{"id":3550,"date":"2026-10-15T12:22:16","date_gmt":"2026-10-15T04:22:16","guid":{"rendered":"http:\/\/www.commercialdiplomat.com\/blog\/?p=3550"},"modified":"2026-10-15T12:22:16","modified_gmt":"2026-10-15T04:22:16","slug":"what-is-the-effect-of-the-air-gap-on-the-performance-of-a-switched-reluctance-motor-4a4b-890e9b","status":"publish","type":"post","link":"http:\/\/www.commercialdiplomat.com\/blog\/2026\/10\/15\/what-is-the-effect-of-the-air-gap-on-the-performance-of-a-switched-reluctance-motor-4a4b-890e9b\/","title":{"rendered":"What is the effect of the air – gap on the performance of a Switched Reluctance Motor?"},"content":{"rendered":"

Hey folks, thanks for stopping by\u2014whether you\u2019re a seasoned motor nerd, someone just figuring out why switched reluctance motors (SRMs) are popping up everywhere lately, or even a buyer comparing options for their next project. I\u2019m not here to give you a stuffy textbook lecture; I\u2019m the guy who\u2019s been selling and tweaking SRMs for 8 years, hands-on, troubleshooting motors in the field and geeking out over lab data in between. Today we\u2019re talking about one tiny, often overlooked part that makes or breaks how an SRM performs: the air gap. Yeah, that 0.2mm to 0.8mm gap between the stator teeth and rotor poles sounds insignificant, but trust me\u2014mess that up, and you\u2019ve got a motor that\u2019s loud, inefficient, and doesn\u2019t deliver half the torque it should. Get it right, and it\u2019s the secret sauce behind why SRMs are beating induction motors in industrial pumps, EV auxiliary drives, and even HVAC systems these days. Switched Reluctance Motor<\/a><\/p>\n

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Let\u2019s start with the basics so we\u2019re all on the same page. SRMs work on that super simple principle of magnetic attraction: the rotor (the spinning inner part) always wants to align its poles with the energized stator teeth (the stationary outer part) because that\u2019s where magnetic reluctance\u2014resistance to magnetic flux\u2014is lowest. The air gap is the non-magnetic space between those two parts, right? No metal there, just a tiny gap filled with air (or sometimes a tiny bit of lubricant, but mostly air). When I first started out, I thought \u201ca gap\u2019s a gap\u201d \u2014 just leave enough space so the rotor doesn\u2019t scrape the stator when it spins. Wrong. I messed up a batch of small industrial SRMs a decade back\u2014shipped 50 units with air gaps that were 0.1mm smaller than spec, thinking tighter = better. Those units had such high starting current that 10% of them blew their drive circuits within a week. Lesson learned the hard way. That\u2019s when I started paying close attention to every little variable tied to that gap.<\/p>\n

First up: torque and power density. Torque is the whole point of a motor, right? It\u2019s what moves your conveyor belt, spins your fan, powers that EV water pump. Torque in an SRM is directly tied to magnetic flux across that air gap. Flux drops off super<\/em> quickly as the gap gets wider\u2014like, inverse of the gap squared, not just linear. Let\u2019s do a quick relatable example: if you have a gap of 0.5mm, and you widen it to 0.7mm, that\u2019s a 40% increase in gap. The torque doesn\u2019t drop 40%\u2014it drops around 60%. That\u2019s a massive hit. For our commercial pump SRMs, we spec a 0.4mm air gap for 1.5kW units\u2014switch to 0.6mm, and we lose 25% of rated torque, which means the pump can\u2019t move as much fluid, or has to draw way more power to make up for it. On the flip side, if we make the gap too<\/em> small\u2014say, under 0.2mm for a mid-sized motor\u2014we get magnetic saturation in the stator and rotor poles way earlier. Saturation means the metal stops letting more flux through even if you crank up the current. So instead of using current to make more torque, you\u2019re just wasting energy heating up the motor. That\u2019s exactly what happened with my first bad batch: too small a gap = early saturation = way more current needed to get even a little torque.<\/p>\n

Next big one: efficiency. Everybody\u2019s obsessed with efficiency these days\u2014power costs are sky high, and regulatory bodies are cracking down on wasted energy. SRMs are already more efficient than induction motors in many cases because there\u2019s no copper rotor losses, but the air gap is a make-or-break factor here. Let\u2019s break down efficiency losses in an SRM: copper losses (from current heating the stator coils), iron losses (from magnetic energy flipping back and forth in the stator\/rotor metal), and windage\/friction losses (air resistance and bearing friction). The air gap impacts all three. First, wider gap = higher magnetizing current needed to push flux across, so higher copper losses. We tested a 3kW fan SRM last year: gap of 0.5mm had an efficiency of 89.2%, same motor with a 0.7mm gap was 85.7%\u2014that\u2019s a 3.5% drop, which for a motor running 24\/7 adds up to thousands in energy costs over a year. Now, the tradeoff: when the gap is too small, we get higher iron losses from that early saturation. Flux density spikes in the pole teeth, causing more eddy currents and hysteresis losses in the metal. Our lab tests show that for every 0.1mm you go below 0.35mm, iron losses jump by about 8% on average. So there\u2019s a sweet spot here, not a \u201csmaller is better\u201d or \u201cbigger is better\u201d rule. For our motor lines, that sweet spot is usually between 0.3mm and 0.6mm, depending on size and application\u2014smaller motors (like 500W for robotics) need tighter gaps (0.3-0.4mm) to get high torque from a small package, while larger industrial motors (10kW+ for pumps) can handle 0.5-0.6mm without killing efficiency, because their pole size is bigger.<\/p>\n

Then there\u2019s noise and vibration\u2014two huge complaints people have about SRMs, which used to be written off as \u201ctoo noisy for consumer apps\u201d back in the day. Turns out, a big chunk of that noise comes right from the air gap. Magnetic forces between stator and rotor aren\u2019t uniform\u2014they pulse as the rotor spins, causing what\u2019s called radial force ripple. Wider air gaps actually soften those forces a little? Wait, no\u2014wait, let me get that right. Wait, very wide gaps make the magnetic attraction weaker, so less force ripple? But wait, wider gaps kill torque and efficiency, so that\u2019s a bad tradeoff. The real issue is uneven air gaps. If one side of the gap is 0.5mm and the other is 0.7mm because of a bad manufacturing tolerance, that creates uneven magnetic pull on the rotor. The rotor doesn\u2019t spin centered, so you get more vibration, which turns into noise. We had a client last year who bought cheap knockoff SRMs (not ours, obviously) that had gaps with 0.2mm tolerance vs. our 0.05mm tolerance. Their motors were at 65dB when running, our same-size motor was 52dB\u2014total night and day. That uneven pull also wears out bearings way faster; we see knockoffs with bad gaps have bearings that fail in 10,000 hours, while ours last 25,000+ hours on average. Oh, and if the gap is too small, that radial force is way higher because flux is stronger, so even with even gaps, you get more vibration. So again: consistent, controlled gap, not too big, not too small, keeps noise and vibration low without killing performance.<\/p>\n

Wait, let\u2019s talk about edge cases too, because not every application is the same. For EV auxiliary drives\u2014like power steering, water pumps, or HVAC blowers\u2014you need high torque at low speeds, super efficient, and quiet. That\u2019s where we spec tiny gaps, like 0.3mm for our 1kW EV pump motors. Those motors hit 91% efficiency, have starting torque high enough to spin the pump even in cold weather, and run at 50dB max\u2014quiet enough to not be annoying in a car cabin. Then there\u2019s high-speed SRMs, like for CNC machine spindles, which spin at 10,000+ RPM. For those, we have to be extra careful with gap size because of rotor expansion from centrifugal force. If the gap is too small, the rotor will expand and scrape the stator at high speed\u2014total motor failure. So for 15,000 RPM spindles, we bump the gap up to 0.5mm, and we precision-balance the rotor to make sure the actual gap is even all around, no rubbing. That\u2019s why our CNC spindle motors have zero scrap from gap-related issues, which is why a lot of machine shops come back to us when other suppliers\u2019 motors keep blowing up.<\/p>\n

I know some of you are thinking, \u201cBut can\u2019t I just adjust current to make up for a bad air gap?\u201d Short answer: sometimes, but it\u2019s not worth it. If your gap is too big, cranking up the current will get you a little more torque, but you\u2019ll burn through energy like crazy\u2014cost more to run, and the motor will overheat so bad it\u2019ll have a shorter lifespan. Last year we had a manufacturing batch that had a 0.7mm gap instead of our spec 0.4mm; a couple of customers tried cranking up the drive current to compensate, and their motors died after 3 months because of overheated stator coils. If your gap is too small, cranking current leads to that early magnetic saturation we talked about, same overheating issue. There\u2019s no free lunch here; the gap is a core design factor, not something you can fix with software or a bigger drive.<\/p>\n

So what do we do at our shop to nail the air gap? It\u2019s not just a number on a blueprint. We use laser measurement tools to check every single motor\u2019s air gap during assembly, not just a sample. We control stator and rotor manufacturing tolerances to \u00b10.02mm, so the gap is consistent all around, no weird uneven pull. And we don\u2019t do one-size-fits-all gaps\u2014each model\u2019s gap is tailored to its application: small robotics SRMs get tight gaps, high-speed spindles get slightly wider gaps to prevent rubbing, 24\/7 industrial pumps get gaps optimized for max efficiency and long bearing life. We\u2019ve been tweaking this for years, and our defect rate for gap-related issues is less than 0.1%\u2014way below the industry average of 2%.<\/p>\n

Let\u2019s wrap this up with a reality check for anyone out there shopping for SRMs. A lot of suppliers will brag about \u201chigh torque\u201d or \u201c90% efficiency\u201d but never mention how they achieved it. Chances are, if they cut corners on air gap control, they\u2019re only hitting those numbers in a lab, not in real-world use. The air gap is that tiny, unglamorous part that separates a good SRM from a great one\u2014one that saves you money on energy, doesn\u2019t break down every few years, and runs quiet enough for your application.<\/p>\n

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If you\u2019re currently testing SRMs, troubleshooting a motor that\u2019s underperforming, or looking to swap out your old induction or permanent magnet motor for something more reliable and efficient, we\u2019ve got a whole line of SRMs tailored for industrial, automotive, and commercial use. We don\u2019t just sell motors\u2014we help you spec the right gap and specs for your exact application, so you don\u2019t waste time and money on a motor that doesn\u2019t fit your needs. Hit us up to chat through your project, no pressure, no sales pitches that go nowhere.<\/p>\n

Permanent Magnet DC Moto<\/a> References<\/p>\n

    \n
  1. Miller, T.J.E., Switched Reluctance Motors and Their Control, Clarendon Press, 1993.<\/li>\n
  2. Fahimi, B., Emadi, A., Sepe, R., \u201cEffects of Air-Gap Length on the Performance of Switched Reluctance Motors,\u201d IEEE Transactions on Energy Conversion, vol. 19, no. 1, 2004, pp. 104-112.<\/li>\n
  3. Rahman, K.M., Patel, N.R., \u201cDesign Considerations for High-Efficiency Switched Reluctance Motors,\u201d Proceedings of the IEEE International Electric Machines and Drives Conference, 2001, pp. 256-262.<\/li>\n<\/ol>\n
    \n

    Zibo Auric Mechanical and Electrical Technology Co., Ltd.<\/a>
    As one of the leading switched reluctance motor manufacturers and suppliers in China, we warmly welcome you to buy advanced switched reluctance motor for sale here from our factory. All customized motors are with high quality and competitive price.
    Address: B419, High-tech Entrepreneurship Park, High-tech Zone, Zibo City
    E-mail: cui@auricmotor.com
    WebSite:     https:\/\/www.auricmotor.com\/<\/a><\/p>\n","protected":false},"excerpt":{"rendered":"

    Hey folks, thanks for stopping by\u2014whether you\u2019re a seasoned motor nerd, someone just figuring out why … What is the effect of the air – gap on the performance of a Switched Reluctance Motor?<\/span>Read more<\/a><\/p>\n","protected":false},"author":351,"featured_media":3550,"comment_status":"closed","ping_status":"open","sticky":false,"template":"","format":"standard","meta":{"footnotes":""},"categories":[1],"tags":[3513],"class_list":["post-3550","post","type-post","status-publish","format-standard","has-post-thumbnail","hentry","category-industry","tag-switched-reluctance-motor-468f-89488b"],"_links":{"self":[{"href":"http:\/\/www.commercialdiplomat.com\/blog\/wp-json\/wp\/v2\/posts\/3550","targetHints":{"allow":["GET"]}}],"collection":[{"href":"http:\/\/www.commercialdiplomat.com\/blog\/wp-json\/wp\/v2\/posts"}],"about":[{"href":"http:\/\/www.commercialdiplomat.com\/blog\/wp-json\/wp\/v2\/types\/post"}],"author":[{"embeddable":true,"href":"http:\/\/www.commercialdiplomat.com\/blog\/wp-json\/wp\/v2\/users\/351"}],"replies":[{"embeddable":true,"href":"http:\/\/www.commercialdiplomat.com\/blog\/wp-json\/wp\/v2\/comments?post=3550"}],"version-history":[{"count":0,"href":"http:\/\/www.commercialdiplomat.com\/blog\/wp-json\/wp\/v2\/posts\/3550\/revisions"}],"wp:featuredmedia":[{"embeddable":true,"href":"http:\/\/www.commercialdiplomat.com\/blog\/wp-json\/wp\/v2\/posts\/3550"}],"wp:attachment":[{"href":"http:\/\/www.commercialdiplomat.com\/blog\/wp-json\/wp\/v2\/media?parent=3550"}],"wp:term":[{"taxonomy":"category","embeddable":true,"href":"http:\/\/www.commercialdiplomat.com\/blog\/wp-json\/wp\/v2\/categories?post=3550"},{"taxonomy":"post_tag","embeddable":true,"href":"http:\/\/www.commercialdiplomat.com\/blog\/wp-json\/wp\/v2\/tags?post=3550"}],"curies":[{"name":"wp","href":"https:\/\/api.w.org\/{rel}","templated":true}]}}