PM motor scaling laws
Hubmotors, such as used for bicycles, belongs to the category small PM motors (permanent magnetic). These motors have by nature a lower efficiency than large ones. These are the relationships with the motor diameter d. The distraction of the formulas is rather complicated, see "Aandrijfsystemen" J.H.J. Boekema.
- Maximum power Pmax ~ d4
- Efficiency η ~ d2
So the efficiency of an electromotor is proportional to the square of the diameter. If we use the motor only at a certain speed and torque, the efficiency can be optimized.
Hub motor weight
Hub motors are too heavy and too large for the little power they deliver. The cause is that manufacturer takes no unnecessary risks to avoid repairs during the guarantee period. Some 250W hub motors have a weight of more than 3kg. To illustrate, here is an example of a high efficiency axial gap dc motor from Lynch Motor Company. The LEM 130 95s model as a weight of 3kg and an output of 3kW. The peak efficiency is 87%.
Here is another example, the "LRP 50920 Vector 8 Brushless Modified" RC car motor: 1.360W, 275g, efficiency 91%.
It should be noted that all these motors are high speed. After considering these motors, I wonder whether it is possible to build ebike hub motors with a higher efficiency and with a lower weight. But often there is no effort to save weight. This is an ebike hub motor with a weight of 3.9kg and a power of just 250-500W.
I made a list of PM motors with their weight. There is a line that indicates the lightweight motors. Motors above the line are unnecessary too heavy.
|Manufacturer||Type||Power [W]||Weight [kg]|
|Lynch Motor Company||95S||4000||3.00|
|Lynch Motor Company||D127||21000||8.50|
|Lynch Motor Company||D135||29000||11.00|
|Perm motor||PGM 132||7200||11.00|
* the motor weight (without cables) is measured by myself.
Tongxin Keyde Cute Goldant
Note that Keyde, Cute and Goldant hub motors all are made by Tongxin.
Hub motors without specifications are suspect
It is noticeable that for many hub motors, the supplier has no data or graphs. They are suspected in advance:
- There are charts, but because insufficient motor performance, these are not published.
- The engine has more power than allowed by law to obtain more power during acceleration and therefore obtain a higher ranking in consumer tests.
The motor data that always must be known are k, Ri and Tf. It is possible to extract the data from the motor chart. See here.
Hub motor history
Wheel hub motors are nothing new. Here is an example of hub motors at the Semper Vivus from 1900:
Using RC aircraft motors for e-bikes
These motors are lightweight, powerful and they have a high efficiency.
See more here: The New and Improved Brushless Electric Scooter Power System Guide