3. Decreasing the Slot/Pole/Phase Ratio (SPP)

The Slot/Pole/Phase (SPP) is a ratio specified by motor design decisions that determines the relationship between the number of stator slots as compared to the number of rotor magnet poles. As the SPP is reduced the overall motor gains more power, per the allotted space given. Since often more power is not required, it is best to consider cost and reduce the motor size until the exactly specified power is reached. This approach is well known and the use of reducing SPP as an advantage applies to both the iron/copper stator as well as the permanent magnet rotor. Motor size is a good indicator of motor cost; therefore, low SPP designs are lower cost than equivalent high SPP designs. This has led many designers to seek lower SPP solutions.

Notice that the portion of space for the windings (not shown) is much higher on the left, due to the thinner yoke on the radius.  The yoke portion does not directly contribute to motor torque and is best minimized. Also on the left note the thinner red-blue magnets on the rotor.

A negative faced by the industry is that lower SPP results in higher operating frequency which is can be a setback in achieving high efficiency. The industry trend is, however, toward SPP 0.5 and lower wherever possible.