One of the motors on our robot is brand new and has been working fine in the last week but when we tested our robot today it wouldn’t turn properly, it appeard to be staling and at first we thought it was an issue with the ports or motor controller however upon further inspection we realized that the motor was really stiff. If you put an axle with a gear in it and tried to turn it it would barley move. This is the first time we have encountered a problem like this. Does anyone have experience with this problem?
If the motor is stiff to turn when not connected to anything then there is probably a short between the black and red wires.
If that is the case, don’t plug it back into your cortex because it may burn it up
Okay, thanks
To me, this sounds like something has gone wrong with the gears that are internal to the motor. There is a way to open up the motor and replace those little gears.
EDIT: Okay, I again tried this with a good motor but this time I made sure I had a very good conductor (a wire this time, not a screwdriver) going across the motor leads, and this time I could notice the difference. Very interesting!
Thanks for the suggestions. At first I did think it was a gear issue but I checked and it was a short, the red and black wires were damaged and touching at the point where they go into the motor housing.
This was news to me. Thanks to TriDragon for the tip! Now I need to figure out the physics behind this.
The physics is simple = turning the motor generates back-EMF (voltage), proportional to the turning speed. When properly shorted, this back-EMF is going only through the winding’s resistance, inducing current proportional, again, to the turning speed. Current equals torque, thus the faster you try to turn it, the harder would it fight back. If you try turning it really slow, no extra counter-torque (besides friction) would occur.
Very interesting. So the motor acts like a little generator this way. Any idea what happens when a motor controller is connected to it and it’s driven backwards like that? How does the motor controller handle that back EMF?
Sorry not to get back with you. In IQ if you spin the motor it will boot the processor 
How it is handled is up to the design, there are lots of ways to handle it including charging the batteries, wasting in heat or use it as a brake.
So a motor and a generator are the same thing. If I took 2 393 motors and plugged them into eachother spinning 1 motor would make the other spin.
DC motors can do both energy conversions, or if you prefer the mathematical definition, a motor is its own inverse function. (my favorite robotics fact)
kinetic -> electrical
electrical - > kinetic
(in the real world they are optimized slightly differently, an optimal generator is not an optimal motor etc)
And the motor controller just passes the electricity back up to the cortex. Try spinning a motor quickly that is connected to a cortex without a battery, it will turn on.
Oh, that explains why the Cortex starts blinking when the robots roll around in the back of my van!
Any idea if enough current could be generated to damage the Cortex when motors are forced to move like that?
Yes, it can definitely damage the cortex or your motor controllers.
I wouldn’t recommend doing it. We always tell the kids to roll the robots slowly and definitely go slow on highly geared sub-systems like a lift. It all depends upon the protection circuitry that is in these components, but if you break down protection diodes in the FETs then they can short the power supply which will burn them up.
It’s a tough thing to teach the little kids in IQ not to play with the robots like this.
Oh, for IQ students, nothing teaches them better than having a motor replaced on the worlds stage seconds before the match…
For unpowered controller, the H-bridge FET body diodes would rectify the back-EMF into the power line,
possibly powering up the brain. For a brain with no battery connected and a really high speed (say 200RPM),
the generated voltage could get quite high (say 14V) with nothing (battery) to keep in in check. Go figure…
Interesting! Thank you for your insights.