Generator

About the electrical generator of the Alco Firefly

The Alco Firefly was designed around repurposed components. The engine, generator, and boiler were based on preexisting products. This was a practical and expeditious use of established manufacturing resources.

In contrast, the contemporary MK814 steam-powered generator set was a ground-up design. Although the result was a more refined product, it likely slowed manufacturing of the MK814.

The generator of the Alco Firefly is a modified Air Ministry Type 46 (or Type 47) Rotary Transformer. (See this link for Air Ministry equipment.) This device, also called a dynamotor, genemotor, or motor-generator, is an electromechanical transformer with direct current input and output. It combines an electric motor and a generator. A low voltage DC current is supplied to the motor armature, rotating the shaft. The generator armature co-wound on the same shaft produces a higher DC voltage and lower current output.

These devices were intended for radio equipment and supplied the high voltages required by vacuum tubes. The Air Ministry Type 46 (or Type 47) Rotary Transformer was used aboard aircraft, possibly the Avro Lancaster heavy bomber.

The 20 Watt-rated Type 46 converts 18 Volts, 3.13 Amps to 450 Volts, 0.05 Amp. The 20 Watt-rated Type 47 converts 9 Volts, 6.26 amps to 450 Volts, 0.05 amps. They differ with respect to low voltage side armature turns.

An Air Ministry Type 46 Rotary Transformer (Sold on eBay and archived by picclick.co.uk)

Another Type 46 is featured at the Museum of Transport and Technology in New Zealand.

An Air Ministry Type 47 Rotary Transformer in Assembly (Sold on eBay and archived by Worthpoint)

Alco Firefly Generator

Modifications were made to the rotary transformer for generator use. The brushes of the high voltage generator side were removed and terminals were replaced with circular sheet metal covers. The low voltage motor side became a 7V generator when the shaft is driven at 1950 RPM. Lastly, the nameplate was changed redesignating the modified device “Battery Charger No. 1”. Based on the serial numbers witnessed on the nameplate of the generator, it was used not just for the Alco Firefly, and was an off-the-shelf component. If anyone knows where else Battery Charger No. 1 generators were used, please write us.

The Type 46 (or Type 47) Rotary Transformer was used as the basis of the generator in the Alco Firefly rather than one of the other types of rotary transformers in existence because its shaft extended from the high voltage side, allowing for a brass drive coupler to be attached. This was unusual. Typically the shaft was flush to the ends of a rotary transformer, because it did not drive anything external. The Type 46 and Type 47 allowed for an accessory like a fan to be attached.

The functionally critical distance from the base to the center of the shaft on the generator was most likely matched on the engine by controlling the height of the redesigned sump on the Stuart Sirius engine. The rag joint couplers took care of slight mismatch.

Other Generators

“Battery Charger No. 2” is a hand crank generator also designed for recharging the battery that powers the B2 radio.

Towards the end of World War II, the British developed a pedal-powered generator, but its naming did not follow the No. 1 and No. 2 Battery Chargers.