# Hub generator calculations

Discussion in 'Spare Parts, Tools & Product Developement' started by duivendyk, May 8, 2008.

1. ### duivendykGuest

In a previous post I directed members of this forum to the informative website www.pilom.com, which I chanced to come across when searching for info on bicycle generators
In the literature list,which by the way,is easy to miss,reference is made to a paper written by Matthias Magdowski from Magdeburg University in Germany,in which he modeled an equivalent circuit for a hub generator,based on observed test results.Since the computed results closely matched actual test results,it would appear that this is indeed a valid equivalent circuit, that would permit meaningful calculations of generator performance to be performed
The hub generator in question was a Novatec EDH-1.
I have been informed that the electrical innards of most hub generators are supplied by Toshiba to the manufacturers,so that their electrical performance can be expected to be closely matched
With this model I first decided to take look at the generator output for different loads.I chose a road speed of 20 mph (corresponding to a frequency of 63 Hz).See table 1
resistance: 15--- 20---- 25---- 30----- 40---Ohms
-- Output : 4.5---5.2----5.8----6.04----6.3--Watts
It is obvious that for best performance a 30 Ohm or higher load should be used.
The next matter of interest would be to find out how the output changes for different speeds, with a particular loads.I chose 30 and 20 Ohms. These results are presented in table 2 for various speeds.
Road speed :-------15----20----25----30------ mph
Output (30 Ohm) : 5.0---6.04--6.78--no data--Watts
Output (20 Ohm) : 4.5----5.2---5.73--6.04-----Watts
It is seen that the output increases, but not proportionaly to frequency,in contrast to the no load output voltage which does (approximately).This output saturation effect is mainly due to the internal inductance of the device.
One way to counteract this is to put a capacitor in series with the load to try to tune out the inductance,this can increase the output over a certain speed range.Although it will reduce the performance at low speeds it does help in the mid range.Table 3 shows the effect of a cap. of 80 microfarads and of 68 MF (30 resp. 35 ohms at 63 Hz), put in series with a 30 ohm load.
Speed--------------15---------20---------25----mph
Output (80 MF)----6.27------7.12-------8.58--Watt
Output (68 MF)-----5.8-------8.27-------8.78 Watt
It can be seen that this improves the output substantially at 20 mph, from 6.04 to 8.27 W or close to 40 %,but the output at 15 mph drops off a bit.
Summing up,these computations show that the optimum ac. load is in the 30/50 ohm range and that capacitive compensation can significantly improve the output power, the question is now how to be cognisant of this in the rectifier & charger design,what is the best voltage 6V or12 V,what capacity ? and what are suitable IC's?.Any opinions on these subjects?
I am pretty good in German,if anyone is keen on getting more info from the Magdowski paper,I could anotate certain pages and explain the captions,let me know.
I gather it is a university dissertation so rather detailed.These computations are tedious and laborious to do by calculator, esp. those with the capacitive compensation,that's why I restricted myself to what I considered the cases of most interest,JJ

Last edited by a moderator: May 9, 2008