04/01/20
Electric vehicles were in fact introduced before internal combustion engine vehicles. Similarly, the wheel hub motors have been invented as early as in late XIX century. What were the obstacles which suppressed the innovations until their rediscovery in the recent years?
It is crucial to know history related to the ones area of expertise, especially being a design engineer or an inventor. It is so for one particular reason – it is wiser to build on others work and avoid problems already experienced by the pioneers. This kind of knowledge combined with theoretical knowledge and practical expertise, all gained by hard work, enables one to create true, useful innovations that last.
With that motivation we have reviewed patents and historical records related to wheel hub electric motors for vehicles (also known as in-wheel motors). Most important records are described and our conclusions and insights are presented.
The article deals only with wheel hub motors and for readers not familiar with electric vehicle history it can be summed up - Electric drives have all output parameters superior to internal combustion engines (nominal efficiency ~90% vs. >40%, much higher stall torque and wide operating speed range, etc.). The sole reason for their fail was much lower energy density and life-time of all know battery types (just until recent years) compared to fossil fuels. Even though, the electric drives did succeed - they are used in common trains with electrification grids ever since. Now, with much better batteries getting cheaper each year, the future of electric vehicles is changing. See the great timeline of EV developement.
US300827: W. Adams 1884
The first patent with wheel hub motor can be traced back to the year 1884 and the inventor W. Adams probably was the first person to introduce the concept.
The first impression viewing the electric drive illustration is, that it is much sophisticated than modern wheel-hub motors. (sic) The machine is based on a brushed DC motor with a planetary gear (4-planets)
We have color-coded the parts of this complex machine to easier grasp its working principle. The drawings presents single axle of a rail-car, where the axle is denoted by letter (A) fixed to the wheel (B) and the rack (V) – both used as the casing. The field rotor assembly (P) is mounted inside the described wheel (B,V). The axle is supported by the tapered bushing which can be adjusted to compensate for its wear (marked by the two arrows next to lubrication channel).
The motor is not an ordinary DC machine, as field (P) and armature (H) windings both rotate and the power is transmitted thru two commutators (W and X). Both windings are connected in series to the battery (B’).
The wheel assembly and field windings rotate with the angular speed \(\omega_1\). The armature (H) connected to the hub (C) with spokes (G) rotates with the angular speed \(\omega_2\)– the hub is used as the sun gear.
The described sun gear is meshing with the planetary gears (K) located on the planet carrier and fixed to the centrifugal clutch assembly (J). The centrifugal clutch pads (N) are placed in the groove (U) and their engagement depends on springs (Q). The outer ring of the clutch is controlled at operation with shaft (I).
The ring gear (K) is fixed to the wheel assembly described earlier and meshed with planetary gears. When the clutch is disengaged the armature and sun gear located on the hub rotate freely, as well as the planet carrier. When the clutch engages the torque is being transmitted to the wheel assembly with field fixed on the axle and the wheel starts to rotate inversely to the hub.
It can be observed that this solution allows operation at optimum, nominal speed using constant supply voltage. The operator can adjust the torque by regulating the clutch pressure with the regulation shaft (I). The main drawback of the solution is the fact that there are now two commutators instead of one – the parts of the electric machine most prone to wear and failure.
However the main drawbacks if the design are its complexity and the fact that the machine is based on Pacinotti-Gramme type electric machine which due to the large air gap performs very poorly. It can be observed that such complex design might be not as durable as common electric motors, especially considering harsh conditions which wheel hub has to withstand. Also the machine was probably harder to manufacture at the time of its invention.
While the design did not succeed commercially it is fascinating that such sophisticated machine was designed as first (or on of the first) wheel hub motors. Many ideas and solutions incorporated in the design can be borrowed and reused in a modern electric drive. Using better embedded electric motor, getting rid of commutators, using power electronics and so on can produce interesting results.
In the next parts of the article we will describe other interesting patents and technologies of wheel hub motors such as these created by famous inventor F. Porche and his distant relative J. Ledwinka involved in Hub Motor Company.