# what does low reactance mean physically

What does "lower" reactance (higher susceptance) mean for a physical system, and when is it "good"? I see that it has less opposition to a change in voltage or current, but what does that mean? Lower resistance makes sense to me in that current can travel more easily, but what about reactance?

• more stored reactive energy – analogsystemsrf Mar 15 '19 at 2:34
• @analogsystemsrf, what is reactive energy? – Chu Mar 15 '19 at 8:35
• energy stored in capacitors, incuctor, or resonant systems. – Jasen Mar 15 '19 at 10:17
• @Jasen, That's just 'energy'. – Chu Mar 15 '19 at 11:29
• Reactance is the impedance presented by a capacitor or inductor. Lower reactance can mean either less inductance or more capacitance. Do you understand how inductors and capacitors work. If not you need to make this clear. – Andy aka Mar 15 '19 at 11:33

Lower resistance makes sense to me in that current can travel more easily

$$I = \frac {V_S} {R}$$

For an electric circuit, Ohm's Law states current is directly proportional to electro-motive force (voltage) and inversely proportional to resistance. If voltage is constant, less resistance will mean a greater current.

For a magnetic circuit, Magnetic Circuit Ohm's Law states flux is directly proportional to magneto-motive force and inversely proportional to reluctance. Similarily, if the magneto-motive force is constant, less reluctance will mean a greater magnetic flux.

$$\phi = \frac {\mathscr {F}} {\mathscr {R}}$$

As copper is a good conductor of current, carbon is a poor conductor. Carbon offers increased resistance.

Iron is a good conductor of flux, while air is a poor conductor. Air offers increased reluctance.

This drawing shows a reduced representation of a switched reluctance stepper motor. When the stator is magnetized by a current flowing in the stator coils, a N pole is formed on the right and a S pole on the left (Right Hand Rule - Conventional current).

The magneto-motive force is constant, $$\\mathscr {F} = NI\$$. N is number of turns on coil and I is current in coil.

Iron is a conductor of flux, so the magnetic field forms in the iron stator. The rotor is made of low reluctance iron, so it will conduct flux easily. But the air gap between the rotor and stator will have a higher reluctance than iron. It is also skewed between the stator pole and rotor pole, so the flux is concentrated/compressed (increased reluctance).

The magnetic field wants to be as small as possible (lowest reluctance), so the rotor is forced to rotate counter-clockwise. Air gap is minimized, decreasing reluctance. Lower net reluctance, so greater flux.

Not shown in the drawing, is other stator poles, which will be magnetized to continue this rotation.

But the rotation is due to the magnetic field wanting to be as small as possible.