Brightspark Magnetos

Affectionately known as the 'DB' (or dog's bollocks), the rig pictured above has evolved over a long time and enables us to conduct cold and hot running tests on magnetos, dynamos and magdynos. It has a main platform on which the machine under test can be mounted, directly or using various spacer plates or mounting brackets.

The DB rig has an annular gap. This is particularly good for prolonged high-speed testing of a magneto. We can fit rings with different internal diameters to vary the nominal voltage at which the gap discharges.

The annular gap can be used with a 'utility factor' circuit that lets us switch in various slightly-conductive loads in parallel with the annular gap to simulate sparking plugs with various degrees of fouling. (E.g. 4 microsiemens = 250,000 ohms.)

With a magneto for a single-cylinder engine, the output has a single polarity (positive or negative relative to ground/earth). Negative polarity is preferred with a single magneto, because the negative HT output goes to the central electrode of the sparking plug, which runs hotter than the earth electrode. Sparks jump from negative to positive. Due to the thermionic effect, a spark can jump more easily from a hotter electrode to a cooler electrode than the opposite way round. The polarity of the spark can be swapped if necessary in many ways, the simplest usually being to flip the polarity of the magnet.

With a magneto for a twin-cylinder engine, one HT output should be negative, and the other positive. However, if the insulating part of the slip ring track has become caked with carbon (e.g. from pick-up brushes that are too soft), it is possible, when the magneto is running on an engine, for a negative spark to go to the positive sparking plug and vice versa. This is because the HT current may find it easier to track around the caked-up carbon on the insulation of the slip ring and jump the gap of the sparking plug in the incorrect cylinder (which is on its exhaust stroke and therefore at relatively low pressure), rather than go direct to the gap of the sparking plug in the correct cylinder (which is on its compression stroke and therefore at relatively high pressure and more difficult to fire). The result is a misfire. Using the polarity indicators and the pair of 3-point test gaps on the DB rig, we can detect this, even at very high speed, by reducing the size of one of the test gaps, and seeing whether that then gets sparks of both polarities.

A strobe lets us freeze the view of the magneto at any position of the armature and at any speed. This is particularly useful for investigating problems with the contact breaker assembly, such as points that jitter as they begin to open (e.g. due to a sloppy pivot). The strobe controls let advance and retard the strobe flash point, and also adjust the balance between brightness and sharpness of the view.

(Click play button to view video clip.)

HT current

LT voltage

For dynamos, the DB rig has what we call a 'Dynalyser' unit. The Dynalyser has its own internal dynamo voltage regulators (supplied by Dynamo Regulators Ltd) and battery bank.

Various loads can be switched into circuit. A voltmeter can be switched to display the voltage of the dynamo output 'D', the field input 'F' or the regulator output 'A'. An ammeter can be switched to display the D, F, A, load or battery current. These are particularly useful for checking the operation of a dynamo regulator over all of its three phases (cut-out, full-field, and regulated) with various loads.

There are oscilloscope outputs so that the D, F or A voltage waveforms can be viewed. For detecting faulty sections in a dynamo armature winding, viewing the D voltage waveform on an oscilloscope is far simpler than using a device such as a 'growler.' By viewing the field voltage waveform, we can see how cleverly or idiotically various designs of regulator operate.

Dynamo D output voltage

Field  F input voltage during the regulating phase

The DB rig has an inbuilt 400 W heater, with circulating fan; a removable, transparent enclosure; and a thermometer. We can therefore conduct any of the above tests at elevated temperatures. We usually test at temperatures up to 50°C, which can be attained in about ten minutes, although it does take longer for the heat to soak through to the centre of the armature. Sometimes we go up to 70°C. Many faults with dynamos and, more particularly, with magnetos appear only when the machine is up to temperature. How many times have you heard the complaint, "It won't start when it's hot"? It is therefore very important for workshop tests to include hot testing.

Workshop

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