Thursday, 18 July 2019

Bicycle Hub Dynamo maintanance - Project plan notes

 Changed; 19-08-2019 to 17-08-2019
Objective;
To discuss the electrical issue with some textbook motor and generator theory.  I do not have a dynamo to experiment with yet.  All so-called dynamos are really AC generators they have no brushes and they are identical to many electro-mechanical mains voltage timers motors such as used to be used in central heating timers.  Some such as a hybrid stepper motor which produces alternating current in a number of phases and at low speed.  This type of motor or generator is called a Permanent magnet synchronous motor or generator.

Graph - Current can be drawn up to the same limit irrespectively of the generator speed and then the 
voltage will drop sharply.
Small graph - With no load, the output voltage increases linearly proportionally to the speed of rotation.

Risk of the magnet demagnetising; 
The magnet will demagnetise if separated from the armature so if you remove the armature you need to replace it with a keeper such as another armature.  A magnet keeper is a piece of soft iron that conducts magnetism very efficiently.  Mu-metal that transformers,  motors, generators and the screen on old colour TV CRT use is ideal.  The metal should not be bent or hammered because this hardens the metal and reduces the metal's desired qualities although working the metal is necessary and should be done by a minimum amount and gently.

A dynamo hub bearings can be serviced without separating the magnetic parts the magnet and the winding so a keeper may not be required.  Although the amount of cleaning will be limited either way.

 Colour TV, CRT magnetic field screen made of mu-metal.

The Plan;
To cut a piece of mu-metal and carefully bend it into a radius.  In more detail like a C shape but more like a G shape with a small gap so that the metal can be shrunk to fit it then expand into the cavity where the armature was.  The G shape so that there is a tale that can be pulled on to unstick and remove the keeper.

The estimate of the dimensions;
Apparently, there are many different size hub dynamos.
  • BSA and others - many sizes I don't know. 
  • Sturmey-Archer
      • The picture on my PC gives OD=82mm, between centres of the bolt heads = 60mm.
      • The real OD = 111mm.  Therefore 111/82 = real spacing/60 302
      • Therefore the real inside diameter is; 81mm = (111 x 60) / 82 
      • The length of mu-metal required is the circumference; 255mm = pi.D.
*** It turns out the estimate from photos is wrong and a GH6 or GH8 hub distances between the bolts is 75mm. So the keeper will need to be about 70mm diameter. ***

The estimate of magnet inside diameter 63mm was wrong for the GH6 or GH8 the correct figure is 75mm.  That figure to scale  the figure for the GH12 to 96.5mm = 81mm * 75mm / 63mm. Therefore a length of metal; 302mm.
    • GH8 (1938-41),  90mm OD, 8V
    • GH6, 90mm OD and are also the same as the AG and FG gear hub dynamos.  6V.
      • Magnet bolts dia; 75mm.
      • The length of mu-metal required is the circumference;  235mm = pi.D.

The width of the metal looking at pictures the hub dynamo looks as if is less than half the width of the hub.  That is less than <25mm = 50mm / 2.

I understand that all post-world war two dyno hubs and gear dyno hubs are the same and parts interchangeable. The external appearance changed from time-to-time though.

Picture - Making the keeper by finding a glass jar of about the right 
diameter, wrapping it in a rage then bending the mu-metal around the jar.

Dynamo magnet keeper's;
I have some mu-metal and have cut a piece 30mm x 260mm x 0.5mm - it looks messy because I used kitchen scissors and it would be better cut with a guillotine to make a magnet keeper for a GH12 hub-dynamo. My plan is to carefully bend into a C shape radius. Actually more like a G so that there is a tail facing in to pull it out with.
I will cut more pieces for other old dynamo types but I need to know the internal circumference (diameter x pi) so if you tell me the diameters. Otherwise, if you have a way of cutting soft steel I shall bring the whole sheet to cut.  Then I can return home with what is left.
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Testing the dynamo;

 1. With the bike wheel off the ground so that it can be spun freely;
  • GH 12 - I believe the circuit is two 6V filament lamps in series rated at 150 mA.  Short one lamp to see if the other lamp illuminates then short the other lamp this will determine if there is a simple lamp failure.
  • GH 8  - I believe the circuit is two 4V filament lamps in series rated at 250 mA. If the circuit is the same as the GH 12 then the test is the same.
  • GH 6 - The circuit is two 6V filament lamps in parallel rated at 100 mA and 200 mA. If one lamp fails then the other lamp will fail so therefore turn the wheel slowly at first to see if there is some light from one or both lamps.
2. With the wheel removed from the bike and the bearings set properly;
  • Turn the shaft and notice the dynamo detent force - which feels springy sort of lumpy and is very light.  It does not matter which direction the wheel is rotated in the output is alternating current in any case.
  • Short the dynamo output with a wire.
  • Turn the shaft and notice the dynamo stronger magnet resistance force - which feels springy lumpy but is significant.
If the force was different as described the magnet is magnetised and the dynamo is good or the magnet is could be week.

If there was no difference then;
  • If the force is week then the winding may be open circuit or the magnet is demagnetised.
  • If the force is strong then the winding may be short circuit but the magnet is good. 
If there is doubt about the lamp and the dynamo circuits test the lamp circuit with a battery instead of the dynamo.  Of cause, you can test with a multimeter rather than traditional methods but a multimeter in its time like an AVO might cost more than a new bike in the 1930s.  The current from a resistance meter if connected to the dynamo will cause a small high voltage spark when you disconnect it (called back-EMF).
The second tool rematerialising the magnet;
This is the same process that happens with a computer hard disk.  A very high pulse of magnetism is applied to the hard iron material to magnetise it.  Such a pulse will also switch a magnets polarity north and south poles. This is done with the dynamo fully maintained and assembled then briefly connecting a high current from a voltage power source to the armature.  The risk is the high voltage could cause the armature winding if it is weak to break down and fail but in that case, the armature was probably going to fail anyway.

AL-0030-01A C1 is trickle charged from a high voltage.  When S1 is closed a pulse of increasing
current is connected to the dynamo winding represented by L1, 1H.  D1 prevents the circuit from
 oscillating (ringing) and thereby demagnetising the magnet. The voltage and component values 
are to be determined.  The Ground connection is really a ground but a reference point for simulation.

Allow the small detent force (the magnetism) to pull the armature to align the poles ready for the pulse of current to re-magnetise.  You can test the magnetism by turning the armature to feel the lumpy pull of the magnets to the armature sections.  This detent force is much bigger if you short circuit the armature and doing that will confirm that the winding has continuity and is not short circuit.
    In due cause, I will make a re-magnetising circuit using a high voltage trickle charge circuit with a capacitor and a diode and a switch. The diode prevents the circuit oscillating and demagnetising again. How it works is that it produces a high current pulse - standard physics stuff but I need to experiment to find out how high the current needs to be?
    The device will of-cause deliver a lethal voltage. This will turn into a blog page.

Simple alternative using a DC power supply and diode;

The high voltage discharge circuit above could be replaced by a DC power supply such as a car battery charger.  The charger is connected to the dynamo then in a different method, the dynamo shaft can move freely so that the magnet poles align when the pulse of magnetism is applied.

Next step is as before switched on for 1/10 second then off but 1 second would be okay.  Take care to switch off quickly so that the dynamo winding does not heat up.  Do not disconnect the dynamo whilst the power is switched on because a high voltage, called back-EMF, will be generated that will put the winding under stress unnecessarily.  See diagram below; 
AL-0030-01A The car battery charger has an internal rectifier diode so another diode is not required.
Connect up then switch on for 1/10th second to 1 second then turn off.  The setting of the low or 
high charge switch will make little difference other than to limit the armature heating.

What happens; The armature (generator winding) of the dynamo (generator) will move and the magnet poles and armature segments will align or re-align as the current and the magnetism increases. The field magnet normally moves but this is stationary and will be magnetised.

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