The most common used electrical formulas – Ohms Law and combinations
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 = ۷۴۶ Php / Winput (5)
where
μ = efficiency
Php = output horsepower (hp)
Winput = input electrical power (Watts)
or alternatively
μ = ۷۴۶ 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
Electrical Motor – Amps
I3-phase = (746 Php) / (1.732 E μ PF) (7)
where
I3-phase = electrical current 3-phase motor (Amps)
PF = power factor electrical motor
