Gyuláné Vincze, Gergely György Balázs
Budapest University of Technology and Economics Department of Electric Power Engineering
Traditional cylindrical rotating machines are designed for high rotational speed so that their diameter would be the smallest possible. This rotational speed is 6-7000/min for vehicle motors, or 6-700 rad/s as angular speed. The fixed ratio r=ω m /ω w between angular speeds of motor ω m and wheel ω w must be set so that maximal motor rotational speed corresponds to final speed of the vehicle.
Calculating speed of vehicle v from angular speed of motor is shown in (2.1.a), where r w is radius of the wheel. This calculation is for ideal conditions, assuming that there is no slip, spin, so circumferential speedof wheel r w ω w equals to vehicle speed.
The expression between the torque of motor M m and its motive force is shown in equation (2.1.b). Losses of the drive can be taken into account with efficiency η<1. One motor can drive several wheels, in this case motive force (2.1.b) is distributed among the wheels, for example among the two driven wheels in Figure 2.1.
If several motors drive the vehicle then motive forces of the motors are summarized in a way that angular speed ω w of the wheels are constrained through the road. A problem can be to distribute the load among the motors equally, there are different solutions for this in different vehicles.
Variable transmission gearbox used for internal combustion engine drives can be eliminated if the mechanical characteristic M m -ω m of the electric drive fits to traction need F-v of the vehicle without modification. Fitted characteristic M m -ω m of the electric motor means that all of its points, calculated as above, suits to ideal traction characteristic, as shown in Figure 2.5. Load torque characteristics calculated from running resistance to motor axle is also indicated in Figure 2.5.b.
Electric power P m =M m ω m required for traction can be calculated from tractive power P=Fv, taking into account drive losses:
Power P m is the sum of the powers of the motors in case of a multimotor solution. In this case, we have to take into account that the distribution of loads among the motors are not even, for example load can be higher than average because of the wear of the wheel. Differences must be taken into account when designing the motors.