Magnequench Technical Resource Website, a subsite of

Factors Affecting Size of Motor – Part 1

Motor output power:

Pout=KBav acD2 LNs


Pout = Output power (W)
K = Constant
Bav = Average airgap flux density (T)
ac = Electrical loading (amp – turn/m)
D = Rotor diameter (m), L= Length of rotor (m)
Ns = 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 Br and hence the Bav)
  2. Increase in motor speed (Ns)
  3. Increase in winding current or no. of turns/coil (i.e. Increase in ac)

Increase In Magnetics/Magnetic Loading (Br)

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