BASIC ELECTRICAL ENGINEERING FORMULA

Electronics is an engineering discipline that involves the design and analysis of electronic circuits. Originally, this subject was referred to as radio engineering. An electronic circuit is a collection of components through which electrical current can flow or which use electromagnetic fields in their operation.

The electronic circuit design and analysis rests primarily on two Kirchoff's laws in conjunction with Ohm's law modified for AC circuits and power relationships. There are also a number of network theorems and methods (such as Thevenin, Norton, Superposition, Y-Delta transform) that are consequences of these three laws.

In order to simplify calculations in AC circuits, sinusoidal voltage and current are usually represented as complex-valued functions called phasors. Practical circuit design and analysis also requires a comprehensive understanding of semiconductor devices, integrated circuits and magnetics.

I = current(amps.), V = voltage(volts), R = resistance(ohms), P = power(watts)

CURRENT:

I = V/R or I = P/V

VOLTAGE:

V= P/I or V = IR

POWER:

I2R or VI

RESISTANCE:

R = V/I

ALTERNATING CURRENT(AC):

Il = line current(amps.), Ip = phase current(amps.), Vp = phase voltage(volts), Vl = line voltage(volts), Z = impedance(ohms), P = power(watts), f = power factor(angle), VA = volt ampers

CURRENT(single phase):

I = P/(Vp cos(f)

Common electrical units used in formulas and equations are:

Volt - unit of electrical potential or motive force - potential is required to send one ampere of current through one ohm of resistance

Ohm - unit of resistance - one ohm is the resistance offered to the passage of one ampere when impelled by one volt

Ampere - units of current - one ampere is the current which one volt can send through a resistance of one ohm

Watt - unit of electrical energy or power - one watt is the product of one ampere and one volt - one ampere of current flowing under the force of one volt gives one watt of energy

Volt Ampere - product of volts and amperes as shown by a voltmeter and ammeter - in direct current systems the volt ampere is the same as watts or the energy delivered - in alternating current systems - the volts and amperes may or may not be 100% synchronous - when synchronous the volt amperes equals the watts on a wattmeter - when not synchronous volt amperes exceed watts - reactive power

Kilovolt Ampere - one kilovolt ampere - KVA - is equal to 1,000 volt amperes

Power Factor - ratio of watts to volt amperes

Electric Power Formulas

W = E I (1a)

W = R I2 (1b)

W = E2/ R (1c)

where

W = power (Watts)

E = voltage (Volts)

I = current (Amperes)

R = resistance (Ohms)

Electric Current Formulas

I = E / R (2a)

I = W / E (2b)

I = (W / R)1/2 (2c)

Electric Resistance Formulas

R = E / I (3a)

R = E2/ W (3b)

R = W / I2 (3c)

Electrical Potential Formulas - Ohms Law

Ohms law can be expressed as:

E = R I (4a)

E = W / I (4b)

E = (W R)1/2 (4c)

Example - Ohm's law

A 12 volt battery supplies power to a resistance of 18 ohms.

I = (12 Volts) / (18 ohms)

= 0.67 Ampere

Electrical Motor Formulas

Electrical Motor Efficiency

μ = 746 Php / Winput (5)

where

μ = efficiency

Php = output horsepower (hp)

Winput = input electrical power (Watts)

or alternatively

μ = 746 Php / (1.732 E I PF) (5b)

Electrical Motor - Power

W3-phase = (E I PF 1.732) / 1,000 (6)

where

W3-phase = electrical power 3-phase motor (kW)

PF = power factor electrical motor.