The paper presents a completely analytical formula to determine the phase back emf of a slotted BLDC motor with interior or exterior rotor and parallel or radial permanent magnet (PM) magnetization and concentrated winding. The formula is based on integration of an analytical solution for the PM magnetic field in the motor air gap. Six combination of the number of slots and number of poles (Ns/Np) are considered: 6/4, 9/6, 6/8, 12/10, 9/12 and 12/14. The results are verified using a finite elements software. © 2007 IEEE.