My problem is the following: I had an unfortunate crash and one of the cables of a motor (the red cable, which I suppose that is the power cable) broke. The rest of my quadcopter is fine, I tested everything and it works.
Due to living in Spain, ordering a replacement motor from 3DR (the motor is the blue one from 3DR) would cost me 70$ thanks to shipment costs. What I thought was, could I solder the cable with tin and see if it works fine?
I haven't done it and I am asking for your opinions. Is doing this viable, will the motor receive proper amperage so that it goes more or less equal with the other 3 motors.
I also want to ask if it's possible to replace the motor with another different 850kv motor from EMAX.
Thanks in advance to anyone giving it's own experience, advice or thought about this.
Looks to me like you can just solder the wires together and insulate the splice with heat-shrink or electrical tape. You should have no problems with it if that is all that happened.
A problem sometimes - not always - is if the motor wires (usually on cheaper motors) are coated with insulation lacquer or such (the wires inside the motor are treated like that). If so, then that must be cleaned off chemically and/or manually or the solder won't work.
You can test for this if you cannot tell visually, simply twist the wires together and see if the motor runs (or check for continuity if you have a meter).
Other than that it is like any other electrical connection so if the joint is good there will be no effect on performance.
By the way, the three motor wires are all the same, there is no meaning to the colors - to reverse a motor, switch any two wires, it does not matter which ones.
Replacing a motor with one that does not match but has similar specs should not create a problem, so long as the ESC is happy with it. The flight controller does not know what is out there on the arm, it only knows the results of its commands -- there could be a hamster in a wheel out on an arm, it wouldn't matter if it does the job.
Remember to confirm rotation direction and change if needed as described above. I would compare temperature of the motor and ESC after a test flight to see that they are more or less the same as the others.
Follow this guide for proper soldering:
you can resolder easily you motor wire, based on the picture you posted.
1-strip about 5mm of the red silicon on each piece of wire
2-Tin each tip with solder : do not hesitate to put lots of solder on the tips
3-place the two 5 mm tips parallel to each other and overlapping (ask a second person to hold the two tips for you , so your own two hands remain free : one to hold the solder wire, the other to hold the solder iron)
4-Melt the pre-tinned tips with the solder iron. Once you see the solder melts , remove your solder iron and let it cool
5-wrap the solder with insulation tape
Most solder contains flux, if I'm not mistaken.