Basic Understanding of PMDC Motor

No-load speed is the speed at which the motor turns when running at nominal voltage and without a load.

Stall current  is how much the motor will draw when power is applied but it is not spinning.

A DC motor's speed is directly proportional to the input voltage. 

No-load speed and stall torque are proportional to the input voltage.

At high temperature, the stall torque (Ts) decreases, the no-load speed (No) increases, and stall current (Is) decreases.

At low temperature, the stall torque (Ts') increases , the no-load speed (No) decreases, and stall current (Is') increase.

Based on the PMDC motor equivalent circuit, backEMF should be zero, the rotor should be locked.

R = V / Is

Is : Stall current

A DC motor's torque/speed curve, also known as a performance curve, shows the tradeoff between torque and output shaft speed:

• Torque: Usually represented on the horizontal-axis

• Speed: Usually represented on the vertical-axis 

The curve slopes downward to the right, with speed at its highest when there is no load. As the load increases, speed decreases until it reaches zero at maximum torque. The curve can be approximated by connecting two points:

Stall torque

The point where torque is at its maximum but the shaft is not rotating. This is indicated by the intersection of the speed line and the horizontal-axis.

No load speed

The maximum output speed of the motor when no torque is applied to the output shaft. 

The curve shows that torque is inversely proportional to output shaft speed. It's important to ensure that the force required from the motor is much less than its stall torque capacity, otherwise it may not work properly and could be damaged. 

Other information that can be found on a DC motor's datasheet includes:

Power curve

Shows the motor's mechanical power output, which peaks at exactly half of the stall torque

Efficiency curve

Shows how efficiently the motor converts electrical power to mechanical power