Gyuláné Vincze, Gergely György Balázs
Budapest University of Technology and Economics Department of Electric Power Engineering
Most of the land crafts roll on wheels and are driven by internal rotating motor. The number of wheels and mechanical solutions for traction are very different in vehicle constructions. Regarding the number of motors the vehicle can be driven by a single motor or multimotor.
Single motor vehicle can be designed if requirements for motion force and traction power can be fulfilled with one motor.
Single motor vehicles can be divided into three groups:
electric cars, hybrid buses, trolleys,
electric bicycles and other low power electric vehicles,
linear motor vehicles, as a special case.
The construction of the vehicles in the first group is usually similar to vehicles with internal combustion engine, and electric motor is connected to front or back wheel axles with cardan shaft and differential gear (Figure 2.1.a). Gearbox with variable transmission and clutch is not needed in electric vehicles.
If there is a fix transmission between the angular speed of motor and wheels then this transmission is required to fit the angular speeds. In low power vehicles even simpler, v-belt or chain-drive is used, or a wheel hub motor placed on the wheel with flat, disc-type shape.
Vehicles with linear motor drive can be considered as special single motor vehicles where motor is along the whole body.
Multimotor drive can be used in low or high power vehicles, too.
An example for multimotor low power vehicle is an electric car with separately controllable wheel hub motors in every wheel (Figure 2.1.b). Usually, rpm of the motor and the wheel are the same. Motors drive the wheels directly and not the axle. Such a drive has mechanical problems as mass of wheels increase, and there is a flexible connection between rotor and stator of the motor.
There are several reasons to design high power multimotor vehicles:
One reason is electrical and is based on conventions. Voltage on one motor can be changed with serial or parallel connection of the motors. Changing serial and parallel connections of two motors is used in two-motor trams and underground trains, for example.
The other reason is mechanical. When using more motors construction can vary. Motive force can be divided to several wheels, power required for traction can be divided between motors, and more, smaller motors are easier to place inside the body.
Typical multimotor vehicles are electric locomotives where motors are placed on bogies, in several variations. A common sign system is used to indicate the mechanical solution. For example, B’B’ sign means vehicle with two bogies with two-two pair wheels and one motor per bogie, BoCo sign means two bogies with two wheel pairs on one bogie (sign B) and three on the other (sign C), and every wheel pairs driven by separate motors (index o), which means 5 motor drives altogether.
Figure 2.2 shows driving motor placed on wheel axle. There is a connection with rubber core and cardan shaft between the motor and axle which enables flexible displacement.
Point of interest of this solution is that there is a separate brake axle to place the brake discs which connects to the axle of the motor through the “big gear”. In this way, motor is not influenced by the heat of brake discs.
Novel multimotor drive solutions can be found in low floor urban/suburban vehicles. Because of low floor (350 mm or below step-in height), wheels on the left and right cannot be connected with axle, contrary to locomotives. Novel types axles are required.
A low floor vehicle with two-two wheels behind each other with common drive can be seen in Figure 2.3. Examples for this solution are Swiss made COBRA and Combino trams running in Budapest. There are electric motors on left and right sides which drive two wheels on the same side (indicated with yellow in the figure) with two-side cardan axles. There are seats above the wheels. A specialty of vehicle COBRA shown in figure 2.3 is that its axles are steered mechanically (automatically from the side of the vehicle), and this solution provides exceptional advantages in curves, regarding to wear and noise.
Low floor vehicle with wheel hub motor is shown in Figure 2.4. An example for this solution is Variobahn vehicle developed by ABB(Adtrans). Wheel hub motor has outer rotor, its stator is inside (Figure 2.4.b).
A special type of multimotor vehicles is a hybrid-electric vehicle, where internal combustion engine can also participate in driving besides the electric motor, at the same time.