pic: Plywood 8WD Concept

[AdamHeard]

Originally Posted by s_forbes

This doesn't sound quite right... you see max current and torque at a stall condition (motor speed = 0 rad/s). Are there by chance any experiments that confirm this is the case, or theory to back it up? Where's Ether?

I would expect to see very little torque on a motor pinion if the robot is going full speed and full voltage in the opposite direction was applied. Although, I've been wrong many times before...

I design motors, it's right

I'll break it down a little more thoroughly. Regardless of how it got that way, if a motor is rotating at some speed, it will generate a voltage (the back EMF). So lets say you were going full speed one way, and then decide to go full voltage the other way. Right before the switch, the Back EMF opposes the battery voltage and you get a very small voltage differential to drive current (which is equivalent to the current required to generate the torque required to match the friction).

Let's say the battery is at 12V, and the back EMF is 11.8 (made up numbers), some current calculated by V=IR would flow.

When you switch the polarity, the 12V and 11.8V no longer oppose each other, you now have 23.8V across the motor. You once again find I from V=IR, and find that this I is ~ double the I that would be calculated at usual stall conditions. Since Torque is proportional to current, you will therefore see about double stall torque.

Another way to visualize this is to extend the classic torque/speed graph to include negative free speed, you'll see the torque there is 2x the stall torque.

Extending it past full speed positive shows that you need to apply external torque to get a motor going faster than free speed as well.

An important takeaway from all this is most FRC systems are capable of seeing 2x stall torque if they reach full speed one way, and negative full voltage is applied.