Test results on rebuilt generator

In order to go from complete guessing to actual planning, I had to run the generator at some known speeds and discover what the output was, at least in voltage at various speeds. The following picture shows how I did this. I secured the generator facing up and used 2" and 2 " pulleys on my drill press. Knowing what speed the drill press was running at allowed me to calculate the speed of the generator shaft using 2 ", 9" and 10" pulleys on the generator. The setups, speeds and results are shown in a table and graph below.

Predictably, after 25 years of great service, the spindle of the drill press chose this day to seize up. A few minutes dissasembly time and some oil fixed the problem. Results are shown below.


The test setup for finding voltage at various rpms.

The following table shows the results of this testing. The three positions (pos 1,2 & 3) are slightly different positions of the brushes in relation to the magnets. Although the positions are not all that different from each other, there are marked differences in performance.

RPM @ generator

Voltage pos. 1

Rpm/volt pos.1

Voltage pos. 2

Rpm/volt pos. 2

Voltage pos. 3

Rpm/volt pos.3

132

9

14.7





146

10

14.6





176

12.5

14.1

13.9

12.7

13

13.5

196

14

14

15.3

12.8

15

13.1

236

16

14.75





260

17.9

14.5





314

22.2

14.1

24.8

12.7

23.5

13.4

350

24.9

14

27.5

12.7

26

13.4

452

29.5

15.3





500

32.9

15.1





603

39.9

15.1

37

16.29

42

14.3

672

42

16

37

18.2

46

14.6

700

44

15.9



49

14.3



The rpm/volt ratio suggests that this generator is most efficient below 450 rpm no matter where the brushes are placed, but is better in the latter two test positions. I could get more volts for lower rpms in the second position, but the generator sparked dramatically and had a loud whining sound I didn't like. In addition, the sparking became so violent it appeared to hamper voltage flow into the brushes above 600 rpm. I backed it off slightly and settled for the values shown in red above (#3).

The graph below shows extrapolations based on the speeds tested during testing. The colors of the lines match the colors in the table above


The green line indicates test position 1. The blue line indicates test position 2. The red line indicates test position #3, which is the position I chose to go with. It is the closest to the straight line theoretical output, and sounds the best.

It should be noted that had I converted this to an alternator by installing slip rings and removing the commutator, I would not have had to do any adjusting of the brushes, although I would have had to change them to match the slip rings. Dynamos are fascinating devices, but are quite complex compared to alternators.

It would appear that as far as speed goes this rebuild was a success. The cut in speed went from 1700 +/- rpm down to 162 rpm for 12 volts. It would appear that it will now generate some usable voltage at wind turbine friendly speeds without gearing.

Matching this generator to the 7' rotor and low wind conditions

The rotor I built was per Hugh Piggott's specifications. His math is better than mine, so I'll refer you to his "Windpower Workshop" book pages 148 and 149. The math is all there. Assuming a 3 blade 2 meter (6.5 feet) diameter rotor with a tip speed ratio of 6, in a 10m/s (22 mph) wind we can get 573 rpm from our rotor with 283 watts of available power. But a 3 m/s (6.7 mph) breeze it will only get us to 172 rpm with only 7.6 watts of available power. Looking at the tests for the generator, It is possible for it to generate some power at 3m/s using these blades. Not much, but it's something.

Now the site I'm looking at is on the eastern plains of Colorado (see below). In the fall and spring and when storms are moving around, there can be some pretty hefty winds, however during most summer days it is dead calm with occasional slight breezes. During the evenings there is usally a small breeze. Over all it is a better site for photovoltaics than wind turbines. Checking the graph above, I see that I could get power from this generator for a 12 volt load above 168 rpm. If I use more than a 12 volt load I won't get much power out of this generator during a good part of the summer. Small though the power generated may be, trickle charging beats not not charging at all. Plus there's all those solar panels I'll be installing, right ?