Thus the efficiency of DC motor depends on the back EMF (Eb) and the motor has higher efficiency if back emf is more.
Condition for Maximum Power
Thus the motor will deliver maximum power when the back EMF is equal to half of the applied armature voltage.
However, this condition can not be realized. If the back EMF is half of the applied armature voltage, the motor will draw more current than its rated current and efficiency of the motor will be below 50 percent.