Electro-Magnetism for AC/DC engines

Tl;DR

Post electro-magnetism 101 and electronics

Intro

Ive been doing additional electronics recaps before going all in with more electronics ideas

git clone /electronics-101/samples-motors

Circuits Recap

Electric Engines

Wondering about buying a car?

DC

AC

Modelling Electrical Engines

Is the classical R-L model enough?

enough for what, right?

These are not powered by slider cranks and powerful combustions

But arent they simple, beautiful, elegant and massively efficient?

AC Engines

DC Engines

These are the kind of motors propelling your DJi Tello Dron.

Yea, the same for which I was recently making a nicer desktop app with computer vision plugged in.

#git clone /dron
cd ./dron
uv sync
uv run main.py

Yep, private video :)

The L-R

Conclusions

Why all of this?

Well, you can use this knowledge for fpv/drons:

See also this one.

The drone brushless DC motors will have present Faraday law with their Back EMF, same principle of the EMF kickback of the watering project.

$$\nabla \times \mathbf{E} = -\frac{\partial \mathbf{B}}{\partial t}$$

Remember: A changing magnetic field creates an electric field!

Or to understand electric cars before buying one

Total Energy Spent: Approximately 211 kWh. Efficiency: $211\text{ kWh} / 16.3\text{ (units of 100km)} = \mathbf{12.9\text{ kWh/100km}}$.

Note: You actually drove more efficiently than your initial 15 kWh estimate!

Unit Cost Comparison ($ USD per 1 kWh)

FAQ

AC vs DC Power Transmission

As experimented and summarized here:

Converting Electrical Energy

1. Rectifier (AC $\rightarrow$ DC)

A Rectifier converts Alternating Current (AC) into Direct Current (DC).

• How it works: It uses diodes (which act like one-way valves) to block or redirect the “backwards” part of the AC wave so the electricity only flows in one direction.
• Common Example: Your phone charger. It takes the AC from your wall and rectifies it into the DC your battery needs.

2. Inverter (DC $\rightarrow$ AC)

An Inverter converts Direct Current (DC) into Alternating Current (AC).

• How it works: It uses high-speed switches (transistors) to “chop up” the flat DC signal and flip its polarity back and forth to mimic the wave shape of AC.
• Common Example: Solar panels. They produce DC, but your home appliances need AC, so a “Solar Inverter” sits in the middle.

Home pv setups tend to have one of these!

3. Transformer (AC $\rightarrow$ AC)

You are exactly right—a Transformer stays within the same “lane” (AC to AC).

• How it works: It uses magnetic induction to change the voltage and current levels, but it cannot change the nature of the current. It cannot work with DC because it requires a changing magnetic field (which only AC provides).
• Common Example: Those big grey cans on utility poles. They take high-voltage AC from the power lines and “step it down” to the 110V/230V AC used in your house.

Pro Tip: If you want to go from DC to DC (like changing the voltage of a battery), you use something called a DC-DC Converter (often a “Buck” or “Boost” converter).

What is KV in motors

You are completely right to catch that! I snuck it into the component chart a few messages back when we built the initial blueprint for the indoor Whoop.

I wrote down: “0802SE 19500KV (Brushless)” for the motors.

Since KV is a highly specific and slightly confusing number in the drone world, let’s look at exactly what it means and why that specific number matters for your build:

What does “KV” actually mean?

In FPV drones, KV does not stand for kilovolts, and it has nothing to do with television.

It stands for the Velocity Constant of the motor.

Let’s look at the math for the 19500KV motors on a fully charged 1S drone battery ($4.2\text{V}$):

$$19,500 \text{ KV} \times 4.2 \text{ V} = 81,900 \text{ RPM}$$

Those tiny brushless motors are spinning the propellers at over 81,000 times per minute when you punch the throttle!

Why such a high KV for a small indoor drone?

• Small Propellers = Less Leverage: Indoor Whoop propellers are tiny (only 31mm wide). Because the blades are so small, they don’t grab a lot of air. To create enough thrust to lift the drone, they have to spin at astronomical speeds. High KV motors give you that raw speed.

• Low Voltage (1S): Since you are only using a single-cell battery ($3.7\text{V}$ to $4.2\text{V}$), the voltage is quite low. To get high RPMs out of low voltage, the motor needs a high KV rating.

• The Contrast: If you were building a massive 5-inch outdoor camera drone that runs on a heavy 6S battery ($25.2\text{V}$), you would use low KV motors (like 1700KV to 1900KV) because the larger propellers grab massive amounts of air and would burn out the motor if spun too fast.

When you go to buy your motors, sticking around 19500KV to 22000KV in the 0802 size is the absolute sweet spot for a snappy, controllable indoor flyer!

KV x Dron size

Choosing the right motor KV isn’t just about the drone’s size—it is a balancing act between Propeller Size and Battery Voltage (Cell Count / “S”).

The fundamental rule of thumb is: Big propellers and higher voltage need lower KV (more torque). Small propellers and lower voltage need higher KV (more raw RPM).

If you pair a big propeller with a high KV motor, the motor will try to spin way too fast, overheat, draw massive current, and likely burn out your ESC or the motor itself.

The drone’s “size” is typically measured by its propeller diameter (in inches).

💡 Understanding the Stator Numbers

When you see a motor size like 2207, the numbers stand for the internal dimensions in millimeters:

• 22 = Stator Width (Wider = more torque and efficiency at low RPM)
• 07 = Stator Height (Taller = more raw power and bite at high RPM)

DC vs BLDC vs AC Engines

Using a ClampMeter