FPV Motor Specs Decoded: Stator Size, KV and Magnet Grade

Every week a distributor sends us a competitor's motor spec sheet and asks one version of the same question: "why is this one $9 and yours is $22?" Sometimes the answer is that we're expensive. Usually the answer is that the two motors share a stator dimension and almost nothing else — different magnet grade, different bearing, different copper fill, different balance standard. The four numbers on the box don't tell you which one you're buying.

This article decodes what those numbers actually mean, where the money hides in a brushless motor, and what a B2B buyer should put in an RFQ so that the sample you approve is the production you receive. It's written from the bench, not the brochure.

Reading the four numbers: 2306.5, 1750KV, N52, 32g

A motor name like 2306.5 is a stator dimension, not a size class. It means a 23mm stator diameter × 6.5mm stator height. The first two digits are diameter, the next two are height in millimetres. That's it — no power rating, no torque figure, no quality claim.

KV is the RPM per volt of applied DC bus voltage, measured unloaded. A 1750KV motor on a 6S pack (22.2V nominal) spins at roughly 38,850 RPM with no prop attached. N52 is a neodymium magnet grade (BH_max ≈ 52 MGOe), and 32g is the bare motor weight without wiring or connectors, or sometimes with — which is why weight claims are only comparable when the measuring convention is stated.

Those four numbers are genuinely useful. They're also insufficient to predict how a motor flies, and they say nothing about how long it lasts.

Stator volume is the real currency

Thrust comes from the stator's ability to convert electrical power into torque. Bigger stator cross-section and taller stack mean more magnetic material in the air gap and more copper to push current through. Approximating stator volume as a cylinder (πr²h) gives a comparison figure that tracks bench thrust far better than the marketing name does:

Two things follow. First, 2306.5 and 2207 are near neighbours — about 1.5% apart in volume. If a supplier tells you one is dramatically more powerful than the other, ask for the thrust stand data. What actually differs is the shape: a wider, shorter stator produces torque more smoothly and tends to run cooler at part throttle, while a narrower, taller stack favours peak RPM. That's a feel difference, not a power difference.

Second, volume comparisons only work inside the same cell count and prop class. A 2806.5 is not "better" than a 2207 — it's built for a heavier propeller turning slower, and on a 5" prop it will be heavier and less responsive.

KV is a gearing ratio, not a power rating

This is the single most misunderstood number in the category, and it causes real purchasing mistakes. KV describes how fast the motor tries to spin per volt. It does not describe torque, quality or maximum power. Higher KV with the same prop and battery means more current draw, more heat, and usually less efficiency — not "more power" in any useful sense.

Practical arithmetic: a 5" freestyle quad on 6S wants roughly 35,000–42,000 RPM at full throttle under load. Because a loaded prop pulls the motor down 25–35% from its unloaded figure, a 1750KV motor (38,850 RPM unloaded) lands in the right window on 6S with a 5" tri-blade. A 1960KV unit gives you roughly 43,500 RPM unloaded — better for racing and aggressive freestyle where you want top-end recovery, at the cost of about 8–12% more current at the same thrust. On 4S, the same 1960KV motor behaves like a mild 6S setup; the same 1750KV on 4S feels flat.

For OEM buyers sizing a BNF lineup, the rule we use is: pick the prop first, then the cell count, then let KV fall out of the target RPM. Choosing KV first and then hunting for a prop is how you end up with a product that overheats in customer hands and generates warranty claims you can't argue with.

Magnets: grade, temperature class, and the arc question

Neodymium magnets are where spec sheets get vague and margins get made. Three parameters matter:

• Grade (N42 / N48 / N52). Higher grade means higher residual flux density, so more torque per amp for the same geometry. The gap between N48 and N52 is a few percent of torque — measurable on a bench, invisible in a spec sheet that doesn't name the grade at all. Many budget motors are N42 and simply don't say so.
• Temperature class (no suffix / H / SH / UH). Standard N52 starts losing magnetic strength permanently around 80°C; SH grade pushes that to ~150°C, UH to ~180°C. A 5" freestyle motor on a hot day with a heavy prop can see bell temperatures well past 90°C. This is why a motor can feel strong when new and noticeably softer after twenty packs — irreversible demagnetisation, not bearing wear. For any product sold into hot climates or heavy-payload use, specify SH or better.
• Arc vs. block segments. Arc magnets are curved to match the bell radius, which keeps the air gap uniform and lets you run a thinner gap — worth real efficiency. Block magnets are cheaper to source and leave a variable gap. "N52 arc" on a spec sheet is a genuine cost signal; "N52" alone often isn't arc.

The verification method is unglamorous: a gauss meter at the bell inner surface, plus a magnet grade certificate from the magnet supplier. We keep both on file per batch, and we will send them with a sample order on request. Any factory that can't produce either is buying magnets on price alone.

The parts of a motor nobody puts on the box

Given identical stators and magnets, two motors can still differ by 2× in landed cost and 5× in field failure rate. The variables:

• Bearings. Japanese-brand (NMB, NSK) or quality Chinese equivalents versus unbranded. Bearing is the number-one warranty replacement item on FPV motors, and it's a $0.60–$2.00 delta per motor. Ask for the brand, not "high-quality bearings".
• Winding. Single-strand windings give higher copper fill and better heat path but need more skilled labour; multi-strand is cheaper and more forgiving. Copper fill factor differences of 10% show up as 5–8°C after a hard flight. Ask whether the winding is machine or hand, and what the fill target is.
• Bell material. 7075 aluminium resists bell deformation in crashes far better than 6061. On a freestyle product this is the difference between a bent bell and a written-off motor.
• Shaft. Titanium alloy (often hollow for weight) versus hardened steel. Weight and bend resistance both improve; cost goes up.
• Balancing. Dynamic balancing to a stated residual unbalance is what separates a smooth motor from one that makes your gyro work overtime. Specify a limit — we balance to a documented residual and reject over-limit units at the bench, the same way we treat the rest of the QC flow described in our factory QC article.
• Potting and stator coating. Epoxy on the windings and a proper stator coating are what keep a motor alive after a wet crash or a prop strike that nicks a winding.

What our bench actually shows

On our thrust stand, running our own Storm 2306.5 1750KV and Storm 2207 1960KV against a 5×4.3×3 tri-blade at 22.2V, the pattern is consistent and it's the pattern we'd advise any buyer to expect:

• Both motors land within about 3% of each other at 50% throttle — the region where most flying happens.
• The 1960KV pulls ahead at the top end by roughly 6–9%, and runs 8–12°C hotter after a 60-second continuous full-throttle hold.
• Efficiency at hover sits around 6–7 g/W for both; at full throttle both fall to roughly 4 g/W. Anyone quoting 10 g/W on a 5" freestyle setup is measuring a different prop or a different throttle point.

That's the honest picture, and it's why we don't market one of these as "more powerful". They're the same class of motor tuned for different pilots, and a distributor's catalogue should carry both for that reason rather than pushing the higher KV as an upgrade.

The five lines to put in an OEM motor RFQ

If you take one thing from this article, take this list. It converts a vague spec sheet into something a factory can be held to:

1. Stator dimensions and KV, plus target cell count and prop size — so the factory can sanity-check your choice instead of silently building it.
2. Magnet grade and temperature class (e.g. N52SH arc, 0.8mm air gap), with a grade certificate required per batch.
3. Bearing brand and size, and the replacement interval you expect in the field.
4. Weight with a stated measuring convention (bare motor vs. with 100mm leads and connector), tolerance ±1g.
5. Acceptance criteria: dynamic balance limit, no-load current window, KV tolerance (±3% is realistic; ±5% is loose), and a thrust/efficiency sample from the first production lot.

Add your own branding requirement — laser-marked bell, custom anodise, retail box — and you have a complete OEM brief. Our ODM/OEM programme runs from 50 pcs with the spec sheet locked before tooling, described in more detail on the factory page; if you're also comparing quotes at this stage, the pricing logic in our wholesale pricing guide explains why the cheap quote usually isn't the same motor.

Bottom line

The FPV motor category has converged on a handful of stator sizes because those sizes work. What's left to compete on is the boring stuff: magnet grade and temperature class, bearing brand, copper fill, bell alloy, balance standard, and whether the factory will put any of it in writing. A buyer who specifies those six things gets a product they can defend to their own customers. A buyer who orders "2306 1750KV, good quality" gets whatever the line was running that week.