PMSM - Sizing

Motor Sizing

For high torque motor its better to use higher diameter which gives more room for magnet around the rotor

As motor dimension is fixed, air gap flux density and ampere turns are responsible for maximising the torque output

Low cost motor airgap shear stress varies from 0.5 to 2

High cost motor airgap shear stress varies from 1.5 to 3

Very high-performance motor airgap shear stress varies from 2 to 10

Large liquid cooled machines airgap shear stress varies from 10 to 20

Fundamental Implication

Power is directly proportional to torque

Volume, mass, inertia is few constrains we need to consider while designing

The volume of the motor increases with square of radius or diameter

The ratio of output power to volume cannot be increases by increasing motor diameter

Once diameter is chosen there are two ways to increase power developed. First is by increasing the operating speed. If speed is dictated by application, we need to use some form of speed reduction to increase torque

Other way to increase power output is by increasing magnetic and electrical loading.

Magnetic loading can be increased by using high performance magnet

Surface mount machines and Interior permanent magnet machines

Surface mount machines – low speed operation, less complexity

IPM used for three reasons

1. Flux concentration
2. Rotor structural strength increases – high speed
3. Drive over wide speed range – field weakening mode