Drone-Motor Selection

Motors are crucial for aerial photography!!! So be extra careful when choosing motors for your drone. Always buy motors from reputable brands to ensure quality. For the safety of your drone, splurge a little! Reputable brands provide complete efficiency charts and full-throttle temperature ratings. Ignore any motors lacking efficiency charts or those that won't disclose full-throttle temperatures.

1. Essentially, motors within the KV1000 range exhibit nearly identical thrust efficiency when driving the same propeller, provided magnetic saturation does not occur (the only difference being minor variations due to internal resistance heating).

Explanation: Motors with different KV ratings driving the same propeller at identical RPM exhibit identical magnetic hysteresis and eddy current losses. The only difference lies in internal resistance heating power consumption. Since the effective power (propeller drive) remains constant, the total input power is essentially the same. Consequently, thrust efficiency is identical.

2. So how do you select the motor? See below:

First, determine the maximum pulling force required for each motor;

Next, search on Taobao to find which motor operates near this maximum force without magnetic saturation.

At this maximum force, a lower magnetic field strength B is preferable, minimizing magnetic saturation and eddy current losses.

Finally, check if the price is suitable; if not, select another motor.

Explanation 1: I haven't addressed the motor's KV value here because it has negligible impact on force efficiency (its only effect is copper loss, i.e., internal resistance heating—motors with lower KV generate less heat under identical wire gauge conditions).

However, motors with excessively high KV should still be avoided, as high KV requires significantly higher current, making internal resistance heating a major factor.

Explanation 2: What kind of motor can generate high torque even with a weak coil magnetic field?

Motors with a small air gap between the rotor and stator, a large product of stator diameter and height, and high magnetic permeability in the iron core. The most intuitive factor here is the product of stator diameter and height—the larger this value, the better, but keep in mind the motor's weight.

The above selection method has one issue: how do we determine if the motor will saturate magnetically under maximum load? This is indeed difficult to ascertain.

An estimation method exists:

First, find two motors from the same manufacturer and series but different models, such as Langyu's V4014 and V3508. Based on my previous explanation, the V3508 is clearly more prone to magnetic saturation when achieving the same torque (it has a smaller diameter and height, the same electromagnetic force multiplied by a larger radius yields greater torque). Therefore, when the V3508 reaches magnetic saturation, the V4014 will certainly not have saturated yet. As mentioned earlier, when magnetic saturation hasn't occurred, the force efficiency is essentially the same. If you look up the force efficiency of these two motors, you'll find that up to a certain pull force, their efficiency is nearly identical. Beyond that pull force, you'll notice the V3508's efficiency drops significantly below the V4014's. That pull force is the point of magnetic saturation. As long as this maximum pull force meets your requirements, it's sufficient. If it doesn't, you'll need to choose a motor with a larger product of stator diameter and height.

To facilitate data comparison, I've listed several motor models I've used. You can search for them on Amazon or our website:https://www.starterstock.com/company/65_Drone/.

V4014 motor, V4010 motor, V3508 motor

Specifically, the V3508 motor exhibits significantly lower torque efficiency than the V4014 beyond 1250g of load. This indicates that the V3508 experiences noticeable magnetic losses and eddy current losses after 1250g, with magnetic losses constituting the majority.

Some retailers include test data on their websites; you can compare specifications. Remember to use identical propellers for comparison—voltage can vary since torque efficiency is independent of voltage.

Explanation 3: Why do helicopter engines require reduction gearing to drive large rotors, while motors can drive large rotors without reduction?

Because without reduction, an engine cannot drive a large rotor. This forces the engine to operate at very low RPM, resulting in low power output—which further hinders large rotor operation. Only reduction gearing allows the engine to spin faster while increasing torque to drive the rotor.

However, electric motors operate differently. While large rotors reduce motor speed, this decrease lowers counter-electromotive force, increasing the effective voltage across the windings and thereby boosting current. This allows the motor to still deliver substantial power output—provided that hysteresis losses and eddy current losses do not increase significantly. Once these losses become pronounced, the motor's output power will notably decrease.

When the motor is not significantly magnetically saturated, larger propellers are generally better (though not infinitely larger, relatively speaking), as larger propellers yield higher power efficiency.