I would say (not knowing anything about your particular motor) that you exceeded the amperage rating for the step-down converter. When you convert 12 volts back to 6 volts, you have to "waste" some energy that wants to be sent through the wires. It gets "wasted" as heat, plain and simple. So, you can forget using the converter you bought. You will need one capable of handling twice the amperage your motor was designed to use (for the sake of safety). In other words, if your motor normally uses 10 amps, you'll need a converter capable of handling 20 amps. That one will have a finned aluminum heat sink on it. It might look like this one:
https://www.amazon.com/Areyourshop-...;psc=1&smid=A38Z6HY60XAMH7&gQT=1 If on the other hand you bought one of the converters some people use for a fuel gauge, that one is only rated for 1 or 2 amps and that is why it is smoking.
Okay, so much for converters. The best of all worlds is you get a 12 volt fan (two or three speed is going to be the most pleasant to use) and a simple 3 or 4 position switch. Please go look in the Freebies section. You can see an honest-to-goodness American made heavy duty 12v three speed motor that is being offered for free (aside from cost of shipping which USPS Priority will be around $16).
I hope this helps. Your fan motor is currently a 6v motor and the only way to make it work safely on a 12v system is with the proper converter.
