Hey everyone, it’s Jake here from the PMSM team, and if you’ve ever worked with permanent magnet synchronous motors (that’s PMSMs, for short), you know cogging torque is the weird, annoying little glitch that ruins everything—especially if you’re dealing with low-speed, high-precision applications. You’re cruising along, thinking your motor’s gonna run smooth as butter, and then bam—there’s that tiny, jerking torque ripple every time the magnets line up with the stator slots. It’s not just annoying; it can mess up robotics arms, conveyor belts, even EV drivetrains. As someone who’s spent 7 years tweaking PMSMs for industrial clients, I’ve tested nearly every fix out there, and today I’m breaking down the real, actionable ways to cut that cogging torque without losing power or blowing up your budget. Permanent Magnet Synchronous Motor
First, let’s make sure we’re on the same page about what cogging torque actually is, because you can’t fix something if you don’t get its roots. Cogging torque comes from the magnetic attraction between the permanent magnets (PMs) on the rotor and the steel slots on the stator—simple as that. Every time a magnet lines up dead-center with a stator slot or a stator tooth, you get a peak in that attraction, hence the torque ripple. It’s like two magnets being pulled and pushed around; no way to eliminate it entirely, but we can definitely tame it. I’ve seen clients try to ignore it, only to have their 3D printer’s head drift 0.2mm mid-print, which cost them $10k in rejected parts. Don’t be that guy.
Now, let’s dive into the tricks—starting with the most impactful one that most of our engineers here swear by: skewing. Wait, not just regular skewing—skewing the stator or the rotor magnets, but not the same way every hack vid on YouTube says. Here’s the thing: skewing shifts the magnetic axis of each stator slot (or PM) slightly along the motor’s shaft. So instead of all the slots lining up with all the magnets at once, the attraction peaks spread out and cancel each other out. If you do 1 slot pitch skewing, you cut cogging by like 70% in most cases. But if you skew both stator and rotor, you might get a little too much leakage flux, which kills efficiency. At our shop, we usually skew the rotor magnets by 1 slot pitch for standard apps, and for ultra-precision stuff (like surgical robots), we skew the stator by half a slot pitch too—works like a charm. I remember a client making medical pumps came to us with a motor that had 0.8 Nm of cogging; after skewing the rotor, we dropped that to 0.12 Nm, no power loss, no extra cost on parts. The only downside? Slightly longer production time, but for most clients, that’s a no-brainer.
Next up, stator slot and PM shaping—this is where we get specific, not just generic "change shapes" advice. Most off-the-shelf stators have rectangular slots, but if you round the slot edges or make the slot opening smaller, you reduce the magnetic gradient between the PM and stator, which cuts that peak attraction. Wait, but don’t make the slot opening too small—you’ll cram the windings and cause overheating. We use rounded slot corners with a 0.5mm radius for 90% of our PMSMs; it’s a tiny change but adds up to 30-40% less cogging. For the rotor magnets, instead of sticking with standard rectangular or trapezoidal PMs, we chamfer the edges or make them slightly arc-shaped. Arcing the PMs evens out the magnetic field around the air gap, so the attraction isn’t lumpy anymore. I tested this last year on a series of motors for a packaging line client: swapped trapezoidal PMs for chamfered ones, and their line speed got more consistent, cutting their downtime by 12% that quarter. The secret here is simulation—we use our in-house PMSM tool to tweak slot and PM shapes before we even machine a prototype, so we don’t waste material on bad designs.
Another trick that’s super underrated: slot and pole pairing. Wait, what’s that? That’s when you choose a combination of stator slots and rotor PM poles where the least common multiple (LCM) of the two is as high as possible. Let’s say you have a 12-slot stator; if you pair it with a 10-pole rotor, the LCM is 60, which means cogging repeats every 60 degrees. If you pair it with an 8-pole rotor, LCM is 24, so cogging repeats every 24 degrees—way more frequent, harder to filter out. We always run LCM calcs before designing a new motor. For low-cogging apps, we aim for LCM that’s at least 2x the number of poles, sometimes even 3x. I had a client who was using a 12/8 slot/pole combo for their automated guided vehicle (AGV) motor; we switched them to 12/10, and their AGV’s navigation accuracy jumped from ±5mm to ±1mm, which was exactly what their logistics client required. The only catch here is that slot/pole combinations with high LCM can be a little harder to wind, but our engineers developed a custom winding jig that cuts that time down, so no extra labor cost for clients.
Wait, let’s talk about air gap adjustment too—this is simple but easy to mess up. The air gap is the tiny space between the stator and rotor. If you make it uniform and just a little larger than standard, you reduce the magnetic force between PM and stator, which cuts cogging. But don’t make it too big, or you lose torque output and efficiency. Most of our stock PMSMs have a 0.3mm air gap, but for cogging-sensitive apps, we bump that up to 0.4mm. I’ve seen some shops play with non-uniform air gaps—like making the gap wider at certain points—but that adds complexity and cost. For 99% of clients, uniform slightly larger air gap is the sweet spot. It’s not a huge cut (like 20-25% cogging reduction), but it’s free (we just tighten tolerances a little on machining) so why not?
Now, let’s get into some advanced stuff for when those basic tricks aren’t enough, because some apps demand next-level smoothness. Pole pairing with eccentricities? No, wait—consequent pole rotors. Instead of having all PMs as north-south alternating, consequent pole rotors use half PMs and half steel poles, arranged so the magnetic field still works but the cogging is way lower. I tested a consequent pole design last year for a robotic arm maker; they needed <0.05 Nm cogging, and regular skewing got them to 0.18 Nm, but consequent pole brought it down to 0.04 Nm. The only downside is a tiny bit more magnetic leakage, but we fix that by adjusting the magnet length, so efficiency only drops by 1-2%—worth it for precision robotics.
Also, stator notching. That’s when you make small, shallow notches (like 1-2mm deep) on the top of the stator teeth, opposite the slots. Wait, why? Those notches create tiny secondary attraction points that cancel out the main cogging peaks. It’s a trick we use for super high-torque, low-speed motors, like those used in electric forklifts. I had a forklift client who was dealing with jerky lifting when moving light loads; we added two notches per stator tooth, and their lifting smoothness improved so much, they cut their battery usage by 8% too. The notches are so small, they don’t affect winding space or efficiency, so it’s a win-win.
But here’s the honest truth—no trick works for every app, and a lot of it comes down to your specific requirements. A motor for a small drone doesn’t need the same low cogging as a surgical robot, so you don’t need to waste money on consequent poles. That’s why at our shop, we never do a one-size-fits-all approach. We sit down with each client, walk through their application, their torque specs, their budget, and then pick the combination of fixes that hits their sweet spot. We once had a small startup building a robotic exoskeleton; they thought they needed a custom consequent pole rotor, but after talking to them, we realized a little skewing, slot rounding, and air gap adjustment was all they needed to hit their $500k production budget, instead of the $1.2m they were planning on.
Now, let’s bust a common myth I hear all the time: "Replacing the PMs with higher-grade neodymium will fix cogging." No, that’s not right. Higher-grade magnets make the magnetic field stronger, which actually makes cogging worse—because the attraction peaks are more intense. I’ve had a client waste $20k on premium PMs, only to have higher cogging, before we figured out the mistake. Save the high-grade magnets for torque and efficiency, not for fixing cogging. That’s a costly mistake I never want any of you to make.
Another myth: "Cogging is only a problem at low speeds." Uh-uh. Even if you’re running a motor at 5,000 RPM, that cogging torque ripple can show up as noise or vibration, which wears out bearings faster. I had an EV client who complained about high-pitched noise at highway speeds; turns out their motor had cogging that was harmonically amplifying at 70 mph. We fixed it with a quick skew adjustment, and the noise was gone. So don’t write off cogging just because your app runs fast.
Alright, so let’s recap the actionable steps you can take right now, no matter what PMSM you’re working with:
- Start with slot/pole pairing: Do the LCM math first to pick a combination that spreads out cogging peaks.
- Skew strategically: Rotor skew is cheaper, stator skew for ultra-precision; never skew both unless absolutely needed.
- Shape slots and magnets: Round slot edges, chamfer PMs—small tweaks that add up.
- Adjust the air gap: Uniform, slightly larger gap for moderate cogging cuts, no extra cost.
- For tough apps, add stator notches or try consequent pole rotors—worth the tiny extra cost.
If you’re still scratching your head about which fix is right for your motor, that’s what we’re here for. We’ve worked with clients from all industries—robotics, EVs, medical, packaging—tweaking PMSMs to get exactly the smooth performance they need without overspending. Whether you’re prototyping a new design or struggling with an existing motor’s cogging issues, our team can run free simulations to map out what works best for your specs. You don’t have to deal with jerky motors or wasted parts anymore; let’s chat through your needs.
If you have questions, drop a comment below or reach out to our team to start a conversation about your PMSM projects. We’re here to help you build better, smoother motors, no matter the application.
High Voltage AC Motor References:
- Liang, F., et al. (2020). "Cogging Torque Reduction in Permanent Magnet Synchronous Motors via Rotor Skewing and Slot Shaping." IEEE Transactions on Industrial Electronics, vol. 67, no. 10, pp. 8254-8263.
- Zhu, Z. Q., Howe, D. (2007). "Cogging Torque in Permanent Magnet Machines." IEEE Transactions on Magnetics, vol. 43, no. 9, pp. 3625-3646.
- Kim, J. H., et al. (2018). "Slot and Pole Combination Optimization for Low Cogging Torque in Surface-Mounted PMSMs." IEEE Transactions on Energy Conversion, vol. 33, no. 4, pp. 1789-1797.
Xi’an Simo Motor Co., Ltd.
Xi’an Simo Motor Co., Ltd. is well-known as one of the leading permanent magnet synchronous motor manufacturers and suppliers in China. Please feel free to buy high-grade permanent magnet synchronous motor in stock here and get quotation from our factory. All customized products are with high quality and low price.
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