I recently bought two 12 V DC motors from Amazon. After I received them today, I tried testing them. One of the two motors run fine but the other motor shows erratic behaviour. It gets very hot within 10-15 seconds of running, and then stops. It also doesn't run as fast as the other motor.
During testing I directly connected a 8.6 V LiPo battery to the motor and it had sparks coming out of it. I then soldered wires to the motor's contacts and added another cell to the pack making it 11.98 V. The sparking stopped, and the motor started working, but I noticed this behaviour. There's also very high resistance when I try to rotate the motor shaft by hand, while on the other motor, it turns very easily.
I'm not sure if I damaged my motor, or if it was already damaged when I received it. How can I determine the cause of the problem?