Brushless Permanent Magnet Motor Design- P3: You've just picked up another book on motors. You've seen many others, but they all assume that you know more about motors than you do. Phrases such as armature reaction, slot leakage, fractional pitch, and skew factor are used with little or no introduction. You keep looking for a book that is written from a more basic, yet rigorous, perspective and you're hoping this is it. | Design Variations 115 the distribution factor McPherson and Laramore 1990 Nasar 1987 Liwschitz-Garik and Whipple 1961 _ sin Nspp6se 2 d Nspp sin flse 2 where _ 7iZV n se Ns 77 77 N N N sppiy ph iysm is the slot pitch in electrical radians. For Nspp 1 kd is equal to 1 as expected and for the case Nspp 2 Np 3 kd equals . Thus for this latter case the magnitude of the back emf is reduced to percent of what it would be if the same number of turns occupied just one slot per pole per phase. Despite the fact that applies only when the back emf is sinusoidal is commonly used to approximate the back emf amplitude reduction for other distributions as well. The underlying reason for using this approximation is that it is better to have some approximation and be conservative rather than have none at all. Pitch factor When Nspp is an integer the distance between the sides of a given coil . the coil pitch rc is equal to the magnet pole pitch tp as depicted in Fig. . However when Nspp has a fractional component as in Fig. the coil pitch is less than the pole pitch and the winding is said to be chorded or short-pitched. In this case the relationship between the coil pitch and the pole pitch is given by the coil-pole fraction rc int Nspp acP T N p spp where int - returns the integer part of its argument. As a result of this relationship the peak flux linked to the coil from the magnet is reduced simply because the net coil area exposed to the air gap flux density is reduced. The degree of reduction is given by the pitch factor kp which is the ratio of the peak flux linked when rc rp to that when tc tp. Because the peak flux linked determines the magnitude of the back emf through the BLv law the pitch factor gives the degree of back emf reduction due to chording. For the square wave flux density distribution considered in Chap. 4 the pitch factor is easily computed with the help of Fig. . When rc Tp the Aux linked to the winding is
