This is part of a back-to-the-basics series that outlines the general process of building a mini-quad from the ground up. To view all the articles in the process, please check out our assembling a miniquad article. The first step to any miniquad build is getting your arms built up and ready to go. This article outlines the process of connecting the motors and ESCs to the PDB and adhering them all to your chosen miniquad frame.

Set-up

You will need the following components:

  1. 01-arms_setup

    You should have all these parts out and ready to go for this portion of the build.

    Frame

  2. 4x Motors
  3. 4x ESCs
  4. Power Distribution Board
  5. Motor and arm hardware (screws and washers)
  6. Heatshrink

In addition, you will need the following tools:

  1. Soldering iron (with tip cleaner and solder)
  2. Multimeter
  3. Heat gun or lighter (for heat shrink)
  4. Hex drivers or screwdriver (depends on what hardware you are using)
  5. Wire trimmers or scissors
  6. Stubby pliers (for holding hot wire)
  7. Threadlocking compound

Prepare ESCs

The first thing I like to do is prepare the electronic speed controllers. What you do here depends a lot on what ESC you bought. If you took our suggestion and bought a BL Heli ESC that supports passthrough, like the Littlebees or the ZTW pro (or a lot of others..), skip to the next paragraph. If you bought some Afro ESCs or discount ESCs from a Chinese retailer, you may need to do some extra work. Specifically, you’ll want to figure out how to connect the ESCs to the motors in such a way so that 2 of your motors spin clockwise, and 2 of them spin counter-clockwise. One way you can affect motor rotation direction is by swapping any two wires connecting the ESCs to the motors. So, what you should do is lay out your motors and ESCs and ensure that two of the motors are connected to the ESCs in one wire configuration, and two of the motors are connected to the ESCs with two of the wires swapped. Again, this is not necessary if you are using a BL Heli ESC that supports passthrough, like the Littlebees, KISS, ZTWs, etc.

Most ESCs come from the factory with wires and heat shrink installed on them. However, the wire is often not the right size or length for my application. For this reason, I like to remove all of the wiring from the ESC as a first step, then cut everything down to the desired size later.

Read more: choosing wires for your miniquad
Wire gauge sizes.

Wire gauge sizes.

Wires are purchased by using their “gauge” as a measurement unit. This is a funky unit because a lower number means a bigger wire. Smaller wires have more electrical resistance, and thus limit your power output as your current draw increases. However, much like trying to divert water from a large PVC pipe into a smaller one, the smallest wire in an overall circuit will always be the electrical current bottleneck in that circuit. Since we build our quads to be as light as possible then, it makes sense to determine what the smallest wire in your system is, and build around that. For the ESC power connection wires, you should choose wires that are the same size as the wires coming out of your motor – since you cannot change the motor wires, you may as well match them. You can often find the size of these wires simply by reading text printed on them. Look for something like “20ga” – which means “20 gauge”. If you don’t see anything, you may need to do some research online.

Similarly, when choosing battery connector wires, you should match the wire size to that of the batteries you are using. You wont be able to draw any more current from the batteries than the wires that are attached to them allow, so why add the extra weight?

When purchasing wire for miniquads, always get the wiring with silicone coating. This wire is extremely flexible and easy to solder and will serve you well. Here is an example of 20 gauge wire I recently bought for a miniquad build.

 

The steps of preparing the ESC are:

  1. Remove the plastic wrap insuation on the outside of the ESC. I do this using a hobby knife with gentle pressure to slowly cut through the plastic. A sharp knife is essential for this. I really like the disposable snap-off knives as they are super cheap and last for a long time.
  2. Remove the 3 motor wires and the power and ground (red and black) wires from the top of the ESC by pressing a hot soldering iron onto the silver solder to melt it while tugging the wire with some pliers. Never leave the soldering iron pressed into the solder for more than 5 seconds after it has fully melted.
  3. Flip the ESC over and remove the signal and ground pins (black and white) in the same way. Save this wire.
05_escprep

The steps of preparing a Littlebee ESC for installation.

Mount the Motors

After the ESCs are prepared, I like to screw the motors on the arms. This will keep them secure while you measure out wire lengths and will give you a nice place to rest the ESCs while you are soldering to them.

The motors should be mounted to the arms using the screws provided with the motor if possible. You can tell if they will work by sticking the screws through the mounting holes in the arms and seeing how much extra length they have on the screw end that protrudes from the arm. I would feel nervous using a screw that was protruding less than 1/8″ or 3mm from the carbon fiber. If your screws aren’t long enough, you’ll probably need to go buy some. Most hobby shops carry them, or you can get a whole set on Amazon here that should cover most of your needs.

Once you’ve selected your screws, mount the motor on the arm, line up the screw holes on the bottom of the motor with the holes in the arm, and screw it down. You will need at least 2 screws for each motor, but if you are not looking to build a top of the line racer where you are trying to save every gram, I recommend putting a screw in all four holes. I can’t tell you how many times I’ve landed and found a missing screw in one of my motors. It pays to be safe here. Make sure to use threadlocking compound on these screws.

Read more: Threadlocker
02-loctite

Putting some threadlocking compound on a plastic baggie for use during an entire build is a handy trick that makes it much easier to use.

Threadlocking compound is a special type of adhesive that cures when squeezed between metal joints (e.g. in the threads of screws or bolts). Once a threadlocker has cured, it helps to ensure that the screws or bolts, which are normally held in place simply with friction, do not come free.

There are several different types of threadlocker, rated by the amount of adhesion the glue has once cured. Blue threadlock is the most common you will use on quadcopters, and provides a great adhesion while remaining pretty easy to remove. Red threadlock is on the other side of the spektrum: in some situations it will be impossible to remove a screw that was locked in with red threadlock without stripping the screw head.

A little bit of threadlocking compound goes a long way. I have been using this bottle of thread-locker for several years now, and it still has plenty left.

Threadlocking compound provide value on your quadcopter on any metal-metal screw that does not have a nylon locknut. That being said, you can get away with not using it on most screws. The screws that bolt the motors to the arms, however, are the exception. The motors and props exhibit vibrations at many frequencies that can rapidly unscrew a non-locked screw. Once a motor comes off of the arm, you will be looking at serious damage. Always use threadlocking compound on your motor screws.

 

When I’m using threadlocking compound on a miniquad build, I will generally squirt some of it on a spare plastic baggie, then dip the screws into it before I drive them home. Threadlocking compound lasts for a very long time when not in screws so this works fine. When dipping the screws in the compound, a little goes a long way. Often just dipping the very tip of the screw into the compound is sufficient.

Check the little circle on the bottom of the motor lines up perfectly in the center of the arm. Also verify that your screws are not contacting the wiring inside of the motor.

Check the little circle on the bottom of the motor lines up perfectly in the center of the arm. Also verify that your screws are not contacting the wiring inside of the motor.

Once the motor have been screwed onto the arm, you’ll want to check that the bottom of the motor is centered inside of it’s hole in the miniquad arm. Neglecting this can cause the c-clip that holds the motor together to rub against the carbon fiber and possibly fail. Also, take out a flashlight and double-check that the screws are not contacting the wiring inside of the motor. This can cause electrical shorts and fry your motor in short order.

Repeat this process for all 4 (or 6!) motors and arms. Once they are all complete, you’ll want to go ahead and attach your arms to your lower frame before you continue on to the next step. You should consult your frame’s manual or build logs for more information on how to do this.

ESC re-assembly

miniquad layout

Now that your motors are bolted onto your arms, it’s time to lay out all of your components so that you can measure out the wire lengths you will need. Begin by placing the ESC on the arm where you want it to stay. You may want to tape it down with some electrical tape to hold it in place. Next, lay the wires you will be using to connect it to the rest of the quadcopter, similar to how I am doing it in the picture to the right.

What do all these wires do?

A common question from beginners to the hobby is what all of the wires we are hooking up do. It is very important to understand this as it will help you figure out where you are having problems later on in your career. Here is an explanation for each of the labeled wires in the image to the right:

Motor Leads – These 3 wires are controlled by the ESC. Current is sent through 2 of these 3 wires to cause the motor to rotate some fraction of a full rotation. It is then changed so that it flows into 2 different wires, causing more rotation. This operation is completely managed by the ESC. Like the AC circuitry in your home, it does not matter how you plug these wires into the ESC, just that each wire goes to a single pad on the ESC. In fact, one of the easiest ways to reverse the direction of rotation of your motor is to swap two of the motor leads on your ESC. If you’re the DIY type who wants to know all about how these wires work, search Google for “How do brushless DC motors work” or check out this RCGroups thread.

ESC Power – These 2 wires provide electrical power to your ESC and should be connected to the Vcc and Ground of your battery. On a quadcopter build, they will generally be connected to your power distribution board, which itself is directly connected to yoru battery terminals.

ESC Signal – These 2 wires provide the “control” circuitry that allow your flight controller to tell the ESC how fast to spin the motor. The white wire contains the control signal and will generally be connected to what is described as the “motor output” pins of the flight controller, which will be numbered 1-4 or 1-6. The black wire simply contains a ground reference (and should be connected to any “GND” pin on your flight controller). Some ESCs have a red wire in this bundle as well, which will carry a regulated 5V. For the purposes of building a quadcopter, you should cut the red wire at the connector end and remove it from the ESC end, as it will not be used.

 

09-solderedThe first thing you will want to do is measure out the length of your ESC signal wires. You want to make sure you have enough length on these wires to reach your flight controller, and then about 1-2 inches (2-4cm) of extra length. You may want to place your flight controller on the frame for this, or eyeball it if you feel confident. Once these wires have been clipped, re-solder the ends to your ESC.

Now, clip your motor leads so they just reach the pads of your ESC. This will leave them very short and stubby, but this is OK – even preferable from an efficiency standpoint. Once clipped, strip off the wire insulation and solder the motor wires directly down on the ESC pads.

Finally, solder on your ESC power wires. You don’t necessarily have to size these exactly correct just yet, since you can remove extra wire length when soldering them to the PDB on the other side.

 

Look at all those extra wires.

Look at all those extra wires.

Pictured is all four ESCs soldered up on the frame. Notice how much wire was removed in the process. This is a ton of weight savings when it comes to miniquads.

Now that you’ve soldered all your wires back onto your ESC, it is time to cover it back up. I use 1″ heat shrink for this purpose on my Littlebee ESCs. Cut the heat shrink a little bit oversize since it (surprise) shrinks.

More info on heatshrink

The primary purpose of heatshrink is to insulate uncovered wires from each other and the carbon fiber quadcopter frame. Larger heatshrink can be wrapped around entire circuit boards, like ESCs to insulate them, just like wires.

The secondary purpose is simply that it looks good. A quad that has all of it’s wires properly covered with heat shrink will win in the looks department over one that was built using electrical tape any day of the week. Plus, heat shrink comes in many different colors which can be used to personalize your miniquad.

Heatshrink is used just like it’s name implies. You cut off an oversized section of heatshrink and slip it over the bare wires or PCB, then apply heat to shrink it down over the component to be protected. The heat can come from any source, but generally needs to be hotter than 250 degrees Fahrenheit (120 Celsius). Specialized hair dryers called “heat guns” are often used in the professional industry for this. I actually prefer to use a cigarette lighter though most of the time: it is much faster. Just make sure you don’t use it around anything that is explosively flammable.

For the purposes of building a miniquad, you will need an assortment of different sizes of heat shrink. I would recommend getting a heat shrink kit with many different sizes, such as this one. You will also want some heat shrink to re-wrap your ESCs once you have worked on them. I bought this 1in heat shrink for that purpose: it slips over most miniquad-sized ESCs such as the Littlebees or KISS.

 

Hooking up to the Power Distribution Board

Fully assembled PDB, ESCs and Motors

Fully assembled PDB, ESCs and Motors.

The final step is to install the Power Distribution Board (PDB) on the frame and start connecting the ESC power wires and battery leads to it. First, connect the battery wires to the PDB, then solder on a XT-60 connector (or whatever other connector you are using) to the wire ends. Next, clip the ESC power wires and solder the red leads to the positive voltage pads on the PDB and the black leads to the negative pads. If you don’t know which pads are which, consult the documentation for your PDB or use a multimeter to determine which pads are shorted to which.

As a last “sanity check”, I like to see if there are any shorts in the system I just put together. I do this by setting the multimeter in resistance mode (1000-10000Ohm works fine), and touching the multimeter probes to the positive and negative battery terminals (without the battery plugged in). If a resistance of 0 is read, you probably have a short somewhere in your system and you should go and check all of the positive and negative wires to find where they are contacting each other.

Once you have completed all of these steps, you are ready to move on to the next procedure, which can be found in the “Assembling a Miniquad” parent page.

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