Motor coil

Dear every body, I have a permanent magnet DC motor 12Vdc, the motor has 3 wires!!. I wondered why
there is 3 wires instead of the usual 2 wires. So I opened the motor body and found that:

2 wires are connected to the two brushes.

the third wire is connected to a coil of about 100 turns and has a diameter less than the
motor diameter and placed in the same horizontal plane of the brushes, and the second end
of the coil is connected to one brush..



Now, what is the acyual function of this coil?????

I wait for your help..

Thanx a lot

Shendy

I could be wrong... but I have seen this with CPU fans. It is a "sense" cable to know how fast the motor is going. PC mainboards use it to check the fan is not failing, or going to slowly.