The 500mA must stay within USB spec (4.75v). The D1 can't do it at 50Hz. The Plug 3 can't do it at 250Hz. I can't say for other units.

Test with a 6V transformer and a high VA rating. 10ohm across the converter. Mains is 50Hz in Europe.

Use a pure sine wave rather than test results based upon a specific dynamo design.