The MGB Electric Sports Car

Analysis by Warren Winovich

Page 7

Appendix

Definition of Terms

aacceleration of the vehicle, ft/sec2
a0initial rate of acceleration, ft/sec2
Across section area, ft2
ctime constant of an accelerating vehicle, seconds
CDdrag coefficient
Ddistance traveled during an acceleration, feet or miles
eeconomy of energy usage, W-hrs/mi
Evoltage, Volts
ffrequency of applied stator voltage, Hz
gca constant for use with English System of Units: 32.174 lbm-ft/#-sec2
Goinitial value of the acceleratinq G-force, #/lbm
Icurrent, Amperes
nmotor efficiency
nccontroller efficiency
nmmechanical drivetrain efficiency
Nsynchronous rotational speed of magnetic field, rpm
pPeukert's constant for the battery system
pnnumber of magnetic poles of the stator of an induction motor
Pdpower component ascribed to aerodynamic drag, Watts
Pipower input to the induction motor, Watts
Popower output of the motor, Watts
Prrated power of the electric motor, Watts or kW
rrolling force, #/lbm
r0initial value of the rolling force ( V = 0), #/lbm
Rradius of the tire/wheel combination, ft or inches
Rbresistance of the battery pack, Ohms
Smotor speed under load, rpm
ttime, seconds or hours
Vvelocity, ft/sec or mph
Vffinal velocity or top speed, ft/sec or mph
Wweight of the vehicle, pounds (lbm)
&rho (rho)air density @ standard pressure and temperature: 0.0745 lbm/ft3


The analysis uses the English System of Units. In this system an ambiguity exists in that the word “pounds” is used to designate both force and mass. To distinguish between force and mass, force is labeled “#”; mass is labeled “lbm”. The relation between force and mass is embodied in Newton's Law:

force = mass x acceleration

To satisfy the requirement of consistency of units in the English Units System, weight in pounds must be converted to mass units by introducing a conversion constant: “gc”. Then, Newton's Law becomes (ESU)

force, # = [(W, lbm)/gc] x (a, ft/sec2)

The value of the constant is determined by considering a pound mass is placed on a spring scale.

By Newton’s Law, the value of “gc” is found as

gc = 32.174 lbm-ft/#-sec2

( The numerical value is identical to the acceleration induced by Earth's gravity.)

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