If you could measure the current draw of these motors at both voltages you may determine if they are 6 or 12 volt motors. I've checked the current draw of my 6 volt fan motor simply with the indicator of my switchable 6/12V battery charger.
As a rule of thumb (no need for scientific precision) doubling the voltage will result in twice the current and gives 4 times the power. Half the voltage will result in half the current and hence one quarter of the power. Power (Watts) is roughly calculated by voltage (Volts) times current draw (Amps).
Any excessivly high or low power number will indicate a wrong voltage. It is of course a bit difficult to determine which values might be reasonable and what is too high/low in regard to them. But if you get a good airflow with about 30-40 watts, then 100 watts will be quite obvious way too much.
My 6 volt motor was going (the term running just didn't apply then) with 8 amps when I took it off the truck. After cleaning and greasing it was running with less than 5 amps. This would give power values of 48 watts before cleaning and about 30 Watts after cleaning/greasing. (All readings with the fan cage as load).
I accidently applied 12 volts one time (cat had tapped the switch) and it read over 8 amps, hence 12V times 8.x amps resulting in about or over 100 watts, more than 3 times the power it was rated for. The stronger air flow in my opinion didn't justify this raise in power.
I am sure it would not have kept up with this abuse for a long time. There are two main problems which will over time lead to failure:
- The first one is that the windings will at some time overheat and burn off the insulation and thus short out the windings.
- The second may be the excessive current flow which will provoke more sparking at the brushes and the commutator which will in the long run damage the commutator and lead to excessive brush wear.
