Saliency Ratio

What makes a good coilgun? Some good clues can be taken from those who design rotary reluctance motors. Let's describe saliency ratio - a concept so basic and widely used in motor design that it is seldom defined.

Saliency Ratio

Salient pole motor The term "salient" refers to visible, or exposed, or sticking out. So a "salient pole motor" is one which exposes the N and S poles to the rotor.

"Saliency ratio" applies equally well to both permanent magnet synchronous motors (PMSMs) and switched reluctance motors (SRMs). And it applies to poles from a permanent magnet, or generated by an electromagnet.

"saliency ratio" = L_d / L_q

L_d = inductance in d-axis
L_q = inductance in q-axis

Direction of D and Q axis in a motor The d-axis is when the rotor is aligned with the poles. It is also the orientation with highest inductance.

The q-axis is when the rotor is aligned with the gaps. It has the lowest inductance.

It can be shown that reluctance motors work best when you maximize the saliency ratio.

In other words, a good figure of merit is to measure the maximum and minimum inductance in your drive coil. You can compare this coil's saliency ratio with another coil, and have a good estimate of their relative performance. And you don't even need to fire the coilgun!