FPV Motors

Motor KV

KV means motor speed constant. It describes the unloaded motor RPM per volt.

\[ RPM_{no-load} = KV \cdot V \]

Example:

• 1750KV on 6S at about 22.2V: \(1750 \cdot 22.2 \approx 38850 RPM\)
• 2450KV on 4S at about 14.8V: \(2450 \cdot 14.8 \approx 36260 RPM\)

This is no-load RPM. With a propeller installed, the real RPM is lower because the prop loads the motor.

High KV:

• More RPM per volt
• Sharper throttle response
• More current draw with the same prop
• More heat if the prop is too large or too high pitch
• Usually paired with lower voltage or smaller props

Low KV:

• Less RPM per volt
• More suitable for higher voltage
• Easier to run larger props
• Usually more efficient for long range or heavy builds
• Often feels smoother and less aggressive

KV, Torque, And Efficiency

KV alone can show the theoretical torque-per-amp trend. Lower KV gives more torque per amp, and higher KV gives less torque per amp.

\[ K_t \approx \frac{60}{2 \pi \cdot KV} \]

Where:

• KV is in RPM per volt.
• Kt is the motor torque constant in N m/A.

xychart-beta
    title "Motor KV vs Torque Per Amp"
    x-axis "Motor KV" [900, 1300, 1750, 2450, 3500, 5000, 8000]
    y-axis "Kt, torque constant (mN m/A)" 0 --> 12
    line "Theoretical Kt" [10.6, 7.3, 5.5, 3.9, 2.7, 1.9, 1.2]

This graph does not show motor efficiency. Efficiency needs the real working point:

flowchart LR
    A[Motor KV] --> E[Efficiency]
    B[Battery voltage] --> E
    C[Prop diameter and pitch] --> E
    D[Current and RPM] --> E
    E --> F[g/W or thrust per watt]

For a useful efficiency graph, use real thrust-test data:

• X-axis: throttle, RPM, or current
• Y-axis: efficiency in g/W
• Separate lines: different propellers on the same motor

Motor Parts

FPV drones use brushless outrunner motors. Outrunner means the outside bell rotates around the fixed stator.

Main parts:

• Shaft: The propeller mounts to the shaft.
• Bell: The outside rotating shell of the motor.
• Magnets: Fixed inside the bell and rotate with it.
• Stator: The fixed iron core inside the motor.
• Copper windings: Coils wrapped around the stator. The ESC energizes these coils to create a rotating magnetic field.
• Bearing: Supports the shaft and lets the bell spin smoothly.
• Base: The fixed part that bolts to the frame.
• Motor wires: Three phase wires from the ESC to the motor.

The ESC switches current through the three motor wires. This creates a changing magnetic field in the stator windings. The magnets in the bell follow that field, so the bell and shaft rotate.

Motor Number

FPV brushless motors are usually named by stator size and KV.

Example:

2207 1750KV

Meaning:

• 22: stator diameter in millimeters
• 07: stator height in millimeters
• 1750KV: unloaded RPM per volt

The stator is the metal part inside the motor that creates torque. Larger stator volume usually means more torque, but also more motor weight.

Measure only the stator, not the outside motor bell. The bell, magnets, shaft, and base make the complete motor larger than the number printed in the motor name.

Common examples:

Motor size	Meaning	Typical use
1103	11 mm diameter, 3 mm height	tiny whoop, micro builds
1404	14 mm diameter, 4 mm height	2.5-3.5 inch light builds
1804	18 mm diameter, 4 mm height	4 inch lightweight
2207	22 mm diameter, 7 mm height	5 inch freestyle and racing
2306	23 mm diameter, 6 mm height	5 inch freestyle
2806.5	28 mm diameter, 6.5 mm height	7 inch long range
3115	31 mm diameter, 15 mm height	heavy lift and large props

KV, Voltage, Prop, Current, And Thrust

The same motor can behave very differently with different voltage and propellers. Higher voltage, higher KV, larger diameter, higher pitch, and more blades all increase motor load.

The table below is a practical starting point. Current and thrust are approximate ranges because real values depend on the exact motor, prop model, battery sag, ESC timing, air density, and test stand.

Frame size	Motor example	Battery	Prop example	Typical current per motor	Approx thrust per motor	Notes
2 inch	1103 8000KV	2S	2x2.5x3	4-8 A	80-160 g	light micro quad
2.5 inch	1204 5000KV	3S	2.5x2.5x3	6-12 A	150-300 g	responsive micro freestyle
3 inch	1404 3500KV	4S	3x3x3	10-18 A	300-550 g	common 3 inch setup
3.5 inch	1604 2800KV	4S	3.5x2.8x3	12-22 A	450-750 g	efficient small freestyle
4 inch	1804 2450KV	4S	4x3x3	15-28 A	650-1000 g	light long range or freestyle
5 inch	2207 2450KV	4S	5x4.3x3	30-45 A	1200-1700 g	classic 4S freestyle
5 inch	2207 1750KV	6S	5x4.3x3	25-40 A	1300-1800 g	common 6S freestyle
6 inch	2507 1500KV	6S	6x4x3	30-50 A	1800-2600 g	heavier freestyle or cruiser
7 inch	2806.5 1300KV	6S	7x3.5x2	20-40 A	1800-2800 g	long range efficiency
10 inch	3115 900KV	6S	10x4.5x2	35-70 A	3500-6000 g	heavy lift or large cruiser

Warning

Do not size the ESC only from average current. Use full-throttle current and add margin. If a motor can pull 40 A with your prop, use an ESC that can handle more than that.

Selecting Motor To Frame Size

Start from prop diameter. The frame decides the largest prop you can safely use. The prop size then decides the motor torque requirement.

flowchart TD
    A[Choose frame size] --> B[Find max prop diameter]
    B --> C[Choose target battery voltage]
    C --> D[Select motor stator size]
    D --> E[Select KV for voltage and prop]
    E --> F[Check current with prop data]
    F --> G[Select ESC and battery]

General rules:

• Small props need high RPM, so they usually use higher KV.
• Large props need more torque, so they usually use larger stators and lower KV.
• Higher battery voltage usually needs lower KV.
• More prop pitch or more blades needs more torque and current.
• Heavy frames need more thrust margin than light frames.

Frame Size Guide

Use these as starting points, then check real thrust data for the motor and prop you plan to buy.

Frame / prop size	Typical motor size	Typical KV on 4S	Typical KV on 6S	Common prop
2 inch	1103, 1104	6500-9000KV	not common	2x2.5x3
2.5 inch	1204, 1303	4500-6500KV	not common	2.5x2.5x3
3 inch	1404, 1505	3000-4500KV	1800-2500KV	3x3x3
3.5 inch	1604, 1804	2500-3600KV	1500-2200KV	3.5x2.8x3
4 inch	1804, 2004	2200-3000KV	1400-2000KV	4x3x3
5 inch	2207, 2306	2300-2700KV	1600-1900KV	5x4.3x3
6 inch	2507, 2408	1700-2200KV	1300-1600KV	6x4x3
7 inch	2806.5, 2807	1300-1700KV	1100-1400KV	7x3.5x2
10 inch	3110, 3115	not common	700-1000KV	10x4.5x2

Thrust To Weight Ratio

For FPV, motor choice should give enough total thrust for the aircraft weight.

\[ Thrust\ Ratio = \frac{Total\ Max\ Thrust}{All\ Up\ Weight} \]

Where:

• Total Max Thrust is thrust per motor multiplied by motor count.
• All Up Weight is the complete drone with battery.

Typical targets:

Flying style	Good thrust ratio
Smooth cruising	2:1 to 3:1
Long range	2:1 to 4:1
Cinewhoop	2:1 to 4:1
Freestyle	4:1 to 8:1
Racing	6:1 to 10:1

Example:

• Drone weight with battery: 700 g
• Motor thrust: 1500 g
• Motor count: 4
• Total thrust: 6000 g

\[ Thrust\ Ratio = \frac{6000}{700} = 8.57 \]

That is a strong freestyle or racing thrust ratio.

Motor Selection Checklist

Before buying motors, check:

• Frame supports the prop size.
• Motor mounting pattern matches the frame.
• Shaft size matches the prop hub.
• KV matches battery voltage.
• Stator size can handle the prop diameter and pitch.
• ESC current rating is higher than expected full-throttle current.
• Battery can supply the expected current.
• Motor weight fits the build goal.
• Thrust data exists for a similar prop and voltage.

Tip

If the motor is hot, reduce prop pitch, reduce blade count, use a smaller prop, or choose a lower KV motor.