I have got the motor shipped and to test it out I connected it directly to a 8V (motor is marked for 12V) battery pack that I have made by joining 2 Samsung ICR18650 LiPo batteries together (in series). However as soon as the contacts touched, there was sparking on the motor contacts (the place where you put the wires) and the motor didn't move.
Have I damaged my motor? (Please tell me no)
Or was the motor defective?
Ok. No, that wouldn't have damaged the motor. If it doesn't turn, those batteries are not providing enough current or the motor was damaged before you got it.
I test my 12v motors by connecting them to a car battery with the engine off. A charged car battery is a bit too much voltage for the motor's rating (13.5v), but a short run shouldn't hurt it.
SurferTim:
Ok. No, that wouldn't have damaged the motor. If it doesn't turn, those batteries are not providing enough current or the motor was damaged before you got it.
I test my 12v motors by connecting them to a car battery with the engine off. A charged car battery is a bit too much voltage for the motor's rating (13.5v), but a short run shouldn't hurt it.
Yes it did work, I added another battery in series making the total voltage 11.98V . I then again touched the contact and the motor did move, but the sparks were still there, I guess the solder on the contacts is shorting somewhere. I'll sold wires to those contacts and try again.
Its one of the reasons for the trend to brushless motors - less EMI, the only parts that can
wear out are the bearings... Its common practice to add a small value ceramic capacitor across
the motor terminals of a brushed motor to reduce RF interference from the arcing.
I'll go a step beyond MarkT and say it is almost mandatory to put an RC snubber on motor terminals, say .47mfd in series with 100 ohms--and don't stint on voltage or watt rating, say 100v capacitor for your 12v motor and at least 2 watt resistor.
Two things happen when disconnecting a dc motor:
First, it has inductance and when you interrupt the circuit this inductance will initially develop whatever voltage is necessary to keep the current flowing. This will last for a few armature time constants, say 20ms to 100ms time constant. The polarity of the pulse will be such as to keep the current flowing in the same direction it was. In other words what was the negative terminal will become positive, but this will last only, say 50ms to 200ms. On a non reversing drive it is common practice to connect a diode across the motor terminals, polarity to block power supply voltage instead of or in addition to the snubber.
Second, when disconnected, the motor becomes a generator generating the voltage at which the motor was operating (proportional to speed) and polarity same as the supply, ie what was the positive terminal stays the positive terminal. This voltage will slowly decay to 0 as the motor coasts to a stop.
Both these effects can cause sparks and within limits the motor is built for it.