New : Electrical Engineering

 Home Aptitude Gen.Knowledge English/Verbal Engineering Reasoning

 Home -> Engineering - > Electrical -> Important formulas

 Important formulas :-
 :: Read Theory :: Important formulas

 Note:- Share/leave comments & know how to remember it at the end of cheptor

Some Important Formulas [Electrical]
 Ohm's Law for D.C. Circuits
 P = EI = I2R = E2 watts R
 E = IR = √PR = P volts I
 I = E = P = √P/R = Ampere R E
 R = E = E2 = P = Ohm's I P I2

 Power= EMF X Current P = X

 Resistance
 ▷ Resistors in series = R = R1 + R2 + R3 +... Rn
 ▷ Resistors in parallel = 1 = 1 + 1 + 1 +... 1 R R1 R2 R3 Rn

 Capacitors & Electrostatics
 ▷ Capacitors in parallel = C = C1 + C2 + C3 +... Cn
 ▷ Capacitors in series = 1 = 1 + 1 + 1 +... 1 C C1 C2 C3 Cn
 ▷ Capacitance of capacitor C = Q V
 ▷ Energy stored in capacitor W = 1 CV2 2
 ▷ Coulomb's law F = Q1 Q2 4πԐ0Ԑr r2
 ▷ Electric field strength E = Q 4πԐ0Ԑr r2
 ▷ Electric flux density D = Q 4πr2
 ▷ Electric potential at a point due to point charge q: V = q 4πԐ0Ԑr r
 ▷ Potential due to number of charges V = 1 [ q1 + q2 +... ] 4πԐ0Ԑr r1 r2
 ▷ Charging of capacitor i = V e-t/RC R
 q = Q(1 - e-t/RC)
 ▷ Discharging of capacitor i = V e-t/RC R
 q = Qe-t/RC

 Electromagnetic Induction, Magnetic circuit, Inductance
 ▷ Faraday's law electromagnetic induction e = -N dΦ dt
 ▷ Dynamically induced emf = B l v Sinθ
 ▷ Self induced emf = -L di dt
 ▷ Mutual induced emf = M di1 dt
 ▷ Self inductance L = NΦ l
 ▷ Mutual inductance M = N2 Φ2 l1
 ▷ Coefficient of coupling k = M √LxLy
 ▷ Time constant for RL circuit T = L R
 ▷ Energy stored in magnetic field = 1 L I2 2
 ▷ Magnetomotive force mmf = NI
 ▷ Magnetic field strength H = NI l
 ▷ Reluctance H = l µ0µr a
 ▷ Ohm's law for magnetic circuit mmf = flux X reluctance

 ▷ Hysteresis loss Ph = ηV.f(Bmax)1.6

 ▷ Eddy current loss Pe = k.(Bmax)2f2t2V  [note here V is volume not voltage]
 ▷ Ampere turn for iron parts ATi = Bi .li = Hili Hi
 ▷ Ampere turn for air gap ATg = 0.796 Bglg X 106
 ▷ Leakage factor = ΦT Φ
 ▷ Force experienced by conductor F = BIlsinθ
 ▷ Magnetic field due to an infinite linear conductor H = 1 2πr
 ▷ Flux density due to circular loop Bz = μIR2 2(R2 + Z2)3/2
 ▷ Flux density at the centre of solinoid Bc = μNI √4R2 + I2
 ▷ Force between two parallel conductors F = μ0 I I' .I 2πr
 ▷ Magnetic Flux density B = Φ A
 ▷ Ampere's circuit law ∫H dl = ∑ I
 H = N.I 2πr
 ▷ Biot-savart's law dH = I Idl sinθ 4π r2

 Measuring Instruments
 ▷ For extension of range of ammeter, shunt resistance Rsh is given by: Rsh = Rm/(m-1)

 ▷ For extension of range of voltmeter, series resistance Rs is given by: Rs = (m-1)Rm

 ▷ The unknown resistance Rs to be measured by wheatstone bridge method is: R = (P/Q)S

 AC Fundamental
 ▷ RL series circuit ν = VmaxSin ax i = ImaxSin (ax - Φ) rms value of current = I = V/Z where Z = √R2 + X2L
 In inductive circuit, the current lag the applied voltage by an angle Φ

 ▷ RC series circuit ν = VmaxSin ax i = ImaxSin (ax + Φ) rms value of current = I = V/Z where Z = √R2 + X2C
 In capacitive circuit, the current leads the applied voltage by an angle Φ

 ▷ RLC series circuit Z = √R2 + (XL - XC)2

 ▷ Active power = V I CosΦ ▷ Apparant power = V I ▷ Reactive power = V I SinΦ
 ▷ Power factor CosΦ = Active power = R Apparant power Z
 ▷ Resonance in RLC circuit occurs at a frequency given by fr = 1 2π√LC
 ▷ At resonance XL = XC Impedance is minimum Current is maximum Power factor is unity
 ▷ Bandwidth = R 2πL

 Transformer
 ▷ For ideal transformer IpNp = IsNs ; VpIp = VsIs
 Transformer ratio k = Ep = Vp = Np = Is Es Vs Ns Ip
 Only current I is secondary/primary here

 Synchronous Motor
 ▷ Synchronous speed of rotating magnetic field, Ns = 120 X f P
 ▷ Percentage slip, S = Ns - N Ns

 3 Phase System
 ▷ For 3 phase star connection with balanced load IL = Iph VL = √3 X Vph P = √3 VLILCosΦ
 ▷ For 3 phase delta connection with balanced load IL = √3 Iph VL = Vph P = √3 VLILCosΦ

 DC Machine
 ▷ EMF of a DC Machine, Eg = PΦIaZ A
 ▷ Constant losses = Iron losses + Mechanical losses