Back-emf, Counter electromotive force, Back-emf constant

The back (or counter) electromotive force (emf) E is the voltage generated by a running motor that acts to counter the supplied voltage. In the simple case of a permanent-magnet DC motor, it is proportional to the rotational speed ω, and written as E = K_Eω. The proportionality coefficient K_E　is called the back (counter) emf constant. In a brushless motor, the back emf is an alternating voltage, which can be measured by a setup as shown below. The example here contains the fifth harmonic. Back-emf is often called induced voltage.