NEWS FLASH - December 2013
We are now selling magneto bearings, insulators, oil seals and armature shims from our on-line shop.

Brightspark Magnetos

NEW ... Take a look at some of the equipment we use in our workshop for magneto servicing and overhauls.

 

 

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Our Coil Testers

  Known as 'Magalysers', our coil testers are loosely based on traditional instruments such as the Eisemann, but using a modern semiconductor switch, instead of motor-driven contact-breaker points with arc-suppressing condenser, and using modern instrumentation.

LT current

The Magalyser pumps suddenly-breaking pulses of current at mains frequency into the low-tension winding. The current at which the pulses break can be adjusted, and the break current is displayed on a meter.

 


HT voltage
The high-tension output of the windings is connected to a 3-point test gap, with the gap between the main electrodes typically set to 5.5 mm.  

The Magalyser has a detector circuit which gives an indication of the proportion of sparks that have recently been missed, and also a tell-tale indicator of whether even a single spark has been missed since the circuit was last manually reset. So, if we wanted, we could leave it running unattended for 24 hours, and then come back and see whether even a single one of the expected 4.32 million sparks had been missed.

  The Magalyser also has an earth probe, which can be touched on the bobbin body to test for earth leakage (if the windings are otherwise isolated from the bobbin body). The earth probe can also be scanned over the outer surface of the windings to check for flaws in the outer insulation.


(Please click the play button)

  We look primarily at the LT break current at which the windings just start to produce consistent sparks. That can vary for different makes and models of armature, and even for the same make and model, particularly if it has been rewound, because different rewinders use different 'winding recipes.' But from the required current at room temperature, we can start to get a feel for whether or not the windings and their insulation are good.  


(Please click the play button)

 

We test windings not only at room temperature, but also at elevated temperature (usually 50°C). With a good set of windings, the required break current always increases slightly with temperature, typically by up to 10% between 20 and 50°C. Good insulation maintains its dielectric strength over this sort of temperature range. However, if the insulation is faulty, the fault is usually very temperature dependent. Therefore, if the break current at 50°C is much more than 10% higher than the break current at 20°C, it is an indication that the insulation around the windings is breaking down, and that a rewind is required. In the example shown in the videos, the windings needed a break current of 1.7 A when cold, and 2.1 A when hot. That's 22% more when hot. In this case, we would probably advise the customer that the windings would probably work OK as is, but we could give no guarantees as to how long they would last. It would be the customer's choice whether or not to have a rewind.

 

These tests on the windings are, of course, combined with tests for continuity of the HT winding cold and hot using an ohm-meter. A lack of continuity indicates a break in the HT winding and the need for a rewind.

Although the majority of "it won't start hot" magnetos have a fault with the condenser, especially if it is an original Lucas paper condenser, or a later substitute paper condenser, a significant proportion of these problems are additionally or alternatively caused by winding insulation, pick-up(s) or a slip ring that goes leaky when hot.

 
  Footnote:  We do not test for breakdown between the windings and the bobbin body using an insulation tester operating at several kilovolts. That voltage is applied not only between the HT winding and the body, but also between the LT winding/earth tail and the body. Although it would be nice for the LT end to be able to withstand such a high voltage, there is absolutely no need for the dielectric strength of the insulation between the LT end of the windings and the bobbin body to be any more than a kilovolt. Applying several kilovolts risks initiating a breakdown of otherwise perfectly serviceable insulation. Our aim with our coil testers is to detect bad windings - not to destroy a customer's good windings.  
 

Workshop

Magneto/dynamo test rig
Coil Tester
Pick-up and slip ring tester
Magnetisers
Other Useful Tools

 
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