RC Project How to Choose Motors and ESCs (Speed Controls)

Introduction

Go to Part 1 Project XFire Introduction

Electronics are the most expensive part by far for something like my RC project XFire. Like most, I am on a tight budget. I only want to build something if it is fun (fast and tough). But it also needs to be cheap. With all electronics the cost is usually related to the power that the electronics need to handle. P=IV so we need to consider both the current and the voltage. Like any product, availability is also a huge factor in cost. If China is making a million size x motors and less of size y then size y motors will probably not be the best value. Sometimes the availability will determine the cost more than the actual cost to manufacture the component.

Motors and kV (RPM/V)

In order to use a variable speed electric motor you need something that can moderate the voltage going to the motor. The current draw will be based on the wire setup, the motor design and the resisting load on the motor output. For a given voltage you need to check that the motor can handle the power/current. There are a few basic types of electric motors; outrunners, inrunners, stepper and servo. Those can also all be brushless or brushed. Stepper and servo motors aren’t really used to drive high speed components but are more used for controlling position generally. Brushed motors are generally cheaper to make because they don’t have the more expensive permanent magnets that brushless motors have. Brushed motors also don’t need a complicated voltage frequency to run them like brushless motors do. Brushless motors are typically a lot more efficient and powerful for their size than brushed motors. Inrunners run better at higher RPM (2000-10000 RPM/V roughly) and outrunners run better at lower RPM (1000-2000 RPM/V roughly). Inrunners tend to be bigger for lower RPMs than outrunners. You can guess that the first thing you need to decide then is what speed (RPM) you want your motors to run at. You can get the RPM from the theoretical max speed you want your RC vehicle (theoretical max thrust if it flies). Once you have the output RPM then we can start the tricky part of choosing gearing and voltage options. Some important factors that affect what options are available are cost and size/weight. For my project I want it to go roughly 50 km/hr as a theoretical top speed. From experience I have found that 30+ km/hr is needed for it to be fun but much more than 50 km/hr generally just means things get broken more and are harder to control. Fun for me with RC cars is more about acceleration than top speed. Keeping a lowish top speed will make sure I am right in the sweet spot for acceleration to the speed I think is the funnest.

rad/s=RPM*2*pi/60
Speed=rad/s*radius

Large Wheels
I have these tires already with an OD (outer diameter) of 5.85in. I know from my research that it is hard to find large 1S motors and harder to find small high voltage RC motors than small low voltage motors. To calculate the speed we need the motor kV (RPM/V), battery V, and motor-tire drive ratio. To get about 50 km/hr these are the relevant voltage, gear ratio and RPM/V values needed. You need to know that #cells*3.7 V nominal/cell=battery voltage/potential.

-1s direct drive -> 500 RPM/V
1000RPM/kV-> gear ratio = 2
10000RPM/kV-> gear ratio = 20
-2s direct drive-> 250 RPM/V
1000RPM/kV-> gear ratio = 4
10000RPM/kV-> gear ratio = 40
-3s direct drive -> 167 RPM/V
1000RPM/kV-> gear ratio = 6
10000RPM/kV-> gear ratio = 60

Looking at that I see that if I am going to try small motors (high kV motors because of availability) then I will need 1S motors and ESCs (speed control or motor driver) and a gear ratio of 20-60. I can also see that regardless of size I am probably not going to be able to do direct drive like I want to. That means I will need gearing of some kind to gear down from the high RPM motor to the RPM the large wheels need to spin at. A practical gear ratio in my mind will be about 1-10. This is because I don’t want complicated and/or heavy gear boxes for every wheel. I am trying to reduce the number of parts and keep the sizes small to increase the clearance between my body shell and the ground. The larger a gear ratio required the more space the gears will need. This is because gear ratios are based on the difference between two sizes of gears (generally) and it is only practical to make gears so small. So to get a larger gear ratio (step down or step up) you need larger gears. We will look in my next post about gearing how I estimated the 1-10 practical gear ratio range for my project. For large wheels I will primarily look for 2S electronics in order to keep my gear ratio as small as possible.

Small Wheels
Thinking about a small rc car with huge wheels is pointless so lets do the numbers again with small wheels about 2in in diameter. This is actually pretty big for a small rc car but I want to make sure mine has enough clearance without suspension and can flip over on either side and keep going like my Tyco rc car. I won’t even include 3S as I haven’t seen any small 3S motors. I also won’t include 1000 kV as I haven’t been able to find any low kV small motors.

-1S direct drive -> 1500 RPM/V
10000RPM/kV-> gear ratio = 7
-2S direct drive-> 750 RPM/V
10000RPM/kV-> gear ratio = 14

This tells me that if I am doing a small rc car then I need to stick to 1S electronics in order to keep my gear ratio in a somewhat practical range.

*Update – I got my 1000 kV motors in and I really want to try them with direct drive so I am going to to build a smaller maybe 1/10 or 1/16 scale on-road/rally type of car planning on drifting the wheels. With the wheels spinning then 2S, direct drive (final drive ratio of 1), 2in wheels gives me a wheel speed of 70 km/hr. This will probably still feel like a 50 km/hr max car speed acceleration since the wheels will be spinning most of the time. I might even get away with making this work without 3D printing since my Tiko 3D printer is still probably a couple months away from shipping. In my first post I mentioned that my funnest car was the Tyco RC car but my second funnest was the Traxxas Rally car. Drifting a car rarely breaks it compared with big off road tires, lots of grip and lots of flipping. I think I will try these tires here for the direct drive version. They are 2.5 inches in diameter but look cool and are probably close enough to what I want.

ESCs (Speed Controls) and Motor Drivers

Brushed motors are pretty straight forwards to control using either ESCs or motor driver shields. Brushless motors require an alternating current and are more complicated to use with motor drivers. Now that we have calculated our kV, V and gear ratio combinations we are interested in, we know what voltage we need for an ESC roughly. For a small rc car we need 1S/3.7V.

Adafruit motor driver shield can do 4 dc motors at 1.5A any more than that and I will probably need separate escs.

Small 1S motors are typically used in quadcopters and can be had at a reasonable price for 4. Small 1S brushless ESCs however are too expensive for a small rc car at $10 each when I need 4. I could get larger brushed ESCs but they can only handle 2S-3S. I basically can’t find a 1S brushed ESC. I think that is because they are usually integral to the control chip at that size. That works well if you know how to design and manufacture your own chip… but I don’t. This basically tells me that my cheap small rc car is out of my skill set right now. The motor driver shields might be an ok option but the low power ratings worry me and I would also have to get a full size Arduino instead of an Arduino nano or similar in order to use the driver shields.

Looks like in this case I might actually need to make a larger rc car just to make the components a reasonable size. So with the larger tire size I found these motors, and these ESCs which look like they should work. That gives me a gear ratio needed of 4. Choosing motors and ESCs isn’t always this obvious so I will go over a few other factors to consider below.

Duty Cycle

It is important to point out that the duty cycle for quadcopters is significantly different than for RC cars or trucks.  Duty cycle is perhaps the most significant factor in correctly designing and electronics system. A duty cycle means how often in a given time period will an electronics item be used to x capacity. For industrial equipment this is specified. A welder I bought is only good for 80% duty cycle. That means it can only be used at full voltage 80% of an hour otherwise the electronics will not last properly. This is why for quadcopters there are tiny ESCs that are super cheap and say they handle 30A and why for the same motor I need more beefy car ESCs with heat sinks. Quadcopters will probably not be run at full power consistently but more importantly air doesn’t resist the blades as much as ground and momentum resists RC car wheels. Remember P=IV? In motors and the connected ESCs this means that if you have a higher resistance on the motor output the motor will draw more power from the ESC/battery at a given voltage. That is why, while I was tempted to just buy the cheaper ESCs that are normally sold with the motors I bought, I instead made sure to buy ones that would be designed more for my intended use. Without full engineering specs/charts that list torque, power, efficiency and duty cycle, motors should be chosen by scale and type of project. For a given style of motor (brushless, inrunner or outrunner) then you can estimate the appropriateness for an intended use by weight. So if you find a brushless inrunner that weighs 100g in a 1/8 car you probably don’t want to stray too far from that for another brushless inrunner for the same weight class of car. If you have the engineering specs available check out this post for a more engineering fundamentals type of motor selection process.

Max Power Available

The easiest way to choose a motor peak power rating is simply based on the battery size. If you have experience in RC then it is pretty easy to predict battery run time based on the size of car and your driving style (or just check the forums). All cars of a given scale with the same style of motor will have similar run times. For a 1/10 truck I know that for 2S and a roughly 20min run time I will need about a 3500mah battery. This isn’t super accurate but it is another check to make sure you are on the right track. The max power the battery can give is the mah*C* rating*V. So 3.500Ah*25C*7.4V=647.5W. My motors are 4*133.2W=532.8. This means that my battery (assuming an accurate C rating) should be able to give all the power my motors can draw continuously without wrecking themselves. I am running my motor ESCs in parallel so they all get the same voltage and the power consumed is the sum.

 

Custom RC Car Parts List So Far

Here is a more detailed list of the electronics I have ordered for my custom RC car project so far. This is absolutely as cheap as I could go for a 1/10 project like mine. All prices include shipping to Canada and are in CAD.

1 meter each of red and black 14 AWG wire for connecting the ESCs, motors and battery
5.3

5 pair XT60 connectors
4.08

1 3500mah lipo 2S 25C battery
25.75

4 brushless 25A 2-3S RC car ESCs 
43.74

1 arduino nano compatible controller with mini-usb cable
3.21

4 1000kV A2212 brushless outrunner motors
24.43

1 FS GT2 FLYSKY Transmitter and Receiver
36.7

Total = 143.21

Things still to order

1 lipo battery charger 
50

4 12 tooth 48pitch pinion gears if my nylon 3D printed ones fail/aren’t durable enough
23.2

4-8 bearings (1-2 for each axle). I am hoping my planetary gear system on each drive line will double as 1 bearing so I only need 1 by the wheel.

Things I have that you would need for this project

soldering gun
soldering wire
grounding mat
maybe desktop power supply for testing arduino configurations
3D printer with nylon and PLA or pet filaments

My next post will be on modeling the gears and designing the car structures. I will describe how I chose the gear style, diametrical pitches and tooth #’s for each gear.

*Disclaimer, If you find an error just let me know and I will correct it 🙂 Any questions just let me know!

Steve

 

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