3W vs 2.4W 6V generators
05-21-09, 02:29 PM
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3W vs 2.4W 6V generators
So Sturmey Archer sells the X-FDD 6v dynamo in two varieties: 2.4w and 3w
Does anyone know what the difference is? The 2009-2010 catalog shows some graphs:
https://www.sturmey-archer.com/pdfs/S..._Catalogue.pdf
but it's hard to say what the difference is, if there's any difference at all.
Does anyone know what the difference is? The 2009-2010 catalog shows some graphs:
https://www.sturmey-archer.com/pdfs/S..._Catalogue.pdf
but it's hard to say what the difference is, if there's any difference at all.
05-21-09, 05:47 PM
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I mean, electrically speaking, aren't we're talking about a simple voltage source? If they both put out 6V and the loads were the same, then wouldn't they both be delivering the same wattage?
05-25-09, 06:28 AM
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The voltage is dependent on the load.
The wattage may well be dictated by the size of the windings.
A 2.4W hub may have less drag than a 3W hub.
The wattage may well be dictated by the size of the windings.
A 2.4W hub may have less drag than a 3W hub.
05-29-09, 07:05 AM
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An ideal voltage source would put out its rated voltage no matter what the load resistance. The bicycle "dynamo" is not like this. It has a high internal resistance (actually more of a reactance to be precise). This means the voltage will change if you change the load resistance. To prevent premature bulb burn out, you should not use the 3W dynamo with a 2.4W load. If you use a 2.4W dynamo with a 3W load the bulbs will last a long time but will be dim. Best is to match dynamo to load; that is why they offer you the choice.
Last edited by bikingbrit; 05-29-09 at 07:10 AM. Reason: correction
01-25-10, 10:14 PM
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A 6V 3W load is 12 ohms resistance. A 6V 2.4W load is 15 ohms resistance. Judging from the graphs, the 3W unit is designed to produce 6V at 15km/hr with a 12 ohm load while the 2.4W unit puts out 6V at 15km/hr with a 15 ohm load.
An ideal voltage source would put out its rated voltage no matter what the load resistance. The bicycle "dynamo" is not like this. It has a high internal resistance (actually more of a reactance to be precise). This means the voltage will change if you change the load resistance. To prevent premature bulb burn out, you should not use the 3W dynamo with a 2.4W load. If you use a 2.4W dynamo with a 3W load the bulbs will last a long time but will be dim. Best is to match dynamo to load; that is why they offer you the choice.
An ideal voltage source would put out its rated voltage no matter what the load resistance. The bicycle "dynamo" is not like this. It has a high internal resistance (actually more of a reactance to be precise). This means the voltage will change if you change the load resistance. To prevent premature bulb burn out, you should not use the 3W dynamo with a 2.4W load. If you use a 2.4W dynamo with a 3W load the bulbs will last a long time but will be dim. Best is to match dynamo to load; that is why they offer you the choice.
If you're getting an LED dynamo light, there's no reason to go with the 2.4 version, get the 3w version.
