Transformer / Generator (I)
KVA X 1.4
Impedence of Transformer (Z)
5 %
Short circuit rating of Transformer
Current Rating / % Impedence = I / Z
No Load Current of Transformer
< 2 % of rated current of Transformer
Impedence of Generator
15 %
Short circuit rating of Generator
Current Rating / % Impedence = I / Z
Full load current ( FLC ) of 3 Phase Motor
HP X 1.5
No load current of 3 Phase Motor
30% of FLC
Full load current of Single Phase Motor
HP X 6
KW rating of Motor
HP X 0.75
OLR selection
STAR & DELTA STARTER :- FLC X 0.6
Fuse Rating for Motor protection
STAR & DELTA :- FLC X 1.5
SLIP RING MOTOR :- FLC X 1.25
ELECTRICAL FORMULAS AT A GLANCE - 1
3 phase A.C POWER (KW) = (1.732 x V x I x PF) / 1000
V - Volts
V - Volts
I - Current in Amps
PF - Power Factor
1 HP = 746 Watts
Power Factor ( PF ) = KW / KVA
Synchronous Speed ( Ns ) = ( 120 x F ) / P
F - frequency
P - No. of Poles
Motor Slip ( % Slip) = (( Ns - Nr ) / Ns) x 100
Ns - Synchronous Speed
Nr - Actual Speed
In STAR connection :-
I L = I ph
(Line current = phase current)
V L = 1.732 x Vph
(Line voltage = 1.732 x Phase voltage)
In DELTA connection :-
I L = I ph
(Line current = phase current)
V L = 1.732 x Vph
(Line voltage = 1.732 x Phase voltage)
In DELTA connection :-
V L = V ph
(Line voltage = phase voltage)
I L = 1.732 x I ph
(Line current = 1.732 x Phase current)
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