What happens / why do motors stall?

[Tom Schindler]

Originally posted by SuperDanman
Thanks for all this, but a lot of it I got from the "Power, speed, and torque... AGH" thread. My question still stands - exactly why does the back EMF sink the input voltage? Is EMF a voltage that travels the opposite direction the incoming voltage travels, or is it just a force that reduces the pressure of the incoming voltage? If the latter, then I don't see the relationship between this force and the voltage. I basically need some help on the physics of the EMF.

The easiest way for me to think of it is as follows:

Current (I) = Voltage(V)/Resistance(R)

using this formual, we can get the following:

I = (Vbattery - Kv*Ang)/(Rbattery + Rmotor)

where:
I = current
Vbattery = battery voltage
Kv = motor's velocity constant - (rad/s)/V
Ang= motor's angular-velocity (rad/s)
Rbattery = battery's internal resistance + wiring or speed controller resistance
Rmotor = motor's armature resistance

from this, we can see that since the battery voltage will be 12V, once the RPM reaches a certain speed the top part of the equation, the (Vbattery - Kv*Ang) will be negative. This will make the current negative. the sign of the current tells which way the current is flowing. If the current is negative it means the motor is generating more voltage than the battery, which causes the current flow to reverse direction and the battery will start to be charged, instead of drained.


Thats the only way i understand it, the EMF stuff was jibberish to me too

Tom