FLASHING THE DYNAMO
From an article by Mike Sharp in the PWA7C Magazine with many thanks
The Austin Seven dynamo is classed as a ‘self-excited’ type. This means it doesn’t need current provided by the battery to start it working. In fact, it will even start generating without a battery being connected. There would be a problem if operated this way, however — the dynamo needs a battery as a ‘load’ to prevent its voltage rising well above its nominal six volts and thus blowing the field-coil fuse and doing other damage.
When a battery is connected (as it always should be) it acts as a ‘current weir’, allowing excess current to flow away by electrolysing water in the battery when the dynamo voltage rises above about 7.5 volts.
The Austin Seven dynamo relies upon residual magnetism within the iron of the field poles to start the generation process. Various members seem to have found the residual magnetism in their dynamos is below the critical level needed to start generating without outside help — despite mightily revving their engines, the ammeter stays stuck on zero. Some members find the current flowing into the dynamo from the ignition warning light is sufficient to start things off, and if they put in a lower wattage ignition bulb, or the bulb blows, then the dynamo won’t work.
The illustration shows how the small current for the ignition light flows to the dynamo field coils, even before the cut-out contacts close (via the thick wire of the cut-out coil and passing either via the resistor for summer or directly via the switch for winter charge but a large proportion of the current bypasses directly into the armature via the D+ brush!) Other Members find that even the current through the ignition warning light is insufficient to get things going and resort to the risky practice of manually closing the contacts of the cutout while the engine is revving. This sends a monster current surge to the dynamo from the battery, helping to re-magnetise the field poles. It’s risky because the cut-out armature may stick down with smoke soon billowing from the wiring! The ideal is to have a good level of permanent magnetism is the field poles. If your car is wired negative earth this is easily achieved by disconnecting the wire at the F terminal, taking a new wire from the battery positive terminal, and briefly touching the other end to the dynamo F terminal.
This process is known as ‘flashing’, but be careful not to touch the dynamo body or you’ll short the battery to earth and get more of a flash than you bargained for! It’s only necessary to make the connection to the F terminal for a fraction of a second but, as less than 3.5 amps will flow into the field coil, there’ll be no harm from even a more prolonged connection.
With a dynamo low on permanent magnetism, the degree of restoration depends upon the current passed through the field coil during the attempt to restore it. If the current is too low, full permanent magnetism won’t be restored. But 3.5 amps is found to be more than enough — about 3.5A flows when using a 6V battery for flashing. My experimentation shows that even one amp is enough to restore the maximum permanent magnetism the pole pieces can hold, and a higher current brings no additional benefit.
Should you suspect your dynamo problems are caused by low residual magnetism, one method of testing is to remove the dynamo from the car and to exclude the field coil from the current-generating loop. Then measure the voltage between the armature main brushes when rotated on ‘open circuit’, i.e. not connecting the D+ and F terminals together, and not connecting either terminal to a load. This requires clamping the dynamo on say a Workmate and driving it with an electric drill. It is best to use a modern drill with speed control and the ability to reverse. Ensure a positive drive with no slippage between the drill and the dynamo - use a socket spanner and extension piece on the nut that secures the straight-toothed gearwheel to the dynamo. Rotate clockwise as looked at from the driven end (i.e. in the direction indicated on the body of the dynamo). Lock the drill speed at a fixed rpm by a plastic cable-tie. About 1,000 rpm is fine, but the figure is not critical as long as you keep it constant between tests — so switch the drill on and off only at the plug, rather than fiddling with the tied speed trigger.
The voltage generated on open circuit is directly proportional to the strength of the magnetism and the rpm — for example if you double the rpm you double the voltage produced. By the way, if you convert to 12V using the existing dynamo, you need to double the revs before the cut-out pulls in and the battery starts to charge. As an example, a suspect dynamo was tested as above. The VOC (‘volts open circuit’) was 0.57 volts, as received. The field coil was then flashed with 0.41 amps, and the VOC, at the same rpm, had risen to 0.7V confirming that the original permanent magnetism was indeed down. After further flashing with 1.58A the VOC showed no improvement, remaining at O.7V. To be absolutely sure I zapped the field coil briefly with 7.3A (using a 12V battery) but the VOC hadn’t risen above 0.7 volts.
An alternative to removing the dynamo from the car is to test ‘in situ’. Disconnect the wires to the dynamo. Jack up the rear of the car so both rear wheels are free to rotate and run the engine in gear. You can set the engine revs using the throttle stop-screw on the carb, or using the hand-throttle, to give a convenient reading on the speedometer. Measure the VOC, ideally using a digital voltmeter, then flash the field coil using the car’s battery, and see if the VOC has increased. If it has, this confirms your dynamo’s permanent magnetism was less than optimum.
Your question may be: why has my dynamo less than optimum permanent magnetism - when it was generating OK last time out? Referring to the diagram you’ll appreciate there must be about seven volts at the dynamo-side of the cut-out’s fine-wire coil before it pulls in. This means that there must also be 7V pushing current through the field coil, either directly or via the resistor (when the charge switch is in the summer position). This current is more than enough to ensure optimum permanent magnetism. So why isn’t it still there when starting up some days later? The magnetism is supposedly not lost by standing, except if the magnet is knocked about or is subject to a reversal in the field current flow. Some members may have been surprised to find when they stripped down a working dynamo to give it a good clean, that it wouldn’t work upon reassembly. This is because much of the permanent magnetism is lost when the armature is removed.
For example, at a fixed speed (lower than that previously used) the ‘as received’ VOC was O.36V. When the armature was removed, then replaced, the VOC had fallen to 0.2 V. Flashing restored the VOC to 0.35V. So armature removal had reduced the permanent magnetism by 44%! So remember to flash after dismantling