Motor output power:

P_out=KB_av acD² LN_s

where,

P_out = Output power (W)
K = Constant
B_av = Average airgap flux density (T)
ac = Electrical loading (amp – turn/m)
D = Rotor diameter (m), L= Length of rotor (m)
N_s = Motor speed (rpm)

For the same output power, the size of the motor can be reduced by:

1. Increase in magnetics/magnetic loading (i.e. Increase in magnet B_r and hence the B_av)
2. Increase in motor speed (N_s)
3. Increase in winding current or no. of turns/coil (i.e. Increase in ac)

Increase In Magnetics/Magnetic Loading (B_r)

Reducing the size of the motor by simply reducing the armature/rotor is not viable, as this would limit the space for winding thus hinder ac (electrical loading). By increasing Br of a magnet, however, the magnet size can be reduced, enabling a smaller motor size while maintaining space for ac.

Factors to be considered while using magnet with higher Br:

• Possibility of magnetic saturation in armature/stator teeth, yoke and back iron/housing
• Possibility of increased core loss, as core loss is proportional to the square of flux density
• Higher no-load current