# Switched reluctance motor inverter

I've been doing some research on switched reluctance motors, and the papers are always talking about driving them with inverters. Why would you use an inverter as opposed to just using a single switching element to turn the coils on and off?

• If you are saying that you've found something in papers, then please provide at least a couple of links or references to these papers. Commented Oct 1, 2014 at 0:25

The thing about SR machines is you need to decay the current in the coil fast at the point of commutation otherwise you will be generating torque that will attempt to impede rotation.

If you were to fire each phase of an SR machine with a simple chopper circuit then the current will freewheel via the anti parallel diode and decay slowly (relatively speaking) via the zero-volt-loop you establish.

Exaggerated decay. Now consider the other 3 phases being fired to facilitate torque generation.

There are periods where there would be torque (as current would still be flowing) that would reduce the shaft torque.

(NOTE: it can be done with a single switch per phase as a form of a chopper but you need a split supply with split caps and a higher voltage rail to pull the energy out of the coil)

If you were to use a single phase H-bridge per phase you now have the ability to not only create positive volt loop (to charge the coil), a zero volt loop (to maintain the current) but a negative voltage loop (to decay the coil fast).

If you then realise an SR machine doesn't care about current polarity when the coil is engaged (+ve current and -be current both setup flux that the rotor will react to) you then could remove 2 switches and 2 diodes from a classic single phase H-bridge.

Why is it called an inverter? Well the resultant waveform is AC

• That makes sense. Thank you.you said it could be done with a single switch per phase. How would you go about doing that? Commented Oct 1, 2014 at 22:26
• Its not really advisable and has alot of limitations (like rotor speed and current). The asymmetrical bridge is advisable. There is an n+1 topology which does work reasonably well BUT the sizing of that +1 needs to take into account almost continuous conduction & again has limitations as you cannot independently setup a negative volt loop
– user16222
Commented Oct 1, 2014 at 22:38

In this context "Inverter" just means a DC to AC converter. A specific implementation is not implied.

When talking about electric motors, people in the industry often refer to motor drives as "inverters". Generally these are AC motors (induction, brushless, PMSM, switched reluctance, etc.) and generally the drives convert DC to AC. AC in this case could mean sinusoid, square, or a modified square wave (eg, a "trapezoidal" shape used by BLDC motors).