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.
