Motor Talk and RC Racing 101

Hey guys I’m gonna to try and give you a very good explanation that hopefully will help a lot of people in regards to how Motors work. I do need everyone to read this because I can’t do it in just a few words.

First we need to understand what power is in relation to an electric motor. Power is measured in watts, Watts is equal to voltage of the battery times amps which is current. (W=V*I) V= Volts, I= current In amps (A) Now due to most rules, voltage is going to be somewhat fixed to a battery of 8.44 volts max.

Current on the other hand is the variable that we play with a lower turn motor that has less electrical resistance than a higher turn motor therefore a lower turn motor is physically capable of flowing more current through it. So for a fixed voltage a lower turn motor will naturally produce more power because it can flow more current than a higher turn motor. IE: If Volts are fixed then (power) W = 8.44 (V) x I (A) As Current (I) goes up power will go up, there is no debate there.

Here’s where things get tricky, The higher the current flow the greater amount of energy passes through the motor and because the motor is not 100% efficient, there will be a negative result in the form of excess heat. The longer the motor runs at a state of high current, the more heat will be generated. This is where gearing takes effect and why gearing is the most important thing you can do for your electric motor of any turn.

Now we need to understand that an electric motor will want to get to its natural state as quickly as possible. Its natural state is it’s KV wherever it’s set up. The Motor will try to achieve it's KV by pulling as much current as it can (limited by the speed controller) and when it gets closer to its natural speed it will pull less current. This is why full throttle bench motor tests pull between 2-12 Amps, but in the car on track you will see much more, especially in the early part of acceleration.

So for instance a 7700 KV motor will try to reach that state as quickly as possible. The gearing and weight of the vehicle slow this process down. So here’s the trick, the trick is to find a gear ratio that will allow the motor to accelerate to its natural state in the least amount of time. While achieving your desired trap speed in the desired ET all while trying to be controllable. If you are drag racing (hot topic these days) There is a limit though, geared too low and the motor will just rev out at max speed the entire duration of the pull and create heat and loose torque.

I know it's tempting but sometimes the lowest turn, highest KV motor is not gonna be desirable. If you’re set up, driving style, or whatever it is cannot manage or control the vehicle, then you will ultimately have a better ET with a motor set up that you can control. Everything comes down to gearing!

At the end of the day there are limitations, going too far in any direction will not be ideal. A happy balance is needed. Just as a very low turn motor might not always be the correct way to go, there is going to be a point where a very high turn motor will just not be able to accelerate regardless of whatever gearing you put on it.

Again balance is needed, in order to achieve the speed you’re looking for whether that is drag racing, on road or off road RC racing motor and gear combinations are key to having a successful outing.

So again the trick is to find a balance between a very powerful motor which for fixed battery voltage is always going to be in the direction of lower electrical resistance which equals lower turn rating. But that doesn’t necessarily mean that you and your car will be quicker with that set up. Going for the biggest fastest motor is not always the way to win!

I think understanding electrical motors and how they work will be very beneficial to everyone who’s trying to race an electric RC car. I hope this was informative to you guys. So remember at the end of the day it’s always best to just work up on gearing with whatever motor you have defined the balance between speed, ET, laptime and heat.

Key points, the higher the gearing the more time and current the motor is going to need to accelerate to its natural state. And because it wants to achieve its natural state it will pull more current and result in more torque! If you ever have a chance to Dyno an electric motor you will see torque fall off with current drop and this happens as it approaches its max speed. Which I think is opposite of what a lot of people believe, but a higher gearing will look to produce more torque out of the motor But it does come at the expense and penalty of heat and acceleration.

 

Article by Gary Karamikian CEO, GK Systems Inc