system
Given the following post, respond as if you're commenting on Reddit:
human
Why do electric motors have maximum torque at low rpms, while combustion engines have to be at high rpms to get maximum torque?

I know that combustion engines get their torque from piston firings, and at low rpms, there simply isn't enough firings to get any torque. Why are electric motors able to have torque at very low rpms?
gpt
Not all electric motors have maximum torque at low RPM's.  Some AC induction motors don't, for example.  But let's assume we are talking about a permanent magnet DC motor.  

In a DC motor, when the armature rotates it causes a back-EMF (coils in the armature rotating in the magnetic field of the field magnets causes the back-EMF).  Essentially this just means that a voltage is generated that is opposite in polarity to the supply voltage.  This voltage is proportional to speed, so at high speeds the voltage is big and at low speeds, the voltage is small.  

What this means is that when you are at your highest speed your voltage from your back-EMF is going to be almost as big as your supply voltage.  Because the 2 voltages are opposite in sign this means the effective voltage across the coils in the motor's armature is going to be the difference between the two and that means it will be small.  And because the effective voltage is small, this means that the current in the armature is going to be small (I = V/R).  And because torque in a DC motor is directly proportional to current in the armature, the torque is also going to be small.  

Now, as the speed decreases, the voltage due to the back-EMF is going to decrease and the effective voltage across the armature coil is going to increase.  This means the current in the armature will increase and the torque will increase.  Therefore, the lower the speed the higher the torque.