Engineering Formulae

Quantity	Symbol	Unit	Symbol	Example
apparent power	S	Voltage x Current	VA	120kVA (1000 x V x I )
current	I	Ampere	A
efficiency	η	per unit		0.95 (i.e. 95%)
frequency	f	Hertz	Hz	50 Hz
poles	p	number		2 pole, 4 pole
pole pairs	pp	number		4 pole is 2 pp
active power	P	Watt	W	kW = 1000 x W
power, input	P_in	Watt	W
power, output	P_out	Watt	W
power factor	pf	per unit		0.86
reactive power	Q	Voltage x Current reactive	var
slip	s	per unit
speed	N	revs per minute		Not SI unit
torque	T	Newton . metre	Nm
voltage	V	Volt	V

Quantity	DC	AC 1 Phase	AC 3 Phase
Current (I)	$\frac{P_{out}}{V\times\;Efficiency}$	$\frac{P_{out}}{V\times\;Efficiency\times\;Power\;Factor}$	$\frac{P_{out}}{\sqrt{3}\times\;V\times\;Efficiency\times\;Power\;Factor}$
Efficiency	$\frac{P_{out}}{V\times\;I}$	$\frac{P_{out}}{V\times\;I\times\;Power\;Factor}$	$\frac{P_{out}}{\sqrt{3}\times\;V\times\;I\times\;Power\;Factor}$
Power Factor		$\frac{P_{in}}{V\times\;I}$	$\frac{P_{in}}{\sqrt{3}\times\;V\times\;I}$

Formula

Quantity	Formula
Fan load power	$Hp=\frac{ft^{3}/min\times\;pressure\;(lbs/ft^{2})}{33,000\times\;Fan\;Efficiency\;(p.u.)}$
Fan load power	$kW=\frac{Flow\;(m^{3}.s^{-1})\times\;pressure\;(kPa)}{Fan\;Efficiency\;(p.u.)}$
Ohm's Law (Resistive circuits)	$V=I\times R$	$I=\frac{V}{R}$	$R=\frac{V}{I}$
Power (kW)	$kW=\frac{T(N.m)\times rpm}{9550}$
Power (Hp)	$Hp=\frac{T(lb.ft)\times rpm}{5250}$
Power Factor	$Power\;Factor=\frac{P_{in}}{V\times I}$
Pump load power (Hp)	$Hp=\frac{Galls/min\times Head(ft)\times Specific\;Gravity}{3960\times Pump\;Efficiency(p.u.)}$
Pump load power (kW)	$kW=\frac{9.81\times Head(m)\times Specific\;Gravity}{1000\times Pump\;Efficiency(p.u.)}$
Slip, s, (p.u.)	$s=\frac{N_{s}-N}{N_{s}}\times 100$
Slip, (%)	$s(_{o}/^{o})=\frac{(N_{s}-N)\times 100}{N_{s}}$
Synchronous speed, N_S, (rpm)	$N_{s}=\frac{Hz\times 120}{p(poles)}$
Torque (N.m)	$T=\frac{kW\times 9250}{rpm}$
Torque (lb.ft)	$T=\frac{Hp\times 5250}{rpm}$

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