The MGB Electric Sports Car

Analysis by Warren Winovich

Page 2

The specifications of the electric MSB sports car are shown in figure 2. Curb weight of the vehicle as an electric car is 2530 pounds - which is 500 pounds heavier than the internal-combustion-driven version. However, the weight is still somewhat below most home-built and production electric cars that feature curb weight of 3000 - 3500 pounds. The light curb weight of the MGB electric car is due to the rather small lead-acid battery pack consisting of eight sealed units with a stored energy of 9.6 kW-hrs ( at the 20-hour discharge rate)

The motor rating adopted for the present study is a 15-kw induction motor. Actual rating is not specified by the manufacturer; however, the stator windings are specially wound to accept the high currents associated with the low-voltage Curtis inverter. The specifications of the Curtis inverter are listed in figure 3. Direct current input is nominally 100 Volts; it is available with a d.c. input of 120 Volts. In contrast, most electric cars powered by induction motors have three-phase a.c, voltages of 220 Volts that require battery voltage of 312 Volts - 26 unit 12-volt batteries. The high voltage induction motor, then, operates with current levels of 30 A ( d.c. ). The Curtis three-phase inverter with 96 Volts, d.c. input, has an output a.c. voltage of 70 Volts; current levels, then, are of the order 100 A (d.c.). For starting conditions associated with the acceleration of the electric car, the current draw by the stator windings can be from 3 - 4 times the rated motor current - which for the Curtis inverter/controller is of the order of 400 Amps. The short-term rating of the motor as well as that of the inverter is 45 - 60 kW.

The battery pack in the MGB electric sports car consists of eight 12-Volt sealed units. Each unit battery is rated: 12 V @ 100 A-hrs. The battery type is shown in figure 4. Stored energy is 9.6 kW-hrs. Battery capacity falls as the current draw increases. At the power levels associated with highway driving, say, battery current can be about 150 Amps and the available capacity falls below 50 percent of the 20-hour discharge rate. That is, for highway driving, available energy is below 4.8 kW-hrs for the MGB electric car. The fall-off in capacity is given by Peukert1s discharge law:

I x tp = 63 for the battery pack in the MGB

The Peukert constant ( applied to the time term here ) is taken as 0.845 for this study. For a specified current, Peukert’s law gives the discharge time; and the product of current and discharge time establishes the actual capacity of the battery pack.

Performance calculation for the MGB electric sports car. The specifications of the MGB electric car shown in figure 2 are the basis for calculating the performance. Performance for an electric car is for driving at steady speed over level roads. Values for city driving with frequent stop-and-go's or for driving over grades are higher for a specified speed. Electric car performance can be summarized by just two factors:

  • 1. Economy of energy usage ( or, economy). Economy is the energy required to travel one mile. It is given by “W-hrs/mi”. It is the actual energy draw from the battery pack that accounts for motor efficiency, controller efficiency, and the efficiency of the drivetrain.
  • 2. Range between charging. Range is the more-often cited performance factor. It is merely the product of velocity and discharge time.

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