The MGB Electric Sports Car

Analysis by Warren Winovich

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Introduction. A vintage MGB, about a 1967 model, has been converted to an electric car using a three-phase induction motor and a small lead-acid battery pack. The unique feature of this conversion is the application of a low-voltage Curtis inverter/controller - nominal voltage: 100 - 120 Volts.

The MG line is the product of Morris Garages, Ltd. of Abingdon, England. These popular sports cars were introduced into the United States during the 60's and 70's. In the early 1940's, the MG Car Company was absorbed by Austin and Nuffield to form British Motors Corporation. It was the BMC that imported the line of MGs into the United States.

A feature that supported the MGB's popularity was its liveliness and acceleration as well as its steering responsiveness. These features were - in part - due to the light-weight construction incorporated in this sub-compact design. The body is of unitized construction with no steel chassis. It has an independent front suspension with disc brakes. The rear suspension has a rigid axle that is supported by semi-elliptic springs. The differential has a gear ratio of about 4:1. An in-line 4-cylinder engine in the front of the car was rated 95 hp (71 kW) @ 5400 rpm. Maximum torque was 110 #-ft @ 3000 rpm. With the standard 4-speed transmission, shifting at appropriate velocities would correspond to a maximum (somewhat constant) accelerating torque of 440 #-ft delivered to the wheels. For the maximum engine speed of 5400 rpm, the top velocity of the MGB was 104 mph as manufactured. Curb weight - as manufactured with the I.C. engine - was 2030 pounds. Performance during accelerations with the I.C. engine is given below. (Test data for the I.C. MGB is given on p.16.)

city driving0-30 mph:5.461 Seconds
highway driving0-60 mph:12.10 Seconds
1/4-mile competition19.269 Seconds @ 81.21 mph

These performance factors can be compared with those predicted for the electric car conversion with the direct-drive transmission, the nominal 70-volt hree phase induction motor, and the (rather heavy) lead-acid battery pack derived below. As a matter of interest, the 1967 MGB had a listed price of $ 2825 for west-coast delivery. Specifications of the 1967 MGB are shown in figure 1.

The MGB electric sports car. The principles incorporated in the conversion of the MGB to electric drive represent the modern, 21st-century, methods that are to be adopted for the home-built electric car. The principle driving force to use a three-phase induction motor is the introduction of the Curtis inverter/ controller. This inverter is a low-voltage design with a nominal rating of 120 Volts. It is compact and light weight. A major factor in its adoption is the programmable regenerative-braking feature. For a 120-Volt limit, the three-phase output is about 70 Volts that requires that the windings of the induction motor must be wound to accommodate the relatively high currents for normal driving and during regenerative braking. A further advantage of the induction motor is that reversing the car can be accomplished electrically by merely switching two leads of the motor windings. When the inverter/controller is programmed, the starting torque can be maintained to maximum value throughout (almost) the entire speed range of the vehicle. The use of an induction motor eliminates the necessity for a transmission; and a direct-drive is appropriate.

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