BLDC Motors, KV Ratings, and “Magic” Numbers Explained!

Welcome to the Mall of Aviation blog! If you are a student building your first quadcopter or a startup designing a new delivery drone, you have probably stared at a motor specification sheet and thought, “What on earth does 3115 mean?” or “Why do I need an ESC?”

Don’t worry. We are going to break down the tech jargon into plain English—no boring lectures—just the stuff you need to know to get flying.

1. The Mystery Numbers in BLDC Motors: 3115, 2805, 2207…

You see these numbers on almost every drone motor. It looks like a secret code, but it’s actually just a measurement.

The number is usually split into two parts: XX and YY.

• The first two digits (31) = Stator Width (diameter) in millimeters.

• The last two digits (15) = Stator Height in millimeters.

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• A 3115 motor has a stator that is 31mm wide and 15mm tall.

• A 2805 motor is 28mm wide and only 5mm tall (very flat!).

  

Why does this matter?

Think of the motor shape like a pizza vs. a burger.

• Wide & Flat (Pancake style, e.g., 5010): Great for torque. These spin big propellers slowly and efficiently. Perfect for camera drones that need to hover for a long time.

• Tall & Narrow (e.g., 2207): Great for speed. These can change RPM very quickly. Perfect for racing drones that need to zip around corners.

2. The KV Rating in BLDC Motors: 980KV vs. 1800KV

This is the most misunderstood part of a motor. KV does NOT stand for Kilo-Volts!

It stands for Constant Velocity (rpm per volt).

It tells you: “How fast will this motor spin for every 1 volt I feed it?”

• 980KV Motor: If you use a 10V battery, it spins at 9,800 RPM (980 x 10).

• 1800KV Motor: If you use a 10V battery, it spins at 18,000 RPM (1800 x 10).

The Golden Rule:

• High KV (1800KV+) = Spins very fast, but has less “muscle” (torque). It needs small propellers. (Think of a Ferrari: fast, but can’t tow a trailer).

• Low KV (300KV – 900KV) = Spins slower, but has tons of muscle. It swings huge propellers. (Think of a Tractor: slow, but pulls heavy loads).

  Start-up Tip: If you are building a heavy lift drone, go for Low KV and big props. If you are building a racing drone, go High KV and small props.

3. Does Higher KV Mean a Smaller Motor?

You asked a great question: “How does increasing the KV decrease the size of the motor?”

Actually, it’s a bit of a trick! Increasing KV doesn’t physically shrink the motor. However, there is a pattern you will notice:

Smaller motors usually have Higher KV.

Why? Because a tiny motor doesn’t have the strength to spin a giant propeller. Since it uses a small propeller, it needs to spin it really fast to generate thrust. So, manufacturers wind small motors to have High KV (2000KV+).

Big motors usually have Lower KV.

A giant motor has the strength to swing a massive 20-inch propeller. If you tried to spin that huge prop at 20,000 RPM, the motor would burn out or the prop would explode! So, big motors are wound for Low KV.

4. The “Binding” (Windings): The Science Inside

How do manufacturers decide if a motor is 900KV or 2000KV? It’s all about the copper wire windings inside.

• Fewer Turns of Thick Wire = Electricity flows easily –> Fast Speed –> High KV.

• More Turns of Thin Wire = More magnetic resistance –> More Torque but Slower Speed –> Low KV.

It’s a trade-off. You can’t have both super high speed and super high torque in the same size motor.

5. The Magic of 3-Phase: Why do we need an ESC?

Here is where students often get confused.

• Your Battery is DC (Direct Current). It has a Plus (+) and a Minus (–).

• Your BLDC Motor runs on 3-Phase AC (Alternating Current). It has 3 wires.

Every serious motor manufacturer provides this table (often called a Thrust Table or Bench Test Data) so you don’t have to guess. It helps you answer the two biggest questions in drone building:

1. “Will this lift my drone?”

2. “Will this burn my battery or ESC?”

Here is a breakdown of what each “magic number” in that table is actually telling you:

 Throttle [%]

• What it is: This is the position of your joystick.

• What it tells you: 30% means you are barely pushing the stick. 100% means you are going full speed (Full Throttle).

• Why it matters: You want your drone to hover (stay steady in the air) at around 40-50% throttle. If you need 80% throttle just to hover, your motor is too weak!

 Voltage [V]

• What it is: The pressure of electricity going into the motor.

• What it tells you: In this table, it says 24V. This confirms the test was done with a 6S LiPo battery (since 1 cell = ~4.0V average, so 6 x 4 = 24V).

• Why it matters: If you use a smaller battery (like 4S), all the thrust numbers will be lower.

 Current [A] (Amps)

• What it is: The “amount” of electricity flowing. Think of this like water flowing through a pipe.

• What it tells you: At 100% throttle, this motor pulls 63.87 Amps.

• Why it matters: This number decides which ESC you need.

  • Since the motor pulls ~64A at max, you need an ESC that can handle at least 70A or 80A. If you use a cheap 40A ESC, this motor will melt it instantly at full speed!

Input Power [W] (Watts)

• What it is: The total energy being used. (Calculated as Voltage $\times$ Amps).

• What it tells you: How “hungry” the motor is.

• Why it matters: It helps you estimate battery life. Higher Watts = Battery drains faster.

RPM (Revolutions Per Minute)

• What it is: How fast the propeller is spinning.

• What it tells you: At full speed, this motor spins at 14,579 times per minute.

• Why it matters: It confirms the KV rating.

  • Math Check: 14,579 RPM / 24 Volts ~ 607 KV.

  • Wait! The image says “900KV”. Why the difference? Because under the heavy load of a propeller, motors slow down. 900KV is the speed with no propeller (free spinning). The table shows the real speed with a propeller attached.

Thrust [gf] (Grams-force)

• What it is: The lifting power. This is the most important column!

• What it tells you:

  • At 50% throttle, one motor lifts 1535g (1.5 kg).

  • At 100% throttle, one motor lifts 3884g (3.8 kg).

• Why it matters:

  • If you have a Quadcopter (4 motors), your total max lift is $3.8 \text{ kg} \times 4 = \mathbf{15.2 \text{ kg}}$.

  • Rule of Thumb: Your drone’s total weight should be half of the max thrust. So, with these motors, you can build a drone that weighs up to 7.5 kg safely.

Efficiency [gf/W] (Grams per Watt)

• What it is: The “Gas Mileage” of your motor.

• What it tells you: How many grams it lifts for every watt of power it eats.

  • Higher number (e.g., 5.4) = Very efficient (Cruising mode).

  • Lower number (e.g., 2.5) = Very wasteful (Racing/Turbo mode).

• Why it matters: If you want a long-range drone, pick a motor that has high efficiency numbers (above 4.0 or 5.0) at the throttle range where you plan to hover (usually 40-50%).

MAX Temperature [98°C]

• What it is: The danger zone.

• What it tells you: At full power, this motor gets HOT (boiling water is 100°C!).

• Why it matters: It warns you not to fly at 100% throttle for too long, or you might damage the magnets inside the motor.

 

Wait, how do we get AC from a DC battery?

That is the job of the ESC (Electronic Speed Controller).

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The ESC is basically a super-fast computer switch. It takes the straight DC power from your battery and chops it up. It fires electricity into the motor’s 3 wires in a specific sequence:

• Phase A fires… then Phase B… then Phase C…

This rapid firing creates a rotating magnetic field that drags the motor magnets around.

• To fly faster, the ESC switches the phases faster.

• To fly slower, the ESC switches the phases slower.

So, the ESC isn’t just a connector; it’s the brain that converts your battery’s simple power into the complex 3-phase rhythm the motor needs to dance!

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Ready to Build?

Understanding the numbers is the first step to building a drone that doesn’t just crash, but soars. Whether you need a beastly 3115 motor for lifting heavy cameras or a zippy 2207 for racing, we have got you covered.

** Check out India’s largest collection of Drone Motors here:**

👉 Shop Drone Motors at Mall of Aviation

** Need an ESC to match?**

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Happy Flying!

– The Mall of Aviation Team