Bicycle Dynamos

[russell_w_b]

I did some mucking about with my old bicycle bottle dynamo back in 2016 and came up with the following. It was a quiet day at work.

The bicycle bottle dynamo (Miller, made in India) is designed to deliver 3.3 Watts at 6V a.c. It consists of a cylindrical permanent magnet rotating within an 8-pole single-coil stator. I know this because I took it to bits to repair an open circuit which, fortunately, was right on the end of the wire where it attaches to the terminal.

After I’d repaired it, I took the opportunity to do a few tests, just to get the feel of the values involved… For this, I had to make a few assumptions, as I can only drive it at one speed from a mains-powered Black and Decker drill!

My oscilloscope showed a sinusoidal waveform of 180Hz. As speed = frequency x 60 / No. of pole-pairs, it was evident that the actual speed was 2700 RPM, which is reasonable, given the slip of the drill under load.

This equates to a bicycle speed of about 6 MPH (9.6 km/H). German standards demand 95% output at 15 km/H (9 MPH) so my tests are a bit under-speed.

However…

Assumption 1: That the resistive part of the generator impedance is deliberately lossy (eddy currents, hysteresis) so as to wind back current as speed increases.

Assumption 2: That the inductance of the quadrature part of the generator impedance will decrease slightly as frequency (speed) increases, but will increase slightly with the increase of current. This is what happens with a Type 59A telephone magneto-bell movement, anyway. The inductive reactance, of course, will increase with frequency (speed) and will negate the increase in current, becoming self-limiting.

First thing was to separate the inductive reactance from the a.c. resistance with ‘real’ current (as opposed to an impedance bridge). This was done with a series resistor, a 50Hz supply, an AVO 7 II for current and a Fluke 25 for voltage. The resistance and reactance were extrapolated using the Cosine rule.

At 50 Hz 600mA, Z = 2.94 + j3.53 (4.59 @ 50.2 degrees).

The next thing was to derive an equivalent circuit. Thevenising, open-circuit voltage (at 2700 RPM) was 8.68V and short-circuit current was 600mA. This gives an equivalent circuit, for 2700 RPM, of an 8.68V generator in series with an impedance of 14.47 Ohms Z.

To separate the in-phase and quadrature components, I made a further assumption: that the inductance stays reasonably constant at 11.24mH (from 50Hz measurements above). This equates to an equivalent circuit of an 8.68V generator in series with a 6.91 Ohm resistor and an inductor with a reactance of +j 12.712

Running the dynamo at 2700 RPM with my drill into a 50 Ohm Bercostat variable resistor load I measured the following values:
3.5 Ohm 540mA 1.89V
10.7 Ohm 415mA 4.5V
33.6 Ohm 210mA 7.03V
The currents, when calculated using the Thevenin-equivalent circuit were reasonably close, suggesting what I’m doing is not a million miles away from what’s actually happening…

3.5 Ohm gives a total impedance of 16.43 Ohms Z, at 8.68V gives 528.3mA, an error of -2.16%.

10.7 Ohm gives a total impedance of 21.718 Ohms Z, at 8.68V gives 399.7mA, an error of -3.7%.

33.6 Ohm gives a total impedance of 42.46 Ohms Z, at 8.68V gives 204.4mA, an error of -2.7%

I’ll do further experimentation when I get a suitable ‘power’ impedance extrapolator source set up*. I’d’ve thought that a reasonable assumption to make would be that the 3.3W 6V delivered to an optimum load of 10.9 Ohms would correspond to the impedance of the dynamo (generator) and that this 10.9 Ohms Z would be achieved when the resistance and the inductive reactance was the same so most power would be delivered, i.e: a phase-angle of 45 degrees.

This would give an impedance of 7.71 + j7.71 at a bike speed of about 14 MPH (22.4 km/H) – a dynamo speed of 6341 RPM, or a frequency of 422.7Hz.

The resistance looks about right but the inductive reactance looks to be a bit low. Maybe optimum phase-angle for output isn’t that important?

*Did this the following day... Assumption 2 is indeed correct. I drove a constant 600mA through the dynamo (generator) from 100 Hz to 600 Hz and the inductance varied from just below 7mH at 100 Hz to about 5.3mH at 600 Hz. Resistance (at a.c.) increases over this range from 3.5 Ohm to 12 Ohm as eddy current and hysteresis losses increase.

Note that these values are for a stationery generator with the magnet in place.

[Jon_G4MDC]

My hub dynamo acquired a full bridge of 1N4001s which charged a NiFe battery slung under the crossbar. It worked very well and ran a LW/MW/SW radio too.

[cmjones01]

Originally Posted by kalee20

Sorry, there's one moving part - the magnet! There has to be movement in a generator... anyone who can make a dynamo with no moving parts has invented perpetual motion!

Of course, there's got to be movement! Point taken. But what I like about the hub dynamo is that there are no moving parts above and beyond what the hub would normally have, so there's no additional complexity or unreliability introduced in order to generate electricity. I think that's very neat from an engineering perspective.

Chris

[G4_Pete]

I have included this as it links back to the recent discussion on the use of Variacs for soft starting circuits so I hope in that respect it is still on topic as it links ancient dynamo systems with modern circuitry.
Essentially the bicycle dynamo, really a generator, is soft start in voltage and frequency it is also normally grounded on one side as were all the available lighting systems. This poses several restrictions for running circuits off of a dynamo of this type.
The attached circuit Wireless World dec 1994 was a circuit I made that has the following features to overcome these issues.

The first part is a voltage doubler which solves the rectifying problem of maintaining the ground for the dynomo and the standard lights.
The second part keeps the switch mode circuit shut down until the dynamo is in a region where it can supply the current otherwise it will not start up due to the high current draw at low voltage of the switch mode. That is this circuit is not able to operate in soft start just like some of the circuits in the variac discussion.
The really fascinating part is that because the switch mode current draw goes down on increase in voltage and the efficiency of the dynamo goes up on lower loads the system flips into a high efficiency mode at a certain pedal speed where there is noticeably less drag on the wheel .

p.s ignore the top speed limit in the picture that was a magazine typo.
Pete

[Andrew B]

I have a SA Dynohub front hub here, I keep meaning to make it into a wind generator, rated output is 6W at 6v, so there is an amp*ish of MF AC to rectify and charge batteries, lights, USB adapters, the world is your oyster with 6 watts.

[Roger Ramjet]

Originally Posted by Andrew B

I have a SA Dynohub front hub here, I keep meaning to make it into a wind generator, rated output is 6W at 6v, so there is an amp*ish of MF AC to rectify and charge batteries, lights, USB adapters, the world is your oyster with 6 watts.

I like your wind generator idea Andrew.... takes me back to the one I saw in our village in the 60's. Plus of course & as you note, plenty of kit out now that is compatible.

Be ideal for going off grid.... if only my daughter would let me have a shed at the bottom of her garden !

Rog

[Nuvistor]

Originally Posted by Herald1360

Dynohubs did have the advantage of being more efficient (less rolling resistance) though less effective (less power output) than bottle types.

That’s was exactly my findings from use of both in the 1950’s.

[Brigham]

As a kid, I always wanted to find a way to power the dynohub!

[russell_w_b]

Originally Posted by Andrew B

’I have a SA Dynohub front hub here, I keep meaning to make it into a…’

Bicycle wheel mounted vertically on a horizontal hub with a vane and thin aluminium vanes fixed and aligned with the spokes?

[Jonster]

Originally Posted by Andrew B

I have a SA Dynohub front hub here, I keep meaning to make it into a wind generator, rated output is 6W at 6v, so there is an amp*ish of MF AC to rectify and charge batteries, lights, USB adapters, the world is your oyster with 6 watts.

I'm afraid the newer SA Dynohubs are only rated at 1.8W, not 6W. The earlier ones (pre 1970's) were slightly more at around 2W You'll be lucky to get more than around 300mA out of one.

[G6Tanuki]

Using bike dynamos [and vehicle alternators] with just a propeller mounted on the end is a fool's errand; these sorts of generating devices are all designed with bearings engineered to handle side-thrusts [from drive belts, the weight of the rider in the case of dynohubs, or the spring-pressure holding the dyno against the tyre for bottle-dynos] but a propeller-on-the-end and pointed into the wind generates end-thrust which the bearings are not designed to take.

The sort of electric-windmills sold to the boating community have proper end-thrust management. They're not cheap.

A friend in the US built a homemade windpower thing for his hilltop ham-radio repeater; it knackered the bearings in one winter because of the end-thrust. Half an inch of end-float and smashed-up slip-ring brushes....

[Andrew B]

The bearings on a dynohub are grease packed cup & cone, more akin to a taper roller, I would think they handle side thrust OK

Make it a vertical axis windmill they you wouldn't need to mess about getting power down from the generating head??

[Roger Ramjet]

Originally Posted by Andrew B

The bearings on a dynohub are grease packed cup & cone, more akin to a taper roller, I would think they handle side thrust OK

Agreed Andrew, plus of course they don't have slip rings as mentioned above ?

Rog

[GMB]

Our tandem was supplied with a bottle-type generator which lit a halogen light, but also stored energy in a super-capacitor to power an LED so the lights never go right out when stopped. The capacitor lasted about a minute I think (not used it for a long while).

When I was a kid I combined a battery-power light with the bottle generator - I forget the detail but it was hampered by the generator and lights using the frame as earth. I ended up with the effect that the light was good when stationary or going fast, less bright at intermediate speeds.

[G6Tanuki]

Originally Posted by GMB

When I was a kid I combined a battery-power light with the bottle generator - I forget the detail but it was hampered by the generator and lights using the frame as earth. I ended up with the effect that the light was good when stationary or going fast, less bright at intermediate speeds.

I did something vaguely similar using a couple of those square 4.5V Ever Ready batteries with screw terminals [126?] a Germanium diode and a Post Office type relay to change over between battery and dyno power.

I blew up a lot of bulbs in the process of experimentation. I also fitted a 'brake light' to my bike by way of a little bit of cut-from-a-baked-bean-tin metal fitted to one of the brake blocks and arranged so when you applied the brakes it provided a ground-path to the brake-light [which used a 6V car sidelamp bulb]

[Silicon]

Has anyone connected a loudspeaker to a dynohub?

It could be used as a bird scarer at seed planting time.

[LyntonP]

I collected a number of dynohubs during my not too distant cycling career. I built several into wheels for my bicycle collection. I remember buying a rear light which in addition to providing light would accept a charge in order to keep the light illuminated for a short period of time when the dynamo was stopped. Very handy at road junctions etc. The downfall of many a dynohub was taking it to pieces without a keeper for the magnets.
Lynton

[nicam49]

The downfall of many a dynohub was taking it to pieces without a keeper for the magnets.

I was going to ask about this! There is a warning on my dynohub to that effect which has deterred me from ever taking it apart to regrease it. Does anyone have advice what to use as a keeper?
Also, my dad used to work at The Raleigh and would sometimes bring home 'stuff' acquired during the working day, one of those things *was a new type of bottle dynamo set that I think was Japanese, and by gum it was good! It used to sing when going fast, and the headlamp had the dimpled bulbs and a white dipswitch mounted on the top, whereas the Miller lamps had a bottom mounted switch.
*mid 1960s, I'm thinking....

[LyntonP]

Sturmey Archer actually supplied a keeper as an accessory.
One hub I never had was the type that was combined with the 3 speed gear mechanism for the rear wheel. I bet that was fun to service!
Lynton

[Red to black]

Didn't have much to do with the dynohubs but I could certainly strip and rebuild SA three speed epicyclic hubs as used on the Raleigh Chopper and Grifter bikes from the '70s early '80s