My 28-turn motor is attached to a ducted fan. When I put 8.4 volts and 3000 mAh into it, it starts emitting an unpleasant odor. I was wondering if I should lower the voltage and/or current and what would happen?
Motors are mainly driven by current. The current produces a torque. Then, as the motor speeds up, it produces a back-EMF (EMF = voltage) that causes the current to decrease.
The motor also has a certain resistance. The resistance does not help to spin the motor. The resistance produces heat, which is equal to \$I^2R\$ - or current squared times resistance. The resistance limits the current even when the motor is stopped there is no back-EMF, but usually it limits it way too high.
When you connect a motor to a voltage source (such as a battery) a lot of current flows (because there is no back-EMF) which causes the motor to start turning. The motor speeds up, back-EMF increases and current decreases, until the back-EMF is only a bit lower than the battery voltage. At that point the current is quite low.
While the motor is speeding up it gets hot because of the high current. If used as designed, the current will decrease before the motor gets hot enough to cause any problems. If the voltage is too high or the motor is jammed so it can't spin, that might not happen and the motor can overheat.
We don't know what voltage your motor is designed for, but the most likely problem is that it's designed for less voltage than you connected to it. Or maybe it was already damaged by something else.
If the motor doesn't spin at all when you connect power, it's permanently damaged. Get a new one that you actually know the specs for.
Neglecting losses, applying a voltage to the motor makes it go at a certain speed, applying a load torque makes it draw a certain current from the power supply. Obviously if the load current the motor tries to draw is above what the supply can deliver, then the supply's output voltage will drop, which can cause confusion, in the ideal case assume the power supply can deliver any required current.
If you don't like a burning smell from your motor, then you should reduce the load torque, which will reduce the current the motor draws from the supply. The motor current is the principal component of any motor heating. You can reduce load torque by fitting a smaller load. If the smell is something else, then perhaps you should just get used to it.
If the load cannot be changed, and depends on the speed of the motor, which is the case with a fan, then you should reduce the voltage. This will reduce the speed, which reduces the load torque, which will reduce the current drawn.
If you limit the current from your power supply to less than the motor wants to draw at any instant, then you are effectively reducing the voltage, until the speed drops enough so the torque has dropped enough so the current the motor tries to draw is low enough to equal to your power supply limit. Which is to say, you can put 8.4v into your motor, but not 3A, it ends up drawing 3A.