Video of how DC Motors work:
Video of DC Motor (click this link if video does not show below)
This video was taken in the Design Studio at the motor display (in front of the Lasercamm room).

Click below to view Animation of how DC Motors work:

Useful Definitions:

• No Load Speed: Speed of a motor (ω [rad/s]) with no load applied. This can also be described as the maximum speed of the motor.
  
• Stall Torque: The amount of torque (τ [Nm]) needed to fully stop the rotation of the motor.
• Power Output: The power output of a motor can be calculated as τ*ω = P [Nm*rad/s = Nm/s = J/s = W].
• Maximum Power Output: The power output of a DC motor is parabolically shaped and it's maximum is at half of the no load speed (which is also at half of the stall torque). The max power output is given by P_max = ½ *(τ_stall)*½ (ω_{no load}).
• Current Draw: As the motor rotates, it creates a back emf voltage that reduces the amount of current flowing through the motor coils. However, when the motor is stationary there is no back emf voltage and a large amount of current could flow through the motor. A designer needs to be aware that operating a motor during startup and during stall will attempt to draw a large amount of current from the power supply. If the current demands are too high, the motor could overheat or the power supply could provide less voltage than it is designed for. It is a good practice to measure the voltage across the motor leads if high current demands are a concern.
• Efficiency. The green efficiency curve shows in the plot above describes the efficiency of converting electrical power into mechanical power. For battery operated devices this efficiency can be important.

Measuring Motor Performance

 

Stall Torque Measurement	No Load Speed

 

For further details, see: Primer on DC Motors by David Gowden